Corrosion Handbook on Stainless Steel

Sandvik Steel Corrosion Handbook Stainless Steels

Sandvik Steel Corrosion Handbook Stainless Steels

Sandvik Steel Corrosion Handbook Stainless Steels

AB SANDVIK STEEL S-811 81 Sandviken, Sweden Phone +46 26 26 30 00 Fax +46 26 25 17 10 www.steel.sandvik.com

Corrosion Tables © 1999 Page 1 – 45 AB Sandvik Steel Page 1 – 88 Avesta Sheffield AB and AB Sandvik Steel ISBN: 91-630-2124-2 Photoes front page: Pär Hedqvist Printed in Sweden Sandvikens Tryckeri

Corrosion Handbook for stainless steels PREFACE

When first introduced in 1994 this “Corrosion handbook for stainless steels” replaced an earlier edition published by Jernkontoret, Stockholm, Sweden, which was jointly produced by the Scandinavian manufacturers of stainless steel in 1979. Being a unique source of information for material specialists and designers, it has been highly appreciated. Continued materials research has resulted in new grades and improved properties of the existing grades. New corrosion tests are continuously being carried out, often reflecting the more aggressive environments to which the materials are being exposed. A combination of these factors has motivated a revision of the corrosion tables. The present revised and extended edition is the result of a cooperation between Avesta Sheffield AB and AB Sandvik Steel in Sweden. As an introduction, a series of papers are presented on the corrosion theories in connection with stainless steels. Corrosion testing, different steel types and grades, general aspects on different applications, as well as fabrication aspects are also discussed. These papers are then followed by corrosion tables and graphs describing the resistance of various materials to different environments (in alphabetical order), concentrations and temperatures. We are pleased to see that this work, which is based on more than 70 years' experience in solving corrosion problems with stainless steel, is made available to industry. It is my belief that this corrosion handbook will be a valuable tool for all material specifiers when designing the process plant and equipment of today and tomorrow.

Sandviken, March 1999

Per Ericson President, AB Sandvik Steel

SANDVIK STEEL CORROSION HANDBOOK

Contents Introduction Corrosion of metals Different corrosion types and test methods

General corrosion Galvanic or two-metal corrosion Intergranular corrosion Pitting corrosion Crevice corrosion Stress corrosion cracking High temperature corrosion

Introductrion Oxidation Catastrophic oxidation Sulphidation Carburisation and nitration Molten metal corrosion Halogen corrosion Erosion-corrosion Applications Composite tube applications Stainless steels

Introduction Austenitic stainless steels and Duplex stainless steels Manufacturing programme

Special stainless steel grades Nickel base alloys Titanium Zirconium Applications for stainless steels

Chemical industry Urea production Oil and Gas industry Corrosion in petroleum refining and petrochemical applications The pulp and paper industry Fabrication Constructional design Bending Expanding into tube sheets Surface properties Steel grades – Manufacturing programme

I:8 I:9 I:10 I:10 I:11 I:12 I:14 I:16 I:17 I:21 I:21 I:22 I:22 I:22 I:23 I:23 I:23 I:23 I:23 I:25 I:26 I:26 I:27 I:28 I:28 I:29 I:30 I:30 I:31 I:31 I:35 I:37 I:40 I:43 I:46 I:46 I:48 I:48 I:49 I:50

Corrosion tables

Isocorrosion diagrams: Acetic acid Chromic acid Citric acid Fluosilicic acid Formic acid Hydrochloric acid Hydrochloric acid with chlorine Hydrofluoric acid Lactic acid Nitric acid Oxalic acid Phosphoric acid – with chloride additions – with fluoride additions Sodium hydroxide Sulphuric acid – deareated – naturally areated – with chloride additions – with iron sulphate – with chromic acid – with copper sulphate Tartaric acid Physical tables

II:1 II:2 II:16 II:16 II:20 II:22 II:24 II:24 II:26 II:30 II:35 II:39 II:41 II:42 II:42 II:54 II:59 II:59 II:60 II:60 II:61 II:61 II:70 II:74

Density, modulus of elasticity and coefficient of linear expansion of stainless steels II:74 Thermal conductivity of stainless steels II:74 Physical properties of certain chemical elements II:75 Temperature conversion table II:76 Chemical elements II:78 Degrees Baumé II:79 Vapour pressure of water II:79 pH values: alkaline solutions II:80 acid solutions II:80 foods II:80 substances in human body II:80 hydrochloric acid, nitric acid and sulphuric acid solutions II:81 Relationship between weight-% and density, molarity, volume-%, kg/litre and degrees Baumé: acetic acid II:81 ammonium hydroxide II:82 formic acid II:82 hydrochloric acid II:83 nitric acid II:83 phosphoric acid II:84 potassium hydroxide II:84 sodium hydroxide II:85 sulphuric acid II:85 Glossary II:86 Disclaimer II:88

7

S-037-ENGHA ISBN: 91-630-2124-2

AB Sandvik Steel, SE-811 81 Sandviken, Sweden, Phone +46 26-26 30 00 www.steel.sandvik.com

SANDVIK STEEL CORROSION HANDBOOK

1. The first part of this handbook constitutes an introduction to corrosion together with a brief description of stainless steel grades and special metals. Different corrosion types are discussed comprehensively in connection with some relevant test methods, some of which have been used to gather the data in the latter part of the handbook. Fabrication of stainless steel products is described and some advice in this area is given. Finally, a number of applications are reviewed for which the corrosion problems are discussed in more detail and materials selection for especially demanding environments is suggested. The corrosion tables comprising the last half of this corrosion handbook have been produced in cooperation between AB Sandvik Steel and Avesta Sheffield AB. They are intended to constitute a guide to the corrosion resistance of the included stainless steel grades. Different levels of corrosion rates are shown together with indications of local attack, such as pitting. A large number of different environments are included and in many cases also a wide range of concentrations and temperatures. You are hereby provided with a useful key in

Introduction the choice of material for a certain application. The data can also be used when the corrosion resistance of used or recommended grades is discussed with regard to changes in concentration or temperature. To further illustrate the corrosion resistance of stainless steel grades several diagrams have been included, showing isocorrosion curves etc. The subject of stainless steel corrosion is a vast area and cannot be completely covered in this handbook. For detailed information about corrosion types in different environments several references may be given [cf ref 1-4]. Standard tests of corrosion resistance are thoroughly described in reference 5. Corrosion research is an ever ongoing process with innumerable articles being published every year. The state of the art can be found in the journals focussing on corrosion, of which reference 6 to 9 are recommended here. Lectures are also continuously being published within AB Sandvik Steel concerning the properties and experiences of Sandvik steel grades and special metals. A catalogue of titles can be requested from your nearest Sandvik Steel sales office.

1.

M.G. Fontana, Corrosion Engineering, 3rd Edition, McGraw-Hill, 1987.

2.

Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar, Eds., Marcel Dekker Inc, 1995.

3.

H.H. Uhlig and R. Winston Revie, Corrosion and Corrosion Control, 3rd Edition, John Wiley and sons, 1985.

4.

G. Wranglén, An Introduction to Corrosion and Protection of Metals, Institut för metallskydd, 1972.

5.

Corrosion tests and standards: Application and Interpretation, John Baboian, Ed., American Society for Testing and Materials, 1995

6.

Corrosion Science

7.

British Corrosion Journal

8.

Corrosion, NACE

9.

Werkstoffe und Korrosion

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SANDVIK STEEL CORROSION HANDBOOK

2.

Corrosion of metals

C o r r o s i o n i s g e n e r a l ly defined as a dissolution of a material due to a reaction with the surrounding environment. Most metals are in a thermodynamically unstable form, and corrosion often means that there exists a thermodynamic driving force for recombination of the unstable elemental form to the chemically stable oxidized form found in nature.

rosion and has become what is usually called stainless. The reason is that the chromium forms an oxide layer on the surface, and this layer sufficiently protects the metal from the surrounding environment when the chromium content is about 13% or more. Other common elements that are used to improve the ability to passivate are molybdenum and nitrogen.

Many metals can maintain their unstable form, in spite of the thermodynamic driving force, thanks to their ability to passivate. Passivation means that the surface of the metal is covered by a, usually very thin, layer of corrosion product. This so called passive layer, which in most cases consists of an oxide film, separates the metal from the surrounding environment and hence the corrosion resistance is considerably increased. The ability to passivate for pure iron is limited, which means that iron will show a relatively low corrosion resistance in most media. The ability to passivate is however increased by alloying with chromium. At a chromium content of about 13% the alloy shows a considerably better resistance to cor-

When passivity cannot be maintained, due to a too aggressive environment, the metal will be exposed to the surrounding environment and a dissolution of the metal may take place. The attack initiates at weak spots on the surface, such as scratches or contaminated areas, and is then continued as local or uniform corrosion. The rate at which the dissolution takes place is usually measured in terms of e.g. mm/year. In the case of local attack, such as pitting or stress corrosion cracking, the corrosion rate is not a relevant parameter. Instead, any signs of local attack should be seen as a warning not to use the material under those specific circumstances.

Heat-exchanger tube of Sanicro 29 heavily attacked by erosion corrosion on the inside of the bend. The cause was sand in the cooling water.

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SANDVIK STEEL CORROSION HANDBOOK

3.

Different corrosion types and test methods

W h e n c o r r o s i o n o c c u r s the attack is characterised by the way in which the metal is dissolved. Many forms of corrosion exist, and here the most common types occurring for stainless steels are described.

General corrosion General corrosion is characterised by a uniform attack over the surface of the material when exposed to a corrosive medium. It is therefore possible to define a corrosion rate (r), often stated as a mass loss per unit surface area (g/m2h), or as a mean metal loss per unit time (mm/year). The latter unit is used for tabled data as well as in the diagrams presented in this book. It is, however, sometimes desired that the corrosion rate should be expressed in mils/year (milli-inch per year or mpy). Corrosion rates in mm/year are easily translated to rates in mpy from the relationship: r (mpy) = 39.4 x r (mm/year)

Isocorrosion diagrams illustrate the resistance of metallic materials to general corrosion. Each isocorrosion line represents a fixed corrosion rate and the dependence of concentration and temperature on the corrosive medium can be shown. F -, % 1.5

The most common environments where general corrosion occurs on stainless steels are strongly acidic or alkaline solutions. The specific composition of the environment is crucial for the corrosiveness, and may be drastically changed if oxidising or reducing compounds are added. The performance of stainless steel grades can vary considerably in the same environment and to different additives. It is therefore of great importance that the environment where a product is to be used is thoroughly characterised. When this is done a suitable material can usually be selected. The economic advantages of choosing a grade with high corrosion resistance, sometimes acquired at a higher price per kilo, can be illustrated by estimations of the life cycle cost. T E S T I N G O F G E N E R A L C O R RO S I O N

Testing of general corrosion is usually performed by exposing samples to the corrosive environments over specific time intervals and calculating the corrosion rates from weight losses. To avoid irregularities from the initiation period the sample is weighed after each period and the first value is disregarded. At least two samples of each material should be used and the corrosion rate is determined as the calculated mean value for two or more periods.

2302b

H3PO4 = 70 % H2PO4 =4 % Fe3+ = 0.45 %

1.0

Sanicro 28

0.5 Alloy 20Cb3 Alloy 904

Figure 2. Sandvik 5R60 (middle) and 5R10 (right) tested in 60% H2SO4 at 100°C for 24 hours. Unexposed test coupon to the left.

Alloy 825

0 200

400

600

800 Cl- , ppm

Figure 1. Curves representing a corrosion rate of 0.3 mm/year for Sanicro 28 and three other alloys in wet-process phosphoric acid at 100°C. The combined effect of chloride and fluoride is shown.

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SANDVIK STEEL CORROSION HANDBOOK

Another way to calculate corrosion rates is to use an electrochemical technique. The corrosion current, I, is proportional to the amount of oxidised metal if no competing oxidation reaction is taking place. By this method changes in corrosion rate can be registered whilst adding corrosives or inhibitors, raising the temperature etc.

Galvanic or two-metal corrosion When two different metals are used in the same environment they often obtain different potentials. If they are in contact or otherwise electrically connected a sufficient potential difference might produce a flow of electrons between them. The more noble material becomes cathodic and the less corrosion resistant anodic. This often results in increased corrosion of the anodic material and a decreased attack of the cathodic. This phenomenon is used for cathodic protection, where a so called sacrificial anode is connected to the material to be protected. When coupling different stainless steel grades potential differences are generally too small to cause galvanic corrosion problems. The ranking of materials with regard to potential can be found in galvanic or EMF-series (electromotive force). Low EMF values are found for magnesium and zinc, among others, whereas copper, platinum and gold have high values. Standard EMF values have been calculated for standard conditions but the order between metals often differs depending on the environment. For metals such as titanium or aluminium, which form very protective oxide layers even at room temperature in air, the low standard values of potentials can seldom be reproduced. For a more realistic ranking of metals galvanic series have been determined empirically for special environments, such as sea water (see table 1). When evaluating the risk of galvanic corrosion it is of great importance that the correct potential values are used, determined for the right solution and temperature. In the same way, when sacrificial anodes are used, and thus galvanic corrosion is desired, the material which functions as an anode can become cathodic to the material to be protected in certain environments. This is the case when zinc is used for the protection of steel, if the concentrations of carbonates become too high. Galvanic corrosion of stainless steels in the passivated state is unusual. When in connection to other metals, such as copper or carbon steel, stainless steels will generally be cathodic. The relative area of the anode compared to the cathode greatly influences the corrosion rate. The larger the cathode area is compared to the anode area the faster the corrosion will be.

Table 1. Approximate EMF-values in sea water Material

Free corrosion potential / volts SCE*

Magnesium

-1.6

Zinc

-1.0

Aluminium alloys

-1.0 to -0.75

Mild steel, cast iron

-0.65

Copper

-0.34

Admiralty brass

-0.30

Ferritic stainless steel, 430 -0.24 Nickel 200

-0.2 to -0.1

304-type stainless steel**

-0.08

316-type stainless steel**

-0.10 to 0

Titanium

-0.05 to +0.05

Platinum

+0.2

*Saturated Calomel Electrode ** For passivated material values well above +0.1 can be obtained

As an example of this steel bolts in a more noble copper sheet corrode more quickly than steel sheet with copper bolts, in the same environment. When the metals are in contact with one another the rate of anodic corrosion often decreases with increasing distance from the cathode, i.e. the material loss is greatest close to the cathode. This is especially pronounced when the surrounding electrolyte has low conductivity. There also exists a related corrosion type sometimes called indirect galvanic corrosion. In this case the metals need not be in direct contact. Instead the more noble metal corrodes by uniform corrosion and its metal ions are transported, e.g. in solution, to the surface of the anodic metal. They are there reduced and at the same time enhanced oxidation of the anodic metal occurs. The corrosion of the anodic metal can accelerate even further if the noble metal atoms on its surface can act as sites for more efficient reduction of other species, e.g. from the electrolyte. C O R RO S I O N T E S T S F O R G A LVA N I C C O R RO S I O N

Tests for galvanic corrosion are rarely performed as this is more for the designer's concern. It might, however, be very useful to check the metal combinations in the environment to be used. The corrosion rates for the metals in contact can then be compared to the rates of uniform corrosion of the isolated metals in the same environment. Another possibility is to measure the potentials of each metal against a reference electrode, or the potential difference between them, in the environment in question. It is important, however, to remember that increased corrosion does not automatically follow from dissimilar potentials. Generally a potential difference of several tens of volts is required and in addition to this a decrease in galvanic corrosion rate is often observed if passivation occurs.

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SANDVIK STEEL CORROSION HANDBOOK

Intergranular corrosion Intergranular corrosion is characterised by preferential dissolution at the grain boundaries. Usually intergranular corrosion occurs in stainless steels that have been exposed prolonged time to certain temperatures (typically 600-900°C), so that a formation of chromium-rich carbides in the grain boundaries has taken place. Immediately adjacent to these carbides, in the outer parts of the grains, there will be a chromium depleted zone. These regions, which consequently have a lower corrosion resistance than the rest of the alloy, will suffer from preferential dissolution.

To avoid intergranular corrosion, resulting from chromium carbides, one can choose a grade with lower carbon content, one alloyed with stabilising elements like titanium or niobium (typically in grades like 321, 347 etc), or avoid heating in the sensitising temperature range. The latter can be difficult to achieve in practice when, for example welding is carried out. Nitrides may also precipitate in grain boundaries during heat treatment, and in molybdenum-bearing stainless steels also the intermetallic phases, such as χ and σ. In a TTT-diagram (timetemperature-transformation) the time required for the formation of precipitates during heat treatment is illustrated. T E S T I N G F O R R E S I S TA N C E TO I N T E R G R A N U L A R C O R RO S I O N

Several methods exist for the testing of intergranular corrosion. Generally an oxidising, acidic solution is used, but pH, potential and temperature depend on the method used. Because of their differences one must choose a method which is suitable for the steel grade and grain boundary composition to be tested. The applicability for some ASTM tests to austenitic stainless steels are summarised in table 2.

Figure 3. Microstructure of a material surface with intergranular attacks.

Intergranular corrosion in stainless steels may result from precipitation of carbides, nitrides or intermetallic phases. Only in the most highly oxidizing solutions can intergranular attack be caused by intermetallic phases. When a test is to be restricted to carbides, in a material containing nitrides or intermetallic phases, a less oxidizing solution should therefore be chosen. Corrosion potentials of wrought stainless steel in different test solutions and the detectable phases are summarised in table 3.

Table 2. Applicability of some ASTM standard practices in A 262 for testing of intergranular corrosion in austenitic stainless steels PRACTICE A

TEST

TEMPERATURE

TIME

APPLICABILITY

EVALUATION

Oxalic Acid Etch

ambient

1.5 min

Chromium carbide

Microscopic: classification

sensitization only

of etch structure

Chromium carbide

Weight loss/

Screening Test B

Ferric Sulphate

boiling

120 h

50% Sulphuric Acid C

D

E

65% Nitric Acid

corrosion rate Chromium carbide

Weight loss/

and sigma phase

corrosion rate

Chromium carbide

Corrosion ratio

3% Hydrofluoric

in 316, 316L, 317

compared to solution

Acid

and 317L

annealed specimen

Chromium carbide

Examination for

10% Nitric Acid

6% Copper Sulphate

boiling

70°C

boiling

240 h

4 h

24 h

16% Sulphuric Acid

fissures after bending

Metallic Copper F

Copper Sulphate 50% Sulphuric Acid Metallic Copper

I :12

boiling

120 h

Chromium carbide

Weight loss/

in cast 316 and 316L

corrosion rate

SANDVIK STEEL CORROSION HANDBOOK

HUEY TEST

STRAUSS TEST

The Huey test (ASTM A262, practice C) means that the samples are boiled for 5 periods of 48 hours each in 65% nitric acid. The corrosion rate is calculated for each period from weight losses. For further information the maximum depth of attack may be measured, but this is not included in the standard evaluation. The environment is strongly oxidising and should only be used as a check on whether the material has been correctly heat treated. The Huey test can therefore not be used to compare the corrosion resistance of different steels to other, less oxidising, environments. This test is suitable for the detection of chromium depleted regions as well as intermetallic precipitations, like sigma phase, in the material.

A common way of investigating the resistance to intergranular corrosion is to heat treat the sample in the sensitising temperature range and carry out a Strauss test (SIS 117105, DIN 50914, ASTM A262 practice E etc.). The samples are boiled in a solution of copper sulphate and sulphuric acid with copper turnings. The test time (15, 20 or 24 hours) depends on the standard used and the evaluation consists of a visual examination for cracks originating from intergranular corrosion attacks. The samples are usually bent before examination. If cracks are suspected to arise from poor ductility a similar but unexposed sample should be used for reference. Temperature, °C (°F) 800 (1470)

1302b

AISI 304

700 (1290)

Sandvik 3R12

600 (1110)

500 (930) 0.05

0.1

0.5

1.0

5.0

10

50 100 Annealing time, h

Figure 5. Time-temperature-sensitisation curve for AISI 304 and Sandvik 3R12 (AISI 304L). Results from testing in boiling Strauss solution (12% H2SO4, 6% CuSO4). There is a risk of intergranular corrosion to the right of the curves.

Figure 4. Intergranular corrosion testing by the Huey method.

Table 3. Corrosion potentials and detectable phases for wrought stainless steels in some acid solutions SOLUTION

CORROSION

AUSTENITIC STEELS

FERRITIC STEELS

POTENTIAL

Cr-carbide

Carbides and Nitrides

Sigma

(VSCE)

Fe-Cr

Fe-Cr-Mo

Intermetallics

65% HNO 3

0.75-1.0

yes

316, 316L, 317, 317L, 321

yes

yes

yes

Fe 2 (SO 4 ) 3 H 2 SO4

0.6

yes

no (321 possible exception)

yes

yes

yes (not σ or χ in unstabilised Fe-Cr-Mo alloys)

yes

no

yes

yes

no

yes

no

yes

yes

no

CuSO 4 0.35 H 2 SO 4 As above but 0.1 with metallic Cu 10% HNO 3 3% HF

-0.1-0.3

yes

no

yes

yes

no

5% H 2 SO 4

-0.6

yes

no

no

yes (not σ or χ in unstabilised grades)

no

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SANDVIK STEEL CORROSION HANDBOOK

Whenever the origin of cracks is questionable a detailed metallographic examination should be performed to determine the absence or presence of intergranular attack. This test method can detect chromium depleted regions in the material but cannot detect other possibly detrimental inhomogeneities, like precipitations of sigma phase. Figure 5 shows a so called time-temperature-sensitisation curve, where the minimum time for heat treatment before intergranular attacks appear is shown as a function of temperature. STREICHER TEST

The Streicher test (ASTM A262 practice B, ASTM G28) requires the samples to be immersed in a boiling solution of ferric sulphate and sulphuric acid for a period of up to five days. The test can detect chromium depleted regions in stainless steels but cannot be used to detect susceptibility to intergranular attacks associated with sigma phase in wrought stainless steels. The evalution of samples is done by calculating the corrosion rates and may also be completed with the measured depths of attack.

the pit. Often a lid of corrosion products is formed leaving a very small hole at the surface which prevents dilution of the pit contents. Finally, the metal surface surrounding the pit mouth becomes cathodically protected through electron migration from the pit. In many cases pitting corrosion is not detected until it has caused severe damage, such as a complete penetration in sheet or tube material. This is due to the very small pit holes formed on the surface and to the fact that the metal surfaces in many applications become covered with precipitates from process fluids or with thick layers of more or less protective corrosion products. The corrosion products from a pit attack are often found to create a lid on top of the pit, with only a very small opening. When examining the metal surface for pits it should therefore be thoroughly cleaned in order to reveal the pitted areas.

Pitting corrosion When pitting corrosion occurs the attack is localised to small areas on the surface where a break through of the passive layer takes place. This will create pits and possibly eventually holes in the metal. This form of corrosion is often more detrimental than general corrosion, due to the local dissolution which can cause rapid penetration of the metal thickness. The nucleation time for pits depends on factors such as the oxidising character of the environment, the concentration of aggressive ions such as chlorides, pH and the alloy composition of the metal. The properties of the surface, for instance the presence of initiation sites at defects and inclusions, also effect the nucleation time. During the attack, however, several mechanisms act together to result in an autocatalytic progress of pit growth. The environment within the pit becomes increasingly aggressive, due to anion selective diffusion into Corrosion products H2O+O2

OH

_

_ Cl

_ e

H+

Fe2+ Fe2+ _ Cl + H

OH

film

e_

Passive _ Cl

_

H2O+O2

P R E - VA L U E S

The chromium content of stainless steel grades is important and alloying with molybdenum and nitrogen has proved very beneficial for the pitting resistance. From experimental data, relations between elemental composition and pitting resistance have been developed. These values are generally called PRE, pitting resistance equivalents, and can be used for an approximative ranking of stainless steel grades. Several forms are known of which one often used expression is showed below. PRE = %Cr + 3.3% Mo + 16%N

Metal

7326

Figure 6. A corrosion pit with some possible chemical reactions.

I :14

Figure 7. Test coupons after pitting corrosion test according to ASTM G48. From the left to the right: Sandvik 3R60, Sandvik 2RK65, SAF 2304, SAF 2205 and SAF 2507.

In the duplex austenitic-ferritic stainless steels the nitrogen content is high, which promotes the pitting resistance. The high nitrogen content in SAF 2205 (nominal value: 0.18%) compared to some other 2205 type grades with a minimum of 0.08% N (for UNS S31803), is therefore beneficial. In the

SANDVIK STEEL CORROSION HANDBOOK

duplex grades the PRE might, however, differ between the two phases. To avoid this duplex steel grades must be developed with a balanced composition so that the elements of importance are partitioned to equal benefit in the two phases. This is the case for SAF 2507, for which PRE in both phases is greater than 40. Table 4. Minimum PRE-values for some Sandvik stainless steel grades Steel grade

PRE (as defined above)

Sandvik 3R12

18.7

Sandvik 3R60

25.6

Sandvik 2RK65

33.0

SAF 2304

24.1

SAF 2205

35

SAF 2507

41

Sanicro 28

37.7

T H E E F F E C T S O F A L L OY I N G E L E M E N T S O N P I T T I N G R E S I S TA N C E

The effects on pitting corrosion resistance of alloying with for instance molybdenum or nitrogen have been investigated, but the picture is not yet completely clear. In the case of alloying with molybdenum improved metal passivation has been found. When pitting attack occurs the molybdenum assists in repairing the passive layer so that pit nucleation is stopped. According to one theory molybdate ions are formed from dissolved molybdenum. The molybdate ions then remain at the outer surface of the diffusion layer so that it becomes cation selective. The aggressive anions, such as chlorides, are thereby prevented from reaching the surface. At the interface between the oxide and the diffusion layer anion selectivity prevails so that oxide growth can continue. After the initiation of attack increased amounts of molybdate ions have also been detected in pitted areas. The effects of molybdenum seems to be enhanced by nitrogen which influences the molybdate concentrations at the surface. This has been explained by the production of ammonium ions which increases the pH which, in turn, makes the formation of molybdate ions more likely. Surface analysis has also proved that iron dissolution is increased with increased nitrogen amounts, whereas the dissolution of chromium and molybdenum decreases. In alloys with increased nitrogen amounts the passive films have been found to contain higher ratios of chromium in the outer layer. Below this exists a thin layer enriched in nitrogen, nickel and molybdenum.

T E S T I N G F O R P I T T I N G R E S I S TA N C E

Ordinary methods using weight losses are not recommended for pitting tests because the overall material loss may be very small even though severe pits have developed. Evaluating the number of pits, their depths or localisation can give useful information about the mechanisms of initiation and growth, but is not adequate for evaluation of pitting resistance. It is therefore preferable to simulate conditions which result in pit initiation to provide a relevant means of assessing resistance to this type of corrosion attack. This is done by exposing the test material to an aggressive environment for a certain time. If corrosion pits are observed the material has failed. THE ASTM G48-TEST

One immersion test method for pitting corrosion is the ASTM G48A-test, where samples are immersed in a 6% FeCl3 solution. This solution is very corrosive due to the simultaneous presence of chloride ions and oxidising ferric ions. The temperature is held constant (e.g. at 22±2 or 50±2°C) and the recommended time for exposure is 72 hours. After cleaning the samples are weighed and the surfaces are examined for pits. C P T D E T E R M I N AT I O N S

The temperature and the presence of oxidising agents are important parameters for pitting corrosion. A number of test methods have therefore been developed to determine the critical values for these. The critical pitting temperature, CPT, is a value often requested. It should be noted that CPT values are specific for the environment and the test method used and these must therefore always be reported. CPT can be determined by a modified version of the ASTM G48 test described above. A sequence of test periods are used at increasing temperatures. Between immersion periods the samples are removed, cleaned and examined for pits. The minimum temperature at which the material experiences pitting corrosion is defined as the CPT. CPT can also be determined electrochemically. The most common ways are either to use a potentiodynamic method and measure at which temperature the break-through potential drops, or to use a constant potential (potentiostatic method) and measure the temperature at which the current increases drastically. A discrepancy exists between results from different methods but they are both frequently used to study differences in pitting resistance of stainless steels. Figure 8 shows the critical pitting temperature for different alloys measured potentiostatically at +300 mV SCE and with varying chloride concentrations. The potential +300 mV SCE is often found in natural sea water. Figure 9 shows the same

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SANDVIK STEEL CORROSION HANDBOOK

figure for other alloys and with a higher applied potential (+600 mV SCE), which corresponds to a potential often found in chlorinated sea water.

CPT,°C (°F), 600 mV SCE 100 (210)

6185b

90 (195)

CPT, °C (°F), 300 mV SCE 100 (210)

SAF 2507

4159b

80 (175)

80 (175)

SAF 2205 70 (160)

60 (140)

AISI 316

25 Cr – Duplex 60 (140)

40 (105) AISI 304

20 (68)

40 (105)

0 (32)

SAF 2205

50 (120)

0.01

0.02

0.05

0.10

0.20

0.50

1.0 2.0 Cl–, weight-%

Figure 8. CPT-values for SAF 2205, AISI 304 and AISI 316 at varying concentrations of chloride. A potentiostatic determination at +300 mV SCE and pH=6.0.

Cl–,% 3 5

6

9

10

15

12

15

20 25 NaCl, weight-%

Figure 9. CPT-values at varying concentrations of sodium chloride, from 3 to 25%. A potentiostatic determination at +600 mV SCE.

Crevice corrosion This form of corrosion is in principle the same as pitting corrosion, but occurs in crevices, e.g. between flange joints, under deposits on the metal surface or in welds with incomplete penetration. A concentration cell is created with the anode in the crevice and the cathode on the outer surface. This corrosion form may be hard to spot, in the same way as for pitting, as it occurs in concealed places. When examining

Figure 10. Schematic illustration of crevice corrosion under a washer.

I :16

the material on location crevices should be opened and the surface be thoroughly cleaned. In practice it is very difficult to entirely eliminate crevices in constructions. Crevice corrosion often occurs at lower temperatures and at lower chloride concentrations than for pitting corrosion. Up to a certain limit, the risk for attack increases the more narrow the crevice is.

SANDVIK STEEL CORROSION HANDBOOK

T E S T F O R C R E V I C E C O R RO S I O N

Evaluation of crevice corrosion resistance may be done according to the ASTM G48B test. As in the pitting test (practice A) samples are immersed in a 6% FeCl3 solution. Before immersion crevice formers with specified properties are mounted on the samples. The critical temperature for crevice corrosion, CCT, may be determined in the same way as for CPT. One method for this is described in the MTI-2 standard, where the temperature is increased by 2.5°C. For reasons of investigating specific crevice formers, e.g. when the extraction of aggressive ions may be suspected, modified tests are sometimes performed.

The mechanism of stress corrosion cracking is not well understood. This is mainly due to the specific features of SCC being the result of a complex interplay of metal, interface and environment properties. As a result of this different combinations of solution and stress are seldom comparable and the most reliable information is obtained from empirical experiments. During SCC the material does not undergo general corrosion and the phenomenon is sometimes considered to be one of activation/passivation interaction. It has been found that cracks often initiate in trenches or pits on the surface, which can act as stress raisers. The isolated times for pit initiation, pit growth, crack initiation and fracture may, however, differ considerably between different materials.

Temperature (°C) 6247b

CPT (°C) 90 80

6Mo+N austenitic

CCT (°C)

SAF 2507

*25Cr - 3Mo - .2N

70 25 Cr Duplex*

60 50 40 30

904L

SAF 2205

20 10

Figure 11. Comparison of CPT- and CCT-values for some stainless steels (obtained by the modified ASTM G48 method).

Stress corrosion cracking Stress corrosion cracking (SCC) is an environmentally assisted cracking process, where a specific environment combined with tensile stress induces cracks on the metal surface. Stress corrosion cracking often occurs at increased temperatures, i.e. above 60°C, but cases where SCC has occurred at lower temperatures exist. The most common media where stress corrosion cracking occurs are chloride containing solutions, but in other environments, such as caustics and polythionic acid, problems with SCC may also appear. Some enviroments that may cause stress corrosion cracking of stainless steels are listed below. Table 5. Some environments where stainless steels are prone to stress corrosion cracking.

Figure 12. Stress corrosion cracking of a tube.

In some cases crack initiation has been associated with the formation of a brittle film at the surface. The film developed at grain boundaries might, for instance, have lower ductility due to a different metal composition than the bulk material. At a certain film thickness and under stress this brittle film will crack and expose the underlying metal. New film growth will proceed with subsequent continued crack growth and so forth. The developed crack tip has a small radius and will develop a very high stress concentration. Even so, the stress condition alone is not sufficient for crack growth, but corrosion still plays a very large part. It has been shown experimentally that stress corrosion cracking can be stopped when applying cathodic protection, i.e. when corrosion is stopped but the stress conditions remain unchanged.

Acid chloride solutions Seawater Condensing steam from chloride waters H 2 S + chlorides Polythionic acid (sensitised material) NaCl-H 2 O 2 NaOH-H 2 S

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SANDVIK STEEL CORROSION HANDBOOK

Crack growth direction

σ

Crack growth direction

σ

σ

H++ eH H H H H

n+ Me n+ Me

Figure 13. Transgranular stress corrosion crack in Sandvik grade 2RE69 after autoclave testing in 1000 ppm chloride at 250°C.

σ

Anodic stress Time to cracking corrosion cracking

Hydrogen embrittlement

Immunity

Cracking may be either transgranular (TGSCC) or intergranular (IGSCC) or, perhaps most usual, a combination of both. The material microstructure and alloying components are of major importance for crack paths as well as for SCC resistance. Alloying with Ni can make materials less prone to SCC and the duplex microstructure of the austenitic-ferritic grades is also beneficial. Standard austenitic stainless steels, like AISI 304 and AISI 316, are generally prone to SCC in chloide containing environments at temperatures above 60°C, except at very low chloride contents, and therefore higher alloyed austenitics or duplex stainless steels should be used. H Y D RO G E N E M B R I T T L E M E N T

Hydrogen embrittlement (HE) is sometimes stated to be a kind of SCC. This might, however, lead to serious misunderstandings as many discrepancies exist. Perhaps most important is that HE cannot be reduced by cathodic protection, but might instead increase under such circumstances. The reason for this is that HE is caused by the penetration of atomic hydrogen into the metal structure. This, in turn, might occur when reduction of H+ is taking place on the metal surface, e.g. during cathodic protection in acidic environments. Several deposition techniques, such as electroplating, also involve reduction processes at the metal surface with the following risk of hydrogen penetration and embrittlement. To avoid this, treated articles are often baked before use to remove the hydrogen. The risk for HE is increased for harder metals, but the tendency to hydrogen cracking decreases with increasing temperature. Some differences between HE and SCC are illustrated in figure 14.

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Anodic current M Men+ + n e-

Cathodic current 2e- + 2H+ 2H 7325

Figure 14. Possible mechanisms of cracking due to SCC and HE respectively.

S U L P H I D E S T R E S S C R AC K I N G

Sulphide stress cracking (SSC) might be defined as a variant of HE, but is sometimes treated as a special corrosion type. Sulphides are hydrogen-evolution poisons and as such prevent the hydrogen atoms formed on the metal surface from pairing up and dissolving as H2 into the surrounding solution. SSC has been found to cause severe problems especially in the oil and gas industry. A standard for material requirements in so-called sour environments has therefore been developed: NACE MR0175. Among the acceptable steel grades are SAF 2205, SAF 2507 and Sanicro 28. New grades can be accepted in NACE MR0175 after successful testing according to one of four methods described in NACE TM 0177. (See also chapter 7, oil & gas industry.) TEST METHODS

The stress corrosion cracking resistance can be tested in a laboratory by different methods. Usually a constant load, a constant elongation or a slow strain rate is applied to the sample in a chloride containing environment. Several standards have been developed regarding the stress application for SCC testing. U-bends, C-rings and bent-beam specimen are some examples of this.

SANDVIK STEEL CORROSION HANDBOOK

Figure 17 shows examples of results from constant load tests according to a modified ASTM G36 method, where 40% CaCl2 at 100°C is used as the corrosive medium. The time to failure versus load is recorded and a threshold stress is determined, below which SCC does not occur. In figure 18 results from constant load tests (stress equals yield strength) are presented for which a spring was used to apply the stress. The mounted samples were tested in an autoclave at different temperatures and chloride concentrations. This method has been verified to correspond to practical cases, as illustrated in the figure.

Figure 15. U-bent sample for stress corrosion testing with constant strain.

Figure 16. Constant load SCC testing in 40% CaCl2 at 100°C, using weights.

Temperature, °C (°F) 1373b

300 (572) SCC 250 (482)

Constant load can be applied to the sample simply by the use of weights as shown in figure 16. Another way is to strain the sample with the aid of a spring. Regardless of method certain requirements must always be met by the testing equipment. Crevices and galvanic corrosion must be avoided and all exposed parts must be resistant to any kind of corrosion in the environment.

200 (392) 150 (302) No SCC 100 (212) 50 (122)

Time to fracture, h 60

4229b

CaCl2

0.0001

0.001

0.01

0.1 SCC

50 SAF 2205

1

10 _ % Cl

No SCC

Laboratory test, aerated _ solution Service experience, Cl_ +S/H2S Service experience, Cl

Figure 18. A compilation of practical experience and laboratory SCC test data of 3RE60.

40 AISI 304L

AISI 316L

30

20

10

0.

0.

0. 0. 1. Stress/Tensile strength 100°C (210°F)

Figure 17. Results of SCC test with constant load in 40% CaCl2 at 100°C (210°F) with aerated test solution.

The drop evaporation test is another common method to measure SCC-resistance. With this method a specimen is electrically heated and subjected to a constant load and at the same time a dilute sodium chloride solution is dripped onto the specimen. At the heated surface the chloride solution evaporates, leaving a highly concentrated chloride environment. The test is continued until the specimen cracks or up to a specified time, usually 500 hours. In the same manner as in the modified ASTM G36-method the time to failure versus load is recorded and a threshold stress is determined below which SCC does not occur.

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SANDVIK STEEL CORROSION HANDBOOK

For the testing of sulphide induced cracking in sour service the NACE standard TM 0177 may be used. A choice between four methods is given, in which stress is applied in different ways. The test solution is selected to give hydrogen absorption conditions equal to that expected of the most severe well environment. It should have a partial pressure of hydrogen sulphide of 1 bar, 5% chlorides and pH=3. The test temperature is set to 20-25°C and the samples must resist a period of up to 30 days without cracking.

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7590

(ultimate) tensile strenght

inert environment corrosive environment

Nom. stress, F/A °

In slow strain rate testing (SSRT) the stress over the sample is continously increased. The strain rate must be chosen correctly so that SCC is induced. Too low values result in repassivation of the attacks, whereas too high values give purely mechanical fractures. This method of loading resulting in continuous breaking of the passive layer and monotonically increasing load is very aggressive and does not reflect the conditions in practical service of the material. The results from an SSRT test are conveniently presented in a stressstrain curve, in the same way as mechanical tests (figure 19). In this way several parameters can be compared for tests in corrosive and inert environments. The ratio between such parameters is often used to rank the susceptibility to SCC for different materials.

fracture

0 0

Nominal strain (I - I0)/I0

Figure 19. Stress-strain curve for corrosive and inert environment respectively.

SANDVIK STEEL CORROSION HANDBOOK

4.

High temperature corrosion

Introduction The temperature region for high temperature steels, ranges from about 400˚C up to temperatures where the mechanical properties of the alloy will be severely reduced. High temperature applications are found in various processes, as for example wire annealing, combustion/incineration, hydrocarbon cracking, air heating (recuperators) and shieldings. In these applications the selection of an alloy becomes difficult, as the material is subjected not only to corrosive chemicals, but also to a degeneration due to the temperature. That is, the alloy may become brittle due to structural changes, e.g. formation of sigma-phase or 475°C embrittlement. This is particularly so in the case of ferritic steel with a Cr-content higher than 15%, and in the temperature ranges 400-800˚C. Furthermore, for materials that are designed for carrying a load, own weight or pressure, the creep properties will become a dominating factor. In most cases a compromise between the desired properties must be made, as no single alloy is likely to have excellent values in all these fields (e.g. high Cr is good for corrosion, but bad regarding structural stability). Most common material temperatures in high temperature applications are in the range 400-900˚C. At these temperatures the alloy will react with the surrounding gas phase and degenerate chemically, forming what is referred to as corrosion products. The formation rates of these products are strongly temperature dependent. In aqueous solutions a 10 times higher corrosion rate is not uncommon for a temperature change of 30˚C. The same 10 fold change (or worse) may occur with a 20˚C change under high temperature conditions. Note that if the temperature is too low it may be difficult for a protective oxide scale to form. This can become a problem particularly in applications where the material is internally cooled, e.g. furnace tubes. Most of the corrosive elements can be found in the upper right corner of the periodic table. The elements in this area have similarities in chemistry and will, thus, have similar reactions paths. The corrosion product formed may be "beneficial" or "detrimental" to the alloy, depending upon the specific product formed. In practice the life time for an alloy relies on it's ability to form a dense, adherent and continuous oxide layer with a low growth rate. If other products are formed, i.e. sulphides, carbides, nitrides, or halides, the scale will be less protective or even more corrosive than no scale at all.

However, a good oxide scale will not last for ever, as the scale will continue to grow until it reaches a certain thickness. At this point stresses due to the differences in volume between the scale and the alloy will become so large that the scale will crack. The cracks will cause the oxide to spall off and then the protection of the scale is gone. The spalling can be measured and is often used to compare and rank the alloys. The spalling temperature is measured by heating the alloy in air at increasing temperature steps for a certain time and the temperature where the weight change of the alloy exceeds a pre-defined value is set as the scaling temperature (usually this value is 1.5 g/m2h). For comparison, the scaling temperature for carbon steel is about 550˚C and for 353MA (25% Cr, 35% Ni) the scaling temperature is about 1225˚C. The maximum service temperature of an alloy is usually set 50˚C below the scaling temperature.

Figure 1. Rapid oxide growth (dark) in 18-8 material caused by too high temperatures. The oxide growth rate has been fast enough to form pieces of metal “islands”. This oxide scale is not protective!

As there are large differences, not only in the volume between the oxide and the alloy but also in the thermal expansion coefficient scaling may become a severe problem when the process includes a temperature cycle. Scaling is not the only mechanism that can disrupt a protective oxide. In many applications a melt deposit may also be formed, especially in combustion processes. This melt can dissolve the oxide layer and open up the alloy for further and rapid corrosion. This particular reaction is often referred to as

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SANDVIK STEEL CORROSION HANDBOOK

“hot corrosion”. Hot corrosion is a well known phenomena in the gas turbine industry. There is an on-going argument on the mechanism of this type of corrosion and so far the only agreement is that there are two types of hot corrosion, which appear at different temperatures. In many cases studied it appeared that the onset of rapid corrosion was dependent on some degree of thermal cycling and that there was an indefined initial period before the corrosion rate increased. (See also catastrophic oxidation below.) In the subsequent text some corrosion reactions will be discussed, but first it should be mentioned that there is a great difference between high temperature corrosion and “wet” corrosion. This means that material developed for high temperature applications may not have as good corrosion resistance at low temperatures and in “wet” applications as it has in the high temperature range.

Oxidation The main corrosion reaction is oxidation. Oxidation of an alloy may occur at any temperature (the oxidation rate will increase with the temperature) if the amount of oxygen is high enough. The advantage with oxidation is that the oxide layer formed may serve as a protection from further corrosion, that is, if the oxide layer formed is dense, continuous, and adherent. Some alloying elements like Al, Si, and Cr may form a dense layer, and of these three elements Al and Si will form the most effective oxide scale. Unfortunately, the amount of Al or Si needed in the alloy for forming this oxide scale will make the alloy rather brittle and it's fabrication difficult. In carbon steel and in low alloyed steel, iron is the main oxide former. Iron may form a rather dense oxide, Fe2O3, which is protective. However, if the temperature is higher than 550˚C, wüstite (FeO) is formed. This phase is porous, it is not protective and rapid oxide growth will occur causing the low scaling temperature of carbon steel. To enhance the protective effects of the oxide scale small amounts of RE (Reactive Elements, = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, (sometimes also called REM, Rare Earth Metals) may be added to the alloy. It is not fully clear why this should occur, but several investigations have shown that these small (< 0.1%) additions result in reduced growth rate and improved adherence of the Cr- and Al- oxides.

Catastrophic oxidation Even if the formation of oxides in general is beneficial for the alloy, there are some oxides that should be avoided. Oxides, such as for example MoO3, have rather low melting points, or may form compounds with other oxides that have low melting points, especially in static (non-flowing) systems. The melts formed may dissolve the remaining oxide layer and

I: 22

Figure 2. Catastrophic oxidation of a Mo-rich duplex alloy, after 24h in 1060˚C

then the material will be destroyed within a very short period of time. This behaviour is often referred to as “catastrophic oxidation”. Other alloying elements that are sensitive to catastrophic oxidation are V, Pb, W, Ta, and Nb.

Sulphidation In many high temperature processes sulphur is present, e.g. in most processes where coal or oil is combusted. The chemistry of sulphur is rather similar to that of oxygen, and sulphur will thus react in competition with oxygen. However, sulphur will not form such a dense layer as in the case of oxides and moreover, the melting point of the sulphides formed may sometimes be several hundred degrees lower than that of the oxide, e.g. Ni3S4, melting point ~650˚C. If Ni3S4 is formed in the grain boundaries this will also decrease the strength of the alloy. Hence Ni-containing alloys should be avoided in sulphurous environments if they are unable to form a protective oxide scale. In a sulphur-containing atmosphere where sufficient oxygen is present to allow an oxide scale to form the corrosion resistance is determined by the properties of that layer, but if cracks (see scaling above) start to form in the oxide layer, then sulphur will attack at these points. This means that the corrosion resistance in sulphur containing atmospheres will depend on the scaling temperature of the alloy, and the maximum service temperature will be dependant upon both the scaling temperature and the amount of sulphur in the gas. On the other hand, if the alloy cannot form a protective oxide the corrosion resistance will be greatly reduced and more dependant upon the alloy composition. Under these conditions it is favourable to use an alloy that is high in Cr and has either no Ni (e.g. 4C54, provided that embrittlement is not a problem) or a lower level of Ni (e.g. 253MA).

SANDVIK STEEL CORROSION HANDBOOK

Carburisation and nitration In some applications the atmosphere is reducing with a high content of carbon (C) or nitrogen (N), e.g. in wire annealing, ethylene furnaces, carbon black production, or steammethane reforming. In these processes the material temperature may be as high as 900-1150˚C. If carbon is used to generate a reducing environment the carbon species in the atmosphere may react with the alloy, forming carbides. Sometimes the carburisation reaction will occur rapidly and this phenomena, “metal dusting” is mainly observed in waste heat boilers in steam reforming processes in the temperature range 500-800˚C. Even if metal dusting does not occur, the formation of carbides is detrimental as this reaction often catalyses coking, and the coke may then block the tube. In some wire annealing processes cracked ammonia is used to create a non-oxidising environment. In this case the nitrogen activity is high, which will lead to the formation of nitrides in the alloy. The chemistry of the alloy may change due to carburisation/nitridation, as it is often one alloying element that reacts with the carbon/nitrogen, e.g. Cr. This element will then be tied up in the precipitates and will not be able to form a protective oxide scale. Furthermore, the bulk material below the surface will be depleted in Cr and this will reuce the possibility of oxide formation. A heavy carbon or nitrogen pickup may also embrittle the alloy as the precipitation of the carbides/nitrides are concentrated at the grain boundaries. Also, the creep properties and the ductility will be affected by the carburisation/nitridation reaction. The resistance to carbon and nitrogen pickup is improved by increased Ni-content. In oxidising environments strong oxide forms such as Cr, Al, and Si are beneficial. Al is of special interest, particularly in environments where carburisation is the main problem.

Molten metal corrosion The main corrosion mechanism here is either massive or selective dissolution. Some general observations are that austenitic Ni-Cr-Fe alloys dissolve more rapidly with increasing Ni-content, and that ferritic Fe-Cr stainless steels tend to be more resistant. Note that the embrittlement caused by the liquid metal is more hazardous than corrosion. This embrittlement is more pronounced for melts containing silver (Ag), copper (Cu), and zinc (Zn). When molten, these metals will penetrate austenitic alloys intergranularly and may cause rupture within a few seconds. Cases where this has been reported are for example, welding of stainless steel to galvanised carbon steel, weld cracking due to copper contamination, and cracking caused by copper containing anti-seize compound.

Halogen corrosion Halogen (i.e. F, Cl, Br and I) containing gases are often very aggressive against all metallic materials. The ones of greatest interest are F and Cl. The former is regarded to be the more corrosive one while the latter is more frequent. If the environment is reducing, Ni and Cr generally improve the corrosion resistance. In oxidising environments, Cr, and especially Mo and W, are detrimental due to their tendency to form volatile oxychlorides. The general mechanism of corrosion is the same as for oxidation and sulphidation. High chloride vapour pressure can result in penetration and disruption of the oxide scale. Voids may be formed under the scale as the chlorides evaporate. There has been limited study of halogen reaction with metals and general material performance criteria are still under development.

Erosion-corrosion Erosion can enhance or retard corrosion attacks; increasing them by removing the protective layer or decreasing them by the removal of corrosive deposits. similarly, corrosion may increase or decrease erosion rates; increasing them by attacking the eroded surface or decreasing them by forming an oxide layer that is more erosion resistant then the parent metal. This means that in addition to having a good resistance against the corrosive atmosphere, materials exposed to erosion must be able to develop an adherent, ductile and selfhealing oxide layer. The addition of Rare Earth Metals has a beneficial influence on all these three oxide layer properties. Application areas where erosion may occur are for example fluidized bed combustion (FBC) and cement production. There are still a lot of work to be done to determine the effects of erosion-corrosion on different materials, hence, no specific data is available on erosion-corrosion of alloys.

Applications The high temperature applications that Sandvik targets are presently recuperators, muffle tubes, thermocouple protection tubes, boiler tubes and ethylene furnace tubes. The selection of the material in each application is very much dependent on the environment. For example, the atmosphere in a muffle tube may be oxidising (air), reducing (cracked ammonia), or carburizing e.g. bundy tube production). Each environment demands specific properties of the alloy. In table 1-3 some Sandvik recommendations on material selection are presented, varying with application and environment.

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SANDVIK STEEL CORROSION HANDBOOK

P OW E R B O I L E R S

Power utility boilers, especially those burning biomass- or fossil fuels which often contain high levels of sulphur and chlorine, represent one of the most demanding applications for stainless steel tubes. The outer tube surface is attacked at high temperature by the combustion products with differing corrosion and/or erosion mechanisms. The inside surface is often subjected to steam oxidation corrosion: and the material is expected to support high loads resulting from high internal steam pressure – often under conditions where creep is a significant factor. Currently, temperatures and pressures are generally being increased, with the intention of improving thermal efficiency and to reduce pollution. Conventional high strength alloys can

be susceptible to the corrosion attack, whilst corrosion resistant alloys often have insufficient strength at the temperatures involved. Tube failures result in having to shut down the power station. A proven solution is to use composite tube. There are two main areas of application: evaporator tubes and the super heater/reheater tubes. In the evaporator section, tubes typically have a highly corrosion resistant outer layer, 25Cr20Ni variant, and a load bearing carbon- or low alloy inner component. In the superheater/reheater elements a typical solution utilises a high alloy outer component, e.g. 310Nb, Sanicro 28, alloy 825, or alloy 625 bonded to a creep resistant steel inner component.

Table 1. Maximum working temperature in different gases. Temp °C

500

600

700

800

900

1000

1100

In air (High humidity may lower temperature 50-150 °C) 353 MA 253 MA 310, Sanicro 31HT, 4C54 304H, 316H, 321H In oxidising sulphurous gases 353 MA 4C54 253 MA 310 Sanicro 31HT 304H, 316H, 321H In reducing sulphurous gases 4C54 253 MA 310 353 MA Sanicro 31HT 304H, 316H, 321H The maximum temperature is depending on the level of flue gas impurities (S, Na, V)

Table 2. Structural stability. Temp °C

400

500

600

700

800

Grade Sanicro 31HT* 253 MA 353 MA 309S 310S 4C54

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γ σ phase σ phase σ phase σ phase 475°C-embrittlement

σ phase

900

1000

1175

SANDVIK STEEL CORROSION HANDBOOK

Composite tube applications In many cases high temperature materials from Sandvik Steel are delivered for applications where both corrosion resistance and pressure vessel approval must be fulfilled. Such applications are black liquor recovery boilers (BLRB): municipal waste incinerators, and power utility boilers. Table 3. High temperature corrosion properties A comparision between Sandvik High Temperature Steels and TP 310. Oxidation

Carburisation

TP 310

0

0

Nitriding* 0

353 MA

+++

+++

++

253 MA

+

+

0

Sanicro 31HT

=

++

++

4C54

0

– **

–

* in cracked ammonia atmosphere. ** 4C54 has very good resistance to metal dusting corrosion. 0 = reference value + = superior to

– = inferior to.

A BLRB is a boiler about 30-70 m high where the floor and the walls are made of panel welded tubes. The boiler is used to burn the residues from the wood cooking and to recycle the cooking chemicals (mainly sulphides). The combustion of the residue is controlled in such way that the sulphides in the fuel (black liquor) will form smelt that accumulates at the bottom of the boiler. This smelt (now called green liquor) is then taken out through smelt spouts at the bottom of the boiler and processed further to be reused in the cooking. The tubes are water cooled, and the water/steam pressure is generally between 60-100 bar. This will correspond to a water/steam saturation temperature of 250-300˚C in the walls and steam temperatures of around 400-480˚C in the superheater. The

actual material temperature may be some 20-50˚C higher than the water, or the steam temperature. This boiler is the heart of the pulp industry, i.e. if the boiler is shut down, the whole plant must stop. There are high safety demands in this process because if a tube bursts, and water contacts the melt, there is a high risk for a boiler explosion. The environment in a municipal waste incinerator boiler (MWB) is much more aggressive to metals that that in the BLRB, and thus the corrosion rates here are high. If a tube bursts or fails here, the plant must be shut down for repair which is costly. For these two applications Sandvik has developed composite tube solutions. Composite tubes consist of two alloys that are co-extruded to form a tube with outer and inner components. The bonding between these components is a chemical metalto-metal bond (sometimes referred to as a metallurgical bond). One of the components is there to serve as the load carrier and the other as a corrosion protection layer. The load carrier is often a carbon steel type 4L7 (SA210-A1) for temperatures below 450˚C, a low alloyed carbon steel like HT8 (T22) for temperatures up to 550˚C, or higher alloyed steel like T91 for temperatures up to 600˚C. In the BLRB's the combinations Sanicro 38/4L7 (floor), 3R12/4L7 (wall) and, 3RE28/HT8 (superheater) are recommended. For waste incinerators the recommendations are 3R12/4L7, Sanicro 28/4L7 or Sanicro 63/4L7, depending on temperatures and type of pollutants in the fuel. There is on-going development of composite tube combinations for various applications.

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SANDVIK STEEL CORROSION HANDBOOK

5. Introduction Stainless steels is a designation for a group of iron-base alloys with such a composition that they are able to passivate, i.e. form a passive layer which protects the metal from the surrounding environment, and thus hinder metal dissolution (corrosion). The chief alloying element in stainless steels is chromium (Cr), which in concentrations above 12-13% forms a passive layer on the metal. Increasing the chromium content leads to a stronger passivity and thus a higher corrosion resistance. Chromium is a so called ferrite stabiliser, which means that chromium does not alter the structure of iron, which has a ferritic structure. The physical properties of an alloy with only chromium added therefore does not differ much from pure iron. These types of alloys are called ferritic stainless steels. Although chromium makes the steel stainless it cannot resist more aggressive environments, and the formability of ferritic stainless steels is limited. Other elements are therefore added to modify the structure, the mechanical properties and the corrosion resistance. Nickel (Ni) is added to alter the structure of the steel but may also improve the corrosion resistance if sufficient amounts are added. Nickel is an austenite stabiliser, which means that an addition of nickel will alter the structure from ferritic to austenitic. In an alloy with 18% chromium about 10% nickel is required to alter the structure to almost purely austenitic.

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Stainless steels These alloys, the austenitic stainless steels, have an improved formability, greater toughness and high temperature properties, as well as improved weldability compared to ferritic stainless steels. The physical properties will also change, e.g. fully austenitic stainless steels are non-magnetic. Molybdenum (Mo) has an effect similar to chromium with regard to structure and corrosion resistance. Molybdenum alloyed steels are what is usually called ”acid proof”, which refers to the beneficial effect of molybdenum on the corrosion resistance in sulphite digester liquor. In some media, like strongly oxidising acids, molybdenum may impair the corrosion resistance, and should therefore be avoided in such applications. Nitrogen (N) increases the strength, the corrosion resistance and also improves the structural stability of stainless steels. In many cases, especially for duplex stainless steels, it also improves the weldability. Copper (Cu) is beneficial for the corrosion resistance in certain acids. Titanium (Ti) and niobium (Nb) are used as carbide formers, which means that carbon is preferentally bound to these elements, thus reducing the risk for precipitation of deleterious chromium carbides in the grain boundaries, and hence decreasing the risk of intergranular corrosion. Four main types of structures exist in stainless steels; ferritic, martensitic, austenitic and duplex (austenitic-ferritic).

SANDVIK STEEL CORROSION HANDBOOK

Austenitic stainless steels and Duplex stainless steels The austenitic family of stainless steels covers a broad interval of alloying elements, from standard 18-9 to super austenitics with up to 7 % molybdenum and matching contents of chromium and nickel. The super austenitics are often also alloyed with nitrogen. Their corrosion resistance is therefore adapted to a great variety of corrosive environments. The super austenitics are most often designed to resist pitting and crevice corrosion in chloride containing environments, e.g. sea water. They also have good resistance to stress corrosion cracking, in solutions containing hydrogen sulphide and in alkaline solutions. The austenitic steels have good ductility, at both low and high temperatures. Their weldability is good. The austenitic steels are readily welded with all normally used techniques. High alloy steels can be welded with over-alloyed filler metals, thus matching the corrosion resistance of the base metal. Duplex stainless steels are stainless steels with a microstructure comprising typically 40-50% ferrite and the rest aus-

Figure 1. Microstructure of austenitic stainless steel.

tenite. These steels combine important properties from both ferritic and austenitic stainless steels. They show good stress corrosion resistance and also good ductility and weldability. All modern duplex stainless steels have a low carbon content. Duplex grades with PRE-number (PRE = Pitting Resistance Equivalent = % Cr + 3.3% Mo + 16% N) greater than 40 are called super duplex. These steels possess very good corrosion properties, especially in chloride containing environments. The duplex structure gives high mechanical strength, approximately twice that of austenitics, combined with low thermal expansion, close to that of carbon steel. The low Ni-content is cost saving and high mechanical strength means lighter construction, which gives cost advantages. The upper service temperature of duplex stainless steels is around 300°C due to the risk of embrittlement and formation of precipitates. The weldability of duplex stainless steels is good. Welding of duplex stainless steels with proper welding parameters and matching filler metals gives good corrosion and mechanical properties.

Figure 2. Microstructure of duplex stainless steel. The dark areas are ferrite and the light areas are austenite.

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SANDVIK STEEL CORROSION HANDBOOK

6.

Manufacturing programme

The most common steel grades manufactured by Sandvik Steel as seamless tube products are listed on page I:43-44. Wet corrosive and high-temperature service alloys and hollow bar materials are shown. The corresponding names in the corrosion table and equivalent standards are shown together with mechanical properties.

Special stainless steel grades Special stainless steel grades have been developed to meet demands for higher corrosion resistance, favourable physical and mechanical properties and good structural stability and weldability. Descriptions of some special steel grades are given below. 904L/2RK65

Steel grade 904L / 2RK65 is a high-alloy stainless steel with low carbon content. It is fully austenitic, and less sensitive to precipitation of ferrite and sigma phase than conventional austenitic grades with high molybdenum content. The grade is intended for use under severe corrosive conditions. It is standardised and approved for pressure vessel use in several countries. Although originally developed to resist corrosion in dilute sulphuric acid, 904L / 2RK65 has been employed in a great variety of applications for many years. It has; – Good resistance to general corrosion, especially in dilute sulphuric acid. – Good resistance to pitting and crevice corrosion. – Very good resistance to stress corrosion cracking. – Good resistance to intergranular corrosion. S A N I C RO 2 8

Sanicro 28 is a multipurpose austenitic stainless extra low carbon content alloy. It is designed for use in highly corrosive environments. Sanicro 28 has very high corrosion resistance in strong acids and very good resistance to stress corrosion cracking and intergranular corrosion, and also very high resistance to pitting and crevice corrosion. Sanicro 28 possesses good weldability. Welding should be carried out without preheating and there will be no need for any subsequent heat treatment. The very low impurity content of Sanicro 28 minimises the risk of hot-cracking in the weld metal.

I: 28

Sanicro 28 is easy to bend and expand using the same methods as standard austenitic steels. Annealing is not normally necessary after cold bending. Machining is easy but requires an adjustment of cutting data compared with AISI 316L. The excellent corrosion properties of Sanicro 28 enable it to be used in the most diverse environments. Sanicro 28 is widely used in the manufacturing of phosphoric acid by the "wet" method. It is suitable for piping and heat exchangers handling sulphuric acid. Other applications are in the oil and gas industry, nuclear power plants, sea water and chloridebearing cooling water and also in the fertilizer industry. SAF 2304

SAF 2304 is a low alloy duplex stainless steel. In acidic environments SAF 2304 possesses good corrosion resistance, and it has a pitting resistance comparable to that of AISI 316L. It has very good resistance to stress corrosion cracking in chloridebearing environments, much better than AISI 316L. SAF 2304 is also characterised by good resistance to general corrosion and pitting. SAF 2304 is a modern duplex stainless steel with the chemical composition balanced in such a manner that reformation of austenite in the heat-affected zone adjacent to the weld takes place quickly. This will give welded joints good mechanical and corrosion properties. The favourable physical properties combined with resistance to stress corrosion cracking and other forms of corrosion means that, in many applications SAF 2304 is a superior alternative to stainless steels such as the austenitics 316L, 321 and 347, the ferritics AISI 430, 444 and the martensitics AISI 410 and 420. SAF 2304 is used in chloride-bearing environments, such as heat exchangers in the process industry where stress corrosion cracking is a problem. The high strength makes it possible to use thinner sections in mechanical constructions and piping systems. SAF 2205

SAF 2205 is a duplex stainless steel with high resistance to stress corrosion cracking in chloride-bearing environments containing hydrogen sulphide. SAF 2205 is also characterised by high resistance to general corrosion, pitting and crevice corrosion. In most media SAF 2205 possesses better resistance than steel of type AISI 316L and 317L.

SANDVIK STEEL CORROSION HANDBOOK

SAF 2205 is a modern duplex stainless steel with the chemical composition balanced in such a manner that reformation of austenite in the heat-affected zone adjacent to the weld takes place quickly. This will give welded joints good mechanical and corrosion properties. SAF 2205 is used in environments containing chlorides and hydrogen sulphide, e.g. in tubing and flowlines for the extraction of oil and gas from sour wells, in refineries and in process solutions containing chlorides. It is also suitable in heat exchangers where chloride-bearing water is used as cooling medium. The steel can be used in dilute sulphuric acid solutions and solutions of organic acids. SAF 2507

SAF 2507 is a high alloy super duplex stainless steel. It has excellent resistance to stress corrosion cracking and localised corrosion in chloride-bearing environments and good resistance in environments containing hydrogen sulphide. SAF 2507 is also characterised by high resistance to corrosion in both inorganic and organic acid. This steel grade is a so called super duplex grade, i.e. a steel with a PRE-number greater than 40 (PRE = Pitting Resistance Equivalent = % Cr + 3.3% Mo + 16% N). The ferrite content is generally between 35 and 50 %. SAF 2507 is a modern duplex stainless steel with the chemical composition balanced in such a manner that reformation of austenite in the heataffected zone adjacent to the weld takes place quickly. This will give welded joints good corrosion properties and a toughness roughly equal to that of the parent metal. SAF 2507 is used in chloride-bearing environments and environments containing hydrogen sulphide, e.g. in process systems in the oil and gas industry and in heat exchangers where the cooling medium is chloride-bearing or chlorinated water. It is also used in the handling of organic and inorganic acids. The higher allowable yield stress levels permitted in e.g. ASTM B31.3 means that substantial reduction in wall thickness is possible for all duplex stainless steels piping systems, resulting in major savings in weight and overall costs. 254 SMO

254 SMO is an austenitic stainless steel which, due to its high molybdenum content, possesses very high resistance to pitting and crevice corrosion. The steel grade was developed by Avesta Sheffield AB for use in halide-containing environments such as seawater. 254 SMO also shows good resistance to general corrosion and, especially in acid containing halides, this steel grade is superior to conventional stainless steels.

The high levels of molybdenum in particular but also of chromium and nitrogen endow 254 SMO with extremely good resistance to pitting and crevice corrosion. The addition of copper provides improved resistance in certain acids. Furthermore, due to it is relatively high nickel content in combination with the high level of chromium and molybdenum 254 SMO possesses good resistance to stress corrosion cracking. The high nitrogen content of 254 SMO gives it a higher mechanical strength than other austenitic stainless steels. In common with the austenitic stainless steels, 254 SMO is characterised by high ductility and impact strength as well as good weldability. A super alloyed welding consumable, designated Avesta P12, is used for welding and the weld metal thus produced has equally good corrosion resistance as the parent metal. 654 SMO

654 SMO is a high alloyed austenitic stainless steel which, due to its high nitrogen and molybdenum content, possesses very high resistance to pitting and crevice corrosion. The steel grade was developed by Avesta Sheffield AB for use in halide containing environments more aggressive than those which can be handled by 254 SMO, e.g. sea water at high temperatures and scrubber solutions. 654 SMO also shows good resistance to general corrosion in acids and especially halidecontaining acids. The addition of copper improves the corrosion resistance in reducing acids. The high levels of nickel and molybdenum contribute to the high resistance to stress corrosion cracking. 654 SMO possesses high yield and tensile strengths. The ductility and impact toughness are also very high. 654 SMO is welded with a nickel-base filler material over-alloyed in molybdenum, designated Avesta P16. The weldability of the steel is good but the welding requires special care. Thorough post weld cleaning is very important in order to obtain optimum corrosion resistance.

Nickel base alloys Nickel base alloys can in general be divided into five different groups according to their major alloying element. The first group is the unalloyed Nickel, the Alloy 200-series, which is suitable for use in alkaline environments. The second group is the Nickel-Copper-alloys, the Alloy 400-series. These alloys are suitable for seawater service and are also used in sulphuric acid. The Nickel-Chromium group of alloys is the 600-series. These are used in highly oxidising environments, like nitric acid, and also for high temperature service. If the 600-series alloys are also alloyed with molybdenum they become suitable to use in weak reducing environments. The next group

I: 29

SANDVIK STEEL CORROSION HANDBOOK

Table 1. Examples of Alloy compositions in the different series Series

Alloy example

UNS designation

Composition Ni

Cr

Mo

Fe

Cu

Other

200

Alloy 200

N02200

99.2

–

–

–

–

–

400

Alloy 400

N04400

bal.

–

–

1.25

31.5

–

600

Alloy 600

N06600

bal.

16

–

8.0

–

–

Alloy 625

N06625

62

21.5

9.0

50

Tantalum or Si-iron

98

>60

Sandvik SX*

* Sandvik SX is supplied by Edmeston AB

Table 2. Impurities in 70% H3PO4 in weight-%. Impurity

SO 3

F-

Cl -

SiO 2

Al2O 3

Fe 2 O 3

MgO

Concentration, %

1-4

0.1-1.5

0.002-0.05

0.01-0.7 0.2-3.0

0.1-2.5

0.1-1.5

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SANDVIK STEEL CORROSION HANDBOOK

P H O S P H O R I C AC I D

Phosphoric acid is extracted from rock phosphate. The most common method of manufacturing phosphoric acid is by the wet method, where the rock is dissolved in sulphuric acid, which yields phosphoric acid and calcium sulphate together with some impurities. The impurity level has a crucial effect on the corrosivity of the acid and influences materials selection. The phosphoric acid is concentrated to the desired level by evaporation. Systems for concentrating phosphoric acid usually include heat exchangers in graphite or stainless steel, or heating elements of the prayon type. The prayon type has the advantage that individual elements (tubes) can be replaced easily. Impurity leveles in the acid vary with the origin of the rock phosphate. Normal concentrations of impurities in concentrated acid are shown in table 2. The impurity level may be higher in phosphoric acid of lower concentration. Table 3 shows the relative corrosivity of acid produced from various sources. Table 3. Corrosivity of phosphoric acid obtained from different phosphate sources. Corrosivity

Phosphate

Low

South Africa (Phalaborwa ) Nauru Senegal ( Taiba ) Florida ( Tampa, Pebble ) Brazil ( Araxa ) Nor th Carolina Kola Morocco ( Khourigba, Youssoufia ) Sahara ( Bu Craa ) Tunisia ( Gafsa ) Togo Syria Jordan Israel Mexico

Medium

High (high chloride phosphates)

Materials of construction

Concentration of phosphoric acid from 28% P2O5 to 52% P2O5 (70% H3PO4) is the normal production route. In these units more or less aggressive conditions appear. As stated earlier, the corrosivity is very dependent on the impurity level. At low or zero concentrations of impurities standard grades such as AISI 316L or 317L may be used, whereas at higher concentrations special alloys such as Sandvik 2RK65, duplex stainless steels or Sanicro 28 have to be used. Table 4 gives limits on fluoride and chloride contents in a typical (“Florida type“) phosphoric acid solution. Superphosphoric acidis manufactured by concentrating 52-54% P2O5 to 70% P2O5 at temperatures up to 200°C. This acid is very aggressive to both highly alloyed stainless

I: 32

Table 4. Allowable limits on combined chloride and fluoride content for various alloys in a solution of 70% H3PO4, 4% H2SO4 and 0.45% Fe3+ at 100°C. The limits are based on a max corrosion rate of 0.3 mm/year. Maximum F - %

Alloy

Maximum Cl - %

Alloy 825

0.3

0.015

Sandvik 2RK65

0.2

0.04

Sandvik SAF 2507

0.6

0.06

Sandvik Sanicro 28

0.8

0.07

steels and to nickel base alloys. It has been found that the corrosivity declines with increasing amount of metallic ions, such as Mg2+, added to the process solution. Addition of 0.25-0.50% Mg2+ decreases the corrosion rate for Sanicro 28 from nearly 3 mm/year down to 0.1 mm/year. N I T R I C AC I D

Nitric acid is produced by an ammonia oxidation process. Liquid ammonia is evaporated, superheated, then mixed with compressed air and passed to a catalytic converter where ammonia oxidation takes place at 850-950°C. The resulting nitrous oxide is cooled and converted to nitrogen dioxide in an oxidation tower, and finally the gas is absorbed by water in a column to form 57-60% nitric acid. The plants include several heat exchangers where the environment is highly oxidising. The severest tube conditions are normally found in the tail gas preheaters, boiler feed water heaters and cooler/condensors where local condensing and reboiling of acid may occur. Materials of construction

In oxidising environment a high chromium content is favourable. Molybdenum has been found to be detrimental to the corrosion resistance. Therefore low molybdenum or molybdenum free materials are normally used as construction materials. The standard material of construction is the austenitic AISI 304L, but in certain critical areas special alloys have to be used. Tail gas preheaters

Tail gas preheaters are normally tubular heat exchangers, where cold nitric acid tail gas from the absorption tower is heated, and boiling occurs when the droplets entrained in the tail gas hit the hot tube wall and evaporate. The conditions are approximately 35% HNO3 at 120-140°C. Some materials of construction are: • AISI 304L • Sandvik 2RE10 • AISI 329 • Sandvik SAF 2304

(less common) (less common)

SANDVIK STEEL CORROSION HANDBOOK

Normally it is sufficient to use standard austenitic grades, but in some plants problems with corrosion occur. In these cases Sandvik 2RE10 is a good choice, and this alloy is therefore recommended for new constructions with no past experience.

AISI 321 W.-Nr. 1.4541

AT = Solution annealed S = Sensitisation 650°C/1h (1200°F/1h)

AISI 304L W.-Nr. 1.4306

AT S

Sandvik 3R12

AT S

AISI 329

AT S/ 5 min

Sandvik

AT S

Cooler/condensors

The cooler/condensors are normally tubular heat exchangers with the process gas on the tube side. Corrosion is common at the inlet where the first condensate is formed. If reboiling of the condensate occurs, the conditions become very severe. An illustration of the hot dew point corrosion is shown in figure 1. In this instance the gas is cooled from the relatively high inlet temperature of 210°C down to 50°C. Since the corrosive conditions at the tube inlet are so high, Sandvik 2RE10 has to be used. If cooling water contains chlorides, the use of the duplex stainless steel Sandvik SAF 2304 or grade Sandvik Sanicro 28, should be condsidered. The experience with these materials is however limited.

1

2 7101b

Process gas

Tube sheet

Figure 1. Illustration of hot dew point corrosion in nitric acid cooler/condensors. 1 = First condensate formed 120-130°C 2 = Reboiling - corrosion increases with increasing temperature. Boiler feed water heaters

Corrosion in boiler feed water heaters can occur at the outlet end. If a condensate is formed there is a risk of reboiling and therefore a risk for corrosion on AISI 304L. Normally the design is made to avoid condensation, but if 304L fails Sandvik 2RE10 should be used. Acceptance testing

Materials for nitric acid service are normally evaluated by a Huey test ( ASTM A262 Practice C, 5x48 h in boiling 65% nitric acid ). Figure 3 shows values from Huey tests of various alloys used in nitric acid plants. A Huey test after a sensitization heat treatment is also shown. Sensitization may occur during welding when tubes are installed.

7100

0.1 (4)

0.2 (8)

0.3 (12)

0.4 0.5 0.6 0.7 0.8 0.9 1.0 (16) (20) (24) (28) (32) (36) (40) Corrosion rate (mean of 5x48h), mm/year (mpy)

Figure 2. Statistical evaluation of Huey-test results. Arrows denote accelerated attack. O R G A N I C AC I D S

Organic acids belong to the most important group of chemicals in the modern chemical industry. In the petrochemical industry these chemicals are common. This acid group includes a large number with differing properties. The corrosivity of different organic compounds is extremely varied. Organic acids have an alkyl group coupled to the acid group (-COOH). As a general rule, the corrosivity of the acid increases when the alkyl group decreases in size; consequently most of the problems with corrosion are related to products or intermediates containing Formic Acid (HCOOH) or Acetic Acid (CH3COOH). Organic acids are generally reducing, but impurities such as chlorides, traces of catalysts, or air, may increase the oxidising power of the process solution. Materials of construction

There are a wide range of materials used for equipment handling organic acids, ranging from the Austenitic 304L to Duplex Stainless Steels, Nickel base alloys and Zirconium. Traditionally, alloys like 304L, 316L and Alloy 20 (UNS N08020) have been used but today high alloyed materials are increasingly used to counter the possible shifting in redox potential over time, which can result in these acids becoming more corrosive. Acetic acid

Generally, AISI 304L can be used for Acetic acid applications up to 60°C regardless of concentration, whereas at higher temperatures up to the atmospheric boiling point, AISI 316L may be applied. The isocorrosion diagram is shown in the corrosion tables section. In Acetic acid above the atmospheric boiling point there is a risk of HCl formation if the solution is contaminated with chlorides. Higher alloyed materials such as Sanicro 28 or SAF 2507 should be considered in these circumstances. Alloy C-276 ( UNS N10276 ) has been used in some cases with extremely severe conditions.

I: 33

SANDVIK STEEL CORROSION HANDBOOK

Contamination by formic acid, catalysts or chlorides can cause severe pitting of 316L, and therefore upgrading to a higher alloyed material is necessary. Anhydrous Acetic acid and traces of Acetic anhydride will also drastically increase the corrosion rate. Alloy selection depends on the amount and type of contaminants in some cases with reducing conditions. Alloy B-2 has been applied in these aggressive conditions. More exotic materials such as Zirconium are also applicable. Figure 4 shows the corrosion rate of various alloys in Acetic acid with chlorides. Figure 4 shows the corrosion rate of various alloys in Acetic acid with Acetic anhydride and chlorides. Clearly, 316L is unsuitable in contaminated acid. Where Acetic anhydride is present even SAF 2205 has a too high corrosion rate. Here either SAF 2507 or a Nickel base alloy like alloy 625 ( UNS N06625 ) has to be used.

Formic acid

Formic acid is relatively corrosive 304L may be used at ambient temperature for e.g. storage tanks, whereas at higher temperatures 316L is preferred, but only below the 10% concentration level. At higher concentrations, materials like Sanicro 28 and SAF 2507 are preferred materials. The isocorrosion diagram is shown in the corrosion tables. Figure 5 shows corrosion rates of various alloys in Formic acid with chlorides. Corrosion, mm/year 1.2

6759

1.0

0.8

0.6

0.4

Corr. rate mm/year 0.15

6967

0.2

>1 mm/ year

0 C-276

0.10

C-22

C-4

SAF 2507

A 625

SAF 2205

Figure 5. Corrosion rates of various alloys in boiling 40% Formic acid with 2000 ppm chlorides.

0.05

Teraphtalic acid

0 AISI 316L

SAF 2205

Sanicro 28

SAF 2507

254 SMO

Figure 3. Corrosion rates of various alloys in 80% Acetic acid with 2000 ppm chlorides at 90°C. Corr. rate mm/year 0.05 >0.1 mm/ year

Table 5. Various alloys tested in a teraphtalic acid plant.

6969

0 SAF 2205

SAF 2507

A 625

C–4

C–22

C–276

Figure 4. Corrosion rates of various alloys in a boiling solution of 99.8% Acetic acid, 0.1% Acetic anhydride and 200 ppm chlorides

I: 34

Teraphtalic acid is an organic acid which is an intermediate for polyester fibre manufacturing. AISI 317L has been a standard material of construction, but in some cases with limited lifetime due to corrosion. Higher alloyed materials such as the duplex SAF 2205 or SAF 2507 are suitable alternative materials. In table 5 results from in plant testing in a acid plant is shown. Whereas AISI 317L had corrosion rates which limited the lifetime of the equipment, SAF 2205 and SAF 2507 showed very good resistance.

Alloy

75% Acetic acid Traces of Bromine, Cu, Mn, T=175°C mm/year

96% Acetic acid Traces of Bromine, Cu, Mn, T=150°C mm/year

AISI 317L

0.44

0.67

SAF 2205

0.012

0.06

SAF 2507

0.004

0.011

SANDVIK STEEL CORROSION HANDBOOK

Urea production I N T RO D U C T I O N

Urea is a nitrogen based chemical which is very common for use as a synthetic fertiliser in agriculture. It has the highest nitrogen content (46%) of commercial synthetic fertilisers. Urea is also used as raw material for e.g. melamine production. Urea in itself is not very corrosive, but during manufacturing of urea the process fluid becomes very aggressive in the high temperature section. Special stainless steels have been developed for use under these conditions, and the process has to be very carefully controlled, especially regarding the oxygen content in the process fluid, in order to maintain passivity of the materials.

T H E U R E A P RO C E S S

Urea is produced by mixing ammonia (NH3) and carbon dioxide (CO2) at high pressure and temperature; typical conditions are temperatures of 200 °C at 200 atm pressure. Under these conditions ammonia and carbon dioxide react to form ammonium carbamate, which is very corrosive. The ammonium carbamate then partly reacts to form urea. In modern recycle processes the remaining carbamate is stripped from the process fluid downstream from the reactor and recycled to the reactor. Oxygen is added to the process fluid to control the

2RE69, 3R60 U.G.

CW

A flow sheet of a urea plant of Stamicarbon design is shown in figure 6. In the reactor the urea and carbamate mixture is formed. The solution is fed to a stripper, which is a falling film heat exchanger where the solution is fed in the direction of gravity, and a stripping gas (CO2) is fed in the other direction. The carbamate is here separated from the urea, and the carbamate is fed to the high pressure carbamate condensor and recycled to the reactor, whereas the urea is depressurised downstream and finally solidified to urea prills, usually in a prilling tower. During the depressurisation any remaining carbamate is condensed in the low pressure carbamate condensor, followed by compressing and recycling to the reactor. M AT E R I A L S O F C O N S T R U C T I O N

In the high pressure part of the urea processes special stainless steels have to be used. In table 1 the most common alloys used today are listed. Table 2 lists the most common materials of construction in the various parts of the high pressure section of urea plants.

Tubes

Scrubber Plates

corrosivity of the ammonium carbamate and to maintain passivity of the construction materials. Today, there are a few big licensers on the market with slightly different designs. The most common are Stamicarbon, Snamprogetti and Toyo but other actors such as Urea Casale and UTI are also very active, especially for revamping plants of older design.

HP Carbamate Condenser

3R60 U.G., 2RE69, SAF 2205, SAFUREX™

Steam Reactor

LP Carbamate Condenser

Water

6.6 kg/s NH3 3R60 U.G., 3R69, 2RE69 SAF 2205, SAFUREX™

Steam Condensates Tubes 2RE69, SAFUREX™

Stripper

Urea Carbamate

H.P. Piping

CO2 8.74 kg/s Urea Solution

Prilling Tower Air 11.57 kg/s Urea Prills Urea Solution Storage Evaporator

Figure 6. Flowsheet of Stamicarbon urea process. Materials choice for critical parts are indicated.

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SANDVIK STEEL CORROSION HANDBOOK

Stripper

The stripper is a tubular heat exchanger with a carbon steel shell. The tube plates and vessel heads are overlay welded with stainless steel and the tubes are of seamless stainless type. The overlay is usually of 2RE69-type and the tubes are made of 2RE69 (Stamicarbon process) or bimetallic tubes with an outer layer of 2RE69 and an inner liner of Zr702, mechanically bonded together ( Snamprogetti process ). Alternative materials of construction are SAFUREX™ (Stamicarbon) or Titanium (Snamprogetti). Titanium is however hardly ever used for new Snamprogetti plants, instead the more resistant bimetallic tubes are used. The corrosivity of the process fluid is very high in the stripper, and corrosion problems occur at the top part of the stripper if the amount of oxygen added is too low.

Figure 7. Solidified urea prills. Table 6. Common alloys used in urea plants Sandvik designation

UNS Number

Remarks

Sandvik 3R60 U.G.

S31603

< 0.6% ferrite

Sandvik 2RE69

S31050

< 0.6% ferrite

Sandvik SAF 2205

S31803

40-60% ferrite

Sandvik SAFUREX™-

S32906

40-60% ferrite

Bimetallic 2RE69/Zr702

S31050/R60702

-

Carbamate condensor

Table 7. Materials selection in the high pressure part of urea plants Section

Product form

Alloy

Reactor

Plate

Stripper

Seamless tubes

3R60 U.G. 2RE69 SAFUREX™ ( Stamicarbon 2RE69 ( Stamicarbon ) SAFUREX™ ( Stamicarbon Bimetallic 2RE69/Zr702 (Snamprogetti ) 3R60 U.G. 2RE69 SAF2205 ( Stamicarbon ) SAFUREX™ ( Stamicarbon 3R60 U.G. 3R69 2RE69 SAF2205 ( Stamicarbon ) SAFUREX™ ( Stamicarbon

Carbamate Condensor

Seamless tubes

H.P. Piping

Seamless pipes

) )

High pressure piping

)

High pressure piping connecting the vessels is usually made in 3R60 U.G. or 2RE69. Materials such as the duplex grades SAF 2205 and SAFUREX™ are excellent alternatives due to their high mechanical strength.

A C C E P TA N C E T E S T I N G )

Reactor

The reactor is a vessel made of carbon steel lined on the inside with stainless steel. The reactor also contain trays designed to make the reaction more effective. 3R60 U.G. is frequently used in the reactor, but sometimes problems with corrosion occur, especially in the lower part of the vessel. Relining is then often made with 2RE69 which offer better long term corrosion resistance. Nowadays 2RE69 is frequently specified also for new plants. An alternative to 2RE69 for the Stamicarbon plants is Sandvik SAFUREX™.

I: 36

The carbamate condensor is a tubular heat exchanger with a carbon steel shell. The tube plates and vessel heads are overlay welded with 2RE69 and the tubes can be either 2RE69 or 3R60 U.G. Corrosion problems that occur are usually from the cooling water side if chlorides are present. Even chloride contents as low as 5 ppm may cause stress corrosion cracking of the alloys 2RE69 and 3R60 U.G. New materials of construction for the Stamicarbon plants to solve stress corrosion cracking problems are SAF2205 and SAFUREX™. These alloys have a duplex (austenitic-ferritic) microstructure and have shown good resistance to both the water side and the process side conditions.

Materials for urea service have to be corrosion tested in order to ensure that the materials are of good quality. Usually the intergranular corrosion resistance has to be determined and the most common test method is the Huey test (ASTM A262 Practice C, 5x48 h in boiling 65% Nitric Acid), but for the duplex stainles steels the Streicher test (ASTM A262 Practice B, 120 h in boiling ferric sulphate-sulphuric acid solution) has been adopted. An illustration of intergranular corrosion testing is shown in figure 8. Table 8 lists the acceptance tests for various alloys used in urea service.

SANDVIK STEEL CORROSION HANDBOOK

Table 8. Acceptance tests of materials for use in urea service. Alloy

Test

Max corrosion rate mm/year

Max selective attack Long. Trans.

Sandvik3R60 U.G.

Huey

0.60

200

70

Sandvik 3R69

Huey

0.60

200

70

Sandvik 2RE69

Huey

0.12

70

70

Sandvik SAF 2205

Streicher

1.78

100

100

Sandvik SAFUREX™ Streicher

0.78

70

70

Figure 8. Intergranular corrosion.

Oil & Gas industry Corrosion problems and materials selection I N T RO D U C T I O N

and gas extracting units. It will include presentations of corrosion in process fluids and in sea water and materials selection for the different systems involved.

C O R RO S I O N

Process fluids

Although hydrocarbons are not themselves corrosive, the process often contains H2S and CO2 in varying amounts. A well that does not contain H2S, or at least does not have a partial pressure of hydrogen sulphide above 0.05psi, is often referred to as sweet, irrespective of the CO2 content. If the partial pressure of hydrogen sulphide exceeds 0.05psi the well conditions are referred to as sour. The conditions vary from almost totally free from H2S in parts of the North Sea to very sour in e.g. some of the Middle East fields. The risk of cracking due to hydrogen sulphide is often the main factor to consider when selecting materials for exposure to process fluids. Sulphides attached to the metal surface catalyse the absorption of hydrogen atoms by the metallic material. Atomic hydrogen diffuses readily into the steel to regions where some discontinuity, e.g. a non-metallic inclusion, is situated. Such an inclusion is not atomically bonded to the steel so there can be room for two hydrogen atoms and the possibility to create a hydrogen molecule, H2, is created. The hydrogen molecule has a very large volume compared to that of two hydrogen atoms, which causes an extremely high pressure between the inclusion and the metallic material. This pressure can be much higher than the strength of the steel and is therefore capable of causing rupture of the material. All of the mechanisms involved in hydrogen embrittlement due to the presence of H2S are however not yet fully understood.

There are basically two corrosive environments within the oil and gas industry; Sea Water and Process Fluids. Whereas sea water can be said to contain more or less the same corrosive elements, irrespective of field development; process fluids vary a lot from well to well. The hydrocarbons themselves are not corrosive, but the process fluids can contain CO2, H2S, which together with chlorides make the environment aggressive from a corrosion point of view.

Carbon steels suffer from uniform corrosion (when exposed to fluids containing carbon dioxide). This would not be a problem for stainless steel grades but because CO2 reacts with liquid water in the production stream to form carbonic acid, the pH value can be significantly lowered by its presence, which in turn makes the environment more corrosive also to stainless steel grades. The carbonic acid can then react with iron in the alloy to form iron carbonate, FeCO3.

Traditionally carbon steels have been used to a great extent in oil and gas production units. Today more than 95% of plants are using carbon steels for their systems. However the use of corrosion resistant alloys (CRAS), from 13Cr and upwardsis increasing significantly as the trend is directed towards deeper wells with elevated H2S contents and higher temperatures, creating more aggressive environments. This survey will discuss the different parts of the systems present in oil

When studying the corrosivity of sea water, the main environmental factors are: chloride content, pH, temperature, oxidizing strength (oxygen and residual chlorine contents) and other factors including fouling, stagnant/flowing solution and galvanic action. The most common corrosion phenomena that can (and do) occur in this environment are pitting and crevice corrosion and, at temperatures above 50-60°C, stress

Sea water

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SANDVIK STEEL CORROSION HANDBOOK

corrosion cracking. All types of corrosion mentioned here are localised attacks. General attacks do not need to be considered for stainless steels since the corrosion rate is very low in sea water. Within the oil and gas industry, materials can be exposed to sea water in three ways. Subsea flowlines are exposed to relatively cold and oxygen poor sea water at certain depths. These materials are often covered with biological species such as barnacles and mussels, creating somewhat more severe conditions. Secondly, the sea water systems on a platform are exposed to sea water internally. These systems are often chlorinated to prevent the build-up of a biological layer on the surface. The chlorination elevates the corrosion potential dramatically thereby putting high demands on the materials used. The third type of exposure is by the splashing of sea water on to the platform and its topside systems. This might create a risk for external chloride induced stress corrosion cracking if the materials used are too low alloyed for the service conditions.

Flowlines

The exctracted process fluids are transported via the wellhead to flowlines, which connect the well with the process equipment. Flowlines may be several kilometers long e.g.from a satellite well to the actual processing site. The flowlines are exposed to both the process fluids (inside) and (if subsea) sea water (outside). Stainless steel grades are increasingly used for these lines. One of the most widely used CRAs has been duplex 22Cr (SAF 2205); a grade that has good resistance to CO2 and to H2S induced attacks and also, to a certain degree, to sea water. Super duplex 25Cr /(SAF 2507) has also been gaining market shares thanks to the higher strength which can reduce the wall thickness, thus reducing the tonnage. Recently, weldable martensitic 13Cr steels have reached the market. These also seem to be a good solution for CO2 containing fluids with low H2S levels. For very severe process fluids higher alloyed stainless steels or nickel base alloys can be chosen. For short flowlines, between e.g. wellhead and floating platform, sometimes flexible pipes are chosen, in order to accompodate the motions created by the sea. The flexible flowlines consist of an inner CRA liner and plastics armoured with carbon steel. The CRAs used vary from grade 316L up to highly alloyed nickel base alloys. Umbilicals

Figure 9. North Sea oil platform.

M AT E R I A L AT O I L A N D G A S E X T R A C T I N G U N I T S

Tubing and Casing

These tubes, production tubing and outer casings, are often referred to as Oil Country Tubular Goods (OCTG). The size of the casing are in the range from 4 1/2" to 13 3/8" and the tubing is in the 2 7/8" to 4 1/2", and positioned at the actual well site. The production tubing will be the first material to be exposed to (often) relatively hot process fluids at high pressure. Traditionally carbon steels have been used for this application but recently martensitic 13Cr and duplex SAF 22Cr grades have been used more frequently. For very severe conditions, with rhigher H2S levels, nickel base alloys, such as Alloy 825 /Sanicro 28 and Hastelloy C-276, are used. The tubes used for this application are often cold worked to specific minimum strength levels. The tubes are connected by couplings and therefore do not have to be weldable.

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An umbilical is operating subsea as a connection between the platform’s control station and the wells on the seabed. The umbilical normally contains electrical and hydraulic lines for well control as well as lines for injection of methanol and other chemicals, typically to prevent coagulation of the oil. A common cross section configuration is to have the larger methanol injection line in the center of the umbilical with the hydraulic and the electrical lines surrounding it. Pressures up to 10 000 psi (700 bar) are typical for the hydraulic and chemical injection lines. The part of the umbilical that is lying on the seabed is called the static part since it is only to a minor extent influenced by the motions of the sea water. The dynamic part of the umbilical is the one hanging from the platform down to the seabed. This part is more influenced by the sea water motions. When a floating production unit is used the movements of the umbilical are further increased and this puts higher demands on the materials used with respect to corrosion and fatigue properties. The materials used for this type of service include

SANDVIK STEEL CORROSION HANDBOOK

thermoplastic hoses (so far the most widely used), austenitic stainless steel type 316L, duplex grades and superduplex Sandvik SAF 2507. Even more exotic materials, such as e.g. titanium and zirconium, can be considered when very corrosive solutions are to be injected. With developments at greater depths the demands put on the materials will increase with regard to mainly mechanical properties such as tensile strength and fatigue limits. Process systems

Process systems, consisiting of various vessels, heat exchangers, separators, compressors etc. for processing of the well fluid, are normally situated on the platforms. Carbon steels have traditionally been used to a great extent. However, the use of CRAs is increasing and today austenitic type 3R60 and duplex grade SAF 2205 are widely used alloys. 3R60 is usually sufficiently corrosion resistant with respect to the oxygen free process fluids.

Figure 10. Oil platform with static umbilicals controlling subsea wells.

With sea water very close to systems, the air surrounding them will be very chloride rich. If the tubes are exposed to this chloride rich air or even splashes of actual sea water, the risk for externally induced chloride stress corrosion cracking of 316L materials is evident. Therefore duplex SAF 2205 should be considered. The upper temperature limit for SAF 2205 is not well defined and for the highest internal temperatures superaustenitics or nickel based alloys can be chosen.

Table 9: Review of CRA materials used for different systems in an offshore oil/gas extracting unit. Function Down Hole (often in a cold worked condition) Flowlines

Umbilicals

Material used Mar tensitic 13Cr Duplex SAF 22Cr Ni-rich alloys such as e.g. Sanicro 28 Carbon Steel lined with CRA ”Super”-13Cr Duplex SAF 2205 Superduplex SAF 2507 Austenitic 254 SMO Ni-rich alloys SAF 2507 Plastic Hoses Exotic Exclusive materials (Ti, Zr) Welded duplex tubes

Process systems, topside

Sea Water systems

Austenitic type 3R60 Duplex type SAF 2205 Austenitic type 254 SMO CuNi alloys Austenitic 254 SMO Superduplex SAF 2507 GRP Titanium

Comment For CO 2 rich wells with no or low levels of H 2 S For wells with reasonable amounts of H 2 S For severe conditions with regard to mainly H 2 S levels Perhaps more used in the future CO 2 containing fluids, low H 2 S contents Reasonable H 2 S levels More corrosive environments Cl - , oxygen, H 2 S More corrosive environments Cl - , oxygen, H 2 S Severe conditions For a wide range of corrosions and high pressure applications. Traditionally most widely used. Unsuitable for many chemicals and in deep water. For ver y corrosive chemicals Slightly less corrosion reistant and lower mechanical proper ties than seamless SAF 2507 If maximum temperature below 60°C If minimum temperature above -46°C If temperature range below -46°C and above 60°C Insufficient resistance to erosion attacks Max temperature 15-30°C if crevices Max temperature 15-30°C if crevices Max 80°C Max 80°C if crevices

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SANDVIK STEEL CORROSION HANDBOOK

Sea water systems

On the platforms, sea water is used mainly for three purposes: for the cooling water systems, as ballast water and perhaps most important for the fire water systems. For these systems carbon steels and copper-nickel alloys have been used to a great extent. Carbon steels do however corrode and have to be replaced after a certain time period and copper-nickel alloys are susceptible to erosion corrosion in sea water. Different stainless steel grades and plastics as well as titanium have therefore increasingly been selected in the last few years. Titanium and GRP (Glassfibre Reinforced Plastics) have satisfactory properties with regard to corrosion resistance in sea water but there have been reports on failures of these materials due to quality problems. High alloyed stainless steel grades such as austenitic 254 SMO (which has been widely used in the Norwegian sector of the North Sea) and superduplex SAF 2507 can be alternatives. In chlorinated systems with these stainless grades the temperature must not exceed 30°C due to the risk of crevice corrosion. For the tightest crevices such as e.g. threaded connections, the upper temperature limit for 6Mo and superduplex materials can be even lower. In Table 9, the use of different corrosion resistant materials is summarised.

Corrosion in petroleum refining and petrochemical applications I N T RO D U C T I O N

Corrosion has always been an unavoidable part of petroleum refining and petrochemical operations. Although certain material problems are caused by other factors, a predominant number are due to various aspects of corrosion. Operating and maintenance costs are substantially increased due to corrosion problems. Scheduled and unscheduled shutdowns for repairing corrosion damage in piping and equipment can be extremely expensive and anything that can be safely done to keep a process unit running for long period of time will be of great benefit. A significant proportion of corrosion problems are actually caused by shutdowns. When equipment is opened to the atmosphere for inspections and repair, metal surfaces covered with corrosive products will be exposed to air and moisture. This can lead to pitting corrosion and stress corrosion cracking unless preventive measures are implemented. When equipment is washed with water during a shutdown, corrosion can be caused by pockets of water left to dry. When the processes are running, there are basically two corrosive environments to consider, corrosion caused by process fluids and, in the case of heat exchangers, corrosion caused by cooling water.

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Among the many metals and alloys that are available, relatively few can be used for construction of process equipment and piping. For practical purposes, corrosion in refineries and petrochemical plants can be classified into low-temperature (wet) corrosion and high temperature corrosion. Low-temperature corrosion is considered to occur below about 300°C and in the presence of water while high-temperature corrosion does not necessarily demand the presence of water. This section deals only with the field of low-temperature corrosion. C O R RO S I O N

Refinery applications

The major cause of corrosion from the process side in a refinery is not the hydrocarbons themselves but the presence of contaminants in the crude oil as it is produced. Although some contaminants are removed during preliminary treating in the fields, most end up in refinery tankage, along with contaminants picked up during the transportation. Examples of crude oil contaminants that affect the corrosion resistance of a steel are CO2, H2S, nitrogen compounds, sulphur compounds and inorganic chlorides such as NaCl, MgCl2 or CaCl2. Often, the actual corrodants are formed during initial refinery operations. An example of this is the hydrolysis of salts during preheating of distillation feedstock, which results in the formation of hydrochloric acid. Wet corrosion on the process side may also be caused by chemicals added to the process such as various alkylation catalysts, certain alkylation by-products, organic acid solvents, stripped hydrogen chloride, caustic soda and neutralisers that, ironically, are added to control acid corrosion. In some production units it is the water used, rather than the process fluids, that causes corrosion. The cooling water can vary in chloride content from virtually nil in de-ionised and fresh water up to 1.5% in seawater. Cooling and process water sources may also be polluted with sulphides, ammonia and carbon dioxide among others as well as carrying entrained solids. These factors, together with temperature and pH dictate the corrosivity of the water and careful consideration must be given to materials selection. Petrochemical applications

Improvements in production economy can involve the use of higher temperatures and pressures, consequently placing higher demands on materials of construction. Important reasons for corrosion on the process side are different aggressive mixtures of chemicals and hydrolysis of organic chlorides, which may lead to formation of hydrocloricacid.

SANDVIK STEEL CORROSION HANDBOOK

The presence of organic acids in some processes may also introduce corrosion problems for several common steel grades. The reactive acid group (-COOH) is often responsible for corrosion attack. The corrosion behaviour of metals in organic acids is characterised by the slightly reducing conditions of the acids. Halide ions are usually present and may cause severe attack on the standard austenitic stainless steels.

SAF 2205 and SAF 2304 have also been successfully applied. Under severe conditions, for instance at high temperatures and at high chloride contents, it might be recommended to use high alloyed grades like Sanicro 28, Alloy 825 or SAF 2507. If the temperature is above 300°C, super austenitic grades or titanium are the safest options. Other materials used to fight corrosion are copper based alloys, brasses and bronzes and nickel based alloys and titanium.

M AT E R I A L S S E L E C T I O N

Refinery applications

Figure 11 shows a simplified flow diagram of a refinery. In the oil refinery process, each step presents different kinds of corrosion problems which sometimes demand different material solutions. The main properties to take into consideration are however similar. Carbon steels are often used but may suffer from general corrosion. Conventional stainless steels like AISI 304 and AISI 316 are prone to stress corrosion cracking (SCC) in chloride bearing environments. The ferritic grade AISI 430 is not sensitive to SCC but suffers from a predominant risk of pitting. The duplex grades are resistant to SCC Grade 3RE60 has been in service for several years. Later

For cooling systems in fresh and brackish water, SAF 2205 is an appropriate material, whereas SAF 2507 is needed where seawater cooling is utilised. Other materials commonly used in seawater are copper based alloys and titanium. Copper based alloys may suffer from erosion corrosion damage at flow rates occurring in seawater cooled heat exchangers whereas SAF 2507 withstands fluid velocities far above those likely to be experienced in heat exchanger applications [1]. Table 10 presents some examples of critical applications in the oil refinery where corrosion resistant alloys may be used to solve specific problems [3].

Table 10. Some examples where stainless steels have solved corrosion problems. Description

Service conditions

Crude oil desalter

Tube side: Waste water with

Materials selection After experiencing rapid corrosion on carbon steel and pitting of

feed water heater

700-900 ppm chloride, pH 6.

the ferritic grade AISI 410, Sandvik duplex stainless steel 3RE60

Temp: Inlet: 190°C, Outlet: 75°C

was installed and lasted for 17 years before excessive corrosion

Shell side: Feed water with 2 ppm

on the carbon steel shell dictated that the whole unit be replaced.

chloride, pH 7.1.

The unit was replaced with heat exchangers fabricated in SAF 2205.

Temp: Inlet: 35°C, Outlet: 145°C Atmospheric distillation

Tube side: Distilled hydrocarbon fractions.

Feed Effluent Exchangers Temp: Inlet: 188°C, Outlet: 196°C

Chloride containing water became trapped under deposits of hydrocarbons and thereby caused corrosion on carbon steel.

Shell side: Crude oil feedstock with

SAF 2507 has now been installed and solved the previous

produced water.

problems.

Temp: Inlet: 149°C, Outlet: 157°C Hydrodesulphurisation Feed/Effluent Exchanger

Tube side: Reactor effluent with 0.1% H 2 S and 10 ppm Cl - . Temp: Inlet: 350°C, Outlet: 200°C

Hydrogen sulphides and ammonium hydrosulphide are aggressive has solved the problems and also enabled a reduction in wall

Shell side: Feed with 10-20 ppm H 2 S

thickness.

to carbon steel, and the 300 series suffer from SCC . SAF 2205

and 10 ppm Cl - . Temp: Inlet: 70°C, Outlet: 230°C Gas Cleaning

Tube side: Steam

The carbon steel condensers failed and corrosion tests indicated

Shell side: Amines, CO 2 , cyanide controlled

that 304L would suffer from both pitting and SCC, while

Vacuum Distillation

by polysulphide addition, NH 3 and H 2 S. Tube side: Seawater

SAF 2507 has been used to replace admiralty brass for which

Surface Seawater

Temp: Inlet: 24°C, Outlet: 35°C

sand entrained in the seawater had caused failure by erosion

Shell side: Hydrocarbons + 3%H 2 , 5.4%N 2 ,

corrosion at flowrates as low as 1.5 m/s.

Cooled Condensers

SAF 2205 would perform well. Finally SAF 2205 was selected.

0.5%CO 2 and 11%H 2 S. Temp: Inlet: 55°C, Outlet: 127°C

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SANDVIK STEEL CORROSION HANDBOOK

LPG GAS FROM OTHER UNITS GAS PLANT

POLYMERISATION

ALKYLATION

CRUDE DISTILLATION

GAS ETHERS

LIGHT NAPTHA

ISOMERIZATION

HEAVY NAPTHA

HYDROTREATER/ REFORMER

GASOLINE

AROMATICS EXTRACTION

CRUDE DESALTER

AROMATICS LIGHT GAS OIL

HYDROTREATERS

CRUDE OIL

JET FUEL DIESEL

HEAVY GAS OIL

CRACKERS

HEATING OILS LUBE PLANT

LUBES

LUBES ASPHALT

ASPHALT RESID

COKE COKER

Figure 11. Simplified flow diagram of refinery processes.

Petrochemical applications

Amongst the vast array of materials in the chemical industry, the iron and nickel based alloys play the most important role. The requirements and aspects for materials selection in a chemical plant is a balancing act between safety, economy, process and product requirements. Stainless steel is often the natural choice of material for vital equipment in the petrochemical industry. The general materials selection considerations described for refineries, for instance regarding cooling water, also apply to a large extent to petrochemical applications. When organic acids are present on the process side, the duplex family of steels have wide application potential. Laboratory tests of duplex stainless steels in different mixtures of organic acids clearly show that SAF 2507 offers an alternative to Ni-based alloys in high chloride containing organic acids, while SAF 2205 is a suitable alternative when the chloride levels are lower [2]. Polymers and plastics like polyethylene or polypropylene, and fibre-reinforced resins such as vinyl esters or epoxy, have excellent corrosion resistance in many aggressive media and

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in cooling water. These materials represent alternatives to sometimes very expensive metallic materials at comparable corrosion resistance. However, the allowable service pressures are limited and for most of them the service temperature should not exceed 100°C. The chemical process industry contains many additional corrosive environments where significant advantages can be realised by selecting DSS for construction materials. One interesting field is the application of DSS under conditions where Ni-rich steels or alloys are attacked by complexing reactions. An example of this is that DSS perform very well in columns for ammonia extraction in waste water treatment plants, whereas austenitic steels corrode severely. CONCLUSION

For efficient and safe plant utilisation in refineries and petrochemical industries material selection is becoming ever more important. The bulk of stainless steels used today are the conventional austenitic grades. In an increasing number of cases, however, economic considerations, such as plant reliability

SANDVIK STEEL CORROSION HANDBOOK

and service life, justify the selection of higher higher alloyed special stainless steel grades. REFERENCES

1. Sandvik Steel R&D lecture S-51-57-ENG: “SAF 2507 for sea water cooled heat exchangers” 2. Sandvik Steel R&D lecture S-51-55-ENG: “Application of duplex stainless steels in the Chemical and Petrochemical Industry” 3. Sandvik Steel brochure S-1541-ENG: “The Role of Duplex Stainless Steels in Oil Refinery Heat Exchanger Applications”

The pulp and paper industry Corrosive environments are found in several stages in the production of pulp and paper. Process modifications, for increased production, as well as for environmental considerations, have in recent years lead to remarkable changes in the demand for improved corrosion resistance. Modified cooking processes and the introduction of new bleaching chemicals are two examples. Sometimes a change from carbon steel to stainless material can provide the solution, but on occasion special consideration should be given to material selction. KRAFT PULPING

Initially wood chips are heated with chemicals in a digester to split the lignin. In the Kraft process Na2S- and NaOH-containing white liquor is used. Chlorides, thiosulphates and metal ions are also present in the digester, emanating from the chips or from recycled chemicals. Black liquor, i.e. used liquor and dissolved wood constituents, is formed during pulping. Pulping actually consists of several steps, including cooking, extraction and washing, with heating steps according to special temperature programmes. The wood chips are added at the top and the white liquor somewhat below. A number of circulating systems are used for heating and washing of the pulp. In batch digesters the pulping programme proceeds simultaneously throughout the digester. Continuous digesters, on the other hand, are divided into different zones in which the different steps occur. Several modified continuous processes have been developed through the last decades, e.g. using counter-current liquor flows or isothermic cooking. Traditionally, carbon steel has been used for pulp digesters but at increased concentrations of hydroxides and sulphides passivation is not successful, with high corrosion rates as a result. Other problems in this environment are intergranular corrosion of sensitised austenitic steel (AISI 304L) and stress

corrosion cracking [1]. Depending on the pulping process used and the amounts of corrosive compounds in the pulp the location and extent of corrosion may differ. Where the corrosion rate of carbon steel in pulp digesters has become excessive, alternative solutions have been tried. Metallization, weld overlay and anodic pro-tection are some methods used to increase the digester life time. Several cases where the intended protection has caused locally increased corrosion rates have, however, been reported. To overcome these problems new digesters are being constructed of duplex stainless steels or compound sheet. The duplex stainless steel grade UNS S31803, corresponding to SAF 2205, has been used successfully for several years in the construction of digesters and surrounding equipment, such as liquor heaters and tubing [2]. W H I T E L I Q U O R R E G E N E R AT I O N

Regeneration of white liquor is performed in a series of steps. Weak black liquor is separated from the pulp in washers and concentrated in multiple-effect evaporators. It reaches a solid content of about 65% and consists mainly of sodium carbonate, thiosulphate, sulphite and sulphate, together with sodium salts of organic acids. In a black liquor recovery boiler (BLRB) combustion of the organic material takes place and the sodium sulphate is reduced to sulphide. A salt smelt consisting of inorganic compounds is formed at the bottom of the boiler. Hydrogen sulphide has also been detected in recovery boilers and the combination of these corrosive species at high temperatures makes this a very aggressive environment. The smelt from the furnace is dissolved in water, forming green liquor, and causticised by the addition of lime and heat. After clarifying the solution white liquor has been regenerated and may be returned to the digester. SAF 2205 is a good choice in the flash tank and the liquor evaporators, where the used black liquor is drawn out from the digester and concentrated before regeneration. Problems with stress corrosion cracking in the evaporators have been solved by the use of this grade [3]. The special environment in the recovery boiler demands the use of high temperature materials with good corrosion resistance. A good solution is to choose composite tubing, which has been used successfully in this application for more than 20 years. The tubes consist of carbon steel on the inside with good resistance against steam corrosion. The shell side material can be chosen to resist different conditions. Ordinary high temperature stainless steel which performs well in wall tubing can be substituted by for instance Sanicro 38 for the bottom tubing. Composite tubing can also be used as superheater tubing

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SANDVIK STEEL CORROSION HANDBOOK

where the most common combination is 3RE28/HT8. The shell side of this material consists of 25Cr/20Ni material whereas the tube side component is a ferritic steel especially suitable for high pressures and temperatures. See reference 4 for further information on composite tubing in BLRB applications. SULPHITE PULPING

Pulping can also be done using the sulphite process in which sulphur dioxide and bisulphate are the active chemicals. The pH of the cooking liquor is chosen to suit the process and wood type to a value between 1 and 10. The corrosion problems found in sulphite mills result from the generation of sulphuric acid, sometimes contaminated with chlorides. Sulphur dioxide for the pulping is produced by the burning of elemental sulphur. Austenitic stainless steel can not be used in sulphur burning due to the formation of low-melting-point nickel sulphides. Carbon steel should be used for the sulphur burners and combustion chambers producing sulphur dioxide. In the sulphite pulping process AISI 316 steel has been the primary choice for construction material. Where higher chloride concentrations are found Sandvik 2RK65, SAF 2507 or other Mo-bearing grades may be preferred. Molybdenumalloyed stainless steels can be used (at lower temperatures) if the chloride concentration in the acidic solution is controlled.

Figure 12. Composite tubes in a black liquor recovery boiler at ASSI Lövholmen, Sweden. New air ports are being installed and the tube height is raised.

Table11. Materials choice in sulphite pulping Equipment

Materials choice

Sulphite digester (low chloride)

Sandvik 2RK65 or SAF 2205

Sulphite digester (high chloride)

SAF 2507 or 654 SMO

Sulphur burners

Carbon steel

Sulphur combustion chambers

Carbon steel

Cooling towers for SO 2 gas Atmospheric absorption towers

Sandvik 2RK65 or Sanicro 28

Acid tanks

Sandvik 2RK65 or SAF 2205

Sandvik 2RK65

Chips in White Liqour in Wash Liquor

Steam

Upper, Spare and Lower Heaters Digester Wash Water Heater

Wash Liquor

Pulp to Blow Tank

Figure 12. Pulp digester with flash tank and heaters.

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Steam Flash Tank Used Liquor to Evaporators

SANDVIK STEEL CORROSION HANDBOOK

C O R RO S I O N I N T H E PA P E R M I L L

P U L P B L E AC H I N G

Bleaching of pulp is actually a continuation of the delignification process which started in the digester. It has traditionally been performed by the use of chlorine containing chemicals with an initial chlorination step (C), followed by alternative alkali extractions (E) and chlorine dioxide treatments (D). This environment can cause severe problems with localised corrosion. For environmental reasons ECF (elemental chlorine free) and even TCF (totally chlorine free) bleaching has been developed. In the ECF process chlorine dioxide is still used, whereas several combinations of steps have been tried to replace this in TCF bleaching. Among the new chemicals used are ozone (Z), peracetic acid and hydrogen peroxide (P). Ozone is highly oxidising and has proved to be a problem for molybdenum-bearing stainless steels which suffer from local attacks, such as pitting. Intergranular corrosion has occurred in 316 ozone tubing due to the formation of nitric acid [3]. Hydrogen peroxide posses no threat for stainless steels, but the corrosion rates for titanium increase with increasing concentrations of the peroxide ion, HO2– , i.e. with increased pH [5]. In chlorination steps titanium, plastic material or rubber is preferred for towers and tubing. Some nickel-base alloys in the 800 series, e.g. Sanicro 30 and Sanicro 41, have also proved successful. Stainless steels up to AISI 317 should be avoided as increased chlorine concentrations have been found to cause crevice corrosion. Super duplex stainless steels, such as SAF 2507, or high-Mo austenitic grades, like 654 SMO or 254 SMO perform well in chloridic environments and can be used in D towers and surrounding equipment. In the environment formed by alkali extractions Sandvik 2RK65 shows excellent performance. For the ozone environments SAF 2304 or other stainless steels which are low in molybdenum are required, whereas any stainless steel can be chosen for the peracetic acid or peroxide environments normally used.

In the process of papermaking from bleached pulp no corrosives are added. Even so, several cases of corrosion failure have been found due to remainders of aggressive ions in the pulp. This illustrates the importance of controlling processes and environments throughout the production. If wet chloride containing pulp contaminates metal surfaces even AISI 316L material has been found to corrode and fail. The attack will begin under the wet pulp, forming shallow pits. These may be the starting point of cracks or deeper pits. In most cases these problems can be solved by controlling the environment, i.e. keeping metal surfaces clean, reducing chloride concentrations etc. If this proves too difficult a change to a more resistant grade such as Sandvik 2RK65, SAF 2205 or SAF 2507 might be recommended. REFERENCES

1. A Wensley, “Corrosion in digester liquors”, Proceedings of the 8th International symposium on Corrosion in the Pulp and Paper Industry, The Swedish Corrosion Institute, Sweden, 1995. 2. P. H. Thorpe “Duplex Stainless Steel Pulp Digesters Fabrication and User Experience in Australia and New Zealand”, Proceedings of the 8th International Symposium on Corrosion in the Pulp and Paper Industry, The Swedish Corrosion Institute, Sweden, 1995. 3. Sandvik Steel R&D Lecture S-54-29-SWE “Materialval som löser korrosionsproblem inom cellulosaindustrin” english ed. in print. 4. Sandvik Steel R&D Lecture S-54-26-ENG “BLRB composite tubes - 15 years of experience”. 5. P Andreasson, “The corrosion of titanium in hydrogen peroxide bleaching solutions”, Proceedings of the 8th International symposium on Corrosion in the Pulp and Paper Industry, The Swedish Corrosion Institute, Sweden, 1995.

Table 12. Materials choice for bleaching equipment Equipment

Steel grade or other

Chlorination tower and piping, chemical lines and sewer lines

Titanium, Sanicro 30 or Sanicro 41

D-step tower, piping and lines

SAF 2507 or 654 SMO

Z-step tower, piping and lines

SAF 2304 or Sandvik 2RE10

Peracetic acid or P step equipment

Sandvik 3R12 or 3R60

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SANDVIK STEEL CORROSION HANDBOOK

8. In this section, advice on design and fabrication of stainless steels is given, mainly related to tube materials.

Constructional design

Fabrication avoid tight crevices in which corrosion could develop. Examples of such crevices are bolted joints, tube - tube sheet, tube isolation material, tube - biofilm etc. Figure 2. Example of crevices in bolted joints and

In many cases corrosion damage can be attributed to unsatisfactory constructional design. Such damage is often unnecessary and could have been avoided if greater account had been taken for the risk of corrosion at the design stage. Another common reason for corrosion damage is unexpected service conditions. It is therefore important to try to predict possible changes from the normal condition. When a material corrodes only by general corrosion it is possible to predict its lifetime relatively well. Local corrosion, like pitting, crevice and stress corrosion, is more difficult to foresee. This kind of corrosion often leads to abrupt failure in a relatively short time and is therefore dangerous. There are several ways to minimise the risk of local corrosion at the design stage. One way to prevent pitting is to avoid stagnant corrosive media in for example horizontal tubes. This is because concentration of aggressive species occurs and pitting preferentially develops in the direction of gravity.

between tube and tube sheet.

In the case of biofilm formation, it can often be prevented by chlorination of the media. However, chlorination has a negative effect because it increases the risk of local corrosion if the concentration is too high. If gaskets are used in joints it is important not to use chloride containing or porous material in the gaskets because this promotes crevice corrosion. Graphite gaskets are not permitted in combination with stainless steels due to the risk of galvanic corrosion.

Figure 1. Example of poor design.

If the medium is flowing the risk of deposits inside the tube is also less. Clean surfaces decrease the risk of crevice corrosion that could develop between deposit and tube wall. One way to solve this problem is through appropriate constructional design that makes it possible to clean the inside of the tube at certain intervals. Another way is to make sure that the medium is able to drain from the tube when a long interruption in production occurs. In chloride containing media it is particularly important to

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When designing equipment in austenitic stainless steel (ASS) it is important not to impose high tensile stresses in the material. The reason for this is that ASS are prone to stress corrosion cracking in chloride containing media. Tensile stresses can also develop when the tube is welded, expanded, bent, assembled etc. The first three items will be considered later on. When assembling tubes it is important that the tube has the final shape before assembling so that no force is applied on the tube during or after mounting (see figure 3).

SANDVIK STEEL CORROSION HANDBOOK

the weld deposit and heat affected zone (HAZ). That gives the best mechanical strength and corrosion resistance possible in the welded joint. There are two main points to be kept in mind when welding duplex stainless steels. A too high cooling rate will give a very ferritic structure containing chromium nitrides, which above all leads to inferior corrosion resistance. Thus, low energy welding processes and autogenous welding should be avoided unless full quench annealing can be done as post weld heat treatment. Figure 3. Example of good and poor assembling.

One advantage with duplex stainless steels (DSS) is that they are less sensitive to stress corrosion cracking compared to austenitic stainless steels (ASS). Another advantage is that they have about twice the strength of ASS. This means that the thickness of the tube wall can be reduced and therefore result in lighter structures. Another thing to keep in mind at the design stage is that DSS have a coefficient of thermal expansion near that of carbon steel. This can be used to reduce stresses in the construction, which could be a problem if ASS are used. One advantage with the ASS is that they retain high impact toughness at low temperatures (below -50°C).

WELDING

If on the other hand, the heat input is too high, precipitations of intermetallic phases can occur, which embrittle the weld and lower its corrosion resistance. Suitable welding methods for duplex stainless steels are: TIG, MIG, MMA, SAW, PAW and FCAW. Spot welding can be done if the nugget is resistance heated 3-5 seconds period of time for good austenite reformation. The duplex filler metal is normally higher in Ni compared to the base material in order to give a correct ferrite content and further improve the austenite reformation at the fast temperature laps that are involved in welding. The austenitic stainless steels of 18/8, 18/8Mo type possess excellent weldability. Generally speaking, however, the weld ability declines with increasing alloy content. As a rule of thumb, high heat input and high interpass temperatures should be avoided. For austenitic steels having a carbon content larger than 0.06% a high heat input will cause sensitisation, which will lead to carbide precipitation in the grain boundaries in the HAZ and thus can cause intergranular corrosion if exposed to corrosive media. In order to give the high alloyed austenitic steels optimum weldability, they are produced with very low phosphorus and sulphur contents. These steels are sensitive to small amounts of impurities, which diffuse to grain boundaries and segregate to interdendritic areas. For this reason, the heat input shouldbe kept low (3550 1890 1492 1083

1487 4827 4827 2482 2900 2595

307

1063

2966

12.3

79

1535

3000

11.34 0.53

28.9 60

35 71

327 179

1744 1371

Mg Mn

1.74 7.50

149 –

651 1244

1107 2097

Mercury Molybdenum

Hg Mo

13.55 10.2

26.1 23 1821) 5.2

8 144

–39 2610

357 5560

Nickel Niobium

Ni Nb

8.90 8.56

13.1 8

92 55

1453 2468

2732 4927

Palladium Phosphorus (yell.) Platinum Potassium

Pd P Pt K

11.9 1.83 21.4 0.86

11.7 125 9.0 84

70 – 71 100

1552 44 1769 64

2927 280 3830 774

Selenium Silicon Silver Sodium Sulphur

Se Si Ag Na S

4.8 2.33 10.49 0.96 2.07

38 2.5 19.0 70 64

– 84 418 134 –

217 1410 961 98 119

685 2355 2212 892 445

Tantalum Tin Titanium Tungsten

Ta Sn Ti W

16.7 7.31 4.50 19.3

6.5 23 8.9 4.3

55 62 19 168

2996 232 1675 3380

5400 2270 3260 5927

Vanadium

V

6.11

8.3

31

1900

3380

Zinc Zirconium

Zn Zr

7.13 6.50

111 21

419 1852

907 3578

1)

30 6

Coefficient of cubical expansion II:75

°F

Temperature conversion table Start with the temperature value appearing in the centre column. If initial value is in Celsius degrees the corresponding value in Fahrenheit degrees will appear in the right-hand column. If initial value is in Fahrenheit degrees the corresponding value in Celsius degrees will appear in the left-hand column. (°C = 5 / 9 (°F -32) and °F = 9 / 5°C + 32). °C

°C / °F

–40 –34 –29 –23 –17.8

–40 –30 –20 –10 0

–17.2 –16.7 –16.1 –15.6 –15.0

1 2 3 4 5

–14.4 –13.9 –13.3 –12.8 –12.2

°F

°C / °F

°F

6.1 6.7 7.2 7.8 8.3

43 44 45 46 47

109.4 111.2 113.0 114.8 116.6

32.2 32.8 33.3 33.9 34.4

90 91 92 93 94

194.0 195.8 197.6 199.4 201.2

33.8 35.6 37.4 39.2 41.0

8.9 9.4 10.0 10.6 11.1

48 49 50 51 52

118.4 120.2 122.0 123.8 125.6

35.0 35.6 36.1 36.7 37.2

95 96 97 98 99

203.0 204.8 206.6 208.4 210.2

6 7 8 9 10

42.8 44.6 46.4 48.2 50.0

11.7 12.2 12.8 13.3 13.9

53 54 55 56 57

127.4 129.2 131.0 132.8 134.6

37.8 43 49 54 60

100 110 120 130 140

212.0 230 248 266 284

–11.7 –11.1 –10.6 –10.0 – 9.4

11 12 13 14 15

51.8 53.6 55.4 57.2 59.0

14.4 15.0 15.6 16.1 16.7

58 59 60 61 62

136.4 138.2 140.0 141.8 143.6

66 71 77 82 88

150 160 170 180 190

302 320 338 356 374

– – – – –

8.9 8.3 7.8 7.2 6.7

16 17 18 19 20

60.8 62.6 64.4 66.2 68.0

17.2 17.8 18.3 18.9 19.4

63 64 65 66 67

145.4 147.2 149.0 150.8 152.6

93 99 100 104 110

200 210 212 220 230

392 410 413.6 428 446

– – – – –

6.1 5.6 5.0 4.4 3.9

21 22 23 24 25

69.8 71.6 73.4 75.2 77.0

20.0 20.6 21.1 21.7 22.2

68 69 70 71 72

154.4 156.2 158.0 159.8 161.6

116 121 127 132 138

240 250 260 270 280

464 482 500 518 536

– – – – –

3.3 2.8 2.2 1.7 1.1

26 27 28 29 30

78.8 80.6 82.4 84.2 86.0

22.8 23.3 23.9 24.4 25.0

73 74 75 76 77

163.4 165.2 167.0 168.8 170.6

143 149 154 160 166

290 300 310 320 330

554 572 590 608 626

– 0.6 0 0.6 1.1 1.7

31 32 33 34 35

87.8 89.6 91.4 93.2 95.0

25.6 26.1 26.7 27.2 27.8

78 79 80 81 82

172.4 174.2 176.0 177.8 179.6

171 177 182 188 193

340 350 360 370 380

644 662 680 698 716

2.2 2.8 3.3 3.9 4.4

36 37 38 39 40

96.8 98.6 100.4 102.2 104.0

28.3 28.9 29.4 30.0 30.6

83 84 85 86 87

181.4 183.2 185.0 186.8 188.6

199 204 210 216 221

390 400 410 420 430

734 752 770 788 806

5.0 5.6

41 42

105.8 107.6

31.1 31.7

88 89

190.4 192.2

227 232

440 450

824 842

II:76

–40 –22 –4 14 32

°C

°C

°C / °F

°F

°C

°C / °F

238 243 249 254 260

460 470 480 490 500

266 271 277 282 288

°F

°C

°C / °F

°F

860 878 896 914 932

393 399 404 410 416

740 750 760 770 780

1364 1382 1400 1418 1436

510 520 530 540 550

950 968 986 1004 1022

421 427 432 438 443

790 800 810 820 830

293 299 304 310 316

560 570 580 590 600

1040 1058 1076 1094 1112

449 454 460 466 471

321 327 332 338 343

610 620 630 640 650

1130 1148 1166 1184 1202

349 354 360 366 371

660 670 680 690 700

377 382 388

710 720 730

°C

°C / °F

°F

560 571 582 593 604

1040 1060 1080 1100 1120

1904 1940 1976 2012 2048

1454 1472 1490 1508 1526

616 627 638 649 660

1140 1160 1180 1200 1220

2084 2120 2156 2192 2228

840 850 860 870 880

1544 1562 1580 1598 1616

671 682 693 704 732

1240 1260 1280 1300 1350

2264 2300 2336 2372 2462

477 482 488 493 499

890 900 910 920 930

1634 1652 1670 1688 1706

760 788 816 843 871

1400 1450 1500 1550 1600

2552 2642 2732 2822 2912

1220 1238 1256 1274 1292

504 510 516 521 527

940 950 960 970 980

1724 1742 1760 1778 1796

899 927 954 982 1010

1650 1700 1750 1800 1850

3002 3092 3182 3272 3362

1310 1328 1346

532 538 549

990 1000 1020

1814 1832 1868

1038 1066 1093

1900 1950 2000

3452 3542 3632

II:77

Chemical elements Atomic weight

Chemical element

Symbol

89 13 95 51 18 33 85

227 26.98 243 121.75 39.95 74.92 210

Mercury Molybdenum

Hg Mo

80 42

200.59 95.94

Ba Bk Be Bi B Br

56 97 4 83 5 35

137.34 247 9.01 208.98 10.81 79.90

Neodymium Neon Neptunium Nickel Niobium Nitrogen Nobelium

Nd Ne Np Ni Nb N No

60 10 93 28 41 7 102

144.24 20.18 237 58.71 92.91 14.01 254

Osmium Oxygen

Os O

76 8

190.2 16.00

Cadmium Calcium Californium Carbon Cerium Cesium Chlorine Chromium Cobalt Copper Curium

Cd Ca Cf C Ce Cs Cl Cr Co Cu Cm

48 20 98 6 58 55 17 24 27 29 96

112.40 40.08 251 12.01 140.12 132.91 35.45 52.00 58.93 63.55 247

Palladium Phosphorus Platinum Plutonium Polonium Potassium Praseodymium Promethium Protactinium

Pd P Pt Pu Po K Pr Pm Pa

46 15 78 94 84 19 59 61 91

106.4 30.97 195.09 244 209 39.10 140.91 145 231

Dysprosium

Dy

66

162.50

Einsteinium Erbium Europium

Es Er Eu

99 68 63

254 167.26 151.96

Radium Radon Rhenium Rhodium Rubidium Ruthenium

Ra Rn Re Rh Rb Ru

88 86 75 45 37 44

226 222 186.2 102.91 85.47 101.07

Fermium Fluorine Francium

Fm F Fr

100 9 87

257 19.00 223

Gadolinium Gallium Germanium Gold

Gd Ga Ge Au

64 31 32 79

157.25 69.72 72.59 196.97

Samarium Scandium Selenium Silicon Silver Sodium Strontium Sulphur

Sm Sc Se Si Ag Na Sr S

62 21 34 14 47 11 38 16

150.35 44.96 78.96 28.09 107.87 22.99 87.62 32.06

Hafnium Helium Holmium Hydrogen

Hf He Ho H

72 2 67 1

178.49 4.00 164.93 1.01

Indium Iodine Iridium Iron

In I Ir Fe

49 53 77 26

114.82 126.90 192.2 55.85

Tantalum Technetium Tellurium Terbium Thallium Thorium Thulium Tin Titanium Tungsten

Ta Tc Te Tb Tl Th Tm Sn Ti W

73 43 52 65 81 90 69 50 22 74

180.95 97 127.60 158.92 204.37 232.04 168.93 118.69 47.90 183.85

Krypton

Kr

36

83.80

Uranium

U

92

238.03

Lanthanum Lawrencium Lead Lithium Lutecium

La Lr Pb Li Lu

57 103 82 3 71

138.91 257 207.19 6.94 174.97

Vanadium

V

23

50.94

Xenon Ytterbium Yttrium

Xe Yb Y

54 70 39

131.30 173.04 88.91

Magnesium Manganese Mendelevium

Mg Mn Md

12 25 101

24.31 54.94 256

Zinc Zirconium

Zn Zr

30 40

65.37 91.22

Chemical element

Symbol

Actinium Aluminium Americium Antimony Argon Arsenic Astatine

Ac Al Am Sb Ar As At

Barium Berkelium Beryllium Bismuth Boron Bromine

II:78

Atomic number

Atomic number

Atomic weight

Degrees Baumé – Density °Be = 145 –

145 density

At a density of