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School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 1 Electrochemical Fundamentals and Pourbaix diagram 1.1.
Write electrochemical equations for the oxidation and reduction processes which may occur during: (a) (b) (c) (d)
the corrosion of aluminium by airfree sulphuric acid. corrosion of iron in aerated sulphuric acid containing ferric sulphate. corrosion of copper in aerated sulphuric acid containing ferric sulphate. the uniform corrosion of zinctin alloy in an oxygen saturated solution of cupric chloride, stannic chloride and hydrochloric acid. (e) corrosion of silver in an aerated solution of cupric chloride, stannic chloride and hydrochloric acid. (f) corrosion of nickel in sea water. (g) corrosion of iron in airfree ferric chloride solution. 1.2.
Express the standard electrode potential, Eo, of a metal in terms of the standard Gibbs free energy change Go. Hence calculate the value of Go at standard temperature and pressure for the corrosion of iron assuming a divalent reaction.
1.3.
Iron is connected to copper and then immersed in a solution containing both Fe 2+ and Cu2+ ions (a) Which metal corrodes? (b) Write equations to describe the reactions that occur at each electrode, assuming each metal has a valency of 2. (c) Calculate the maximum possible potential of the resulting corrosion cell.
1.4.
Calculate the rest potential, versus the saturated calomel electrode (SCE), of a piece of copper in equilibrium with a solution containing 106 M copper (II) ions.
1.5.
Use the data provided to construct a composite FeH 2O Pourbaix diagram showing the areas of stability of solids (Fe, Fe3O4 and Fe2O3) and ionic species (Fe2+ and Fe3+) for 1 mole per litre of dissolved ions. Also draw the areas of stability for 10 6 mole per litre of dissolved ions. Briefly explain how the stability areas are designated to respective species. The standard free energies of formation of various solids and ionic species at 298 K are as follows (value in cal/mol): Fe2+
20300 Fe
0
H+
0
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS Fe3+
1.6.
2530
Fe3O4
242400
Fe2O3
177100
H2O
56690
Using the attached Pourbaix diagram for the tinwater system, answer the following questions: (a) What type of equilibrium reactions is represented by the dashed lines and what are represented by the solid ones in the diagram? (b) Why do we see three types of lines, horizontal, vertical and inclined, in the diagram? (c) What is the meaning of the intercept point of lines 2', 3' and 4'? Write the equilibrium reaction for each line. (d) Would you expect tin to be stable in an aqueous, oxygenfree acid solution with a pH = 1.0 that contains 106 M of tin ion? Explain your answers and write the halfcell reactions and overall reactions that would occur. (e) Tin coated cans are commonly used to contain slightly acidic food products. Using the enclosed Pourbaix diagram, schematically sketch the expected corrosion rate of tin in an aqueous solution as a function of pH at a constant potential of E=0. Write the corrosion reaction for any region of high corrosion rate. (f) Indicate in the diagram, the regions of corrosion, immunity and passivation for tin in water at 25°C.
Figure 1. Pourbaix diagram for the tinwater system at 25 oC
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 2 – Electrochemical Fundamentals of Corrosion 2.1.
Write electrochemical equations for the oxidation and reduction processes occurring during:
(a)
the corrosion of Al in airfree sulphuric acid
(b)
the corrosion of Fe in aerated sulphuric acid containing ferric sulphate
(c)
the corrosion of Cu in aerated sulphuric acid containing ferric sulphate
(d)
the uniform corrosion of ZnSn alloy in an oxygen saturated solution of cupric chloride, stannic chloride and hydrochloric acid
(e)
the corrosion of Ag in an aerated solution of cuperic chloride, stannic chloride and hydrochloric acid
(f)
the corrosion of Ni in seawater
(g)
the corrosion of Fe in airfree ferric chloride solution
2.2.
Draw a kinetic diagram (E vs log i) for metal M corroding in an acid solution under activation control. Label carefully and indicate the corrosion rate of this system (icorr). Will corrosion rate be increased or decreased by the following changes:
(a)
increasing io of the MM+ + e reaction ?
(b)
increasing io of the 2H+ + 2e H2 reaction ?
(c)
shifting the reversible potential to a more noble (+) value ?
(d)
increasing the value of for the oxidation reaction ?
2.3.
A typical activepassive metal is passivated by placing it in a solution containing a large quantity of oxidising agent. For such a system what happens to corrosion rate during
(a)
anodic polarisation and
(b)
cathodic polarisation ?
2.4.
Determine halfcell potential of Zn electrode in a 0.01M ZnCl2 aqueous solution. [0.827V]
2.5.
Determine halfcell potential of H electrode in aqueous solution of pH=7, under 0.5atm partial pressure and at 40C. [0.426V]
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS 2.6.
Given that the cell potential is +0.590V of a ZnH cell in a 0.5M ZnCl 2 solution, determine the pH of the solution. [3.28]
2.7.
A piece of Zn is submerged in CuCl2 solution in a small container. Will corrosion occur? If so will the reaction stop eventually? If so under what condition will it stop? [[Zn2+]/[Cu2+]=1.45x1037]
Tutorial 3 - Uniform Corrosion, Galvanic Corrosion 31.
Although all forms of corrosion are undesirable, uniform corrosion is least objectionable. Why?
32.
Can uniform corrosion be used to the benefit of corrosion protection? Can uniform corrosion be used to the benefit of other engineering purposes? Give some examples if you think yes to the above two questions.
33.
What is rust? Does rust have any effect on corrosion?
34.
A metal, M, of valency z, atomic mass W and density D (kgm ) is corroding uniformly over its exposed surface by an electrochemical process with a current
3
2
density of icorr (Am ). Derive an expression of corrosion rate in mmpy of the metal, assuming that the buildup of corrosion product does not stop the corrosion reaction. 2
If the metal is Cu and the corrosion current density is 0.01 Am , calculate the corrosion rate. (0.0116 mmpy) 35.
The rate of uniform corrosion can also be experimentally determined by measuring weight loss over a period of time. Derive an expression of the rate of corrosion in mmpy using data from such a measurement. (Identify every variable in your equations). Discuss the similarities and differences between this method and the method dealt with in question 3.4
36.
It is important to recognise galvanic corrosion when it happens. Consider the following cases:
(a)
It is noticed that a magnesium drain plug in the steel oil pan of an automobile is visibly corroded on the inside (oil side). Is this an example of galvanic corrosion?
(b)
Equal areas of copper and Monel sheet bolted together (Monel nuts and bolts) are immersed in seawater for one year. Inspection shows some general corrosion of the copper and no visible attack of the Monel. Is this a classical case of galvanic corrosion?
(c)
During animal testing of alloys for surgical implants, most procedures recommend that only one type of metal be tested in each animal to avoid galvanic corrosion even though the specimens are physically separated. Is this a reasonable criterion?
(d)
A 10 m2 lead plate contaminated by a small drop (1 mm 2) tin on its surface is immersed in seawater. Will this produce galvanic corrosion?
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS 37.
What are the essential conditions for galvanic corrosion?
3.8.
If one metal of a galvanic couple is to be coated, which would you choose? Why?
3.9.
Galvanic corrosion is an electrochemical process. Is it possible to determine the corrosion rate in this case by electrical measurement? If so, how?
310. Can a comprehensive EMF table serve the needs for predicting industrial galvanic corrosion? Why or why not? 311. On one hand it is known that relative electrode potentials are transferable, on the other it is recommended that galvanic test is to be carried out for a given coupling in a given environment whenever possible. Is the recommendation really necessary? 3.12
Estimate the critical [Sn2+]/[Fe2+] ratio at which Sn is galvanically corroded by Fe at room temperature.
3.13
A piece of galvanised steel is submerged in a small container containing tap water at room temperature. Will the steel corrode? If so, is it possible to identify the condition numerically? Will the situation be different if it was a large container?
3.14
Sea coastal locations are generally more corrosive than inland areas. Does galvanic effect contribute to corrosion in this case and why?
3.15
A Cu plate has an exposure area of 7430 cm2. In the plate there are 5 steel rivets, each with an exposure area of 0.64 cm2. The assembly is submerged in an electrolyte in a sealed container with purging N2. If the corrosion rate of the steel in the solution is 0.156 mm/y when uncoupled: (1) Determine the corrosion rate of the steel rivets.
[77 mm/y]
(2) If it was a steel plate with 5 Cu rivets, what would be to corrosion rate of the steel plate?
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 4 – Crevice, Corrosion, Pitting Corrosion 41.
Explain how crevice corrosion is accelerated by the migration of Cl into the crevice.
42.
Explain why crevice corrosion is particularly difficult to detect.
43.
Describe the autocatalytic nature of pitting corrosion.
44.
Comment on the maximum and average pit depth measurements as means to evaluate pitting corrosion.
45.
Aqueous storage containers are often found to corrode just under the water line (location (c)). This form of corrosion is commonly known as waterline corrosion.
(1) Explain the mechanisms of this type of corrosion. (2) Write down the possible corrosion reactions (assuming galvanised steel container) and point out the location for each reaction.
a
water line
b c d
(3) How can this form of corrosion be avoided? 46.
Write the possible corrosion reactions for situation (a) and (c) shown below and predict which of the three Zn samples is likely to corrode more quickly? Explain your prediction. What would be your prediction if the samples were Al?
Zn rain water rain water (b)
Zn
47.
Zn rain water
air
(c)
(a)
Two plates of mild steel are bolted together with a small gap between them by a plastic bolt. Explain what is likely to happen if the pair is placed in
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS (a) neutral water (b) seawater If the plates are of an austenitic stainless steel, what is likely to happen? 48.
Two type 304 stainless steel plates are bolted together to form a crevice. The assembly is placed in a container filled with artificial seawater. Explain the corrosion process at location A in the following situations (ignore the oxidation of Fe2+ to Fe3+):
(1)
The assembly is immersed and then the crevice is sealed off by an imaginary gate at the opening of the crevice (a).
(2)
The gate is lifted after a prolonged period of time (b). gate lifted
gate closed
B
A (a)
B
A (b)
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 5 Intergranular Corrosion, Selective Leaching, Erosion Corrosion 51.
What is the sensitisation of austenitic stainless steels? Can it be avoided? Once happened, can it be eliminated?
52.
What is weld decay? Can it be avoided? Once happened, can it be eliminated?
53.
Would you recommend the use of type 347 welding rod to weld type 304 sheet to avoid intergranular corrosion? Why? If it has to be an austenitic stainless steel, which steel would you recommend to avoid the problem of weld decay?
54.
Are type 347 and 321 stainless steels immune to sensitisation?
55.
Describe two examples of metallurgical microstructure having significant effect on corrosion resistance.
56.
What is the graphitisation of grey iron? What an alternative material would you recommend to replace a grey iron underground pipeline to avoid/minimise the problem of graphitisation?
57.
What is the dezincification of brass? Does every brass dezincify equally under a given condition? If it has to be a brass, which brass would you choose to avoid the problem of dezincification?
58.
One argues that a dezincified brass is just a brass with less zinc and it should not behave with any difference from a low zinc brass. How would you comment on this?
59.
Why is most damage suffered by the leading edge of propellers?
510. Discuss the effect of flow velocity on corrosion. 511. List three ways to prevent erosion corrosion. 512. Erosion corrosion is due to the combined action of a corrosive environment and mechanical abrasion. In what way does it differ from (i) mechanical erosion, and (ii) environmental corrosion?
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 6 Stress Corrosion Cracking, Corrosion Fatigue 61.
What effect does stress cycling frequency have on a corrosion fatigue test?
62.
How dose increased relative slip between two metals affect fretting corrosion?
63.
What is the wedging effect of corrosion products?
64.
Stresscorrosion cracking is due to the combined action of a tensile stress and corrosive environment. In what way does stresscorrosion cracking differ from (i) mechanical cracking, and (ii) environmental corrosion? Tutorial 7 Oxidation, Hydrogen Attack
71.
Comment on the use of PillingBedworth ratio as a criterion for oxidation resistance. List desirable properties of a scale for good protection against high temperature oxidation.
72.
High temperature oxidation is regarded in analogue to aqueous galvanic corrosion. Comment on the differences in measures for prevention of deterioration in these two cases.
73.
What are ntype oxides and what are ptype oxides? What is doping effect?
74.
Explain the following observation: (1) The oxidation rate of Ni is observed decrease after being alloyed with Li and increase after being alloyed with Cr. (2) The oxidation rate of Zn is observed increase after being alloyed with Li and decrease after being alloyed with Al.
76.
What are the differences between stresscorrosion cracking and hydrogen embrittlement? Are these differences important so far as protection measures are concerned?
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 8 Corrosion Monitoring and Testing 81.
What is the planned interval test? What information can you get from such a test?
82.
Compare the suitability and limitations of the following corrosion rate monitoring techniques: (1) coupon insertion (2) electrical resistance probe (3) linear polarisation method
83.
How is crevice corrosion measured and how is crevice corrosion tendency evaluated?
84.
What techniques can you use to monitor the wall thickness reduction due to corrosion of a slurry pipeline?
8.5.
Explain different setups for stress corrosion cracking testing.
8.6.
The tendency or resistance of a material toward stress corrosion cracking can be tested in a number of ways, including constant strain test, constant load test and slow strain rate test. List the parameters that can be determined to evaluate stress corrosion cracking tendency/resistance in these tests. Comment on the merits and demerits of the methods.
8.7.
It is important to have a standard, wellplanned procedure to carry out a corrosion failure analysis. List the essential steps of the procedure for a component failed on site for the following three stages of analysis: on site inspection, laboratory test, evaluation and reporting.
8.8.
Can hydrogen attack be detected? If yes, how?
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS
Tutorial 9 Corrosion Control 91.
A steel lockgate is to be protected using aluminium sacrificial anodes. The anodes are bars of 125 mm by 500 mm with a steel core for welding to the structure. Determine the following parameters for the protection system and answer question (4): (1) (2) (3) (4)
minimum number of anodes required. number of years these anodes may provide sufficient protection. minimum number of anodes required to provide sufficient protection for 10 years. Is overprotection a concern if you use the number of anodes as calculated in (3)?
Data: dimensions of lock gate:
92
width: 12 m Waterline height: 8m (upper stream) 3m (lower stream) 2
current required for protection:
130 mA m
maximum current output of aluminium anode:
6.5 A m
volume consumption rate of aluminium anode: anodic efficiency of aluminium anode:
1180 ml A y 65 %
2 1 1
If a metal is corroding in an acidic solution at a rate corresponding to a current density i, by the application of a cathodic current of the same density magnitude will (1) the corrosion of the metal be suppressed (2) the rate of the corrosion be reduced (3) the rate of the corrosion be increased? Explain your answer using a diagram.
93.
Calculate the theoretical capacity of zinc sacrificial anode material. The measured 1
capacity of a zinc anode is 780 Ahkg . Calculate the efficiency of the metal. Calculate the consumption rate of the zinc anode.
1
(819 Ahkg )
94.
Using a graph, explain schematically at what current density will an aluminium anode be consumed if it is used to protect an underground carbon steel pipeline.
3.10.
Explain why only DC is used for impressed current cathodic protection.
96.
You are asked to design for a cathodic protection system using sacrificial anodes for an under sea structure. What information would you need?
9.7.
You are asked to design a cathodic protection system using impressed current for an underground pipeline. What information would you need?
9.8.
A steel pipeline submerged in seawater is to be protected by impressed current cathodic protection. The length of the pipeline is 500 m. If anodes are empirically chosen to be placed at a distance 2 m from the pipeline, estimate the number of anodes required for the protection system, given that the range of current density on the
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS structure for proper protection is 90~160 mAm2. Assume the electrical resistivity of seawater is 20cm. It is desirable to have fewer anodes for easy installation and maintenance. What can you do to reduce the number of anodes? What are the implications and concerns of these changes? 9.9.
A current of 10A is charged into a steel water pipe and leaves it at an interval of 20m. The pipe has an exterior diameter of 50mm and a wall thickness of 1.25mm. (a) Predict the corrosion behaviour of this pipe. (b) Assuming the electrical resistivity of the steel is 10 5cm and the resistivity of water is 104cm, estimate the total annual metal loss due to corrosion. (c) Estimate the total annual metal loss due to corrosion if the media is seawater, which has a resistivity of 20cm.
School of Mechanical Engineering
Industrial Corrosion and Prevention 308
TUTORIAL QUESTIONS other questions 75.
Ni produces a ptype oxide. Li has virtually no resistance to oxidation. Cr is an element invariably used in oxidationresistant alloys. What oxidation behaviour would you expect when Ni is alloyed with a small amount of (1) Li and (2) Cr?
91.
It is planned to place an uncoated steel drilling platform in the sea. The immersed parts of the structure will be protected with an aluminium alloy (Galvalum I) sacrificial anode for cathodic protection for a lifetime of 10 years. The anodes are to be semicylindrical with a diameter of 100 mm, a length of 400 mm and a steel core for welding the plane side to the structure. Calculate the following parameters (in your calculation the ends, the back and the steel core may be ignored): (1) minimum number of anodes required. (2) number of years these anodes may provide sufficient protection. (3) minimum number of anodes required to provide sufficient protection throughout the designed life time of 10 years. 2
Data: wetted surface area of legs: 2000 m
wetted surface area of cross members: current required for protection:
2
500 m 2
110 mA m
maximum current output of Galvalum I:
2
6.5 A m
1 1
volume consumption rate of Galvalum I: 1180 ml A y anodic efficiency of Galvalum I: 60 % 96.
An offshore oil and gas company has a section of a carbon steel pipeline located in seawater. The section is to be protected using semicylindrical Mg anodes. Try to sketch a design of the protection system and estimate the number of anodes necessary to provide adequate protection for a period of 5 years. The protection specifications are as following (assuming pure Mg anodes and ignoring the ends and back of anodes): length of pipeline to be protected: outside diameter of pipe: current required for protection: dimensions of Mg anode: anode maximum current output:
1.5 km 600 mm 100 mA/m2 20x45 mm 4.1 A/m2
If the pipeline was buried underground how would you modify your design (sketch only)?
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