2-Corrosion Rate Measurement

CORROSION RATE MEASUREMENTS

Image source: Alabama Specialty Products, www.metalsamples.com

Basic Corrosion Measurements If a piece of metal:

corrodes to this:

How would we know how much corrosion had occurred? Clearly, we need before and after measurements… Mass? Weigh before, weigh after To be of any use, a weight loss measurement has to be scaled to the size of the specimen … corrosion is a surface related phenomenon.

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Basic Corrosion Measurements Size? Length,, width and/or thickness before… Length before • What about after?

Volume before and after… byy liquid q displacement… p • Sensitive enough?

M Mass? ? Weigh before, weigh after To be of any use, use a weight loss measurement has to be scaled to the size of the specimen … corrosion is a surface related phenomenon.

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Basic Corrosion Measurement Because corrosion is a surface phenomenon, weight loss is related to surface area. For example: a metal “coupon” having a total surface area of 10 cm2 loses 1 mg when h exposed d to t a particular ti l environment i t for f a month; th

1 mg Total corrosion = 10 cm 2 which should be a characteristic amount for that metal in that environment in a month.

mg Rate = 0.1 01 cm 2 month 4

Basic Corrosion Measurement • Corrosion engineers traditionally work in units of dm2, thus: mg cm2 mg 0.1 2 10 ×100 = = 10 mdm 2 2 cm ⋅ month dm dm ⋅ month

• BUT … even though mdms give reasonable numbers for most corrosion situations situations, the month is a bad unit …

• So,

mg dm 2 ⋅ day

are often used …. mdd

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Basic Corrosion Measurement If we know the metal density density, we can convert the weight loss to “penetration”: for ρ in g/cm3:

1g mg cm 3 dm 2 x −5 = × × × = 10 cm / day 2 2 ρ dm ⋅ day g 1000 mg 100 cm ρ x

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Basic Corrosion Measurement • However, we note that 1 cm = 104 μm, thus:

x

ρ

10

−5

cm x μm day d x μm ≡ × 0.1 × 365 ≡ 36.5 day ρ day yr ρ yr

• A Another th common unit it for f corrosion i penetration t ti (in (i the th US) iis the th mil … or milli-inch per year (=0.001 in/year or mpy).

z

μm yr

=

z mil 25.4 yr

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Basic Corrosion Measurement NOTE: these rates are averaged over the exposure time, and are quoted as if corrosion were constant with time … though it often is not …

Corrosion rate 1 =

M1 T1

Corrosion rate 2 =

M2 T2 8

Basic Corrosion Measurement Often, our bit of metal ..

after exposure, looks like this …

It is covered with scale. How would we assess corrosion under these conditions?

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Basic Corrosion Measurement Consider the scaling process… Weight before = Wo [g] Surface area = A [dm2]

Exposure time = θ [days] Find condition; weight after = W1 [g]

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Basic Corrosion Measurement We can only be sure that the remaining metal provides an unequivocal reference, so … Descale

Descaled weight = W2 [g]

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Basic Corrosion Measurement Total corrosion amount =

(W0 − W2 ) 1000 mg ⎛ mg ⎞ × ⎜ 2 ⎟ A 1g dm ⎝ ⎠

(divided by θ for rate … mdd mdd)) We also have a measure of the weight of scale …

(W1 − W2 ) g scale = A dm 2 If we assume the scale is composed of Fe2O3, we can calculate the iron in scale as …

Fein scale Fein scale

(W1 − W2 ) = × fraction of Fe in scale A 1000 mg (W − W2 ) ( 2 × 56 ) = 1 × × A ( 2 × 56 ) + ( 3 × 16 ) 1g 12

Basic Corrosion Measurement Fein scale

(W1 − W2 ) mg = 700 A dm 2

The difference between iron in scale and total amount corroded is either – Iron released to environment… OR… – Iron deposited p from environment

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How Do We Descale? Chemically or electrochemically … usually in weak acids (See Table 1.2). BUT … such descaling can also dissolve some of the remaining metal, so … how h do d we accountt for f thi this?? – Employ an “inhibitor” – Use a blank – Figure 1.24. (ASTM G1)

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Summary • Simplest and most powerful technique for corrosion rate determination is the Weight Loss Technique Corrosion Rate = mass/[(exposed surface area] · [time]) or = Average corrosion penetration depth/time = (mass/density/surface area/time) • Common Corrosion Rate Units – gmd: grams of metal loss per square meter per day (mdd (mdd)) – mm/y: / average millimeters illi t penetration t ti per year – mpy: average mils penetration per year, 1 mil = 0.001 inch)

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Basic Corrosion Measurement (Non--uniform Corrosion) (Non NOTE: such corrosion measurements and units are useful for general or uniform corrosion but they don’t work for localized corrosion (e.g. pitting or cracking). A component can fail by stress corrosion cracking (SCC) with no detectable weight loss …

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Basic Corrosion Measurement • For these situations, we use concepts such as crack propagation rate o or pit p p propagation p g rate.. rate • For example: –

“percent

through through--wall wall” for tubes tubes, pipes pipes, vessels vessels, etc etc.;;

–

“crack depth” or “pit depth” – usually in mil or mm;

– “propagation rate” –

μm yr

,

mm etc. yr

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Industrial Corrosion Measurement • How would you “measure” (estimate) the corrosion of plant components? p p • For example: – – – – –

pressure vessels or pressure tubes; f d pipes feeder i steam generator tubes; feedwater pipes and heaters; etc. etc.

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Corrosion Rate Measurement Technique • Corrosion coupon (weight loss measurement) • Electrochemical methods – – – – –

Potentiodynamic polarization (Tafel analysis) Linear p polarization resistance (LPR) ( ) Electrochemical impedance spectroscopy (EIS) Corrosion potential Electrochemical noise ((EN))

• NonNon-electrochemical methods – – – –

Electrical resistance (ER) Hydrogen monitoring Chemical analysis Microscopy

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Corrosion Coupons Various coupon shapes and sizes

ASTM G58

“C” Ring – ASTM G38

Image source: Alabama Specialty Products, www.metalsamples.com

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Example: Atmospheric Exposure Test

Image source: www.corrosionsource.com 21

Electrochemical Techniques

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Electrochemical Polarization • Involving large perturbation – Potentiodynamic polarization ((Tafel Tafel analysis)

• Involving small perturbation – Linear polarization resistance (LPR) – Electrochemical impedance spectroscopy (EIS)

• Involving g no perturbation p – Corrosion potential (Ecorr) – Electrochemical noise (EN)

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Electrical Resistance (ER) Method • Measures the change in electrical resistance of a corroding metal elements relative to a reference non non--corroding element sealed within ithi the th probe b body. b d

L R=ρ A

Image source: Alabama Specialty Products, www.metalsamples.com

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ER Probe Types

Image source: (1) Alabama Specialty Products, www.metalsamples.com (2) S.Y. Li et al., Mat. Chem. Phys., 103 (2007) 9

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Hydrogen Permeation Monitoring • The generation of atomic hydrogen, as part of the cathodic reaction in acidic environments, can be used for corrosion monitoring purposes. • Hydrogen monitoring sensors are often attached to the outside walls of vessels and piping.

Image source: IonScience, www.ionscience.com 26

Hydrogen Permeation Monitoring Principles

Pot 1

•

Pot 2

As hydrogen passes through the metallic wall of the vessel it enters a probe chamber attached tightly to the outside wall, leading to –

C harge Side + H + e -> H

Oxidize Side + H + OH -> H O

C athodic to p roduce H C ounter electrode

–

2

Measure C urrent μmA/cm 2

– Ref elect

Working electrode Samp le Memb rane

Solutions NaOH 1M

Pressure increase with time within the chamber An electrochemical current resulting from the oxidation of hydrogen under an applied potential A current generated in an external circuit, as hydrogen enters a miniature fuel cell.

Devanathan-Starchurski Technique

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Surface Analysis: Microscopy

SEM image of a cross section of a steel specimen including an iron carbonate film. Exposed for 10 hrs at T=80oC, pH 6.6, PCO2 = 0.54 bar, Fe2+ = 250 ppm, v = 1 m/s

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Surface Analysis: Microscopy

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Radiographic Inspection

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Standards for Corrosion Testing ASTM A90/A90M Weight of Zinc Coatings

G5 Potentiostatic/Potentiodynamic Anodic Polarization

A262 Practice A (Oxalic Acid Etch)

G28 Method A (Ferric Sulfate/Sulfuric Acid)

A262 Practice B (Streicher Test) A262 Practice C (Huey Test) A262 Practice E (Modified Strauss Test) A380 Cleaning and Descaling Stainless Steels

G28 Method B (PEMT) G31 Immersion Testing G36 Boiling Magnesium Chloride G44 Alternate Immersion Testing

A923 SSAT Duplex Stainless Steels

G47 Stress Corrosion Cracking of Aluminum

B813 Liquid and Paste Fluxes for Soldering Copper

G48 Method A (Ferric Chloride Pitting)

B154 Mercurous Nitrate – Copper and Copper Alloys

G48 Method B (Ferric Chloride Crevice)

C871 Extraction of Leachable Ions

G48 Method C (Ferric Chloride Critical Pitting)

D1141 Ocean Water

G48 Method D (Ferric Chloride Critical Crevice Temperature)

D1193 Reagent Water D2240 Rubber Property Durometer Hardness D2583 Barcol Hardness D4340 Pressurized Hot Wall

G49 Constant Load Test G61 Cyclic Potentiodynamic G71 Galvanic Couple p Test G75 Miller Wear Test

E3 Standard Metallographic Practice

G102 Calculation of Corrosion Rates from E/C Measurements

E45 Inclusion Content

G108 E/C Reactivation for Detecting Sensitization

E112 Grain Size

G123 Boiling Sodium/Calcium/Lithium Chloride

F746 Medical Implants F2129 E/C Testing of Surgical Implants G1 Preparation, Cleaning, Evaluating Test Specimens G3 Conventions Applicable to E/C Measurements

G150 Critical Pitting Temperature

NACE TM0169 Standard Immersion

ISO ISO 6509 Dezincification of Brass

OTHER Corrositex™ Assayy

TM0174 Protective Coatings

DuPont SW800M SSAT

TM0177 Sulfide Stress Corrosion

DuPont SW1P Ferroxyl Test

TM0284 Resistance to Hydrogen Induced Cracking (HIC)

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Homework 1) What are the four component of a corrosion cell and list five different cathodic reactions – DO NOT LIST FIVE METAL REDUCTION REACTIONS!!! 2) Problems 1-4 3) Problems 1 1--9 Due: Next week

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