Pitting Crevice Corrosion

Pitting Crevice - Pitting Pitting and crevice corrosion Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate

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Unformatted text preview: Pitting Pitting and crevice corrosion Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion mitigation 1 image source: Corrosion Doctors , www.corrosion-doctors.org 2 A pit Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Causes of pitting Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments general attack (small pits) deep pit (anode) An aluminum A92519 specimen exposed to a 3,5% NaCl solution for seven days. The width of the picture is approximately 1 mm. Corrosion mitigation intact (cathode) image source: Corrosion Doctors , www.corrosion-doctors.org 3 • Localized chemical or mechanical damage to the protective oxide film. • Low dissolved oxygen concentrations and high concentrations of chloride (as in seawater) • Localized damage to, or poor application of, a protective coating • The presence of non-uniformities in the metal structure of the component, e.g. inclusions. Corrosion mitigation 4 1 Passive metals pit Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Morphology of pitting Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Pitting corrosion on a stainless steel bar exposed to an alkaline solution loaded with chlorides. narrow/deep pits elliptical pits shallow pits Corrosion mitigation Corrosion mitigation mesa attack image source: Prof. Dr. H. Böhni, www.ibwk.baum.ethz.ch 5 image source: Corrosion Doctors , www.corrosion-doctors.org 6 Morphology of pitting (continued) Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments subsurface pits Background: theory of differential aeration Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments O2 + 2H 2O + + 4e potential / V undercutting pits vertical grain attack horizontal grain attack image source: Corrosion Doctors , www.corrosion-doctors.org 7 cathode E corr 4OH on c atho tota de l ca thod ic = – - O2 + 2H2O + 4e - E couple corr anode E corr 2e e 2+ + d Fe d catho n Fe node a ic nod on a tal a = to couple I corr 4OH – on anode Corrosion mitigation Corrosion mitigation | log I / A 8 2 Anode acidifies Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Cathode passivates Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments … due to depletion of oxygen anode acidifies: soluble … due to rapid oxygen reduction on the cathode and a more alkaline solution : Fe2+ + 2H2O Fe(OH)2 + 2H+ O2 + 2H2O + 4e … it passivates 4OH – … leading to even more iron dissolution Corrosion mitigation Corrosion mitigation 9 10 An active/passive differential aeration cell Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Under deposit pitting of passive metals Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion on passivated cathode + 2e – + 2+ on p O2 + 2H 2O assiv a ted + 4e - cath ode Fe 4OH O2 + 2H2O + 4e stuff 4OH – – potential / V E couple corr Fe Fe cid on a 2+ + 2e ano Fe – ified de passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments passive film Fe Fe 2+ + 2e - stainless steel Corrosion mitigation couple I corr Corrosion mitigation | log I / A 11 12 3 Under deposit acidification Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Pit: a differential aeration cell Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments around the pit … due to the depletion of oxygen and due to increasing concentrations of dissolved iron under the deposit, the solution acidifies: soluble Fe 2+ + 2e – + O2 + 2H 2O + 4e - 4OH – arou nd t h e pit potential / V Fe Fe2+ + 2H2O Fe(OH)2 + 2H+ couple E corr Fe 2e 2+ + Fe pit the in – Corrosion mitigation … leading to activation (active dissolution) of iron under the deposit and rapid growth of a pit 13 Corrosion mitigation couple I corr | log I / A 14 Under deposit pit growth Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion Chloride induced pitting of passive metals Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments O2 + 2H2O + 4e stuff - 4OH – FeOOH + Cl passive film FeOCl + OH – passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments passive film FeOOH stainless steel Fe Fe 2+ + 2e - Corrosion mitigation Corrosion mitigation pitting aggravated by a large cathode/anode ratio 15 16 4 Chloride induced pit growth Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion Chloride induced pitting of passive metals Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments + O2 + 2H2O + 4e - 4OH – without Cl - passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments passive film Fe Fe 2+ + 2e - | potential / V with Cl - Corrosion mitigation Corrosion mitigation log (current density) / (A m-2) pitting aggravated by a large cathode/anode ratio 17 18 Prevention of pitting Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Crevice corrosion Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments • decrease: – chloride content – acidity – temperature • increase: – flow / mixing – surface cleaning Crevice corrosion is a localized form of corrosion usually associated with stagnant solutions in shielded areas such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints, clamps, etc. • eliminate – suspended solids – dead legs / stagnant sections Corrosion mitigation • add inhibitors • material selection 19 Corrosion mitigation image source: Corrosion Doctors, www.corrosion-doctors.org 20 5 Example of crevice corrosion Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Another example of crevice corrosion Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Crevices corrosion of a S30400 stainless steel washer after 30 days in 0.5 FeCl3 + 0.05 M NaCl solution Corrosion mitigation Corrosion mitigation Stainless steel bolt used in seawater after 5 years of exposure. image source: George Dinwiddie, www.alberg30.org 22 image source: Corrosion Doctors, www.corrosion-doctors.org 21 Crevice corrosion of passive metals Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion Crevice corrosion of passive metals Introduction Electrochemical thermodynamics Electrochemical kinetics Corrosion rate measurements Various forms of corrosion passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments O2 + 2H2O + 4e - 4OH – … due to depletion of oxygen and due to increasing concentrations of dissolved iron in the crevice, the solution acidifies: soluble passive film passivity galvanic corrosion pitting/crevice corrosion cracking erosion corrosion corrosion environments Fe Fe 2+ + 2e - Fe2+ + 2H2O Fe(OH)2 + 2H+ stainless steel crevice corrosion aggravated by a large cathode/anode ratio Corrosion mitigation Corrosion mitigation … leading to activation (active dissolution) in the crevice 24 23 6 ...
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This note was uploaded on 03/17/2010 for the course ME ME78212 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.

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