Crevice Corrosion and Pitting Corrosion Most common forms

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Crevice Corrosion and Pitting Corrosion

Crevice Corrosion and Pitting Corrosion

Most common forms of localized corrosion are pitting – film breakdown often by halides

Most common forms of localized corrosion are pitting – film breakdown often by halides and oxidizing solutions, crevice corrosion - within small gaps and openings and stress corrosion cracking

Crevice corrosion schematics

Crevice corrosion schematics

Electrode potential for crevice is more negative than the open face iron

Electrode potential for crevice is more negative than the open face iron

0. 5 mm is the diffusion layer thickness for oxygen in water, smaller gaps

0. 5 mm is the diffusion layer thickness for oxygen in water, smaller gaps mean oxygen transport is impeded Crevice heights

Evans diagram for the crevice corrosion of iron Crevice corrosion propagation by sustained different

Evans diagram for the crevice corrosion of iron Crevice corrosion propagation by sustained different composition in crevice

Confined anodic reaction produces concentrated cations and cations, which then attract Cl- to maintain

Confined anodic reaction produces concentrated cations and cations, which then attract Cl- to maintain charge neutrality and make a very corrosive local environment

Area effects Area outside crevice drives/accelerates crevice corrosion How?

Area effects Area outside crevice drives/accelerates crevice corrosion How?

Distance from crevice

Distance from crevice

Design considerations to reduce crevice corrosion

Design considerations to reduce crevice corrosion

• • Protecting against crevice corrosion Corrosion inhibitors chromates, dichromates, molybdates, phosphates, etc.

• • Protecting against crevice corrosion Corrosion inhibitors chromates, dichromates, molybdates, phosphates, etc. Cathodic protection Design to eliminate crevices Select resistant materials

halides promote pitting and shift peak effect of changing anions in solution

halides promote pitting and shift peak effect of changing anions in solution

Overwhelming the Cl with SO 42 - reduces its effect

Overwhelming the Cl with SO 42 - reduces its effect

Pit initiation

Pit initiation

Actual measured values in a real pit

Actual measured values in a real pit

Increasing H+ and Cl- increases the corrosion current (conditions found in pits)

Increasing H+ and Cl- increases the corrosion current (conditions found in pits)

2) Dissolved Mo species makes molybdates which acts as a corrosion inhibitor 3) Mo

2) Dissolved Mo species makes molybdates which acts as a corrosion inhibitor 3) Mo species interact with cation vacancies to slow transport 4) Molydate species form a layer which inhibit transport of Cl- ions 5)Mo slows dissolution at pit base

Why? ? ?

Why? ? ?

Demonstration of Cl- increased in film when pitting occurs

Demonstration of Cl- increased in film when pitting occurs

Illustration showing similarities between pitting corrosion, crevice corrosion and stress corrosion cracking There is

Illustration showing similarities between pitting corrosion, crevice corrosion and stress corrosion cracking There is always a transport obstacle for the cathodic and anodic reactions.

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