Pitting corrosion acceleration

Wilde, B. E. and Williams, E., On the Correspondence Between Electrochemical and Chemical Accelerated Pitting Corrosion Tests , J. Electrochem. Soc., 117, 775 (1970)... 

Localised Corrosion (or localised attack) accelerated corrosion at certain sites only of a metal surface, usually induced by spatial separation of the anodic and cathodic sites. Examples include pitting corrosion, stress-corrosion cracking and intergranular corrosion. 

Pitting (pitting corrosion) the formation of small holes in an otherwise passive metal surface as a consequence of locally accelerated corrosion. 

Chlorides in particular present a problem because of their tendency to attack and weaken passive oxide layers and accelerate metal wastage by pitting corrosion and other forms of concentration cell processes. 

On a clean surface of an Fci7Cr alloy in a 0.5M NaCl electrolyte, corrosion is accelerated as pitting corrosion when a potential pulse of 1 s duration extending from the passive region and above the pitting potential is applied. Gugler et al. ° showed by in situ AFM that in this case the pitting corrosion was initiated close to an inclusion on the surface (Fig. 8). Such a surface defect may act as a center for pit nucleation, as was... 

As we saw in the foregoing section, pitting corrosion of passive metals occurs beyond the critical pitting potential, plt. In order to protect passive metals from pitting corrosion, therefore, it is advisable to hold the corrosion potential as far less positive from plt as possible in the passive potential range. The presence of p-type oxides, however, makes Econ more positive and hence enhances the breakout of pitting corrosion. In the same way, metals corroding in the active state will accelerate their corrosion rates when their electrode potential is made more positive (more anodic) by the presence of p-type oxides. 

T.M. Devine and B.J. Drummond, Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitization and Susceptibility to Intergranular Stress Corrosion Cracking in High Temperature Water of Duplex 308 Stainless Steel, Corrosion, Vol 37, 1981, p 104-115... 

Alloys, used at high temperatures, obtain their protection from a dense and adherent oxide layer formed on the metal surface. The corrosive attack of metals and alloys in molten salts is due to the solubility of oxide scales by basic and acidic dissolution. This breakdown of the passive film gives rise to accelerated metal consumption by enhanced oxidation (Hot Corrosion). The phenomenon is closely related to pitting corrosion of metals and alloys in aqueous solutions. 

B.E. WHde, E. Williams, On the correspondence between electrochemical and chemical accelerated pitting corrosion tests, J. Electrochem. Soc. 117 (1970) 775—779. 

Step of initiation of SCC [36], It has been reported that bound water, which occupies active sites for corrosion, initiates pitting corrosion [37]. Zhang et al. [38,39] suggested that the same model could be apphed to explain the inhibition and acceleration of anions [38] and cations [39] for the initiation of SCC. 

Crevice corrosion is another form of localized corrosion that occurs in occluded regions where the electrolyte has limited access. Examples of such regions include under rivets and lap joints. The chemistry of crevice corrosion bears some similarity to pitting corrosion in that aggressive electrolyte conditions can develop in the occluded regions with a resultant acceleration of the corrosion rate. 

Temperature can also be used as an acceleration factor in a fashion similar to potential. Many materials wiU not pit at a temperature below a critical value that is often extremely sharp and reproducible [56-62]. At low temperatures, extremely high breakdown potentials are observed, corresponding to transpassive dissolution, not localized corrosion. Just above the critical pitting temperature (CPT), pitting corrosion occurs at a potential that is far below the transpassive breakdown potential. This value of CPT is independent of environmental parameters and applied potential over a wide range, and is a measure of the resistance... 

Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitisation and Susceptibility to Intergranular Stress Corrosion Cracking in High-Temperature Water... 

Under certain special environmental conditions, the passive films, which were described earlier in this Chapter, are susceptible to localized breakdown. Passivity breakdown may result in accelerated local dissolution (localized corrosion) of the metal or alloy. There are two (related) major forms of localized corrosion following passivity breakdown localized corrosion initiated on an open surface is called pitting corrosion, and localized corrosion initiated at an occluded site is called crevice corrosion. In the presence of mechanical stress, localized dissolution may promote the initiation of cracks. 

In the case of surface corrosion or pitting corrosion not microcolonies but biofilms are involved (Kunzel, 1991). Under oxygen deprivation in the biofilm, fermentative processes take place and carbonic acids (also complexing agents) are formed. Also, sulfate reduction occurs (H2S evolution). These processes cause acid corrosion as well as attack by hydrogen sulfide. In the reaction of HjS with iron(II) ions, iron sulfide is formed, and this speeds up/accelerates the oxygen corrosion (cathodic reaction) Cough-ton et al., 1988). These processes are presented in Fig. 4-6. 

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