Crevice corrosion initiation

Figure 2.4 Crevice corrosion—initial stage in oxygenated water containing sodium chloride. (Courtesy of Mars G. Fontana and Norbert D. Greene, Corrosion Engineering, McGraw-Hill Book Company, New York City, 1967.)...

Figure 18 Schematic of Fontana and Greene model for crevice corrosion initiation of stainless steels in aerated Cl- solution. (After Ref. 21.)...

The production of aqueous sulfide via the chemical dissolution of MnS inclusions was shown to increase the aggressiveness of the low pH, high [Cl ] occluded solution as shown in Fig. 10 such that crevice corrosion can occur at rates on the order of 30 to 50 mA cm . As the alloy sulfur content decreases, the difficulty in obtaining the aqueous sulfide content increases, lengthening the initiation time. Nonetheless, even at the lowest alloy sulfur contents studied (17 ppmw), crevice corrosion initiation due to aqueous sulfide accumulation still occurred. 

The sample shown in Fig. 3 was held several hundred millivolts below its pitting potential [28]. After an incubation period, crevice corrosion initiated. The area immediately inside the crevice was... 

Crevice corrosion initiated immediately upon application of +0.4 V (SCE), as expected for JR -controlled crevice corrosion. An example of the attack observed afterwards is shown in Fig. 19 for a gap of 93 pm. Three regions of attack were observed passive dissolution was observed from the mouth to some distance inside the crevice (termed Xcnt), a region of accelerated attack was then observed at intermediate distances, and finally a region of variable attack was observed at the greatest distances from the mouth. 

Brossia and Kelly [26] studied the effect of bulk solution composition on crevice corrosion initiation and the occluded solution composition. As shown in Fig. 23,... 

Biofilms on passive alloy surfaces can increase cathodic kinetics by way of increasing the propagation rate of galvanic corrosion. Cathodic kinetics increased during biofilm formation on passive alloy surfaces. Crevice corrosion initiation times were reduced when natural biofilms were allowed to form on passive alloys S 30400 and S 31600. 

Crevice corrosion initiation is based on a pH decrease at crevice bottom due to Eq. (7.5). Positive metal ions formed in the absence of oxygen reduction attract chloride into the crevice, initiating acidification (water hydrolysis) ... 

The anodic polarization curve for a specimen with an active crevice will be in principle as shown in Figure 7.17. In this case a very small free external surface is assumed, and any internal hydrogen reduction is disregarded. is the potential as measured with the reference electrode positioned outside the crevice. As explained above, the real potential in the crevice, Ei , is more negative. The lower limit for corrosion in an active crevice is the protection (or repassivation) potential Epr. However, the critical potential that must be exceeded for initiation of the ereviee corrosion process, the crevice corrosion initiation potential, is higher than the protection potential. 

Steel type Resistance to pitting and crevice corrosion Initiation Growth Resistance to stress corrosion ... 

Variations in salinity can also alter the localized corrosion resistance of metals in seawater. As the salinity is increased, the chloride ion activity increases and can result in increased pit and crevice corrosion initiation and propagation [6]. 

One such model was applied to several experimental datasets, including crevice corrosion initiation on stainless steel and active corrosion of iron in several electrolytes [11]. The model was said to break new ground by... 

As a consequence, the polarization curves drawn on a specimen equipped with a crevice former device exhibit (Fig. 7) features very similar to those typical of pitting corrosion. Dining the upward potential scan, crevice corrosion initiates quite abruptly at a critical potential that is generally lower than the pitting potential. Then, during a subsequent backward potential scan, corrosion stops at a critical potential significantly lower than... 

Fe-Cr-Ni-Mo class of steels on crevice corrosion initiation is important. It has been shown that iron, chromium, nickel and Mo improve the resistance of steels to crevice corrosion [7]. Crevice corrosion is not usually observed with 6% molybdenum. The effect of chromium can be harmful if it increases the acidity in the crevice by hydrolysis. At a higher chromium concentration, it generally increases the stability of the passive film, lowers the pH below that required for crevice to initiate, and reduces the passive current density. The rate of entry of chromium ions in the crevice is, therefore, minimized, hence hydrolysis which causes acidic conditions in the crevice is minimized by chromium. 

Figure 4.15 Probability of crevice corrosion initiation in one month. (From Andreason D.B. (1974). R D INCO, Paper presented on ASTM-ASM Symposium in Pitting Corrosion, Detroit, Ml, October 23. Reproduced by kind permission of ASM, Metals Park, Ohio, USA)...

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