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An electrochemical technique has been developed with which the steadystate concentration of adsorbed hydrogen on an iron surface, 8, can be determined. By using values of e determined by electrochemical techniques, a method of evaluating the specific rate constant for hydrogen absorption into iron, kabs, has been developed. Values of ~ determined on a cathodically polarized iron surface in 0.0001N sodium hydroxide ~ 0.1M sodium sulfate were between 5 and 12% of the total monolayer coverage, over the currentdensity range 10-300 ~A/cm 2. This current-density range represents a hydrogen generation rate equivalent to a corrosion rate of 5-150 mils penetration per year. The variation of ~ with cathodic overpotential was in close agreement with theory; values of 5~/5 log10 ~ were 3.5 V/decade measured experimentally and 4.2 V/decade theoretically. Experimental values of 0 permit, for the first time, determination of the specific rate constant for the absorption reaction. * Electrochemical Society Active Member.

Evidence is presented to demonstrate that although the critical pitting potential is qualitatively related to the resistance of a material to passivity breakdown by pit initiation, it is of questionable value in predicting the corrosion performance off a structure containing crevices. Experiments conducted on a 30Cr-3Mo-Fe alloy indicated total immunity to passivity breakdown by pit initiation during anodic polarization in 1M NaCl and under freely corroding conditions in acidified 10% FeCl3. However, on specimens containing an artificial crevice, passivity breakdown by crevice corrosion initiation was observed in the same two media. The long term implications of these data were confirmed by 16 month immersion tests in sea water, where severe crevice attack was noted equal in extent to that observed on A9SS Types 304 and 316 stainless steels whose pit initiation properties were considerably inferior to those of the 30Cr-3Mo-Fe alloy. The results of cyclic potentiodynamic anodic polarization experiments conducted on A8S1 Type 430 stainless steel are presented to demonstrate that the so called protection potential, Ep, is not a unique material parameter. It is shown that Ep varies in a semi logarithmic manner with the amount of localized attack induced by polarization; that is, with the chemistry changes in the growing pit caused by hydrolysis of corrosion products and chloride ion buildup in the pit cavities. On this basis, it is concluded that Ep data (however determined) cannot be used alone as a criteria for protection against the propagation of pre-existing pits or crevices in an engineering structure. Based on cyclic polarization procedures, a method of assessing the susceptibility and resistance of an alloy to crevice corrosion is suggested. It is shown that, for alloys which initiate pits during anodic polarization, the magnitude of the “difference potential” (Ec-Ep) is related to the crevice corrosion weight loss. For alloys that have no critical pitting potential, the presence of hysteresis during cyclic polarization with an artificial crevice indicates susceptibility to crevice corrosion.

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