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Failure of structures and of many types of automatic equipment is often due to crevice corrosion. Differential aeration was advanced some time ago as a major cause of such attack. The present investigation was made to demonstrate other factors in the mechanism of crevice corrosion. Electrochemical methods were used to determine effects and relationships of polarization potentials and corrosion current densities in the main volume of electrolyte (which has free access to the open surface of the metal tested) and in the metal's crevices and clearances. Special laboratory apparatus was used with which crevice corrosion conditions could be simulated and in which “crevice” width was adjustable. Iron, aluminum and stainless steels were tested in 0.5N NaCl. Iron was also exposed to a mixture of Na2SO4, NaCl and NaNO2, to 8.6N HNO3 and to 0.2N H2SO4. Observations were also made of linear selective type crevice corrosion at the metal-dielectric interface perimeter in acid and of comparative corrosion rates above, at, and below the water line of partly-immersed iron. (In the latter case, a “crevice” is considered to exist near the liquid's meniscus.) It was concluded that much destruction of metal in clearances is due to peculiar electrochemical behavior which results in acceleration of the anodic metal ionization reaction and deceleration of the cathodic reaction. Insignificant differences in potential between metal on the open surface and that in the crevices initiate operation of corrosion cells. Linear-selective dissolution of iron in acids, on the phase boundaries of the metal-dielectric-acid system, proceeds according to the crevice corrosion mechanism. In water-line zone corrosion, the crevice corrosion mechanism is also in evidence, rate depending on whether the electrolyte is neutral, acid or alkaline. In inhibited media, metal potential near the water line becomes negative, due to difficulty of access of inhibitor, so that macrocells are activated.

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The mechanism of anodic dissolution and self-dissolution of stainless steels during local corrosion processes is discussed. The dissolution mechanism can be established only by determining actual velocities of metal dissolution at point anodes and their dependence on potential. In stably operating pits the actual rate of metal dissolution is potential independent. A similar relationship was found under uniform dissolution of steel ~n concentrated chloride solutions of low pH for the ability of steel to transform from the active to the passive state. Such an electrode dissolves at a rate similar to dissolution rates observed in the pits (50-60 mA/cm2), and the rate is governed by diffusion kinetics. On the strength of the results, it is assumed that dissolution of steel in pits takes place by the mechanism of "salt passivity," the rate of which is controlled by the diffusion of anode reaction products through the salt film. The repassivation mechanism of steels in pits is also considered. Pits functioning in the kinetic regime repassivate easily, whereas for those operating in the diffusion regime an external effect on the system is necessary. Contrary to the point of view of some researchers an opinion is expressed that the repassivation potential does not correspond to the metal potential at the pit bottom. The repassivation potential is a variable value and, therefore, it cannot serve as a parameter characterizing the ability of steel to undergo pitting corrosion. Certain regularities of stainless steel dissolution and the causes for breakdown of the passive state are considered. Even small shifts in potential in the positive direction from the stationary one (r can bring about activation in crevices and gaps. The corrosion resistance of stainless steels in crevices is determined essentially by their ability to resist activation in acidic chloride solutions.The local breakdown of the passive state of stainless steels has attracted the attention of numerous researchers (1-21). Nonetheless the mechanism of the process is still obscure. In our opinion, there are questions, e.g., the nature of the anodic process in poin: anodes, the repassivation mechanism and the nature of the repassivation potential, the mechanism of stable operation of a two-electrode system (pit-passive surface) unstable from the electrochemical point of view, the existence of the crevice corrosion potential (like that of pitting formation), etc., which are yet to be studied.However, prior to examining these questions, we would like to draw attention to the reliability of the experimental results obtained from electrochemical kinetic studies of local corrosion processes, as well as to the conclusions drawn from them. We feel that the results of polarization measurements frequently published in the literature do not convey quantitative information on the electrochemical kinetics, because they do not reflect the true anodic dissolution rates of metal in point anodes. Moreover, false conclusions may be drawn, if the local dissolution mechanism is ...