Breakdown of the Passive State and Repassivation of Stainless Steels

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 in concentrated chloride solutions of low p H 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/cm 2 ), 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 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.