The Significance of the Protection Potential for Fe-Cr Alloys at Room Temperature

A new electrochemical method has been tested with a series of binary Fe-Cr alloys to provide information regarding the mechanisms by which corrosion current is suppressed at the protection potential in chloride solutions. A deactivation mechanism is said to occur when, as the result of lowering its electrode potential sufficiently, it is made thermodynamically immune to corrosion. Experimentally, deactivation is detected by a rapid increase in current density with little change in electrode potential when a second anodic polarization scan is made immediately after determining the protection potential. A repassivation mechanism is said to occur when, upon lowering its electrode potential, the kinetics of a thermodynamically-favored corrosion reaction, at the metal electrode surface becomes very slow as the result of formation of a reaction product film. Repassivation is detected experimentally when, with a large increase in electrode potential, there is very little increase in current upon anodically re-polarizing the specimen after determining the protection potential. Fe-2% Cr exhibited deactivation. Alloys containing 16.9% or more chromium content exhibited repassivation. Fe-1 2% Cr showed either deactivation or repassivation depending on pH.