Corrosion-electrochemical behavior of iron family silicides in various electrolytes

Basic regularities of the anodic dissolution and cathodic behavior of iron family silicides in acidic and alkaline electrolytes are considered. The effect of external and internal factors on the mechanism and kinetics of the anodic process is studied using a combination of electrochemical, analytical, and structural methods. The silicides are shown to have a high chemical resistance in oxidizing and nonoxidizing acids, except for environments containing fluoride ions. The mechanisms of the anodic dissolution of silicides in acidic and alkaline environments and in fluoride-containing electrolytes substantially differ. In acidic electrolytes, metal selectively dissolves from the silicide sublattice, while silicon remains on the surface and forms a SiO2 film, which has a high resistance in fluoride-free environments. Later, the process is limited by the diffusion of metal atoms from the bulk of the silicide toward the surface layer and the diffusion of oxidized metal through the film of hydrated silicon hydroxide. In alkaline electrolytes, the solubility of silicon and silicon dioxide sharply increases, and the kinetics of the anodic process is determined by the formation of protective films composed of oxides and hydroxides of the metal component of silicides. The films passivate the surface and make the silicides stable against alkalis. In a number of cases, the lower silicides reveal electrocatalytic properties in the cathodic process.