Application of electroactive films in corrosion protection. II. Metal hexacyanometalate films on TiO₂/Ti surfaces

The metal hexacyanometalate films, Prussian blue (PB), Ni hexacyanoferrate (NiHCF), and Cu hexacyanoferrate (CuHCF), were studied for their ability to poise the potential of an oxide-covered metal in a stable, passive region of voltage. This requires the films to catalyze oxygen reduction, and oxygen reduction on PB films was found to be as large as 0.14 mA/cm². However, no net oxygen reduction was observed on NiHCF and CuHCF films. When PB films were galvanically coupled to Ti electrodes, the open-circuit potential was stabilized at 0.38-0.52 V vs. SCE even at area ratios (Ti/PB) as high as 10. PB films directly deposited onto Ti electrodes were also studied. The films could only be reduced at a negative potential (-0.18 V vs. SCE) near the flatband potential for TiO₂, and electrochemical oxidation did not occur under dark, deaerated conditions. Upon exposure of reduced PB on TiO₂ to dissolved O₂, or to illumination, the PB film was oxidized and the coupled system adopted a stable potential between +0.2 and 0.3 V.