Photoelectrochemical processes on anodic layers

Properties of anodic layers formed on various stainless steels and other iron base alloys were studied by means of photopotential and photocurrent measurements. The tests were carried out in boratebuffer solutions and in sulphate solutions with pH ranging from 2 to 11. The study has been extended to layers formed by thermal oxidation in air. The results obtained at pH 8.7 indicated that the various materials can be grossly classified into two groups according to their photoelectrochemical behaviour : those behaving like Fe and those behaving like Ni. The first group comprises Fe-Cr alloys, Fe-Ni alloys below 40 % Ni, and all the examined stainless steels. To the second group belong Fe-Ni alloys with more than 40 % Ni. For Fe-Ni alloys with Ni> 40 % a difference of the semiconducting properties has been established between anodic and thermal oxides. A difference of behaviour was also observed between AISI 3xx type and AISI 4xx type steels in the electrode potential range 500-1100 mV (NHE) when the pH is below 4. Tests were also made in order to investigate the influence of anodic current density on the characteristics of anodic layers during their formation.