Corrosion mechanism of zirconium and its alloys-III. Solute distribution between corrosion films and zirconium alloy substrates

Analyses of stripped corrosion films were performed by wet chemical methods, neutron activation and by electron microprobe analysis. It was found that the amount of tin, niobium, and chromium in post-transition oxide films was directly proportional to the amount in the alloy. There was no measureable difference in film composition of a given alloy as a function of film thickness, and the combination of alloying elements in ternary alloys had no effect on the amount of a given element in the corrosion film. Analyses of a pre-transition film in a Zr-l Cr binary alloy were made by using a tracer, Cr⁵¹. No concentration differences in chromium were found in corrosion films formed in 400°C steam over a range of thickness from 7000 to 18,000 . It was concluded that this alloy did not exhibit concentration changes in the substrate adjacent to the oxide film as found by Wagner for Ni-Pt alloys, and that diffusion of chromium in the metal was unimportant for this particular case. The beneficial effect of chromium and harmful effect of both tin and niobium on post-transition corrosion rates can be associated with the amount of solute in the film, although no suitable mechanism for these effects can be found at present.