State analysis of electrolytic chromate film by XPS and SXS

Abstract

The chemical state of Cr in the interior and on the surface of silica-free and silica-added electrolytic chromate and that at the interface between the chromate films and electrogalvanized (EG) steel sheet were quantitatively investigated by SXS and XPS. Specimens for the interface analysis were prepared by dissolving the Zn coating in mercury after adhering stainless steel sheets to the films. The Cr 3+ ( Cr 3++ Cr 6+) ratio (Cr 3+ ratio) of the interface was substantially the same as that of the interior, and that of the surface was 20% lower than that of the interior. Addition of silica to the film does not affect the chemical state of Cr. The Cr 3+ ratio of the interface as well as the surface increased with the increase in the insoluble Cr 3+ ratio obtained by an immersion test in boiling water, indicating that Cr 3+ is insoluble in water. The surface of the chromate film was Cr 6+-rich and the interior of the film is Cr 6+-poor, which is attributed to the fact that Cr 6+ is easily reduced to Cr 3+ during chromate formation at the interface between the chromate bath and EG surface.