Electrochemical characterization of nitride-coated iron surfaces

Abstract

In the last few years, techniques have been developed that result in surfaces with lower thermal oxidation, and improved wear and corrosion resistance than previous ion-implantation methods. Ion-beam mixing has proved effective in producing highly adherent surface coatings with interesting chemical and physical properties. However, the corrosion behaviour of these films, as determined by electrochemical tests, is poor, probably because of their limited thickness or because of defects in the films. To overcome such difficulties, our laboratories have studied some new coating methods such as reactive ion-beam enhanced deposition (RIBED) consisting of simultaneous or sequential deposition-implantation steps, or ion-beam mixing in which very thick multilayers (about 100 nm thick) are deposited followed by a high energy implantation. In these ways we have obtained, on pure iron samples, highly adherent and hard surface layers consisting of boron or chromium nitrides. These materials, which are particularly suitable for tribological applications, have been examined electrochemically to characterize their corrosion behaviour. The RIBED coatings exhibited an improved passivability in an acetate buffer solution and a lowered susceptibility to chloride ion attack in an NaCl solution.