PHASE-COMPOSITION OF CHROMIUM FILMS DEPOSITED UNDER NITROGEN ION-BOMBARDMENT AND THEIR CORROSION PROTECTION POTENTIAL

Chromium films were deposited onto silicon and iron substrates by electron beam evaporation in an atmosphere of backfilled nitrogen under nitrogen ion bombardment with high ion energy and current density. In the X-ray diffractograms of the films mainly peaks of chromium appear. Rocking curves show that the grains have a preferred crystallographic orientation. In addition to the chromium signals a single small peak of Cr2N is present. According to the diffraction results the nitride may have precipitated as small needle-like crystallites with preferred orientation. Nuclear reaction analysis shows that only 5 at.% nitrogen was incorporated, which is less than expected. This result is attributed to nitrogen loss in the radiation environment. A further effect of bombardment with high power density is strong sputtering. Proton-induced X-ray emission (PIXE) measurements of film thickness reveal that around 60% of the deposited chromium atoms were lost by sputtering. This has a considerable influence on the corrosion protection effect of the films on iron substrates in aqueous environment. If the film is too thin corrosion is enhanced by the formation of a galvanic couple between film and substrate. Films thicker than 1-mu-m showed effective corrosion resistance in electrochemical and long-term immersion tests.