A surface study of the black chromate conversion film on the zinc-iron alloy deposit

摘要

A new type of black chromate conversion film was obtained on the zinc-iron alloy deposit containing 0.58% iron from our proprietary chromating solutions and characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy(SEM). The results showed that the black chromate conversion film is a composite film composed mainly of Cr_2O_3, Cr(OH)_3, Cr(OH)CrO_4 and Zn_2(OH)_2CrO_4, and the Zn, Cr and O elements are comparatively uniformly distributed in depth. However, the content of high valent chromium in the film on zinc-iron alloy deposits is higher. The film contains trace amounts of Fe~(2+) in the depth of it and this is the main reason of its high corrosion resistance. SEM observation indicated that there exist 'dried riverbed' cracks on this film surface. Synergistic effects between chromate conversion film and zinc-iron alloy deposit in corrosion resistance were also discussed.