Abstract:

Two kinds of molybdate/silane composite films were prepared using a one-step method and a two-step one on hot-dip galvanized steel sheets, respectively. The chemical composition and corrosion resistance of the two composite films were then investigated by means of XPS, AES, neutral salt spray （NSS） test and Tafel polarization curve measurement, and were compared with those of single-layer molybdate and silane films. The results show that （ 1 ） both the two films are of the same double-layer structure, and the inner layer, similar to molybdate conversion film, mainly contains O, Mo, Zn and P, while the outer layer, similar to silane film, mainly contains C, O and Si; （2） the chemical composition of the composite films gradually changes from the film to the substrate and from the inner layer to the outer one; （3） as compared with the single-layer molybdate and silane films, the composite films are of an evidently-inhibited cathode polarization process, with a self-corrosion current decreasing to less than 1/5 of that of the single-layer films and a remarkably-improved corrosion resistance; and （4） the two-step method results in a corrosion resistance stronger than that of the conventional chromate passivation film, while the one-step method results in a slightly poor corrosion resistance which is close to that of the conventional chromate passivation film.

Key words: hot-dip galvanizing, molybdate conversion film, silane film, composite film, corrosion resistance