Vapour deposited Zn-Cr alloy coatings for enhanced manufacturing and corrosion resistance of steel sheets

摘要

In the present work, various Zn-Cr alloy coatings were deposited on automobilistic steels by a vacuum process, in which Zn and Cr are evaporated from resistance heated sources. After ion etching, different Zn:Cr evaporation rates were applied to produce several compositions in the films until a final coating thickness of 4 μm was obtained. The coatings were characterized with respect to morphology (SEM), structure (XRD) and hardness (by nanoindentation). The structure changed from the hexagonal Zn-Cr solid solution to a cubic (bcc) phase as the amount of Cr in the coating increased. Nanoindentation tests showed that the presence of Cr can significantly change coating hardness: this effect can be correlated with compositional and microstructural changes. To optimise the adhesion, the necessity of a multistage pre-treatment of the steel substrates was recognized. Further to Ar{sup}+ sputter cleaning of the substrates, the deposition of a thin Cr interlayer is necessary. It was observed that, without suitable pre-treatments, the coatings are poorly adherent. Salt spray corrosion tests revealed that even small Cr additions are able to significantly improve the corrosion resistance of Zn coatings. The time for red rust to appear is very long (at least four times) when compared to pure Zn vapour deposited, or steel sheet electroplated with Zn (reference specimens). The generation of protective corrosion products which can suppress cathodic reaction is considered an important factor for that improvement.