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

In the present work, various Zn-Cr alloy coatings were deposited on automob ilistic steels by a vacuum process, in which Zn and Cr are evaporated from resistance heated sources. After ion etching, different Zn:Cr evaporation r ates were applied to produce several compositions in the films until a fina l coating thickness of 4 mu m was obtained. The coatings were characterized with respect to morphology (SEM), structure (XRD) and hardness (by nanoind entation). The structure changed from the hexagonal Zn-Cr solid solution to a cubic (bcc) phase as the amount of Cr in the coating increased. Nanoinde ntation tests showed that the presence of Cr can significantly change coati ng hardness: this effect can be correlated with compositional and microstru ctural changes. To optimise the adhesion, the necessity of a multistage pre -treatment of the steel substrates was recognized. Further to Ar+ sputter c leaning of the substrates, the deposition of a thin Cr interlayer is necess ary. It was observed that, without suitable pre-treatments, the coatings ar e poorly adherent. Salt spray corrosion tests revealed that even small Cr additions are able t o significantly improve the corrosion resistance of Zn coatings. The time f or red rust to appear is very long (at least four times) when compared to p ure Zn vapour deposited, or steel sheet electroplated with Zn (reference sp ecimens). The generation of protective corrosion products which can suppres s cathodic reaction is considered an important factor for that improvement. (C) 2000 Elsevier Science S.A. All rights reserved.