Alloy design of graded Zn-Cr PVD coatings for corrosion protection of cold formed automotive parts

PVD ZnCr coatings exhibit different metastable crystal structures δ, ζ, Γ depending on the Cr content and the deposition conditions. Here we describe the different mechanical and electrochemical properties of these phases. We show that by introducing an appropriate Cr concentration profile it is possible to produce a ZnCr coating with a thin Γ-phase interface layer optimized for substrate adhesion and a δ-phase matrix with a strong basal texture which possesses excellent cold forming properties. This alloy coating is tailored so that the application of a short low temperature annealing step after forming, such as an automotive paint bake hardening treatment, transforms the textured δ-phase into a nanocrystalline ı-phase with no marked texture. The latter microstructure provides significantly improved primary corrosion protection.