Composition of electrodeposited Zn–Cr alloy coatings and phase transformations induced by thermal treatment

Zn–Cr alloy coatings were obtained in a flow cell, for modeling the process of high speed electrodeposition on steel strips. Alloy coatings, containing between 6 and 18 at.% Cr were annealed at 260 °C in an inert atmosphere. The phase composition and the crystallographic characteristics of “as prepared” and “annealed” coatings, were studied by XRD. It is shown that non-equilibrium δ- and Γ-(Zn,Cr) phases are major constituents of the “as prepared” coatings. On annealing, equilibrium ζ-CrZn13 phase precipitates from δ- and Γ-supersaturated solid solutions. The lattice parameters and the similarities in phase composition of the annealed coatings, deposited onto two types of substrates – low carbon steel and Cr plated (protected) low carbon steel – show that if Fe from the substrate “contaminates” the precipitated ζ-CrZn13 phases, its relative amount do not exceed few tenths of a percent.The influence of the elemental composition, conditions of electrochemical deposition, and post-deposition thermal treatment on phase composition of the coatings is discussed.