Crystal Structure of Electrodeposited Zn-Cr Coatings

Abstract

The crystal structure of the electrodeposited Zn-Cr coating has been analyzed by using X-ray diffraction. The electrodeposited Zn-Cr coating has η′-Zn phase under the content of about 8 mass%Cr, it has γ′-ZnCr phase over about 15 mass%Cr and the two phases co-exist between 8 and 15 mass%Cr. The lattice constant of the a-axis for η′-Zn phases having hcp structure increases and that of the c-axis decreases as a function of Cr content. The crystal structure of the γ′-ZnCr phase is close to that of Cu5Zn8 type structure. η′-Zn and γ′-ZnCr are nonequilibrium phases. After the as-deposited Zn-Cr coatings are heated at 260°C for 5.4 ks, the coating of 0.8 mass%Cr transforms to η-Zn phase and those of more than 5.7 mass%Cr transform to ζ-ZnCr phase. ζ-ZnCr phase which has not been reported in the Zn-Cr binary alloy system has CoZn13 type structure.