Corrosion behavior of the ζ-CrZn13 phase obtained by annealing an electrodeposited Zn-Cr coating

Abstract

Electrodeposited and physically vapor-deposited Zn-Cr alloy coatings consist of several non-equilibrium Zn-Cr phases – hexagonal η-(Zn, Cr), hexagonal δ-(Zn, Cr), and cubic Γ-(Zn, Cr) – which have different corrosion characteristics. Upon annealing, these non-equilibrium phases transform into ζ-CrZn13 phase and η-Zn or α-Cr, depending on the Cr content of the alloys. A layer solely composed of the ζ-CrZn13 phase can be obtained from a certain stoichiometric composition of the starting alloy. In the present work the corrosion behavior of a layer composed solely of the ζ-CrZn13 phase is studied for the first time. This phase is obtained by annealing a Zn-5.6%Cr alloy coating (η-(Zn, Cr) phase) electrodeposited on steel. Electrochemical and corrosion studies revealed a more positive corrosion potential in 0.5 M Na2SO4 solution (at pH 5.9) and less weight loss (at pH 1.3) of the thermally treated coating as well as more regular corrosion compared to the as-deposited layer. The changes in the morphology and phase composition of the alloy layer occurring as a result of annealing, and the effect on corrosion, are studied using SEM and XRD.

Keywords

• Electrodeposited Zn-Cr alloy
• ζ-CrZn13 phase
• Corrosion
• Anodic dissolution