Effect of Proportions of Γ and Iron–Zinc Solid Solution Phases on the Corrosion Prevention Performance of Mixed Coating Layers on Heated Galvannealed Steel Sheets

Abstract

Coating layers having different ratios of a Γ phase to an Fe–Zn solid solution phase were produced on galvannealed steel sheets by heat treatment. The corrosion prevention characteristics of these layers were then investigated using combined cyclic corrosion test, electrochemical measurements and analyses of corrosion products. A coating layer with a larger proportion of the Γ phase showed better corrosion prevention properties along with a smaller corrosion mass loss. The Γ phase exhibited a sacrificial corrosion protection effect with regard to both the Fe–Zn solid solution and the steel substrate while the Fe–Zn solid solution showed a sacrificial corrosion protection effect on the steel substrate. The corrosion products formed from the Γ phase comprised Zn5(OH)8Cl2·H2O, Zn5(CO3)2(OH)6, ZnO and various amorphous compounds, while the Fe–Zn solid solution generated Zn-containing ferrihydrite and amorphous compounds. The presence of Zn evidently stabilized the ferrihydrite and delayed the formation of crystalline iron oxyhydroxide based on comparisons with corrosion products formed on cold-rolled steel sheets. These effects improved the corrosion prevention performance of the Fe–Zn solid solution. The zinc-based corrosion products formed from the Γ phase provided even better corrosion prevention performance by delaying the formation of crystalline iron oxyhydroxide and were also themselves resistant to reduction.