The effect of iron oxide as an inhibition layer on iron-zinc reactions during Hot-Dip galvanizing

Abstract

A study was conducted on the effect of a uniform oxide layer on the galvanizing reaction in 0.20 wt pct Al-Zn and pure Zn baths at 450 °C. In the 0.20 wt pct Al-Zn bath, poor wettability of the oxide layer was observed. No significant liquid Zn penetration of the oxide occurred and, therefore, attack of the steel substrate to form localized Fe-Zn growth did not occur. It was found that the iron oxide acted as a physical barrier or inhibition layer in the pure Zn bath, similar to the Fe₂Al₅ inhibition layer that forms at the steel interface in Al-Zn baths. The inhibition effect of the oxide in the pure Zn bath was temporary, since cracks and other macrodefects in the oxide acted as fast diffusion paths for Zn. Localized Fe-Zn growth (outbursts) formed at the steel/coating interface, and the number of outbursts was generally inversely proportional to the oxide layer thickness at constant immersion times. Increased immersion time for a constant oxide layer thickness led to an increase in the number of outbursts. These results simulate the diffusion short circuit mechanisms for Fe₂Al₅ inhibition layer breakdown in Al-containing Zn baths.