THE EFFECT OF IRON-OXIDE AS AN INHIBITION LAYER ON IRON-ZINC REACTIONS DURING HOT-DIP GALVANIZING

A study was conducted on the effect of a uniform oxide layer on the ga lvanizing reaction in 0.20 wt pet Al-Zn and pure Zn baths at 450 degre es C. In the 0.20 wt pet Al-Zn bath, poor wettability of the oxide lay er was observed. No significant liquid Zn penetration of the oxide occ urred 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 Fe,AI, inhibition layer that forms at the steel interface in Al-Z n baths. The inhibition effect of the oxide in the purl Zn bath was te mporary, since cracks and other macrodefects in the oxide acted as fas t diffusion paths for Zn. Localized Fe-Zn growth (outbursts) formed at the steel/coating interface, and the number of outbursts was generall y inversely proportional to the oxide layer thickness at constant imme rsion times. Increased immersion time for a constant oxide layer thick ness led to an increase in the number of outbursts. These results simu late the diffusion short circuit mechanisms for Fe2Al5 inhibition laye r breakdown in Al-containing Zn baths.