EFFECT OF SUBSTRATE GRAIN-SIZE ON IRON-ZINC REACTION-KINETICS DURING HOT-DIP GALVANIZING

In the galvanizing process, it has been proposed that the grain size o f the substrate steel influences the Fe-Zn alloy phase reaction kineti cs and growth rate during immersion in the liquid Zn bath. Two grain s izes (nominally 15 and 85 mu m) were developed in a decarburized low-c arbon (0.005) steel and hot-dipped galvanized in 0.00 wt pct Al-Zn and 0.20 wt pct Al-Zn baths to study the effect of substrate grain size o n Fe-Zn phase formation. Uniform attack of the substrate steel occurre d in the 0.00 wt pct Al-Zn bath, since an Fe2Al5 inhibition layer did not form. No barrier to nucleation of the Fe-Zn phases exists in this Zn bath, and therefore, the substrate steel grain size had no signific ant effect on the kinetics of phase growth for the gamma, delta, and z eta phase layers. In the 0.20 wt pct Al-Zn bath, discontinuous Fe-Zn p hase growth (outburst formation) occurred due to the initial formation of the Fe-Al inhibition layer. The nucleation of the Fe-Zn phases was significantly retarded in this bath for the large (85 mu m) substrate grain size. Whereas outbursts were found in the 15-mu m grain size su bstrate after 10 seconds of immersion time, it required 1200 seconds t o nucleate just a few outbursts in the 85-mu m substrate. These result s support the mechanism that Fe-Al inhibition layer breakdown occurs a long fast diffusion paths for Zn in the inhibition layer that correspo nd to the location of substrate steel grain boundaries where reaction with Fe can occur.