EFFECT OF PHOSPHORUS SURFACE SEGREGATION ON IRON-ZINC REACTION-KINETICS DURING HOT-DIP GALVANIZING

Phosphorous was ion implanted on one surface of a large grain (10 to 2 0 mm) low-carbon steel sheet in order to study the effect of surface s egregation on the formation of Fe-Zn phases during galvanizing. Both a n Al-free and a 0.20 wt pct Al-Zn bath at 450 degrees C were used in t his investigation. It was found that P surface segregation did not aff ect the kinetics of Fe-Zn phase growth for the total alloy layer or th e individual Fe-Zn gamma, delta, and zeta phase alloy layers in the 0. 00 wt pct Al-Zn baths. In the 0.20 wt pct Al-Zn bath, the Fe2Al5 inhib ition layer formed with kinetics, showing linear growth on both the P- ion implanted and non-P-ion implanted surfaces. Fe-Zn phase growth onl y occurred after extended reaction times on both surfaces and was foun d to directly correspond to the location of substrate grain boundary s ites. These results indicate that P surface segregation does not affec t the growth of Fe-Zn phases or the Fe2Al5 inhibition layer. It was sh own that in the 0.20 wt pct Al-Zn bath, substrate grain boundaries are the dominant steel substrate structural feature that controls the kin etics of Fe-Zn alloy phase growth.