FE-ZN PHASE-FORMATION IN INTERSTITIAL-FREE STEELS HOT-DIP GALVANIZED AT 450 DEGREES-C .1. 0.00 WT-PERCENT AL-ZN BATHS

Interstitial-free alloy steels containing various combinations of solu te additions of titanium, titanium + niobium and phosphorus, were hot- dipped in a pure zinc (0.00 wt% Al) at 450 degrees C in order to study the morphology and kinetics of Fe-Zn phase formation. Uniform attack of the substrate occurred on all of the steels leading to the formatio n of a th ree-phase alloy layer morphology containing gamma, delta and zeta Fe-Zn phases. Titanium and titanium + niobium solute additions h ad no effect on the growth kinetics of any of the Fe-Zn phases. Phosph orus additions were found to retard only the kinetics of gamma-phase g rowth, without influencing the growth kinetics of the other Fe-Zn phas es. In fact, the gamma-phase layer in the phosphorus-containing substr ates was no longer discernable in light optical microscopy after 120 s immersion. The growth kinetics of the total Fe-Zn alloy layer (gamma delta + zeta) was dominated by the growth of the zeta-phase layer whi ch was in contact with liquid zinc during immersion in the zinc bath. The zeta-phase layer followed a two-stage growth process governed by t (1/3) kinetics. The delta-phase layer also exhibited two-stage growth with parabolic t(1/2) kinetics. The gamma phase followed t(1/4) growth kinetics, indicative of grain-boundary diffusion-controlled growth.