Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450 degrees C .1. 0.00 wt% Al-Zn baths

Interstitial-free alloy steels containing various combinations of solute 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 formation of a th ree-phase alloy layer morphology containing gamma, delta and zeta Fe-Zn phases. Titanium and titanium + niobium solute additions had no effect on the growth kinetics of any of the Fe-Zn phases. Phosphorus additions were found to retard only the kinetics of gamma-phase growth, without influencing the growth kinetics of the other Fe-Zn phases. In fact, the gamma-phase layer in the phosphorus-containing substrates was no longer discernable in light optical microscopy after 120 simmersion. The growth kinetics of the total Fe-Zn alloy layer (gamma + delta + zeta) was dominated by the growth of the zeta-phase layer which 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.