FE-ZN PHASE-FORMATION IN INTERSTITIAL-FREE STEELS HOT-DIP GALVANIZED AT 450 DEGREES-C .2. 0.20 WT-PERCENT AL-ZN BATHS

The effect of solute additions of titanium, titanium and niobium and p hosphorus on interstitial-free steels on Fe-Zn phase formation after i mmersion in a 0.20 wt% Al-Zn bath was studied to determine the morphol ogy and kinetics of the individual Fe-Zn phases formed. These results were contrasted to the previous study using a pure zinc (0.00 wt% Al) bath in Part I. It was found that in the 0.20 wt% Al-Zn bath, an iron- aluminide inhibition layer prevented uniform attack of the steel subst rate. instead, localized Fe-Zn phase growth occurred, termed outbursts , containing a two-phase layer morphology. Delta-phase formed first, f ollowed by gamma-phase. Zeta-phase did not form in the 0.20 wt% Al-Zn bath, in contrast with zeta-phase formation in the pure zinc bath. As in the pure zinc bath, the growth kinetics of the total layer was cont rolled by the Fe-Zn phase in contact with the liquid zinc during galva nizing. For the 0.20 wt% Al-Zn bath, the Fe-Zn phase in contrast with the liquid zinc was the delta-phase, whereas the zeta-phase was in con tact with liquid zinc in the pure zinc bath. The delta-phase followed t(1/2) parabolic growth, while the gamma-phase showed essentially no g rowth after its initial formation. Titanium and titanium + niobium sol ute additions, which enhance grain-boundary reactivity, resulted in mo re rapid growth kinetics of the gamma-and delta-phases. Phosphorus add itions, which decrease grain-boundary reactivity, generally increased the incubation time and retarded the growth rate of the gamma-phase. T hese results further confirm the concept that solute grain-boundary re activity is primarily responsible for Fe-Zn phase growth during galvan izing in a liquid Zn-Al bath in which an iron aluminide inhibition lay er forms prior to Fe-Zn phase formation.