Iron-zinc reaction kinetics of hot-dip galvanized interstitial free steels

The objective of the present study was to investigate the effects of interstitial free (IF) steel substrate characteristics (such as grain size and chemistry) on Fe-Zn reaction kinetics and intermetallic phase formation during galvanizing at 450{dollar}spcirc{dollar}C in zinc baths containing 0.00 wt% and 0.20 wt% Al. In the 0.00 wt% Al bath, uniform alloying of the substrate steel led to the development of a three phase Fe-Zn alloy layer containing gamma, delta and zeta phases. In the 0.20 wt% Al bath, however, alloying instead took the form of discontinuous Fe-Zn phase growth (outburst formation) at discrete sites along the steel/coating interface due to the initial formation of an Fe-Al inhibition layer.; Grain boundary reactivity was evaluated by testing ultra low carbon steels with identical chemistries but widely varying grain sizes (15 {dollar}mu{dollar}m and 85 {dollar}mu{dollar}m). In the 0.20 wt% Al bath, the initial formation of Fe-Zn phases occurred at a far more rapid rate on the 15 {dollar}mu{dollar}m substrates than on the 85 {dollar}mu{dollar}m substrates. Only after extended reaction times was localized Fe-Zn growth found to occur on the 85 {dollar}mu{dollar}m grain size steel, and the location of this growth typically corresponded to substrate grain boundary sites.; The effect of phosphorus as a surface segregation layer was studied by ion implanting phosphorus into the surface of selected steel samples, all of which possessed an extremely large grain size (10-20 mm). Phosphorus surface segregation had no apparent effect on the kinetics of Fe-Zn phase growth on low carbon steel substrates in either 0.00 wt% Al or 0.20 wt% Al baths. In the 0.20 wt% Al bath, Fe-Zn phase growth occurred on both the P-ion implanted and non P-ion implanted surfaces only after extended reaction times. Fe-Zn phase growth was found to correspond to the location of substrate grain boundary sites, suggesting that the long reaction times needed to produce Fe-Zn phase growth were due to the very large grain size of the steel. Substrate steel grain size (and not P surface segregation) thus appears to be the dominant substrate surface condition which controls the initial formation of Fe-Zn phases in a 0.20 wt% Al-Zn bath.