Morphology and Kinetics of Interfacial Layer Formation during Continuous Hot-Dip Galvanizing and Galvannealing

A galvanizing simulator with rapid spot cooling was used to obtain a well-characterized reaction times as short as 2 seconds in order to study the short-time microstructural development and kinetics of the galvanizing and galvannealing interfacial reaction layer. It was determined that the incubation and nucleation events of the interfacial layer formation were completed by the 2-second reaction time in all cases. For a 0.20 wt pct dissolved Al bath, FeAl3 nucleates and grows during the initial stages of interfacial layer formation followed by Fe2Al5ZnX formation by diffusion-controlled transformation and growth. The final microstructure of the interfacial layer consisted of Fe2Al5ZnX in a two-layer arrangement comprising a fine-grained, compact lower layer with a coarser, noncompact upper layer. The Al content of the interfacial layer increased with reaction time and reaction temperature. Both of the Fe-Al phases formed exhibited a strong preferential crystallographic orientation with respect to the substrate surface. The evolution of the interfacial layer formed in a 0.13 wt pct dissolved Al bath was the result of competing processes. Fe-Al phases formed and grew during the reaction times explored, per the preceding mechanism. However, Fe-Zn phases also nucleated and grew during the reaction times explored via the process of inhibition breakdown, with these phases dominating the interfacial layer microstructures at longer reaction times. In this case, the Al content of the interfacial layer increased for all reaction times explored, but decreased with increasing reaction temperature, due to the more rapid initiation of inhibition breakdown. A model to describe the interfacial layer growth kinetics as a function of reaction time, bath temperature, and inhibition layer microstructure for the case of the 0.20 wt pct dissolved Al bath was proposed. It indicated that the development of microstructure of the interfacial layer had significant influence on the effective diffusion coefficient and growth rate of this layer.