Mesoscale investigation of reaction-diffusion and structure evolution during Fe-Al inhibition layer formation in hot-dip galvanizing

A new mesoscale model based on the lattice Boltzmann method is developed to simulate the coupled reaction-diffusion processes during the formation of inhibition layer (IL) comprised Fe-Al intermetallic compound in the galvanizing. This model considers Fe(l) and Al(l) transport in liquid zinc as well as in the solid IL, dissolution of iron from the steel surface and the precipitation reaction of Fe-Al compound. Evolutions of IL morphology and concentration fields of Fe(l) and Al(l) are predicted and discussed. Three distinct stages during the IL growth are identified, including reaction-controlled, Al-diffusion controlled and Fe-diffusion controlled stages. The evolution of the IL thickness with time agrees with the experiments. Two existing mechanisms in the literature are explored. In the initially complete cover mechanism, effects of the solid diffusivity on the IL growth are investigated. Heterogeneous diffusion in the IL including slow lattice diffusion and relative quick grain boundary diffusion is considered, based on which the interesting IL double-layer structures widely observed in experiments are well captured for the first time. In the initially partial cover mechanism, effects of iron dissolution rate are explored and it is found that the double-layer structures also can be formed even homogeneous diffusion is considered; however, considerable sites of the steel surface are only covered by 10 nm thick IL. A new mechanism for the IL growth is proposed, which recognizes both the heterogeneous nucleation and heterogeneous diffusion.