The addition of alloying elements to the zinc bath is one of the best solution to reduce overthicknesses of the coating when galvanizing silicon-killed steels. The use of nickel addition to the molten zinc offers industrial advantages, especially in the modification of the Sandelin curve related to the reactivity of steels towards the galvanizing bath. However, dress formation in a 0.1 wt % Ni bath is the main drawback of the process. Bending on thermodynamic description of the ternary Fe-Zn-Ni system at 450 degrees C, the reaction between a pure iron substrate and a nickel added zinc bath is modelled at 450 degrees C. The approach of the extension of the liquid phase in the quaternary Fe-Zn-Si-Ni system leads to understanding of the effect of nickel addition to the zinc bath when galvanizing reactive silicon steels. These thermodynamic interpretations lead to understanding of experimental phenomena during the galvanizing process: dress formation, reduction of the coating thickness, smoothing the solid-liquid interface when nickel is added to the zinc bath at 450 degrees C. Based on morphological and kinetic observations, steel-zinc reactions are clearly investigated showing the important effect of allowing elements to the substrate or to the bath.