Steel produced in Electric Arc Furnaces (EAF) contain a high amount of copper that causes a detrimental surface cracking phenomenon called hot shortness. Studies have found that nickel can alleviate hot shortness by increasing copper solubility in the Fe phase, decreasing oxidation rate and promoting occlusion [1-3]. Occlusion is a phenomenon whereby the copper-rich phase becomes incorporated into iron oxides. Nickel promotes occlusion by causing an uneven interface and increasing the number of internal oxides. The uneven interface is likely a result of the two concentration fields resulting from ternary diffusion of nickel, copper and iron in the Fe phase. This work is aimed at explaining why nickel causes wavy oxide/liquid-Cu and liquid-Cu/Fe interfaces. Constitutional super-saturation criterion  was applied to explain uneven interfaces caused by nickel. A model simulating diffusion behaviors of copper and nickel in Fe was developed by coupling Comsol Multiphysics® and Matlab®. Interface concentrations of copper and nickel and perturbation criterion values were calculated as a function of time. Modeling results show that (i) the nickel interface concentration first increases to a peak value then decreases slowly during oxidation process as a result of the change in oxidation rates, and (ii) the alloys with higher nickel contents have more potential for interface breakdown and this occurs within the initial linear oxidation regime.