(MnCu)3O4 spinel coatings are good candidates for Cr-positioning protection on stainless steel interconnect. The spinel coatings can be formed by sputtering MnCu followed by a hot oxidation treatment. To understand how the elements diffuse in the MnCu-steel system, a homogenization diffusion-couple model was built with consideration for Mn oxidation at the coating surface. According to the simulation, the diffusion of Fe from the steel substrate to the MnCu coating occurred while Cr was almost trapped under the MnCu coating. Cu-rich metallic phase formed under the Mn-oxide layer early in the process. The solid solubility of Cr in such Cu phase was very low which can function as a Cr blocker so that Cr external oxidation can be inhibited. The inward diffusion of Mn from the coating to the substrate was caused by the formation of a Mn concentration peak at the interface which, based on thermodynamic simulations, was probably due to the dissolution of Mn with Fe and Cr.