Oxidation of A356 aluminum alloy at molten state is investigated from thermogravimetric analyses. Linear kinetics followed by unexpected logarithmic kinetics is observed during the oxidation process. Scanning electron microscopy imaging coupled with wavelength‐dispersive X‐ray spectroscopy analyses reveal a heterogeneous oxide layer composed of spinel (MgAl2O4) embedded in a nonoxidized alloy matrix. Nonlinear diffusion of magnesium through the heterogeneous oxide layer is the key mechanism proposed to explain the logarithmic kinetics. The growth of spinel phase causes a growing complexification of the aluminum network through which magnesium is diffusing. To model the magnesium transport through the oxide layer, an effective diffusion coefficient, build from the pioneering approach developed by Davies et al. (Davies, D., Evans, U. & Agar, J. (1954) Proc. Roy. Soc. London. A., 225, 443), is introduced in the Deal–Grove model.