Formation of different alumina phases and magnesium aluminate spinel during contact of molten AlSi7Mg0.6 alloy with mullite and amorphous silica

•Corrosion of mullite or fused silica coatings by molten AlSi7Mg0.6 alloy was studied.•The results of microstructural analyses were verified by thermodynamic calculations.•Mullite decomposed to silica and alumina, silica was reduced by the molten alloy.•The products of the corrosion reaction are MgAl2O4 spinel and alumina.•In fused silica, metastable γ-Al2O3 precedes thermodynamically stable corundum.