The development of CMAS (CaO–MgO–Al₂O₃–SiO₂) -resistant thermal barrier coatings is an urgent issue, as the operating temperatures of aero-engines exceed the melting point of the CMAS. Application of alumina as top layer on 7YSZ coatings has shown promising CMAS resistance behavior. It reacts with the CMAS and crystallizes into new stable phases, which seals the CMAS penetration paths in the top layer to save the underlying 7YSZ layer from infiltration. This paper deals with the study of the erosion behavior of porous thermally sprayed Al₂O₃ sacrificial layer on top of EB-PVD 7YSZ coatings before and after the CMAS infiltration and investigates the combined erosion/corrosion regime. The TBC system has been infiltrated with three different CMAS compositions having a different Ca/Si ratio which resulted in different reaction scenarios and products. The change in the erosion behavior of the coating system as a function of CMAS infiltration depth was investigated and an erosion model has been proposed. The key factors that dictate the erosion resistance of such alumina layers were found to be the thickness and morphology of the reaction layer, the porosity of the non-reacted alumina layer underneath the reaction layer as well as the coating adhesion mechanism between alumina and 7YSZ.