In this paper, the effect of heat treatment temperature on the resultant composition of composite Al-Si coatings on ZK60A alloy investigated. The coatings were developed using magnetron sputtering deposition. These coatings can serve as an effective anti-corrosion barrier for Mg alloys in aqueous mediums. The magnetron target composition was 80% Al and 20% Si, while in the deposited coating it was 78% Al and 18% Si, with the rest being atoms from the substrate. The as-deposited coatings had pores and the Al-Si composite was deposited in the form of globules. The sputtered coatings were subjected to heat treatment for 1.5 hours at 420 °C (HT1) and 350 °C (HT2). The ratio of elements in the heat-treated coatings changed significantly due to diffusion between substrate-coating interface and migration through the pores that exist in the coating. It was found that Mg migrates into the coating and Si into the substrate. Al/Mg ratio reduced by 81.2 and 60.6 %, respectively, in the coating after the HT1 and HT2.