The initial pH effect on the dissolution of zinc-magnesium coated steel is investigated with or without chlorides for long immersion periods. The coating is characterized by TEM, SEM and GDOES. With immersion time, the alloy electrochemical behaviour is investigated by EIS and the corrosion products are analysed using ICP-OES and XPS. In alkaline medium, the formation of zincite is inhibited and chlorides do not hinder a passive layer formation. In chloride electrolyte at natural pH, chlorides are not detected on the metal surface. Chemical composition of corrosion products and their protective effect depend on the initial pH and its evolution.