Corrosion is an important issue for cast Al alloy in an engine cooling system, but how the microstructural features affect the coolant-related corrosion behaviour is not well understood. In this research, the evolution of corrosion in an ISO 2379 cast Al alloy was studied in an antifreeze radiator coolant under heat-rejecting conditions. Extensive analyses of microstructures and corroded surfaces were carried out using an optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X-ray diffractometer. Intergranular cavitation corrosion was observed to occur at interfaces between α-Al matrix and intermetallics (Al2Cu and Al5FeSi) or to a less degree at interfaces between α-Al matrix and Si phase. The large area fraction of the cathodic phases (Al2Cu, Al5FeSi and Si) led to the galvanic coupling between them and the adjacent anodic α-Al matrix. The heat-rejecting condition in antifreeze radiator coolant was favourable condition to cavitation process while severe crevice corrosion was predominant at oxygen-depleted regions in the heat-transfer corrosion cell.