No abstract

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 a-Al matrix and intermetallics (Al 2 Cu and Al 5 FeSi) or to a less degree at interfaces between a-Al matrix and Si phase. The large area fraction of the cathodic phases (Al 2 Cu, Al 5 FeSi and Si) led to the galvanic coupling between them and the adjacent anodic a-Al matrix. The heatrejecting 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.

Coolant corrosion is a major drawback for the use of magnesium alloys in engine and cooling system, but the coolant is not normally intended to prevent corrosion of magnesium alloys. This research assessed the corrosion performance of two magnesium alloys, AZ91D and AM50A, in two newly formulated radiator coolants using immersion test, potentiodynamic polarisation test, and corroded surface analysis. Two coolants were named as Irgacool Plus L and Irgacool Plus S. C7, C8-organic acids and polycarboxylic acid were the main inhibitor species in Irgacool Plus L while Irgacool Plus S was formulated with C7, C8-organic acids and sebacic acid inhibitors. Corrosion rates of magnesium alloys decreased twice in Irgacool Plus L compared with Irgacool Plus S. AZ91D alloy had better corrosion resistance than AM50A alloy in both radiator coolants. Both alloys suffered corrosion due to microgalvanic coupling between cathodic b-Mg 17 Al 12 intermetallic and anodic a-Mg matrix, and the presence of Al 8 Mn 5 and Al 11 Mn 4 intermetallics in AM50A led to further microgalvanic corrosion. A continuous network of b-Mg 17 Al 12 phase and higher Al content a-Mg matrix accounted for better corrosion resistance of AZ91D alloy.

No abstract