Effects of the corrosion mechanism evolution of low silicon-cast aluminium alloys in service

The corrosion mechanism evolution of four low silicon-cast aluminium alloys exposed to heat treatment at 200 degrees C for 0-50 h was studied by means of immersion corrosion, intergranular corrosion and electrochemical test, SEM, EDS and EBSD analysis methods. The correlation of corrosion resistance with composition and the thermal exposure time was investigated. The results show that the corrosion mechanism of all aluminium alloys changed after thermal exposure at 200 degrees C. Before thermal exposure, intergranular corrosion was the main corrosion, and after 200 degrees C thermal exposure, serious pitting corrosion and spalling corrosion occurred in the local area. The contents of Si and Mg in the grain boundary increased obviously. The contents of elements in the poor zone of the grain boundary increased, and the electrical potential difference between the grain and the grain boundary reduced. The intergranular corrosion sensitivity of the alloy was reduced. But the degree of pitting gradually becomes serious. The electrochemical test of the aluminium alloy with 3.5 wt% Si showed that its corrosion potential increased with the increase inthermal exposure time, and its corrosion current density increased with the increase inthermal exposure time. After the EBSD test, it was found that the large angle grain boundary decreased gradually and the proportion of large grain size increased with the increase in thermal exposure time to 50 h.