The influence of iron level on corrosion of high-pressure die-cast LM24 alloy

Iron is a detrimental alloying element for aluminium alloys; however, it is essential for facilitating die ejection in high-pressure die-casting process. Hence, studying the effect of iron on the corrosion performance of HPDC aluminium alloys is useful to find an optimal iron content for a specific alloy. Four LM24 alloys with 0.6 wt.% Fe, 0.8 wt.% Fe, 1.2 wt.% Fe and 2 wt.% Fe additions have been examined in this study. In addition to the differences in iron levels, the copper and manganese contents varied in two alloys. The corrosion performance of the alloy specimens was evaluated by electrochemical noise and electrochemical polarisation measurements, and scanning Kelvin probe force microscopy. The microstructures of the individual alloy specimens were observed by scanning electron microscopy prior to and after each corrosion evaluation experiment. The results show that more corrosion products accumulated in the alloys with higher iron additions, and this led to reduction of cathodic reaction rates in the aqueous aggressive chloride solution. An increased Mn/Fe ratio causes the distribution of iron-rich phase particles and cathodic sites across the surface of the alloy. Hence, the compositional potential gradient of the phases throughout the surface decreases. In addition, the potential gradient of the phases also depends on the surface roughness of the phase particles; therefore, the alloys with the larger iron-rich phase particles show the highest potential gradients.