Effect of chromium on microstructure and corrosion performance of ADC12 alloy in static and dynamic 3.5% NaCl medium

The cast Al-12Si (ADC12) aluminium alloys are widely used in various industries including automobiles and home appliances; they often suffer from premature failures due to localized corrosion. In this context, the present study explores addition of 0.3 wt% and 0.6 wt% Cr to improve the corrosion resistance of ADC12. Microstructural characterization revealed that the Cr addition transformed needle-like monoclinic β-AlFeSi phase present in the unmodified ADC12 into a star or cuboids like Cr-rich α-Al(Fe,Cr)Si phases. Furthermore, it refined the eutectic-Si into shorter rods, thereby promoting homogeneous distribution of the phase. Corrosion evaluation using electrochemical polarization and electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution at pH 11 revealed that 0.6 wt% Cr addition reduced passive current density by 50% and increased the charge transfer resistance by threefold in static condition. Similar improvements were observed under electrolyte rotation at 50 and 300 rev.min −1 , though the benefits diminished at 600 rev.min −1 . Corrosion products formed on Cr-modified alloys contain protective Cr-rich oxides, unlike defective aluminium hydroxide observed in unmodified ADC12, as revealed by X-ray photoelectron spectroscopy (XPS) study. Severe micro-galvanic corrosion along β-AlFeSi/eutectic-Si boundaries was observed in unmodified ADC12 but it markedly prevented with Cr addition. The improved corrosion resistance of Cr-modified alloys is attributed to the combined effects of microstructural refinement and reduced micro-galvanic corrosion via minimization of electrochemical potential difference resulted from evolution of α-Al(Fe,Cr)Si phase, together with the formation of stabilized oxide film.