Visualization of Preferential Cathodic Reaction Sites in Die-Cast Al–12Si–Cu Alloy Using a pH Indicator

A 0.1 M NaCl solution film containing bromothymol blue and having a thickness of 0.4 mm was formed on a re-melted die-cast ADC12 aluminum alloy (Al–12Si–Cu). An optical microscope was used to identify cathodic reaction sites by observing the color changes caused by local alkalization. In the alloy, Al2Cu phase, Fe-containing phase, Si-rich phase, and a phase rich in Mg, Al, Si, and Cu were present in addition to the Al-matrix. A color transition from yellow to blue, indicating alkalization, first occurred in the Al2Cu phase and then spread to the Fe-containing phase. Based on the surface morphology of each phase after immersion, the difference in cathodic activities between the Fe-containing and Al2Cu phases were analyzed and discussed.