Insights into the selective phase corrosion of as cast NiAl bronze alloy: Effect of electrical properties of each phase’s protective film

Selective phase corrosion (SPC) is one of the most serious failure modes for as cast NiAl bronze (NAB) alloy. In this paper, the electrical properties of corrosion protective film of each phase in the alloy was investigated using atomic force microscopy (AFM) electrical modes, and its effect on the SPC behavior in different (pH=2, 3.5, 4.2 and 7) 3.5% NaCl salt solution was also scientifically studied. The results show that in neutral solution, the protective films of κ and β´ phases exhibit higher Volta potential and lower-conductivity compared with α phase. But after long time immersion, the film of β´ phase is easy to form micro-cracks, resulting in its severe corrosion. At pH value of 3.5, no SPC occurs, leading to the best corrosion resistance of the alloy. Below and above 3.5, the alloy exhibits completely different SPC behavior. Below 3.5, the protective film was destroyed, and the SPC mainly depends on the Volta potential of each phase. Above 3.5, due to the different electrical properties of protective film, the adjacent α phase around the κ phases is preferentially corroded, which eventually leads to the separation of κⅱ and κⅳ phase and α phase corrosion in α + κⅲ eutectic structure.