Characterization of the Corrosion Product Films Formed on the As-Cast and Friction-Stir Processed Ni-Al Bronze in a 3.5 wt% NaCl Solution

An as-cast Ni-Al bronze (NAB) was subjected to friction-stir processing (FSP). The friction-stir processed NAB (FSPed NAB) exhibited better corrosion resistance in a 3.5 wt% sodium chloride solution. The corrosion product films formed on the as-cast and FSPed NAB were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and electron probe micro-analysis (EPMA). Uncorroded phases were retained in the films. The interfaces between these phases and their surrounding corrosion products allowed the chloride ions in and reduced the film protectiveness. For the as-cast NAB, the un-corroded phases were large in size or continuous in structure, resulting in continuous interfaces. Therefore, the film on the as-cast NAB exhibited relatively inferior protectiveness. The films on the as-cast and FSPed NAB were similar in composition with mainly copper chloride hydroxide Cu-2(OH)(3)Cl in the outer layer, and Al2O3 and Cu2O with the incorporation of Fe and Ni in the inner layer. Stability of the films in the flowing condition was also studied and a film damage mechanism was raised. In the flowing condition, the film on the as-cast NAB was damaged by crack formation and expansion, while the film on the FSPed NAB was damaged by thinning.