The role of microstructure of nickel–aluminium–bronze alloy on its cavitation corrosion behavior in natural seawater

An investigation was carried out in our laboratory to study the effect of the microstructure of nickel–aluminum–bronze (NAB) alloy on its cavitation corrosion behaviour in seawater using a 20-kHz ultrasonic induced cavitation facility. Cavitation tests were made under free corrosion conditions as a function of exposure time in natural seawater. Optical and scanning electron microscopy showed NAB immersed in stagnant seawater suffered from selective corrosion of the copper-rich α phase at boundaries with intermetallic κ precipitates. The κ precipitates and precipitate-free zones did not suffer corrosion. Cavitation made the surface of this alloy very rough, with large cavities or pits, ductile tearing and corrosion of the boundaries of the α columnar grains. In addition, the number of cavities and their size increased with exposure time. Microcracks 5–10 μm long were observed in the α phase adjacent to κ precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were considered to be the cause of the cracks observed.