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 alpha phase at boundaries with intermetallic kappa precipitates. The kappa 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 a columnar grains. In addition, the number of cavities and their size increased with exposure time. Microcracks 5-10 mum long were observed in the alpha phase adjacent to kappa precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were considered to be the cause of the cracks observed. (C) 2002 Elsevier Science Inc. All rights reserved.