Abstract
The role of microstructure on the cavitation behavior of as-cast nickel aluminum bronze was studied in seawater. The cavitation tests were made under free corrosion and cathodic protection conditions, using a 20 kHz ultrasonic vibrator. In the presence of cavitation, the rate of mass loss was 186 times that under quiescent condition. When cathodic protection was applied, the rate became less than that under free corrosion condition by 47 %. This reduction was ascribed to the cushioning of bubble collapse by cathodic gas and elimination of electrochemical dissolution. Microscopic examinations showed that nickel aluminum bronze immersed in quiescent seawater suffered from selective corrosion of the copper rich α phase at the boundaries with the intermetallic κ precipitates. The κ precipitates and the precipitates free areas did not suffer from corrosion. Cavitation made the surface of the material very rough, exhibiting large size cavities, ductile tearing and corrosion of the boundaries of α columnar grains. In the presence of cathodic protection, the number of cavities increased, but grain boundary attack was absent. Microcracks, 5-10 μm in length, were observed in the α phase adjacent to κ precipitates along the cross-section of the material. Selective phase corrosion and cavitation stresses were implicated as the causes of cracking.
