CAVITATION CORROSION BEHAVIOR OF CAST NICKEL-ALUMINUM BRONZE IN SEAWATER

The cavitation corrosion behavior of nickel-aluminum bronze (NAB) was investigated in seawater, using a 20-kHz ultrasonic vibrator. Cavitation tests were made under free corrosion and cathodic protection conditions. Electrochemical measurements were made to elucidate the role of cavitation. The presence of cavitation shifted the free corrosion potential of the material in the active direction by 70 mV. It also increased the cathodic and anodic currents during polarization by an order of magnitude. The corrosion current densities were 2 x 10(-2) mA/cm(2) amd 1.2 x 10(-3) mA/cm(2) for the cavitated and noncavitated specimens, respectively. The rate of mass loss in the presence of cavitation was 186 times that under quiescent conditions. When cathodic protection was applied, the rate was 47% less than that under free corrosion conditions. This reduction was ascribed to the cushioning of bubble collapse by cathodic gas and elimination of electrochemical dissolution. Optical and scanning electron microscopy showed NAB immersed in quiescent seawater suffered from selective corrosion of the copper-rich alpha phase at boundaries with intermetallic kappa precipitates. The kappa precipitates and precipitate-free areas did not suffer corrosion. Cavitation made the surface of the material very rough, with large cavities, ductile tearing, and corrosion of the boundaries of a columnar grains. In the presence of cathodic protection, the number of cavities increased, but grain-boundary attack was absent. Microcracks 5 mu m to 10 mu m 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 implicated as the causes of cracking.