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. Cavitati on tests were made under free corrosion and cathodic protection condit ions. Electrochemical measurements were made to elucidate the role of cavitation. The presence of cavitation shifted the free corrosion pote ntial of the material in the active direction by 70 mV. It also increa sed the cathodic and anodic currents during polarization by an order o f 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 wa s 186 times that under quiescent conditions. When cathodic protection was applied, the rate was 47% less than that under free corrosion cond itions. This reduction was ascribed to the cushioning of bubble collap se by cathodic gas and elimination of electrochemical dissolution. Opt ical and scanning electron microscopy showed NAB immersed in quiescent seawater suffered from selective corrosion of the copper-rich alpha p hase at boundaries with intermetallic kappa precipitates. The kappa pr ecipitates and precipitate-free areas did not suffer corrosion. Cavita tion 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 incre ased, but grain-boundary attack was absent. Microcracks 5 mu m to 10 m u m long were observed in the alpha phase adjacent to kappa precipitat es along the cross section of the material. Selective phase corrosion and cavitation stresses were implicated as the causes of cracking.