FIELD AND LABORATORY STUDY OF CAVITATION CORROSION OF NICKEL-ALUMINUM BRONZE IN SEA-WATER

A case study of a nickel aluminium bronze vertical type pump that failed during service in sea water and a laboratory investigation of the cavitation corrosion behaviour of the cast alloy are reported. The failure of the pump was found to result essentially from cavitation damage and erosion corrosion grooves. Other factors such as sulphide attack and preferential phase corrosion played a secondary role. The preferential attack occurred at the boundaries between the copper rich alpha-phase and the intel metallic precipitates. It had an average depth of penetration ranging from 17 to 48 mu m. In laboratory tests, the presence of cavitation was found to shift the free corrosion potential of the material in the active direction ection by 65 mV. It also increased the cathodic and anodic currents during polarisation by an order of magnitude. The rate of mass loss in the presence of cavitation was found to be 186 times that under quiescent condition. Cathodic protection reduced the rate by 47% with respect to that under free corrosion condition. In a similar manner to the failed pump, laboratory cavitation testing made the surface of the material very rough, exhibiting large size cavities, ductile tearing, and corrosion osion of the boundaries of a columnar grains. In the presence of cathodic protection, the number of cavities increased, but gain boundary attack was absent. Along the cross-section of the material, microcracks, 5-10 mu m in length, were found in the alpha-phase adjacent to kappa precipitates.