CAVITATION CORROSION OF DUPLEX STAINLESS-STEEL IN SEAWATER

A laboratory study was conducted on the cavitation corrosion behavior of a commercial cast duplex stainless steel (DSS) in seawater using an ultrasonically-induced cavitation facility. Mass loss, free-corrosion potential, potentiodynamic polarization, and microscopic examinations were compared in. the absence and presence of cavitation. The rate of mass loss was negligible in quiescent seawater. However, the rate was 0.64 mg/h-cm(2) after testing for 11 h in the presence of cavitation. Cathodic protection (CP) reduced the rate of mass loss by 19%. Cavita tion caused an active shift in the free-corrosion potential by similar to 140 mV. During polarization in the absence and presence of cavitat ion, the alloy passivated spontaneously without an active-to-passive t ransition Cavitation slightly increased the cathodic and anodic curren ts, shifted the corrosion potential in the noble direction by 75 mV, a nd decreased the breakdown potential by similar to 50 mV. Under the fr ee-corrosion condition, small cavities initiated in the ferrite matrix and at the ferrite-austenite boundaries. With the progress of cavitat ion, the attack concentrated in the austenite phase but spread to the ferrite phase and was associated with ductile tearing, cleavage-like f acets, river patterns, and crystallographic steps at later stages. CP decreased the number of cavities slightly. Specimen cross sections rev ealed microcracks initiating from the ferrite matrix at the bottom of cavities. Crack propagation into the bulk of the material was impeded by the austenite islands and branched along parallel slip systems.