THE CORROSION BEHAVIOR OF COPPER-ALLOYS, STAINLESS-STEELS AND TITANIUM IN SEAWATER

Effects of mass transport, corrosion products and biofilm formation an corrosion kinetics of five copper alloys, four stainless steels and t itanium grade 2 exposed to natural and artificial seawater were determ ined. Corrosion current density (i(corr)), anodic (b(a)) and cathodic (b(c)) Tafel slopes were determined from analysis of polarization curv es recorded in the vicinity of the corrosion potential (E(corr)) under controlled mass transport conditions as a function of exposure time. Rotating cylinder electrode experiments demonstrated that E(corr) and corrosion rate for stainless steels and titanium were independent of m ass transport. For copper alloys i(corr) depended linearly on rotation speed r(0.7), while E(corr) was independent of mass transport. These results indicate that both cathodic and anodic reactions for stainless steels and titanium are under charge transfer control, whereas these reactions are under mass transport control for copper alloys. Corrosio n rates of copper alloys were also obtained using linear polarization and weight loss. Corrosion rates obtained with the two methods agreed for exposures in artificial seawater. In natural seawater, corrosion r ates determined with linear polarization underestimated corrosion rate s from weight loss data by a factor of two or more. Environmental and traditional scanning electron microscopy coupled with energy-dispersiv e X-ray analyses were used to document surface topography, bacterial c olonization and form of corrosion after removal of corrosion products. De-alloying of all copper alloys occurred in natural seawater. Interg ranular corrosion was observed for 70Cu-30Ni alloy under elliptical de posits of embedded diatoms. De-alloying and intergranular corrosion di d not occur in artificial seawater. Sulfides produced by bacteria may have accelerated attack of nickel in grain boundaries of 70Cu-30Ni all oy.