The corrosion behavior of copper alloys, stainless steels and titanium in seawater

Abstract

Effects of mass transport, corrosion products and biofilm formation on corrosion kinetics of five copper alloys, four stainless steels and titanium grade 2 exposed to natural and artificial seawater were determined. Corrosion current density ( i corr), anodic ( b a) and cathodic ( b c) Tafel slopes were determined from analysis of polarization curves 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 mass 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. Corrosion 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 rates determined with linear polarization underestimated corrosion rates from weight loss data by a factor of two or more. Environmental and traditional scanning electron microscopy coupled with energy-dispersive X-ray analyses were used to document surface topography, bacterial colonization and form of corrosion after removal of corrosion products. De-alloying of all copper alloys occurred in natural seawater. Intergranular corrosion was observed for 70Cu-30Ni alloy under elliptical deposits of embedded diatoms. De-alloying and intergranular corrosion did not occur in artificial seawater. Sulfides produced by bacteria may have accelerated attack of nickel in grain boundaries of 70Cu-30Ni alloy.