Initiation and propagation of localised corrosion on stainless steels in seawater containing high biocide concentrations

The practice of chlorination whether by electrolytic production of chlorine or by the addition of sodium hypochlorite biocide, is used extensively in industrial systems to control marine biofouling. In contrast to previously reported studies, this work addresses the corrosion implications of high chlorine concentrations (up to 500 ppm total chlorine) which exist at biocide distribution points. Girth welds on UNS S32760 superduplex stainless steel resisted corrosion attack in seawater containing a high chlorine concentration. This study has demonstrated that the principal mode of corrosion attack on the austenitic stainless steel (UNS S31603) and the superduplex stainless steel (UNS S32760) is crevice corrosion. Higher propagation rates in seawater containing high levels of sodium hypochlorite compared to unchlorinated seawater are thought to be primarily due to the observed depolarisation of the cathodic reaction. This effect has also been shown to induce higher deposition rates of CaCO₃ and Mg(OH)₂ on the cathode of galvanic couples of stainless steel and carbon steel.