Effect of Cathodic Polarization on Localized Corrosion of 25Cr in Seawater

Abstract Duplex stainless steel (25Cr) can suffer from corrosion in chlorinated seawater at temperatures exceeding 20°C according to ISO 21457. To prevent corrosion, cathodic protection from attached sacrificial anodes is often used. However, this can cause hydrogen stress cracking (HSC) on the alloy assuming tensile stresses above certain levels occur in the component. Some HSC failures have been observed on submerged seawater pumps assemblies made from 25Cr duplex stainless steel protected by sacrificial anodes. To avoid HSC to initiate, anodes have been removed on some pumps. Despite the fact that seawater temperature is below 20°C, severe localized corrosion attacks have been observed after removal of the anodes. This presentation gives an overview of the outcome from a test program examining the effect of pre-cathodic polarization of 25Cr duplex stainless steel on critical crevice/pitting temperature and the anodic polarization curve. Exposure in 3.5% NaCl solution and natural seawater to different cathodic polarization levels have been examined. The effects of cathodic polarization on the oxide layer thickness and composition have been examined with SEM, EDS and XPS. The test results show that there is a dramatic reduction in critical crevice corrosion temperature due to pre-cathodic polarization. A temperature reduction from approximately 60°C to 25°C was observed on samples exposed to natural seawater after polarization to -1100 mVAg/AgCl. This effect is caused by a change in the oxide layer (composition/thickness) after cathodic polarization.