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

Duplex stainless steel (25Cr) can suffer from corrosion in seawater at temperatures exceeding 20 deg.C. To prevent corrosion cathodic protection from attached sacrificial anodes are 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 are below 20 deg.C severe localised corrosion attacks have been observed after the anodes have been removed.This presentation gives an overview of the outcome from a test program examining the effect of pre-cathodic polarisation of 25Cr duplex stainless steel on critical crevice/pitting temperature and the anodic polarisation curve. Both welded samples and base metal in addition to different cathodic polarisation levels have been examined. The effect of cathodic polarisation on the oxide layer thickness and composition have been examined with SEM EDS and XRD.The test results show that there is astrong reduction in critical crevice/pitting temperature due to pre-cathodic polarisation. This effect is caused by a change in the oxide layer composition after cathodic polarisation.

Product Number: 51319-12870-SG
Author: Roy Johnsen
Publication Date: 2019