A Novel Approach to Characterizing the Corrosion Resistance of Super Duplex Stainless Steel Welds

The effectiveness of electrochemical techniques in characterizing the corrosion resistance of welded super duplex stainless steels has been evaluated. Localized corrosion of the alloy-depleted zone associated with sigma-phase formation is the primary concern, with specific interest in the extent to which various methods could predict the likelihood of sustained propagation of corrosion damage due to connectivity of depleted zones. Testing was carried out on a solution-annealed super duplex stainless steel, on samples isothermally heat-treated for different periods, and on welded pipe sections produced with different heat inputs. In addition, a failed weld from service was investigated. Three electrochemical methods were utilized, viz., critical pitting temperature (CPT) measurement, electrochemical potentiodynamic reactivation (EPR), and a novel galvanostatic test. All three electrochemical methods showed sensitivity to the extent of sigma-phase content in the isothermally heat-treated samples. The CPT and EPR methods were not particularly sensitive to the sigma phase present in the heat-affected zone (HAZ) of the controlled welded specimens and did not distinguish the weld metal from the HAZ in that respect. However, it should be emphasized that the sigma-phase content was relatively low in the welds. The galvanostatic method was more sensitive to the variable quality of the welds and shows the most promise for service application.