Detection of deleterious phases in duplex stainless steel by weak galvanostatic polarization in halide solutions

A simple electrochemical procedure was developed to detect and partially quantify secondary phases in 25Cr duplex stainless steel, isothermally treated at temperatures from 675 to 825 °C. The test is based on weak anodic galvanostatic polarization and uses halide solutions (NaCl, NaBr) to generate a pit-like attack of solute-depleted zones. The electrode potential after a particular period of polarization was correlated with the volume fraction of σ phase via the Charpy impact toughness. The chloride solution is unable to detect mild solute depletion at room temperature, because even the depleted zones are below their critical pitting temperature. Heating the solution to 50 °C gives a better discrimination, but a much more convenient method is to use bromide, which reduces or eliminates the beneficial effect of Mo on pitting resistance, and enables the procedure to be carried out at room temperature with excellent discrimination of the material condition. Both solutions were more responsive to σ phase than to other, smaller, second-phase particles because the spatial extent as well as the composition of the depleted zone is important in stabilizing a pit. However the bromide solution in particular is also able to detect the early stages of nitride and χ phase precipitation. There is some evidence that chloride pitting favours χ sites, which have a higher Mo depletion than σ sites. An initiation test was demonstrated in which the anodic current is reduced to such a low value that no stable corrosion occurs; instead there is a series of metastable events that over time decorate the most susceptible sites, which appear, perhaps surprisingly, to be the nitrides.