Localised corrosion of super duplex stainless steel in synthetic NaOH-Na2S solution at different temperatures

The pitting corrosion resistance of the UNS S-32760 super duplex stainless steel (SDSS) in synthetic NaOH-Na2S solution at different working temperatures of 25, 50, and 70°C has been evaluated in the current investigation to assess the alloy behaviour under the caustic environments used in the pulp and paper industry. The temperature effect on pitting (Ep) and repassivation (Erp) potentials was studied with the Cyclic Potentiodynamic Polarisation (CPP) technique according to the ASTM G61. Results demonstrated that pitting corrosion is prone to occur when increasing alloy temperature since both pitting and repassivation potentials lean towards more negative values while the repassivation zone is reduced. Furthermore, the morphology of the found pits was determined using Scanning Electron Microscopy (SEM). The pitting corrosion mechanism for the studied super duplex stainless steel under the exposed conditions was introduced. The thiosulphate ion is attributed to be responsible for the initiation of the localised corrosion.