Evaluation of selective corrosion in UNS S31803 duplex stainless steel with electrochemical impedance spectroscopy

The precipitation of certain phases, in particular of sigma phase, in duplex stainless steels makes them susceptible to selective corrosion. In the present work, the effect of heat treatments upon selective corrosion of UNS S31803 duplex stainless steel was investigated in 2 M H2SO4 + 0.5 M NaCl + 0.01 M KSCN solution at 30 °C using electrochemical impedance spectroscopy (EIS). For this purpose, samples were solution annealed at 1050 °C for 30 min and, subsequently, heat-treated at 750 °C or 850 °C for times varying from 1 h to 30 h in order to precipitate the sigma phase and other phases. After these heat-treatments the specimens were immersed in the test solution and submitted to EIS tests. These tests, performed at the open circuit potential after 84 h of immersion in the test solution, showed higher impedances for samples tested at 850 °C than at 750 °C, implying lower selective corrosion rates for the former samples. Moreover, for samples tested for longer times at 850 °C, there are evidences of an increase of the high frequency phase angle, which seems to be associated with the recovery of Cr and Mo content in the depleted zone near the precipitated sigma phase. These results are in good agreement with those obtained in tests performed with the electrochemical potentiodynamic reactivation double loop method.