Comparison Between Potentiodynamic and Potentiostatic Tests for Pitting Potential Measurement of Duplex Stainless Steels

Potentiodynamic and potentiostatic polarization techniques were used to investigate pitting corrosion of the solution-annealed duplex stainless steels (DSS) UNS S31500 and UNS S32404. Tests were performed in naturally aerated 3.5% sodium chloride (NaCl) aqueous solution at 23°C ± 2°C. With the potentiodynamic test, it was only possible to measure the pitting potential (E_pit) of UNS S32404, which was equal to 1,007 mV_SCE ± 31 mV_SCE. On anodic polarization curves of UNS S31500, current oscillations were observed, and no sharp current density increase was detected, which made determination of E_pit impracticable. However, with the potentiostatic test, it was possible to determine E_pit for UNS S32404 and UNS S31500 of 985 mV_SCE and 561 mV_SCE, respectively, by extrapolating the potential to the infinite value of the induction time (t). The best performance of UNS S32404 was related to its chemical composition. The potentiostatic technique also was useful in reviewing the pitting nucleation mechanism in both steels. At higher potentials, the oxygen anodic reaction seemed to participate in pitting nucleation through formation of a triple interface between the metal, electrolyte, and oxygen bubbles. Results indicated this was the acting mechanism for UNS S32404. For UNS S31500, conventional pitting nucleation took place at potentials < ~ 800 mV_SCE. At higher potentials, oxygen became involved, which explained the slope change observed in the straight line relating the electrode potential and inverse of τ.