Comparison between potentiodynamic and potentiostatic tests for pitting potential measurement of duplex stainless steels

Potentiodynamic and potentiostatic polarization techniques were used to inv estigate 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 degrees C +/- 2 degrees C, With the potentiodynamic test, it was only possible to measure the pitt ing potential (E-pit) of UNS S32404, which was equal to 1,007 mV(SCE) +/- 3 1 mV(SCE). On anodic polarization curves of UNS S31500, current oscillation s 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 t o the infinite value of the induction time (tau). The best performance of U NS S32404 was related to its chemical composition. The potentiostatic techn ique also was useful in reviewing the pitting nucleation mechanism in both steels. At higher potentials, the oxygen anodic reaction seemed to particip ate in pitting nucleation through formation of a triple interface between t he metal, electrolyte, and oxygen bubbles. Results indicated this was the a cting mechanism for UNS S32404. For UNS S3 1500, conventional pitting nucle ation took place at potentials < similar to 800 mV(SCE). At higher potentia ls, oxygen became involved, which explained the slope change observed in th e straight line relating the electrode potential and inverse of tau.