Comparative Study of the Crevice Corrosion Resistance of Uns S30400 and Uns S31600 Stainless Steels in the Context of Galvele'S Model

The crevice corrosion repassivation potential of austenitic UNS S30400 and UNS S31600 stainless steels was determined in 0.1 mol/L and 1 mol/L NaCl solutions and in 5 mol/L CaCl2 solution at temperatures between 0 degrees C and 90 degrees C. The repassivation potential of UNS S30400 decreased with increasing chloride concentrations and temperatures in the range from 0 degrees C to 60 degrees C, reaching a constant value of -0.430 +/- 0.015 V-SCE in the range from 60 degrees C to 90 degrees C. The repassivation potential of UNS S31600 showed a continuous decrease with increasing chloride concentrations and temperatures. Crevice-corroded spots showed crystalline attack plus pitting corrosion. Crystalline attack prevailed in UNS S31600 and pitting corrosion prevailed in UNS S30400. A solution of 0.5 mol/L HCl was used to simulate the crevice-like environment. Results of repassivation potential were analyzed in the context of Galvele's localized acidification model. The ohmic potential drop was the dominant contribution in determining the repassivation potential for both steels. The contribution of the polarization to sustain the critical crevice was nil. The main effect of the 2.5 wt% Mo addition in UNS S31600 compared to UNS S30400 on the crevice corrosion resistance was the increase of the ohmic potential drop within the corroded area. This effect was especially significant for the more dilute chloride solutions and for increasing temperatures.