Electrochemical and SEM study on Type 254 SMO stainless steel in chloride solutions

Abstract

An investigation was conducted to examine the critical crevice potential ( E crev) and the critical protection potential ( E prot) for Type 254 SMO stainless steel in 4% NaCl solution by using potentiodynamic cyclic anodic polarization (PCAP) technique at temperature ranging from 30 to 90 °C. The critical crevice temperature (CCT) and the critical crevice protection temperature ( T prot) were determined by plotting the values of breakdown potential and E prot versus solution temperature, respectively. The values of CCT and T prot were recorded at the abrupt transition with increasing the temperature from transpassive corrosion to crevice corrosion and were found to be at 55 and 52 °C, respectively. Above CCT (70 °C) the following points were recorded. The E crev and E prot decreased linearly with log [Cl −]. The addition of bromide ions to chloride ions at a fixed halide content of 4% increased both E crev and E prot. The E crev value in 4% NaCl increased linearly with increasing pH in the range 1–10. The addition of 0.5 M bicarbonate ions to 4% NaCl completely removed the crevices effect while increasing the addition of sulphate ions to 4% NaCl increased both of E crev and E prot. The morphology of the crevice corrosion produced on the steel surface was examined by scanning electron microscope (SEM) after PCAP treatment under different test conditions.