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

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 degreesC. The critical crevice temperature (CCT) and the critical crevice protection temperature (T-prot) were determined by plotting the values of breakdown potential and Eprot versus solution temperature, respectively. The values of CCT and Tprot 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 degreesC, respectively. Above CCT (70 degreesC) the following points were recorded. The E-crev and Eprot 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. (C) 2004 Elsevier Ltd. All rights reserved.