The Effect of Hydrogen on the Corrosion Resistance of Stainless Steel in Seawater
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Stainless steels have been used for various applications in the oil and gas industry for a long time due to, among others, their high corrosion resistance. However, immersion in seawater is demanding of the material because of varying temperatures and the presence of Cl- ions. Cathodic protection (CP) in the form of sacrificial anodes is therefore typically applied to protect the stainless steel parts. It has been observed that cathodically protected stainless steels suffer from hydrogen induced stress cracking (HISC), believed to have been caused by the hydrogen development during CP. After the sacrificial anodes were removed, severe corrosion attacks occurred on the materials, suspected to have initiated due to adsorbed hydrogen during CP. The objective of this work has been to investigate the effect of absorbed hydrogen on the corrosion resistance of stainless steel. Samples from two different batches of UNS S32750 have been used as well as samples from UNS S31254. Welded samples of the second batch of UNS S32750 were also provided. Simulation of cathodic protection was performed through cathodic polarisation of the samples at fixed current densities. The effect of different hydrogen charge current densities (HCCD) and storage time after charging were investigated on anodic polarisation curves and the critical crevice corrosion temperature (CCT). The oxide film composition was examined with X-ray photoelectron spectroscopy (XPS), and the electrical properties of the oxide film were studied with Mott-Schottky analysis. Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to evaluate selective corrosion. The results reveal a clear decrease in corrosion resistance of samples charged with hydrogen. A decrease in corrosion potential, Ecorr, and an increase in anodic current density was observed in the polarisation curves. The decreased Ecorr for charged samples was explained by the low equilibrium electrode potential of hydrogen oxidation compared to that of Fe oxidation. The CCT of charged samples decreased compared to uncharged samples. Storage time, however, resulted in higher Ecorr, lower anodic current densities and increasing CCTs, indicating improved corrosion resistance with storage time. During storage, hydrogen diffuses out of the stainless steel, leading to repassivation. SEM and EDS results confirmed selective corrosion of the austenite phase. Results obtained with XPS revealed a higher relative atomic percentage of Cr(OH)3 in the oxide film on hydrogen charged samples, implying lower corrosion resistance due to the higher defect concentration in hydroxides compared to protective oxides like Cr2O3 and CrO3, and thus a higher conductivity in the oxide layer. The Mott-Schottky analysis showed electric properties of the oxide film which were contradicting to previous studies, and the limitations of the method are discussed. The work presented in this thesis shows that hydrogen has a detrimental effect on the corrosion resistance of stainless steel in seawater. The reason for the decreased resistance is likely caused by interactions between hydrogen and the oxide film, resulting in higher concentrations of hydroxides compared to oxides, thereby destabilising the film, and facilitating easier and faster corrosion.