Stress Corrosion Cracking Of E690High Strength Steel Under Thin Electrolyte Layer In Offshore Engineering

High strength steels are of critical importance for the development and utilization of marine resources. As marine exploration has reached to harsher environment such as the South Sea, the risk of stress corrosion cracking (SCC) significantly increases, restricting the future development of offshore engineering. Due to the limited study of SCC in marine environment especially under thin electrolyte layer, the SCC sensitivity and mechanism in these environments have not been clearly understood. The effect of the different factors on the SCC sensitivity is not clarified.In this study, E690high strength steel was tested by electrochemical techniques and constant load tests under wet-dry cycles simulating marine environment. From these tests, the electrochemical corrosion behavior as well as the sensitivity, key influencing factors, mechanism and SCC crack propagation mode was investigated. The results show that the extension of test time not only enhanced the cathode current density and reduced the anode current density, but also influenced the corrosion resistance under the unsteady thin electrolyte layer in marine environment. The SCC sensitivity of high strength steel increased with increasing pH, and was highest at pH=7.5. As the concentration of Cl" increased, the SCC sensitivity of high strength steel first increased and then decreased, followed by a second increase. The highest SCC sensitivity was found in3.5wt%NaCl solution. The SCC sensitivity of high strength steel increased with increasing alternating wet-dry of frequency. The SCC mechanism of E690high strength steel under unsteady thin electrolyte layer in marine environment was a mixture of anodic dissolution and hydrogen-induced cracking (HIC) in terms of transgranular fracture.Electrochemical corrosion behavior and SCC of E690high strength steel in seawater and under thin electrolyte layer in marine environment were investigated by electrochemical techniques, constant load tests and slow strain rate tensile tests (SSRT). The results showed that the SCC sensitivity of E690high strength steel was higher under electrolyte layer than in seawater. The SCC mechanism was a mixture of anodic dissolution and HIC in terms of transgranular fracture.The effect of O2and H on SCC of E690high strength steel was investigated by SSRT and electrochemical techniques under the condition of stress. The results showed that increasing O2concentration of not only increased corrosion current density, the cathode and the anode current density, but also reduced the corrosion resistance. The SCC sensitivity of high strength steel first increased and then decreased with increasing O2concentration of. The highest and lowest SCC sensitivities were found in O2concentration of21%and100%, respectively. The increase in the current density during electrochemical precharging not only enhanced corrosion current density, but also reduced corrosion resistance. The anodic dissolution and SCC of E690steel were promoted by H. The SCC susceptibility of E690steel increased, as the charging current density increased. The SCC mechanism was a mixture of anodic dissolution and HIC in terms of transgranular fracture when concentrations of O2or concentrations of hydrogen in E690steel is different.The influence of electrochemical corrosion behavior and SCC of E690high strength steel under electrolyte thin layer in marine environment was investigated by electrochemical technologies under different strains, constant load tests and finite element analysis. The results showed that increasing strain not only produced higher corrosion current density, but also reduced zero current potential and corrosion resistance. The anodic dissolution and corrosion of E690steel were promoted by strain, especially the plastic strain zone. The SCC sensitivity of high strength steel increased with increasing strain. E690high strength steel exhibited higher SCC sensitivity in the plastic strain zone than in the plastic elastic zone.Fe3O4was the main corrosion product E690high strength steel and α-FeOOH、β-FeOOH、γ-FeOOH、γ-Fe2O3、α-Fe2O3were also produced both under thin electrolyte layer and in seawater. FeOC1and Fe2(OH)2Cl was produced with prolonged exposure. The electrochemical corrosion behavior and SCC of E690high strength steel under electrolyte layer in marine environment were influenced by the structure and species of the rust layer. The SCC of E690steel were promoted by the enrichment of Cl-in the inner layer of corrosion products. Cr contributed to the formation of a dense rust layer, which reduced the uniform corrosion, and enhanced the SCC sensitivity.The corrosion resistance of E690high strength steel first increased and then decreased with the decreasing cathodic protection potential (open circuit potential to-1200mv potential range) under thin electrolyte layer in marine environment. The SCC sensitivity of high strength steel first decreased and then increased. The corrosion resistance was highest and the SCC sensitivity was lowest at the potential of-850mV. An obvious brittle fracture a relatively high SCC sensitivity occurred at the potential of-1200mV. The corrosion resistance of E690steel was increased and the SCC sensitivity of was reduced when protected by sacrificial anodes.