| The exploration and development of marine oil and gas resources puts higher requirements on the mileage and transportation capacity of oil and gas pipelines.As an"energy highway"that guarantees the national oil and gas supply,oil and gas pipelines have long-term work in an extremely complex marine environment containing a large amount of corrosive chloride ions and microorganisms.Therefore,the corrosion problem of pipeline steel is inevitable.05Cr17Ni4Cu4Nb martensitic precipitation hardening stainless steel?17-4 PH?is widely used in the structural components of marine pipelines,and its safety is of great significance for the safe operation of oil and gas pipelines.Three heat treatment processes of single-stage aging,double-stage aging and intermediate treatment were designed.The microstructures were observed and analyzed by optical microscopy?OM?and transmission electron microscopy?TEM?.It was found that the microstructures were mainly composed of tempered martensite and a small amount of ferrite and retained austenite.Significant heterogeneities were noticed in single and double aging treatment samples,which contained a large amount of quenched martensite and tempered martensite,and lath martensite was obvious.Martensite structures were uniform and fine and the orientations of lamellar sheets were clear in intermediate treatment sample.There were a large number of fine precipitate phases.The corrosion resistance of materials treated by different processes was studied by electrochemical methods such as polarization curve and electrochemical impedance spectroscopy?EIS?.The results show that the corrosion potential(Ecorr)of the double-stage aging sample is the smallest,which is-418.26 mV,and the maximum corrosion current density(Icorr)is1.18×10-55 A/cm2.The Ecorrorr of the intermediate treatment sample increased and the Icorrorr decreased,which were-395.46 mV and 4.98×10-6 A/cm2,respectively.The corrosion resistance of the intermediate treatment sample was significantly better than that of single-stage aging and double-stage aging samples.In the simulated marine solution,the stress corrosion cracking?SCC?resistance of samples under constant strain loading was investigated by slow strain rate tension and scanning electron microscopy?SEM?.The anodic dissolution?AD?was found to dominate the stress corrosion cracking mechanism.The yield strength of the double-stage aging sample is the highest,and the cracks in the fracture morphology are deeper and wider,exhibiting brittle fracture characteristics and the SCC susceptibility(Iscc)is the highest.The intermediate treatment samples have the lowest Iscc.In addition,under the same heat treatment process,the applied strain also increases the value of Iscc.The effect of sulfate reducing bacteria?SRB?and stress on corrosion behavior was investigated in SRB-inoculated simulated marine solution.The results of the study showed that the Iscccc values of the single-stage aging,double-stage aging and intermediate treatment samples in the SRB-inoculated solution were increased by 5.2%,9.3%,and 4.4%,respectively,compared with the sterile solution.EIS tests showed that the impedance values of all samples were reduced.The production of FeS enhances the anodic dissolution process and accelerates the corrosion reaction.Double-stage aging samples are most sensitive to SRB due to the coarse martensite in the structure facilitating the entry and accumulation of hydrogen.The SCC mechanism for single-stage aging and intermediate samples is AD,and for double-stage aging samples is hydrogen induced cracking?HIC?. |
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