Study On Sulfur Corrosion Mechanism Of Pipeline Steel

In the acidic oil and gas field development process, the deposition of elemental sulfur causes pipeline blockage and severe localized corrosion, which seriously affect the normal production of oil and gas and the normal operation of related equipment and instrumentation. Severe localized corrosion appearing at elemental sulfur deposition zone causes pipeline perforation, leading oil and gas leak, which not only pollutes the environment but also threatens the safety of life and property of the surrounding residents. For elemental sulfur corrosion problems on the pipeline steel, to understand the law of sulfur corrosion and find out the root cause of sulfur corrosion, this study based on the research findings of the source of the elemental sulfur in the reservoir, elemental sulfur deposition mechanism and sulfur corrosion law in the deposition zone, with reference to the latest research results of the domestic and foreign, we designs experiment plans to study on sulfur corrosion mechanism. When high-sulfur oil and gas is produced and transported, the research results of the paper shall provide a theoretical reference for pipeline protection.The main content of the paper is as follows:(1) By orthogonal experimental method, the degree of hydrolysis reaction of elemental sulfur was studied at different temperatures, pressures and sulfur content conditions. The results showed that:the temperature is the main factors influencing the elemental sulfur hydrolysis reaction. Stress is the second most important influencing factors. Sulfur content has no significant effect on the hydrolysis reaction of the sulfur. By linear regression of the experimental results, fitting equation of degree of hydrolysis of elemental sulfur at different temperatures and pressures is got: pH=8.0238-0.03979T-0.04464P(2) By the corrosion weight loss experiment, the corrosion of L360NS pipeline steel was studied at different sulfur states and different elements sulfur content. The results showed that:Under the condition of sulfur suspension and sulfur precipitation, the corrosion rate of the L360NS steel was below the corrosion rate in the blank solution, which shall consist of5.0wt%sodium chloride and0.5wt%glacial acetic acid dissolved in distilled water. Uniform corrosion is caused by weak acid and pitting corrosion is caused by Cl. Under the condition of sulfur suspension, with the increase of elemental sulfur content, the corrosion rate of L360NS steel dropped to a steady value. Under elemental sulfur melting cover condition, L360NS steel suffered a serious uniform corrosion. (3) Electrochemical methods was used to research the corrosion of L360NS pipeline steel in S/Cl-/H2O system, and the impact of the state and content of elemental sulfur. L360NS pipeline in different sulfur states, different sulfur content had the same corrosion mechanism, i.e. corrosion was controlled by activation polarization. Under elemental sulfur melting cover condition, specimen had a negative capacitive loop in electrochemical impedance spectroscopy (EIS), which meant that the impedance of the electrode process measure did not meet the stability conditions. Mainly because the moisture content of the coating of melting sulfur had a significant change during the measurement. The EIS of L360NS pipeline has the characteristics of after the formation of pitting corrosion in the standard solution, in the precipitated sulfur condition and in sulfur suspension condition.(4) Using slow strain rate stress corrosion tests(SSRT), the stress corrosion cracking of L360NS pipeline steel at different sulfur content, under different sulfur states was researched. The results showed that the presence of elemental sulfur obviously increased the stress corrosion sensitivity of L360NS pipeline steel. Specimen were mainly brittle fracture in high elements sulfur environment. Under the condition of sulfur suspension, with the sulfur content in the experimental range, L360NS pipeline steel stress corrosion susceptibility increased with the increasing content of elemental sulfur. Under elemental sulfur melting cover condition, specimen gauge section suffered significant corrosion. A lot of corrosion pits emerged at specimen gauge section. The black corrosion products was Fe1+xS, which is easy to fall off. At the same time H2S generated.