Corrosion Behavior And Mechanism Study Of Ultra High Strength Pipeline Steels In The Soil Environment

In recent years, with the rapid development of oil and energy industry in China, the construction of buried pipeline have booming, the high pressure, large diameter and high grade pipeline steel is inevitable trend of the development in oil and gas transmission pipeline, while the failure problem of pipeline is more and more serious. Stress corrosion cracking(SCC) and microbiologically influenced corrosion(MIC) are important factors to the safety of pipeline in soil environment. Therefore, for the high grade pipeline steel XI00,it is necessary to study stress corrosion, microbiological corrosion, synergy between stress corrosion and microbiological corrosion.The electrochemical corrosion behavior of XI00 pipeline steel in Yingtan, Korla, near-neutral p H value and Haibin soil simulation solutions were investigated using the electrochemical test, SEM, EDS, XRD analysis and weight loss and so on while the effect of sulfate reducing bacteria(SRB) on the electrochemical corrosion behavior of XI00 steel were studyed. The results showed that polarization curves of XI00 pipeline steel are controlled by activation polarization in four types of soil simulation solutions with SRB or without SRB.The corrosion rate of XI00 pipeline steel decrease at first, then continued increase with the increase of immersed time in near-neutral pll value and Haibin soil simulation solutions without SRB,while increasing initial then stable in Haibin and near-neutral p U value soil solutions with SRB, decreasing initial, then continued increase and satble in Yingtan soil simulation solutions without or with SRB, decreasing initial, then continued increase in Korla soil simulation solutions without SRB and continued increasing in Korla soil solutions with SRB. The corrosion rate of XI00 pipeline steel in 4 types of soil simulation solutions without SRB are basic greater than sterile environment, and more obvious at the beginning of the corrosion. After immersed 35d 60d, the degree of corrosion of X100 pipeline steel belong to severe and moderate corrosion in Yingtan soil simulated solutions without SRB and with SRB, the rest of the environment all belong to moderate corrosion, at the same time, the average corrosion rate of X100 pipeline steel in 4 types of soil simulation solutions without SRB are greater than sterile environment. After immersed 60d the corrosion products of XI00 pipeline steel is mainly iron oxides in sterile environment, iron oxides and iron sulfide in with SRB environment. In 4 types of soil simulation solutions with SRB, XI00 pipeline steel surface have a layer of transparent microbial film, in the early corrosion, microbial film and corrosion products combine together have formed a more compact combination membrane that can prevent corrosion.Stress corrosion cracking(SCC) behavior and sensitivity of X100 pipeline steel and welded joint in Yingtan, Korla and Haibin soil simulation solutions were investigated using slow strain rate tension(SSRT) test, while diccussing MIC caused by sulfate reducing bacteria(SRB) effect on SCC, the experimental results show that time-to-failure(Tf), dependent variable, elongation rate, reduction-of-area(ROA) of X100 pipeline steel and welded joint at sterile environment less than Yingtan, Korla and Haibin soil simulation solutions with SRB. All tensile specimen fracture side of X100 pipeline steel have the secondary crack in 3 types of soil simulation solutions, applied stress axis is approximately a 45 degree angle or vertical at sterile environment, vertical at bacteria environment, crack density of sterile environment are higher than bacteria environment. The tensile specimen fracture side of X100 pipeline steel show a uniform corrosion and roughness in solutions with SRB, and smooth, clean, localized corrosion product, show a local corrosion in sulutions without SRB, the fracture side of base metal have pits in Haibin and Korla soil simulation solutions with SRB. X100 base metal are toughness-brittle mixed fracture, but the number of SCC crack at sterile environment are more than bacteria environment. The microscopic fracture of X100 welded joint show brittle characteristics in Yintan, Korla soil simulation solutions without SRB and ductile fracture characteristics with SRB, toughness-brittle mixed fracture in Haibin soil simulation solutions without or with SRB and the nest of sterile is samller. SRB has inhibited embrittlement of X100 pipeline steel and welded joint, reduces the sensitivity of SCC, in particular, the effect to welded joint is more serious.