Stress Corrosion Cracking Of Pipeline Steels In Soil Environments

Rich experiences of pipeline operating in the world have shown thatstress corrosion cracking (SCC) is one of the main causes leading tofailure of buried pipelines for gas or oil transmission. There are twotypes of SCC that causes failures on pipelines. One is high pH SCC, theother is near-neutral pH SCC. Research into SCC will help to ensure thesafe operation and reduce the risk of SCC of the high-pressure gas pipelineof more than 500 kilometers around Shanghai. In this work, SCCcharacteristics of three types of pipeline steels X70, X60 and SS400 insimulated high pH soil environment and near neutral pH soil environmentfor Shanghai have been studied in laboratory with slow strain ratetest(SSRT). The main results were obtained as follows:Ⅰ. The results in high pH simulated soil solution indicated that:The SCC susceptibility of pipeline steels X70, X60 and SS400 decreasedgenerally with increasing the electrode potential in the wide range from-1500 mV (SCE) to -300mV (SCE)in high pH soil solutions. The greater thestrength of pipeline steel was, the greater the SCC sensitivity was. Inthe range from 30℃to 65℃, temperature had no apparent effect on SCCsusceptibility of the steel X70 in anodic polarization range, but the SCCsusceptibility decreased with rising temperature in cathodicpolarization range. Cracking mechanism analysis showed that when E＞E_H(theequilibrium potential for hydrogen discharge), anodic dissolution shoulddominate cracking process. On the contrary, when E≤E_H, it should behydrogen induced cracking (HIC) which dominates the cracking process.Similar to that of the base materials, SCC sensitivity of the weldsof X60 and SS400 in high pH solution at 30℃decreased with increasing the potential. And their SCC sensitivity was generally higher than thatof their base materials.2. The results in near neutral pH soil solution for Shanghai indicatedthat:The SCC susceptibility of pipeline steel X60 in near-city soil,cyan-yellow soil, cyan-purple soil solution at 30℃and pipeline steelSS400 in yellow soil, yellow sand, coastal saline soil solution at 30℃decreased generally with increasing the potential in the range from-1500mV(SCE) to-300mV (SCE). It was found that the SCC sensitivity ofpipeline steels X60 and SS400 in soil solution had some relation with theelectrode electrical current. The greater the current was, the greaterthe SCC sensitivity was. Soil had same effect on the SCC sensitivity ofpipeline steels X60 and SS400. In anodic polarization range, X60 had thegreatest SCC sensitivity in the near-city soil solution, but had thesmallest SCC sensitivity in the cyan-yellow soil solution. The SCCsensitivity of SS400 steel in the coastal saline soil or yellow sandsolution was significantly greater than that in the yellow soil solutionin the anodic polarization range. Cracking mechanism analysis showed thatwhen E＞E_H, it should be the united effect of hydrogen-induced crackingand dissolution; When E≤E_H, it should be hydrogen-induced crackingwhich dominates the cracking process.Similar to that of the base materials, SCC sensitivity of the weldsof X60 and SS400 in Shanghai soil solutions at 30℃decreased withincreasing the potential. And their SCC sensitivity was generally higherthan that of their base materials. The heat-affected zone near the fusionline in X60 weld and the weld zone in SS400 weld had the highest SCCsusceptibility.3. As the three pipeline steels have shown high sensitivity to SCCin the low-potential range in various simulated soil environments, sufficient attention should be paid to the adverse effect of cathodicpolarization. It is necessary to optimize the cathodic protectionparameters in order to avoid excessive protection. The heat-affected zonenear the fusion line in X60 weld and the weld zone in SS400 weld were themost likely influenced areas by SCC, so that more attention should be paidto the welding area of pipeline steel and to the elimination of the stressconcentration. It should pay more attention to the areas in which soilshowed high SCC sensitivity in tests in order to promote pipeline safety.