The Influence Of The Fluctuating Stress To The SCC Behavior Of X80 Pipeline Steel In Near Neutral Environments

The study takes X80 pipeline as the research object, potentiodynamic polarization technique is employed to investigate characteristics of anodic dissolution of X80 pipeline steel in a mimic localized soil environment(NS4 solution) under different p H and fluctuating stress conditions. Slow strain rate test(SSRT) and fluctuant slow strain rate test are applied to test its stress corrosion cracking(SCC) behavior and characteristics of crack propagation. With the application of optical microscope(OM) and scanning electron microscope(SEM), the fracture surfaces and data are analysed to identify the effect of different applied potential and fluctuating stress to the mechanism of SCC behavior.The results of electrochemical test showed that the corrosion of the X80 pipeline steel is characteristic of active anodic dissolution. The p H-value of the NS4 solution and fluctuating cycle stress have no effect on the mechanism of anodic dissolution. The corrosion current density increases as the value of p H decreases in the range between 3 and 7. Fluctuating stress also enhanced the anodic dissolution, with the applied potential, the average corrosion current density was increased by 66% comparing to original condition.SSRT results showed that X80 pipeline was SCC sensitive to NS4 solution. X80 pipeline showed highly relevant to cathodic protection potential which applied to steel in SSRT. As the catholic protection potential shifted negatively, SCC susceptibility of the steel was promoted and the dimple fracture transformed into cleavage fracture gradually. The brittleness of X80 steel increased gradually and hydrogen embrittlement mechanism played a major role in the fracture process. When the applied potential was-1200 m V, X80 pipeline steel showed highest sensitivity index.Fluctuanting slow strain rate test showed that X80 pipeline would fracture with the fluctuating stress which had not reached the yield limit ss. Under the different maximum load stress and stress radio, the cracking form is transgranular fracture mode.