Study On Stress Corrosion Cracking Behavior And Mechanism Of X80Pipeline Steel Under AC Application In High PH Solution

In recent year, the risk of alternating current (AC) interference on the buried steel pipeline has increased with rapidly constructed high voltage AC transmission lines and AC electric railways. AC interference has resulted in AC corrosion of the pipeline, and accelerated the corrosion of pipeline, which have constituted great threats to the normal operation of pipelines. At present, many researchers have carried out large studies on AC corrosion both in China and abroad, however, the related literature about the effect of AC on stress corrosion cracking behavior and mechanism of pipeline steel is never reported. Thus, in this paper, the corrosion behavior of X80pipeline steel under AC application was systematically investigated in high pH carbonate/bicarbonate solution using polarization curve, corrosion potential test, electrochemical impedance spectroscopy(EIS), Mott-Schottky curves, immersion test, slow strain rate test (SSRT), U-bent immersion test and crack propagation test. In especial, the effect of AC interference on stress corrosion cracking (SCC) behavior and mechanism of X80pipeline steel was mainly studied. The main conclusions are as follows:The results of electrochemical test show that the corrosion potential of the steel shifts negatively with increasing AC current density. A significantly negative shift of corrosion potential can be observed at low AC frequency (30-300Hz), whereas higher frequency (300-1000Hz) AC produces a much smaller negative shift. While the order of the degree of the negative shift in corrosion potential of the steel under various waveforms AC current is:sine wave<triangular wave<square wave. Under AC application, the passivity of the steel in high pH solution degrades severely. Low AC current density causes little damage on the passive film, with the increase of AC current density, the passive region significantly narrows and the critical pitting potential shifts negatively. Meanwhile, the passive current density increases and the donor density of passive film increases. Low frequency AC current can greatly increase the corrosion damage degree of the passive film, in contrast, high frequency AC has less damage to the passive film. Besides, the application of various waveforms AC current severely degrades the passivity of the steel.The results of immersion test show that, at the low AC current density, the steel mainly experiences a slight uniform corrosion. As AC current density continues to increase, corrosion becomes more localized. Under application of high frequency AC current, the steel electrode experiences slight uniform corrosion. When low frequency AC current is applied to the steel, serious corrosion with significant pitting is observed on the surface of the electrode. The localized corrosion of the steel under square wave AC is significant, next the triangular wave, the least sine wave. A superimposed AC current enhances the corrosion rate of X80steel, and the corrosion rate increases with increasing AC current density. In the frequency range from30Hz to200Hz, the corrosion rate decreases sharply with increasing AC frequency. At high AC frequency of200Hz to1000Hz, however, a slower decrease in the corrosion rate can be observed. The order of corrosion rates of steels under various AC waveforms is:sine wave<triangular wave<square wave.The results of SSRT tests demonstrate that, at the effect of pre-applying AC current, localized corrosion (pitting) has occurred on the steel due to the application of AC current, accelerating the initiation and propagation of SCC micro-cracks, and thus increasing the SCC susceptibility of X80steel in high pH carbonate/bicarbonate solution. The SCC cracks are intergranular and the SCC mechanism is anodic dissolution for steels with or without the application of pre-applying AC current. The SCC susceptibility increases with the increase of AC current density above the critical value of30A/m2. High frequency AC current has little effect on the SCC susceptibility of steel. With the decrease in AC frequency, a higher susceptibility to SCC is observed. When the applied frequency is30Hz, the SCC susceptibility of the steel increases significantly. The SCC susceptibility of the steel under square wave is the highest, next the triangular wave, the least sine wave.In SSRT test, when AC is applied, there are a large quantity of wide and deep cracks, and the propagation mode of these cracks is transgranular, which shows the characteristic of obvious brittle fracture. By contrast, in the absence of AC current, there are existing numerous narrow and shallow cracks, and the cracks are intergranular. The SCC mechanism of the steel under AC application in high pH solution is mixed controlled by both anodic dissolution and hydrogen embrittlement, while the mechanism is anodic dissolution for steel without AC application. The superimposed AC enhances the susceptibility of the steel to SCC, and the increased AC current density increases the SCC susceptibility. When iAc100A/m2, the steel has high SCC susceptibility. The order of the SCC susceptibility of steels under various AC waveforms is:sine wave<square wave <triangular wave. The decrease in AC frequency enhances the SCC susceptibility, and the steel at low frequency AC has high susceptibility to SCC.The results of U-bent immersion test verify that, AC application enhances the corrosion of X80steel, accelerating the occurrence of pitting corrosion, and induces the initiation of SCC cracks. The crack propagation tests show that there is an obvious difference in the crack propagation behavior of X80steel between with or without AC application. The average crack propagation rates of the steels tested with or without AC are5.645×10-3mm/cycle(7.056×10-6mm/s),5.99×10-4mm/cycle (7.48×10-7mm/s) respectively. The average crack propagation rate of the steel under AC application is much larger than that of steel without AC. AC application accelerates the crack propagation of X80steel.AC and Cl" generate synergistic effect on the corrosion behavior of steel, resulting in a large amplification negative shift of corrosion potential, and enhancing the electrochemical activity of steel, as well degrading the passivity of the steel. Moreover, the corrosion rate greatly increases and corrosion becomes more localized. The SCC susceptibility increases due to the synergistic effect of AC and Cl-, and the morphology exhibits the apparent characteristic of brittle fracture. The SCC mechanism is controlled by anodic dissolution and hydrogen embrittlement.