During heat treatment processing,microstructures of heat affected zone(HAZ)were formed in X80 pipeline steel.After observation by optical microscopy,scanning electron microscopy and transmission electron microscopy,microstructure of the as-received X80 steel was confirmed to be acicular ferrite,while the microstructures of quenched,normalized and annealed X80 steels were lath bainite,granular ferrite and quasi-polygonal ferrite,respectively.After immersion in the simulated acidic soil solution for 48 h,corrosion rates of these four steels were determined by mass loss measurements and corrosion products were examined by Raman spectroscopy and X-ray photoelectron spectroscopy.Scanning vibrating electrode technique was used to characterize the micro-galvanic corrosion behaviors of the synthetic bimetallic electrodes which were formed by coupling each of the simulated HAZ microstructures with the as-received steel in direct physical and electric contact.It is demonstrated that the as-received steel acts as cathode in the as-received/quenched and as-received/normalized couples,while the annealed steel acts as cathode when coupling with the as-received steel.The distinction of current density between the galvanic couples reduces with prolonging the immersion time.
Susceptibilities to stress corrosion cracking(SCC) of X80 pipeline steel in relatively concentrated carbonate/bicarbonate solutions with different chloride ion concentrations or p H value at a passive potential of-200 m V vs SCE were investigated by slow strain rate tensile test.In order to explore the SCC mechanism and the evaluation criterion for the SCC susceptibility of the steel in passive state,electrochemical measurements were taken.Potentiodynamic polarization curves were obtained at different potential sweep rates,and electrochemical impedance spectroscopy measurements were taken after fast polarization to the passive potential.The effects of chloride ion and p H on SCC behaviors of X80 steel at the passive potential were also discussed.The results showed that the SCC mechanism of X80 pipeline steel was greatly influenced by the passive film formed in these solutions.The SCC behaviors followed the film suppressed anodic dissolution mechanism in these circumstances,because the filming process accounted for a considerable proportion of the overall electrode process.The criteria for evaluating the SCC susceptibility of the steel at passive potential were proposed and validated.Decreasing in the concentration of chloride ion or increasing in p H value resulted in the reduction in SCC susceptibility.The existence of chloride ion greatly lowered the passivation tendency and the film stability,while its concentration determined the dissolution rate of the steel matrix.Higher p H value was responsible for the stable and tenacious passive films and the high repassivation capability.It was also inclined to lower the anodic dissolution rate at crack tips by retarding the cathodic oxygen reduction.
Lin FanZhi-Yong LiuWei-Min GuoJian HouCui-Wei DuXiao-Gang Li
