Characteristics and mechanism of high pH stress corrosion cracking of pipeline steel X70

The characteristics and mechanism of high pH SCC of pipeline steel X70 were studied by means of electrochemical measurements and slow strain rate tests (SSRT) in 0.5M Na2CO3 + 1M NaH CO3 solution (pH similar to 9.3) at various potentials and also in hydrogen-charged condition. The steel exhibited apparent active-passive transition when anodically polarized to about -700mV(SCE) in the solution. The SCC susceptibility in terms of plasticity loss during SSRT in the solution increased generally with decreasing potential in the range from -200 to -1500mV. The steel showed deep and transgranular cracks in cathodic polarization range from -1500 to about -840mV, but exhibited shallow, numerous and mainly intergranular cracks around -700 similar to -600mV, and few cracks at more anodic potentials. The rise of temperature from 30 degrees C to 50 degrees C increased the SCC susceptibility in the anodic polarization range but had no apparent effect on that at cathodic potentials. SSRT of hydrogen-charged specimens showed that the steel was susceptible to hydrogen induced cracking (HIC). The cracking mechanism in different conditions is discussed.