Effect of Electrochemical Potential and Solution Concentration on the SCC Behaviour of X-70 Pipeline Steel in NaHCO3

The stress corrosion cracking (SCC) behaviour of a X70 microalloyed pipeline steel, with different heat treatments under both anodic and cathodic overpotrentials has been evaluated by using of the slow strain rate testing (SSRT) technique at 50°C. Heat treatments included the as-received (annealed) condition and water quenched. Tests solutions consisted of 0.1, 0.05, 0.01 and 0.005 M sodium bicarbonate (NaHCO3). Hydrogen permeation measurements were also carried out. Additionally, experiments using the SSRT technique with pre-charged hydrogen samples and potentiodynamic curves at different sweep rates were also done to elucidate hydrogen effects in this type of steel. Results showed that the steel under both heat treatments was highly susceptible to SCC only in 0.005 M NaHCO3 and with the application of anodic or very high cathodic overpotentials. The quenched steel, with a martensitic microstructure, was always more susceptible to SCC than the steel in the as received condition with a ferritic-pearlitic microstructure. Hydrogen permeation tests showed a much higher hydrogen uptake by the quenched than the as-received steel. Pre-charging tests showed that only the quenched specimen was susceptible to fail by a hydrogen-based mechanism, whereas low and fast sweep polarization curves showed that the steel in the as-received condition was more susceptible to fail by an anodic dissolution-based SCC mechanism. Thus, the SCC mechanism for the as-received steel was dominated by anodic dissolution at the free corrosion potential, Ecorr, or with the application of anodic overpotentials, whereas for the steel in the quenched condition was dominated by a hydrogen-based mechanism. Both steels were dominated by a hydrogen-based mechanism with the application of high cathodic overpotentials.