ELECTROCHEMICAL BEHAVIOR AND SCC SUSCEPTIBLE POTENTIAL RANGE OF X70 PIPELINE STEEL IN NaHCO3/Na2/Na2CO3 SOLUTIONS

摘要

electrochemical behavior of X70 steel in NaHCO3/Na2CO3 solutions with different HCO3- concentration was investigated by potentiodynamic polarization, Cycle Voltammetry (CV) and X-ray Diffraction (XRD) test. Results showed that prepassive (first- time passive) film on surface of the steel is more difficult to form and the prepassive film stability decreased with increasing of HCO3- concentration in the solutions (less than 0.5 mol/L), while passive(second-time passive)film stability increased with increasing of HCO3- concentration. The CV spectroscopes showed that Fe[Ⅱ] species as the composition of prepassive film were generated in the active anodic dissolution potential range, the dissolution reaction of Fe was favored by the presence of HCO3- and it was explained through the formation of unstable complex containing Fe[Ⅱ] and HCO3-. XRD patterns of prepassive film confirmed the presence of Fe[Ⅱ] species on thick surface film grown in the prepassive potential region. Component of prepassive film was changed with the increasing of HCO3-. The SCC susceptible potential range was assessed using boundary condition of Parkins and stress corrosion cracking (SCC) parameter Pi, which incorporate the ratio of current densities obtained in fast and slow potentiodynamic scans. The SCC susceptible potential range of X70 pipeline steel was determined between -640mV to -570mV in 1 mol/L NaHCO3 +0.5mol/L Na2CO3 solution.