POTENTIAL EFFECT OVER THE STRESS CORROSION CRACKING BEHAVIOR OF A X-70 MICROALLOYED PIPELINE STEEL IN DILUTE NaHCO_3 SOLUTIONS

摘要

Studies were carried out to evaluate the stress corrosion cracking (SCC) behavior of a X-70 microalloyed pipeline steel, with different microstructures by means of the slow strain rate testing (SSRT) technique at 50 °C. Tests solutions consisted of 0.1, 0.05, 0.01 and 0.005 M sodium bicarbonate (NaHCO_3) with KCI, CaCl_2 and MgSO_4-7H2_O additions. Different anodic and cathodic potentials were applied. Additionally, experiments using the SSRT technique but with pre-charged hydrogen samples and potentiodynamic curves at different sweep rates were also done to elucidate hydrogen effects in this type of steel. The results showed that the different microstructures in conjunction with the anodic applied potentials shift the cracking susceptibility of the steels evaluated. On the other hand the closest cathodic potentials to their respectively steels rest potentials lower SCC susceptibility no matter the microstructure being tested. Contrary to this behaviour higher cathodic potentials promote SCC in all steel conditions. The fracture mechanisms actuating on the SSRT samples were promoted by anodic dissolution and protective film rupture. The results at different sweep rates indicate that certain microstructures are more susceptible to present anodic dissolution corrosion mechanism. Meanwhile more concentrated solution promotes ductile (no brittle) fracture surfaces on SSRT specimens.