ELECTROCHEMICAL POLARIZATION AND STRESS-CORROSION CRACKING BEHAVIORS OF A PIPELINE STEEL IN DILUTE BICARBONATE SOLUTION WITH CHLORIDE-ION

The electrochemical and stress corrosion cracking behaviour of Grade 5 50 (X-80) pipeline steel with coating disbandments was studied in a ra nge of bicarbonate solutions using a potentiodynamic polarization tech nique and slow strain rate test. The shape of the polarization curves were found to vary with bicarbonate concentration. Increase in bicarbo nate concentration results in a wider passive region and a more noble breakdown potential. Low concentrations of chloride ion in bicarbonate greatly affect the polarization characteristics and can cause the eli mination of passivity in specific solutions. Tensile specimens were pu lled at constant strain rate epsilon=10(-6)/sec. under potential contr ol ranging from -350 mV to -900 mV (SCE) in NS-4 solution (dilute bica rbonate solution with chloride ion). Fracture surface of the specimens were examined under scanning electro microscopy. Overall fracture is ductile dimple, however, transgranular stress corrosion cracking has b een found in the edge of specimen for the sample with potential contro lled at -800 mV, -750 mV and -650 mV (SCE).