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 550 (X-80) pipeline steel with coating disbandments was studied in a range of bicarbonate solutions using a potentiodynamic polarization technique and slow strain rate test. The shape of the polarization curves were found to vary with bicarbonate concentration. Increase in bicarbonate 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 elimination of passivity in specific solutions. Tensile specimens were pulled at constant strain rate epsilon=10(-6)/sec. under potential control ranging from -350 mV to -900 mV (SCE) in NS-4 solution (dilute bicarbonate 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 been found in the edge of specimen for the sample with potential controlled at -800 mV, -750 mV and -650 mV (SCE).