Transition criterion of metastable pitting towards stability for carbon steel in chloride solutions

The current transients during metastable Pitting Of A516-70 carbon steel in bicarbonate- chloride solutions were monitored and statistically analysed. The transition criterion from metastable to stable pits was determined. Two experimental approaches, were used for this purpose: potentiostatic control and galvanic coupling. It was shown that the initiation of metastable pits was indicated by typical current transients: a quick current rise followed by a slow recovery. A pit stabilisation criterion, that the ratio of peak pit current to pit radius must exceed 2 x 10(-2) A cm(-1) during stable pit growth, was established from the potentiostatic measurements. This criterion reflected the critical concentration difference of cation between bulk solution and pit anolyte. Two other pit stabilisation factors, statistical parameters in the galvanically coupled experiments, qualitatively provided an indication of the change in pitting activity. However, these factors had shortcomings due to the errors existing in the measurement system.