Abstract
An experimental study has been conducted on the formation of hydrogen bubbles on a mill-scaled steel surface near the entrance to a crevice simulating a disbonded pipeline coating surrounding a holiday and containing a carbonate-bicarbonate solution. The study has shown that the rate of growth of bubbles and their effects on the potential along the crevice is a function of the potential applied at the holiday and the crevice thickness, whilst the orientation of the simulated crevice and the temperature had little further effect upon bubble formation. Because of IR drops along the crevice, the rate of hydrogen release is less for creviced than for non-creviced surfaces exposed to the same applied potential. Thus, for both situations the highest (least negative) potential for hydrogen release is between -1.05 and -1.1V (SCE), when the potential is made successfully more negative from about -0.8V at daily intervals. However, when the non-creviced surfaces that had previously been held for extensive periods at -1.1V or below were raised to higher potentials, hydrogen continued to be evolved, but at diminishing rates, until -0.8V was reached. This hysteresis in the discharge of hydrogen as a function of potential is suggested as resulting from various reactions on the mill- scaled steel surface, with reduction to metallic iron at -1.1V or below providing the lowest overpotential for hydrogen release.
