Hydrogen Gas Evolution from Cathodically Protected Surfaces^★

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