Galvanic Interactions Between Fe Electrodes in CO₂-Saturated Solutions with Different pH

Localized CO₂ corrosion is a very common problem in the oil and gas industry. Severe damage of the surface is attributed to the formation, and breakdown, of protective iron carbonate (FeCO₃) scales. When the corrosion layer is compromised, the difference between the open-circuit potentials of the FeCO₃-covered and noncovered regions present on the same metallic surface acts as the driving force for a galvanic interaction. Depending on the area ratio of the anodic and cathodic areas, the surface could suffer severe localized damage. The present study was focused on the galvanic interactions between iron samples in solutions with different pH. CO₂-saturated 1% NaCl solutions with bulk pH of between 6 and 8 and temperature ranging from 20°C and 80°C were studied. A split cell allowed for customization of different environments in each of the half cells, along with simultaneous monitoring of the galvanic current and driving force as indicated by the difference in open-circuit potential. Corrosion product layers were characterized using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The results indicated that the pH of the bulk solution plays a major role in the formation of protective FeCO₃ scales. Fe exhibited passive-like behavior when immersed in a solution at 80°C with pH adjusted to 8. After reaching a passive-like behavior, Fe samples were cathodic when coupled to samples immersed in a solution with lower pH. The magnitude of the galvanic current increased with increasing pH gradient.