Abstract
The corrosion protection due to iron sulfide scales has been and continues to be a controversial issue in corrosion science. These scales are very important to understand the mechanism of sour corrosion. For instance, it is known that the point of zero charge (PZC) for mackinawite occurs at pH ~ 7.5. This implies that it is positively charged at pH < 7.5 and negatively charged at pH > 7.5. How this affects corrosion regimes underneath the scale is unknown. This paper describes the formation of a mackinawite scale on carbon steel in a borate buffer pH = 8.4. Then, it was exposed to different buffers with lower pH. The integrity of the scale and the corrosion occurring underneath was monitored with electrochemical impedance spectroscopy (EIS). Two distinct behaviors were identified. At pH values above PZC, general corrosion and low pore resistances were observed. At pH values under PZC, localized corrosion and a high pore resistances were detected. This last behavior also supports iron-sulfides as cathodic fuels for the corrosion process. It can be concluded that mackinawite’s PZC is a new important factor in the corrosion behavior of these scales.
