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.
INTRODUCTION
The influence of the iron sulfide scale on sour corrosion is of paramount importance. However, studies on the electrochemical behavior of such scales are insufficient. Ma et al.1 followed the behavior of iron sulfide scales at solution pH from 3 to 5, they found that a protective iron-sulfide layer forms at low concentrations of H2S. The structure and composition of this protective FeS layer is closely related to H2S concentration, pH of solution and the immersion time of the sample. Menendez et al. warned about under-deposit localized and general corrosion occurring underneath mackinawite deposits.2 Wolthers et al. reported the surface chemistry of disordered mackinawite.3 They found that the point of zero charge (PZC) of mackinawite occurs at pH ~ 7.5. Therefore, pH will surely control the electrochemical behavior of these scales and, in turn, its corrosion behavior. Electrochemical impedance spectroscopy (EIS) was used to monitor the corrosion behavior of a mackinawite scale in buffered pH above and below 7.5. When the mackinawite scale was charged negatively, protection of the metal surface occurred, whereas when it was charged positively, localized corrosion occurred underneath.
