The Influence of pH on Localized Corrosion Behavior of X65 Carbon Steel in CO₂-Saturated Brines

Pitting and localized corrosion of carbon steel is considered to be a complex process influenced by a wide range of parameters such as temperature, bulk solution pH, and chloride ion concentration. Solution pH is known to influence corrosion product characteristics and morphology in CO₂- and H₂S-containing corrosion systems. However, from the perspective of pitting corrosion in CO₂-saturated environments, the extent to which bulk pH of solutions and the presence of corrosion products influence localized attack is still not clearly understood. This paper presents an investigation into the role of pH on the characteristics of corrosion product and pitting corrosion behavior of X65 carbon steel (UNS K03014) in CO₂-saturated brine. Pitting corrosion studies were conducted over 168 h at 50°C in 3.5 wt% NaCl solutions at different bulk pH (buffered to pH values of 6.6 and 7.5 in some cases) in order to understand and correlate the role of pH on corrosion product morphology, chemistry, initiation, and propagation of pits within each distinct environment. Corrosion product composition and morphology are identified through a combination of electrochemical and surface analysis techniques, which include scanning electron microscopy and x-ray diffraction. The extent of corrosion damage of the carbon steel is evaluated through the implementation of surface interferometry to study discrete pit geometry, namely the size, depth, and aspect ratio. Results indicate that the process of pit initiation and propagation of carbon steel in CO₂ corrosion environment is different depending upon bulk solution pH. At low pH (pH values starting at 3.8), pitting initiates faster and propagates steadily along with significant uniform corrosion resulting from the formation of an “amorphous” form of FeCO₃. At higher pH, uniform corrosion is significant, while pitting initiates with increasing protection from crystalline FeCO₃. At a pH value of 7.5, pitting corrosion initiation occurs after and/or during pseudo-passivation is achieved as a result of the formation of a “protective and pseudo-passivating” FeCO₃ film.