Pitting Corrosion of X65 Pipeline Steel in CO2-Saturated Chloride Solutions: Effects of Acetic and Formic Acids with Pit Chemistry Insights from Pure Iron Experiments

The influence of acetic acid (HAc) and formic acid (HFr) on the pitting corrosion behavior of X65 pipeline steel in CO2-saturated 1 wt% NaCl solution was investigated using potentiodynamic polarization, long-term immersion, and pit morphology analysis. To mechanistically evaluate the role of organic acids on pit chemistry, a pure-iron lead-in pencil electrode technique was used, allowing controlled, one-dimensional pit growth and quantification of key chemical parameters inside the pit environment. Both HAc and HFr shifted the corrosion potential to more noble values and enhanced cathodic kinetics by buffering local pH, thereby accelerating overall corrosion rates. Immersion testing and scanning electron microscopy revealed that the presence of organic acids resulted in fewer but deeper and more aggressive pits than those formed in acid-free chloride solutions. Mechanistic analysis revealed that HAc and HFr exert a dual effect, increasing the pit propagation rate while also altering the local chemistry to delay the termination of pit growth by raising the required concentration of metal cations. These findings indicate that although acetic and formic acids do not increase the number of corrosion pits, they substantially increase pit severity, representing a critical risk factor for pipeline integrity in CO2-rich, chloride-containing environments common to oil and gas operations.