New insights on Mechanistic Model for CO₂ Corrosion of Carbon Steel Under Acidic Conditions (RIP2026-00095)

Marcelo Sampaio, jessica Nogueira da Cunha, Tatiana Almeida, Eliane D'EliA, Oscar Rosa Mattos

Universidade Federal do Rio de Janeiro, Universidade Federal do Rio de Janeiro, Universidade Federal do Rio de Janeiro, LNDC/UFRJ

The corrosion of carbon steel in CO₂-containing environments is a major concern in the oil and gas industry, where the exposure of pipelines and equipment to CO₂-rich aqueous phases accelerates metal degradation and has been studied for more than fifty years. From classical studies in the 1970s it became widely accepted that carbonic acid acts as an additional cathodic reactant, thus increasing corrosion rates beyond those observed in strong acids at the same pH. However, subsequent research challenged this assumption. Alternative interpretations suggested that the CO₂ system enhances corrosion through its buffering capability. Despite these advances, inconsistencies persisted in correlating corrosion kinetics with measurable parameters such as pH, electrochemical response, and mass loss.
Considering the above, electrochemical impedance was used to investigate the effect of CO2 on X65 carbon steel corrosion in in 0.17 mol L⁻¹ NaCl solutions at pH 3, 4, and 5, both in nitrogen-purged conditions and in CO₂-saturated media. The steel used was API 5L X65, and the experiments also included steady-state polarization curves and mass loss testing.
The polarization curves revealed a fundamental difference between media with and without CO₂. In nitrogen-purged solutions, corrosion current densities increased significantly as pH decreased. Conversely, in CO₂-saturated solutions, corrosion rates remained essentially constant across all pH values tested.
Electrochemical impedance results further supported this finding. In all conditions, the diagrams exhibited three time constants: a high-frequency capacitive loop corresponding to double-layer response, a mid-frequency inductive loop associated with intermediate like (FeOH)ads, and a low-frequency capacitive feature. In the absence of CO₂, the inductive loop characteristic frequency was strongly dependent on pH, shifting to higher values as acidity increased. However, when CO₂ was present, the frequency of this inductive loop remained independent of pH and instead shifted only when the electrode potential was varied. Mass loss results confirmed the electrochemical trends.
The main contribution of the present work lies in present a new reaction mechanism pathway in the hydrogen evolution reaction, associated to the carbonic species. It was determined that one of the cathodic elementary steps becomes independent of surface H⁺ concentration when CO₂ is present. The newly proposed model successfully reproduces experimental observations in both steady-state and transient regimes.
The work provides significant advances toward a more complete mechanistic understanding of CO₂ corrosion of carbon steel. This refined understanding is critical for the development of improved predictive models and mitigation strategies for pipeline corrosion in oil and gas systems.