Corrosion mechanism of carbon dioxide in oil and gas fields and characteristics of corrosion damage
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I. Corrosion mechanism: Core electrochemical process – Dry CO₂ is almost non-corrosive; when it dissolves in water, it forms carbonic acid. The overall acidity of this acid is higher than that of hydrochloric acid at the same pH level. Through electrochemical processes, it corrodes steel. The overall reaction is: CO₂ + H₂O + Fe → FeCO₃ + H₂. Anodic reaction: Iron loses electrons and dissolves to form iron ions. Cathodic reaction: It is divided into two types – non-catalytic reduction of hydrogen ions and catalytic reduction of hydrogen ions with surface-adsorbed CO₂ – and the hydrogen evolution process is the main factor controlling the corrosion rate. Special mechanisms of localized corrosion: The coverage of the FeCO3 corrosion product film on the steel surface is uneven, resulting in the formation of corrosion couples with strong autocatalytic properties in different areas. This leads to a rapid acceleration of localized corrosion, ultimately causing perforation and failure. II. Characteristics of corrosion damage: General corrosion: It occurs mainly under conditions of temperatures below 60°C and low CO₂ partial pressures; the metal surface thins uniformly, and the corrosion product film is soft with poor adhesion. Local corrosion: It is the most common form of severe damage in oil and gas fields, including pitting corrosion, honeycomb corrosion, and planar corrosion. Under flowing conditions, knife-edge grooves parallel to the flow direction can form, which can easily lead to rapid pipe perforation.
The content shared by the original poster is very professional; CO₂ corrosion is indeed a troublesome issue in oil and gas fields. I have worked on projects related to this before, and I’d like to share a few practical insights for reference: The density of the corrosion product film (FeCO3) is crucial – if the film can cover the surface evenly, the corrosion rate will decrease significantly ； However, once there is local damage (such as high flow rates or erosion), pitting or plateau corrosion is likely to occur, posing an even greater threat. Temperature has a significant impact on the corrosion rate: the range of 60–80°C is when the density of the FeCO₃ film is relatively low, resulting in the most severe corrosion; therefore, this temperature range should be given special consideration during design and material selection. In practical applications, corrosion inhibitors and coatings (such as nickel-based alloys or coatings) are commonly used methods; however, the specific approach must be determined based on the operating conditions (pressure, pH value, Cl⁻ concentration, etc.). It is advisable to conduct laboratory simulations in collaboration with corrosion engineers. The information above is based on my own experience; the composition of oil and water varies greatly depending on the location. It is recommended to conduct further tests or consult a professional agency based on the actual conditions.