CO₂ Corrosion

• CO₂ Information
• Where found
• Prevention / Mitigation
• Pictures

CO₂ Information

Carbon dioxide systems are one of the most common environments in the oil field industry where corrosion occurs. Carbon dioxide forms a weak acid known as carbonic acid (H₂CO₃) in water, a relatively slow reaction. However, CO₂ corrosion rates are greater than the effect of carbonic acid alone. Cathodic depolarization may occur, and other attack mechanisms may also be at work. The presence of salts is relatively unimportant.

Corrosion rates in a CO₂ system can reach very high levels (thousands of mils per year), but it can be effectively inhibited. Velocity effects are very important in the CO₂ system; turbulence is often a critical factor in pushing a sweet system into a corrosive regime. This is because it either prevents formation or removes a protective iron carbonate (siderite) scale.

Conditions favoring the formation of the protective iron carbonate scale are elevated temperature, increased pH (bicarbonate waters) and lack of turbulence. Magnetite scales are also formed in CO₂ systems, and corrosion product scales often consist of layers or mixtures of siderite and magnetite.

The maximum concentration of dissolved CO₂ in water is 800 ppm. When CO₂ is present, the most common forms of corrosion include uniform corrosion, pitting corrosion, wormhole attack, galvanic ringworm corrosion, heat affected corrosion, mesa attack, raindrop corrosion, erosion corrosion, and corrosion fatigue. The presence of carbon dioxide usually means no H₂ embrittlement. For more on the theory and mechanisms for each corrosion type, go to the theory pages

CO₂ corrosion products include iron carbonate (siderite, FeCO₃), Iron oxide, and magnetite. Corrosion product colors may be green, tan, or brown to black.

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Where Found

As stated before, CO₂ corrosion is one of the most common environments where corrosion occurs, and exists almost everywhere.

Areas where CO₂ corrosion is most common include flowing wells, gas condensate wells, areas where water condenses, tanks filled with CO₂, saturated produced water and flowlines, which are generally corroded at a slower rate because of lower temperatures and pressures. For more information on specific equipment corrosion issues, refer to the equipment pages.

CO₂ corrosion is enhanced in the presence of both oxygen and organic acids, which can act to dissolve iron carbonate scale and prevent further scaling.

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Prevention / Mitigation

To reduce or prevent corrosion in an CO₂ environment:
Drilling - pH control with caustic soda
Producing wells - corrosion inhibitors
Flowlines - continuous corrosion inhibitor injection

Prediction of corrosion

In sweet gas wells with a pH of 7 or less,

a CO₂ partial pressure of 30 psi usually indicates corrosion.
a CO₂ partial pressure of 7 - 30 psi may indicate corrosion.
a CO₂ partial pressure of 7 psi is usually considered non-corrosive.

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Pictures - Click on thumbnail to see larger picture

Uniform Corrosion

Pitting Corrosion showing wormhole attack pattern, where pits are interconnected.

Galvanic ringworm corrosion, often occurring four to six inches from the upset, where carbon particles have been spheroidized.

Heat-affected zone (HAZ) corrosion is a type of galvanic corrosion which occurs along a weld seam.

Raindrop attack occurs in gas condensate wells. In areas, water condenses on the metal surface, causing deep pits with tails.

CO₂ corrosion in flowing environments

Mesa attack is a form of CO2 corrosion that occurs in flowing environments, and occurs where a protective iron carbonate coating is worn away in areas.

Erosion Corrosion, or flow-enhanced corrosion, usually occurs in areas where the diameter of the pipe or direction of flow is changing. Severe metal loss can quickly occur.

Corrosion due to fatigue occurs in areas of cyclic stresses. Here we see fatigue corrosion in a drill pipe.

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