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CO2 Corrosion
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,
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a CO2 partial pressure of 30 psi usually indicates corrosion.
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a CO2 partial pressure of 7 - 30 psi may indicate corrosion.
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a CO2
partial pressure of 7 psi is usually considered non-corrosive.
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Pictures
- Click on thumbnail to see larger picture
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Uniform Corrosion |
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Pitting Corrosion showing wormhole attack pattern, where pits are interconnected. |
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Galvanic ringworm corrosion, often occurring four to six inches from the upset,
where carbon particles have been spheroidized. |
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Heat-affected zone (HAZ) corrosion is a type of galvanic corrosion which occurs
along a weld seam. |
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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
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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. |
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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. |
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Corrosion due to fatigue occurs in areas of cyclic stresses. Here we see fatigue
corrosion in a drill pipe. |
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