CO2 corrosion occurs in equipment conveying and processing fluids produced from oil and gas production. Carbon dioxide is a naturally occurring component. The composition of CO2 varied greatly from field to field
CO2 is an acidic gas. It dissolves into the water phase increasing acidity and corrosivity. The corrosion product, iron carbonate, is protective but prone to removal leading to both pitting and general form of metal loss.
Knows
Corrosion of mild carbon steel in CO2
Temperature < 60oC, pH < 4.0, CO2 corrosion is controlled by Fe2+(anodic reaction control)
Temp>70oC, CO2 corrosion becomes cathodically controlled, with lower corrosion rate due to the forming of iron carbonate scale.
Iron carbonate scale’s role in CO2 corrosion
pH in CO2 corrosion
temperature factor, while fluid chemistry is in favour of scale forming, higher temperature decreases the corrosion rate. While fluid chemistry prevents forming of scale, higher temperature increases the corrosion rate.
Top of line corrosion due to water condensation
Unclear:
Localised CO2 corrosion
The role of organic acid, typical for localised pitting
H2S for mild carbon steel corrosion
Other steel grades in CO2 corrosion
Inhibition efficiency in slug flow
Role of glycol/methanol on CO2 corrosion
Rule of Thumbs
A maximum corrosion rate is usually found at temperature around 70oC , with condensation presence.
Oil and gas upstream
A CO2 partial pressure greater than 30 psi generally indicates corrosion
A CO2 partial pressure between 7 to 30 psi may indicate corrosion
A CO2 partial pressure less than 7 psi is generally considered as noncorrosive.
It is important to take water chemistry into consideration: for example, the produced fluid may be more corrosive in less alkalinity water with lower CO2 partial pressure.
Simulation and Modelling
The corrosivity of CO2 is highly depending on:
- partial pressure in gas stream
- temperature
- presence of water
- water chemistry.
The De Waard-Milliams diagram gives you the first approximation on how CO2 partial pressure and temperature impacting on your corrosion rate.
Furthermore, water chemistry plays an important role in CO2 corrosion since aggressive carbonic acid can only produced with water. however, a complication arises from the fact that the carbon always consists in multiple compounds such as carbonates and bicarbonates, while also the presence of gaseous CO2. Nevertheless, the presence of the hardness elements such as calcium, strontium and magnesium make the case more complicate by forming carbonate precipitates ( IAEA.org ). All these reactions will affect the pH of produced water, which eventually determine the corrosivity of the fluid.
corrosion of steel in fluids containing CO2 produces a protective corrosion product iron carbonate film that initially results in the decreased corrosion rate. however, at higher velocity, the film spalls off and therefore increased of corrosion rate.
The CO2 corrosion control is a primary task for upstream oil and gas corrosion engineers.