Carbon steel is thermodynamically unstable in water with dissolved CO2 and the only reason that carbon steel is so attractive and can be so widely used in oil and gas production is that the steel surface becomes covered by a protective layer of corrosion products, oil, mineral scale or inhibitors. It is relatively easy to predict and explain the high corrosion rates on bare steel. The real challenge is to reduce the corrosion and that requires knowledge about the performance of the protective layers, means to predict the breakdown of the layers and methods and techniques to ensure that robust layers form on the surface.
The paper discusses how CO2 affects the water chemistry, the electrochemical reactions on the bare steel surface, and the initiation and growth of protective corrosion product films. As many sweet systems contain organic acids that affect the solution chemistry and the formation and stability of the FeCO3 corrosion product films, organic acids need also to be considered when the effect of CO2 is discussed.
The mechanism of carbon steel corrosion in a CO2 containing environment has been studied and debated for decades. Hundreds of papers related to CO2 corrosion have been published and a large variety of corrosion rates and mechanisms have been reported. Oil companies and research institutions have analyzed the data and developed a number of prediction models1 to take account of the various parameters that determine the corrosion rate. The models give up to two decades difference in the predicted CO2 corrosion rate and it all depends on how the various parameters are treated and how much conservatism that is built into the model.
In order to explain the confusion and the apparently contradictory observations and results that have been seen and reported, it is important to realize that the term CO2 corrosion and the effect of CO2 is not related to one mechanism only. A large number of CO2 dependent chemical, electrochemical and mass transport processes occur simultaneously on and close to the corroding steel surface. The various reactions respond differently to changes in CO2 partial pressure, temperature, water chemistry, flow and other operational parameters. All the reactions should be taken into account when corrosion in a CO2 containing environment is to be quantified and explained.
Many researchers have studied and discussed the electrochemical reactions taking place on the bare steel surface. The mechanisms that control the rate of the electrochemical reactions are of great academic interest, but are less important when it comes to the practical application of carbon steel. When carbon steel is directly exposed to water and CO2 the bare steel corrosion rate will under almost all circumstances become prohibitively high for practical use in oil and gas production. This is illustrated in Figure 1 where the corrosion rate has been predicted for various CO2 partial pressures and pH values as a function of temperature. The corrosion rate predicted up to 40 °C apply for bare steel, while partly protective films are formed at higher temperature. It is seen that the corrosion rates are in the order of several mm/year, even at CO2 partial pressures below 0.5 bar, i.e. pressures where the old rule of thumb says that carbon steel can be applied without any treatment.
In the present paper it is focused on fundamental corrosion mechanisms in sweet systems. Three major effects of CO2 will be addressed: The effect on the water chemistry, the effect on the electrochemical reactions, and the impact on the initiation and growth of corrosion product films. As many sweet systems contain organic acids that affect the solution chemistry an
