Current models often over-predict CO2 corrosion rates for wet gas and oil transport systems. One of the main reasons for this is the fact that the formation of corrosion product scales is not properly taken into account. An evaluation of in-house and literature experiments has been carried out to develop an equation for iron carbonate precipitation kinetics. Modelling the kinetics of this precipitation process offers a number of applications in the area of corrosion prediction and control. The ultimate goal of the work is to establish the conditions under which stable, protective
corrosion product layers form. A good indicator for the formation was found to be the so-called scaling tendency, which is defined as the ratio between the precipitation and corrosion flux. To form reliable scales, the bulk scaling tendency should be high enough for protective film formation and the local scaling tendency, in case of film damage, should be high enough for film repair. Both processes have been modelled and it was found that above approximately 80 °C reliable scales are often easily formed whereas below this temperature, pH values of at least 6 are required. The precipitation model also results in improved prediction of the solution pH which allows better corrosion rate descriptions even if the scales are not protective. In some cases the changes in pH may lead to reductions of 40% in the predicted corrosion rates. Other applications of iron carbonate precipitation modelling are also discussed.
Keywords: carbon dioxide corrosion, corrosion prediction, iron carbonate precipitation, scale repair, supersaturation, scaling tendency.