Active dissolution and natural passivation of carbon steel in carbon dioxide-saturated alkanolamine solutions.

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Corrosion rates of carbon steel specimens were measured in carbon dioxide (CO2) -saturated alkanolamine (amine) solutions by weight-loss tests and by the polarization resistance method. In dilute amine solutions, lower than 10-2M, corrosion rates were controlled by the formation of protective FeCO3 scale as in hot, CO2-H2O environments. Tested amines inhibited CO2 induced corrosion in the concentration range between 10-2 and 2M. In concentrated amine solutions, higher than 3M, primary and tertiary amines showed markedly different effects on carbon steel corrosion. The corrosion rates of carbon steel rose with the increase in primary amine concentration, whereas the corrosion rates of carbon steel stayed low in tertiary amine solutions even at 6M, due to the formation of protective FeCO3 scale.Tertiary amines absorb CO2 as HCO3- in the presence of water. On the contrary, primary amines absorb CO2 by two ways. One is as HCO3- like tertiary amines, and the other is direct absorption of CO2 without water, producing amine-carbamate.The chemical substances produced due to the absorption of CO2 by amines are considered to play important roles not only on corrosion rates but also on the morphology and protectiveness of corrosion products.
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