Corrosion related failures in the oil and gas industry represent a very serious and costly problem. The successful application of carbon steels in oil and gas pipelines and production tubulars in CO2 containing environments depends mainly on either the formation of protective corrosion product film or the use of corrosion inhibitors. Both laboratory experiments and field experience have shown that the protective properties and adherence of the film may vary significantly for carbon steels with apparently the same composition and microstructure. In the present work, characteristics such as morphology, thickness and composition of corrosion layers formed on carbon steel with two different microstructures (annealed, and quenched and tempered) have been studied by means of scanning electron microscopy (SEM), electron dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS). The corrosive media was a deoxygenated 5% NaCl solution, saturated with CO2 at 40 degreesC and pH 6. The effect of the addition of benzimidazol as a corrosion inhibitor has also been analyzed. It is concluded that the microstructure of steel influences the inhibitor efficiency and the properties of the corrosion layers, such as morphology and proportion of the various chemical compounds present. (C) 2002 Elsevier Science B.V. All rights reserved.