THE INFLUENCE OF STEEL MICROSTRUCTURE ON CO_2 CORROSION

摘要

Carbon dioxide corrosion problems in oil and gas production are very common. Carbon dioxide dissolved in the presence of a water phase is corrosive to steel pipes. Corrosion rate depends on the formation of a surface corrosion - product scale. Microstructures of carbon and low alloy steels can influence corrosion resistance. Large performance differences are found in field applications for similar steels with small differences in microstructure. This has been seen in pipelines produced according to the same standards but delivered by different manufacturers, which have shown quite different corrosion rates in field. Inhibitors efficiency is also affected by surface condition and microstructure. At present there is no general explanation for the influence of microstructure on CO_2 corrosion and on inhibitors performance. The aim of this work is to study basic aspects related to the relationship between microstructure, surface films and inhibitors efficiency in order to contribute to the understanding of this issue. Electrochemical and surface analysis techniques were used in order to characterize the corroded surface of the carbon steel with two different microstructures (annealed, and quenched and tempered). The corrosive media was a deoxygenated 5% NaCl solution, CO_2 saturated at pH 6 at 40 °C. Benzimidazol and an imidazoline based commercially available product were used as inhibitors. From the results exposed in this work, it is concluded that when two different heat treatments are applied on steel, the corrosion processes and the efficiency of the inhibitor are greatly influenced.