Abstract
The protective properties of iron sulfide films formed on carbon steel equipment in gas and oil production fields depend upon several environmental factors, such as concentrations of H2S and CO2, salinity, and temperature. A recent article (Paper No. 02223, Corrosion/2002) has reviewed the applicability of various empirical models that are used for estimating corrosion rates of carbon steels in CO2/H2S environments, and has found that each model has limitation in that it is useful only in very specific ranges of the variables from which the model was developed. Therefore, more definitive corrosion rate data, generated from laboratory investigations, are often required to cover the environmental conditions existing in production fields of interest.
The present laboratory investigation, which was performed with AISI 1018 carbon steel in 0.01% NaCl and 10% NaCl solutions with 1% CO2 and 0.03% to 3% H2S, at 27° to 79° C (81° to 175° F), covered virtually three decades of CO2/H2S ratio - 0.0, 0.3, 3.3 and 33. Nitrogen was the balance gas in each gas mixture that was continuously refreshed in the test cell at atmospheric pressure. The corrosion rate of the carbon steel specimens was determined gravimetrically and also by the electrochemical impedance spectroscopy (EIS) technique. The sulfide films formed on the specimens were characterized with the low angle X-ray diffraction technique. A strong correlation was found between the corrosion rate and CO2/H2S ratio for each of the two test solutions and the temperatures investigated. However, no systematic effect of temperature was found on the corrosion rate in either of the solutions. In general, the EIS technique yielded corrosion rates that were much higher than that obtained gravimetrically in the same environment. Mackinawite was found to be the predominant species in the iron sulfide films. Only traces of troilite, greigite and pyrite were detected in some of the specimens.
