Results of a laboratory study of internal corrosion in wet gas pipelines under conditions involving a stagnant or slowly flowing aqueous phase indicate that the corrosion rate is not affected by calcium and magnesium scale-forming tendency of the test solutions, provided that they are buffered. It is possible that the scales formed are sufficiently porous to permit electrolyte contact with steel. However, an increase in pH and decrease in oxygen resulted in significant corrosion rate reduction. Thermodynamic analyses indicate that the formation of metastable iron sulfide (FeS) precipitates is promoted by higher pH and higher dissolved iron concentration, and hindered by the presence of carbon dioxide (CO 2). The presence of oxygen on corrosion may lead to the transformation of a metastable FeS phase, mackinawite (Fe 1+x S), to another metastable phase, greigite (Fe 3 S 4). Electrochemical polarization data indicate that the steel behaves in an active manner symptomatic of a nonprotective corrosion product film. The effect of oxygen in increasing the corrosion rate may be related to the increase in corrosion potential. Surface analysis by Raman spectroscopy confirmed some of the thermodynamic predictions of stable phases. Additionally, Raman spectroscopy indicated the predominance of akaganeite (β-) and repidocrocite (γ-FeOOH). Based on the results of this study, a regression equation developed from nonscale-forming solutions may be adopted for predicting the internal corrosion of gas transmission and gathering lines caused by condensed water and acid gases.

Rotating cylinder experiments demonstrated that corrosion rates of steel exposed to oxygenated aqueous solutions containing hydrogen sulfide (H 2 S) and ammonia (NH 3 ) were dependent upon pH, the presence of oxygen (O 2 ), H 2 S concentration, and flow. Corrosion was more severe than expected based upon a review of published reports considering systems at relatively low ammonium hydrosulfide (NH 4 HS) concentrations and above pH 6. Dependence of carbon steel (CS) corrosion rates on pH and the impact of O 2 was strong, but only prior to reaction between H 2 S and O 2 . The influence of H 2 S concentration was unexpected in that neutral, 1 mM solutions were more corrosive than neutral, 10 mM solutions in short-term tests and on a sustained basis.

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