Unexpected Effect of Small Oxygen Concentrations in Sales Gas on Element Currents between Pipeline Steel and Magnetite from Black PowderWhereas detrimental effects to the natural gas infrastructure caused by low levels of oxygen are widely recognized for wet areas like underground storages trace oxygen has gained awareness by transmission system operators only recently. Indiscriminate changes to the world of gas supply as the injection of gases from regenerative sources or backfeeding of low pressure gases into the high pressure network etc. may contribute to a higher oxygen content in pipeline gas than previously.Concerns about the integrity of the pipeline infrastructure have led Open Grid Europe to launch a project with the objective to systematically evaluate the impact of trace oxygen in high pressure sales gas to pipeline steel. Astonishingly in the presence of black powder an increase of its corrosivity has been detected. This paper reports on experimental data that allow for the first time a correlation between small oxygen concentrations in dry sales gas and the corrosion of pipeline steel in the presence of magnetite the major component of black powder.Due to its high surface area black powder was found to be highly adsorptive towards all kinds of polar gas field chemicals such as dehydration and desulfurisation liquids. These chemicals are transported as trace constituents with the natural gas stream over long distances and may be adsorbed and form thin liquid films alongside black powder deposits. It was found that glycol films can cause significant galvanic element currents between steel as the local anode and magnetite as the local cathode provided the liquid film phase exhibits a minimal electrical conductivity. This minimal conductivity is generally given due to the presence of small concentrations of ageing and decomposition products such as low molecular acids (formic acid acetic acid glycolic acid) water (at and significantly below the 1% level) or even quarternary ammonium salts which are eventually introduced from corrosion inhibitors. At a given conductivity the elements currents between steel and magnetite are significantly increased in the presence of oxygen at pressures below 20 mbar.Electrical conductivity and more significantly temperature have an effect on overall elements currents. The latter also increase with the surface ratio of the magnetite cathode and the steel anode. At high surface ratios – which is generally true for black powder deposits – corrosion rates in the order of 0.1 to 0.8 mm/y can be estimated. The implications for a maximum allowable oxygen content in sales gas specifications are discussed.