Effects of Reaction Kinetics of H2S, CO2 and O2 on the Formation of Black Powder in Sales Gas Pipelines

Carbon dioxide, hydrogen sulphide and oxygen gases dissolve in water condensate in steel pipelines react with the iron to form iron carbonate, iron sulphides and iron oxides as the corrosion products, respectively. It is unknown whether these gases react individually or there is a competition between them leading to a kinetically favorable reaction. The reaction rate is governed by the concentration of gases dissolved in water that is controlled by the gas composition, diffusivity, solubility, and the intrinsic kinetics. The purpose of this study is to look at the reaction rate and products when iron powder is immersed in water exposed to oxygen, methane doped with CO₂ and H₂S, and the mixture of doped methane with oxygen.

Our findings suggest that turbulence in pipelines may induce mixing resulting faster reaction when compared to laboratory experiments in the absence of mixing. Hence, the reaction rate and product is actually controlled by the concentration of CO₂, H₂S, and O₂ in the liquid water phase. In the absence of mixing (stagnant fluid), the mass transfer resistance has to be taken into account which requires higher driving forces (higher pressure or temperature) to corrode the pipeline.