Corrosion off Iron in an H2S-CO2-H2O System

Abstract

Mechanism of formation of iron sulfide film during corrosion of Armco iron by liquid phase H2S-CO2-H2O and H2S-CO2-H2O-iron sulfide systems has been studied. Weight loss, weight of film remaining on the metal, number of blisters formed and chemical composition, as determined by electron diffraction, were measured as a function of time at various H2S concentrations. During liquid phase corrosion of iron by H2S-CO2-H2O, over-all reaction is controlled partially by interface reaction and partially by passage (diffusion) of ions and electrons across film. At low H2S concentrations, reaction mechanism approaches complete diffusion control and at high H2S concentrations mechanism approaches complete interface control. At all H2S concentrations for both systems steady state was reached in 5-15 hours. It is believed that sulfide film was formed initially by iron ions migrating to nucleated arrays of sulfide ions adsorbed on surface of metal, thereby reacting to form discrete crystallites. At 0.00958 psia of H2S film consisted of troilite, pyrite or marcasite and very little kansite. At H2S concentrations of 0.0544 to 3.25 psia, film consisted of troilite, pyrite or marcasite and predominant amount of kansite. All film components formed initially, but rates at which each grew and number of crystallites of each formed probably were different. In H2S concentration range from 3.25 to about 0.09 psia, amount of hydrogen formed during corrosion needed to produce blisters remained constant as H2S concentration decreased. Below 0.09 psia of H2S, more hydrogen was needed to produce blisters than at H2S concentrations above 0.09 psia and amount of hydrogen needed to produce blisters increased as H2S concentration was decreased.