The Formation Mechanism Of Corrosion Scale And Hydrogen Permeation Behavior Of Low Chromium Steel Under H₂S/CO₂Environment

As the high-content of hydrogen sulfide oil-gas field had been widely developed. H2Scorrosion and the hydrogen embrittlement induced by H2S had been one of the obstacle forexploitation of oil and gas field. Corrosion scale had an important impact on the corrosionrate, corrosion types and the behavior of hydrogen permeation of low-chromium underH2S/CO2environment. At present, the theory of ion-selectivity had get more identity, butthe evidences was not sufficient to support it. It was significant to study the formationmechanism and the effect on the behavior of hydrogen permeation of the corrosion scalesystematically.The ion-selectivity effect on H2S/CO2corrosion of scales were studied by measuringthe curve of membrane potential. High temperature and high pressure H2S/CO2corrosionsimulation tests were employed to investigate the influence of temperature on corrosion rate,corrosion type and the formation of scales of80SS, P110-3Cr and P110-7Cr. The influenceof different ion-sulfide scales on hydrogen permeation was researched by Devanathan-Sthachurski method.The results of high temperature and high pressure H2S/CO2corrosion simulation testsat different temperature indicated that: corrosion rate of80SS and P110-3Cr steel decreasedat the beginning and then increased with temperature increasing; corrosion rate of P110-7Crsteel increased with temperature increasing. The corrosion types of three material weregeneral corrosion. There was no siderite（FeCO3） was detected, The corrosion wascontrolled by H2S; Cr was rich in the corrosion scale.The curve of membrane potential of FeS and Cr(OH)3showed that: FeS film wasbipolar ized with cation-selectivity contacting base metal side and with anion-selectivitycontacting solutions respectively. Ca2+and Mg2+would change the ion-selctivity of FeS film from bipolar structure into anion–selective, anion–selective film had no inhibitive oncorrosion. The ion-selectivity of FeS film would diappear when absorbed HCO3-and SO42-.Cr(OH)3film was unpolar structure and applied to pH value. At pH=11, Cr(OH)3film wascation selective, which was protective to base metal. The ion-selectivity would disappearwhen pH value was6. The Cr(OH)3film became anion selective at pH=4.5.The results of hydrogen permeation tests showed that for80SS steel, FeS0.9film couldrestain the behavior of hydrogen permeation evidently, while FeS film had slightlypromotion. Both two kinds film of P110-3Cr inhibited hydrogen permeation, and theinhibitory effect of FeS was stronger than FeS0.9. As for P110-7Cr steel, the two kinds ofscale also both inhibit the hydrogen permeation and inhibitory effect of them was similar.The corrosion scales increased the superficial H adsorption density, decreased the diffusioncoefficient. Furthermore, the superficial H adsorption density of FeS was higher than FeS0.9,the former had the stronger pore adsorption. Compared P110-3Cr and P110-7Cr steel, theinhibitory effect of scales not increased with chromium content increasing, instead ofdecreasing. The mechanism of effect of scale on hydrogen permeation consisted,(1)scaleeffected the superficial H adsorption density,(2)the effect of films on hindering thediffusion of hydrogen to base metal.