Materials Performance

NOV 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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33 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 encountered operational issues, such as corrosion problems on well material s, which have to be solved before large-scale application. When the anthropogenic CO 2 was ap- plied to the EOR system , the corrosion ef fects of impurities on the system still require comprehensive assessment, in- cluding the effects on the capture, trans- port, EOR, and permanent geological stor- age processes. The present work primarily addresses thi s matt er and det ermin ed that the corrosion effects of the impurities in anthropogenic CO 2 on the EOR-CCUS system were complex, unique, and non- negligible. Corrosion Effects of Impurities in Capture Process Alkanolamines are usually employed as absorbents in the CO 2 capture process, and they are also well known for their usage in the oil and gas industries to remove acid gas impurities, such as removing CO 2 and H 2 S from natural gas. During this kind of CO 2 capture process, the impurities, such as O 2 , SO x , and NO x , can promote the degra- dation of alkanolamines in the CO 2 scrub- bing process, leading to the formation of heat-stable salts (HSS) that will remain in the amine solution. The HSS and acid gas impurities them- selves can also exacerbate the corrosion problems within the CO 2 capture units. Although the precipitation of siderites on the carbon steel (CS) surface at the high- t e m p e ra tu re re g i o n s c a n re m a rk a b ly decrease corrosion rates, 7-8 utilization of stainless steel at certain parts of the cap- ture unit is essential in order to control cor- rosion. Recently, it was described that the presence of sulfurous acid (H 2 SO 3 ), owing to the SO 2 impurity, can accelerate the cor- rosion of a valve steel by promoting anodic dissolution and also by introducing a new cathodic reaction, the direct reduction of H 2 SO 3 or bisulfite (HSO 3 – ), into the cathodic process when coexisting with CO 2 . 9 This acceleration effect may also exist in the CO 2 capture process. The stress corrosion cracking of both alloys and CS may occur in the amine solu- tion at higher temperature conditions. The presence of impurities such as oxygen and nitrates owing to the presence of NO x could accelerate this effect. Optimized material selection is needed to control this kind of material failure. Corrosion Effects of Impurities in Transport Process Typically, CO 2 is compressed to the supercritical phase and transported by pipelines to the EOR and storage sites, a process that was thought to be more eco- nomical than other means, such as by truck, rail, and ship. 10 When water and acid gas impurities (SO 2 , NO x , and H 2 S) coexist in the pipeline, the pipeline internal corro- sion problem will occur because of the con- densation or adsorption effect of water on the steel surface and the subsequent disso- lution of corrosive species, even though the water concentration is under the solubility limit in supercritical CO 2 . The uniform cor- rosion rates of the pipeline steels in impure supercritical CO 2 environments are always around several mm/y depending on spe- cific conditions. Meanwhile, pitting corrosion is more harmful to pipeline integrity ; therefore, researchers are more curious regarding pit- ting corrosion in supercritical CO 2 environ- ments, especially its propagation ability. However, Sun, et al. 11 recently reported that the propagation rate of corrosion pits of pipeline steel in the depth direction obvi- ously decreased with time, and the corro- sion type changed from localized corrosion to general corrosion with long-term expo- sure to a static, water-saturated supercriti- c a l C O 2 - S O 2 - N O 2 - H 2 S - O 2 e nv i ro n m e n t . These conclusions might be different when the steel was exposed to the water-rich phase under the same conditions, which might be encountered if there was a large amount of condensed water accumulated at the bottom of the pipeline. Attention must also be paid to the corrosion of com- pressor materials and the degradation of polymer materials owing to the impurities in CO 2 . Therefore, a low concentration of water is required for almost all of the existing CO 2 pipelines to control the corrosion prob- lems. The presence of free water in the CO 2 pipeline can also lead to the formation of hydrates at low-temperature conditions, which could block the pipeline and result in operational accidents. When making a CO 2 quality recommendation for the CO 2 pipeline transport, the impact of impurities on the health , safety, and environment must also be considered , including the impacts of CO, hydrogen cyanide (HCN), Hg, H 2 S, SO x , NO x , and amines. FIGURE 1 Schematic diagram of the EOR-CCUS system.

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