Studies Of Inhibition Performance And Adsorption Behavior Of Inhibitors On Carbon Steel In Near Neutral CO₂-saturated Brine Solution

The change of the metal surface state with the change of pH value not only has influence on the corrosion behavior of the metal, but also can effect significantly on the adsorption behavior, inhibition efficiency and the inhibition mechanism of the inhibitors. Much work about the performance evaluation and mechanism investigation of the inhibitors is carried out in the CO2-containing acidic environment, less work in the near neutral environment such as the produced water containing CO2 in oil gas industries. The performance of the inhibitors is much environment-dependent, the inhibitors which behave well in acidic environment might have no role in neutral environment and vice verse. The investigation about the adsorption behavior, inhibition performance of the inhibitors in near neutral environments will facilitate not only to the better understanding of the inhibition mechanism of the inhibitors but also to the effective selection and successful design of new inhibitors in the near neutral environments.Dodecylamine (organic alkali) and lauric acid (organic acid) which have the simple molecule structures were considered as the model inhibitors in this work. Electrochemcal methods and surface analysis methods were used to investigate the corrosion behavior and the corrosion mechanism of N80 carbon steel in near neutral CO2-saturated brine solution. The inhibition performance, adsorption behavior and the inhibition mechanism of the inhibitors in near neutral environments were also investigated. Furthermore, the interaction between the inhibitors and the corrosion products formed in near neutral environments were studied.The results have shown the corrosion behavior of carbon steel was dependent on pH value of the solution. The cathode reaction of the corrosion was controlled by the electrochemical reaction in the range of pH< 6.8, and the determining process was transfered to a mass-diffusion process when pH> 6.8. With the further increase of pH value, the cathode reaction was controlled by both the electrochemical and mass-diffusion processes. For the anode reaction, the metal was in the active dissolution state in the range of pH< 5.9. The metal state has the characteristics of the transition state at pH 6.5;When pH> 6.8, the typical passivation phenomena was observed, and the complete and continuous corrosion product film formed on the metal surface. However, the higher pH value enhanced the dissolution action of the corrosion products.The inhibition ability of dodecylamine was strongly dependent on pH value of the solution and improved with the increase of pH value. The minumum effective inhibitor concentration reduced distinctly with the increase of pH value. At lower pH value, dodecylamine inhibited mainly the cathode process; while it inhibited both the anode and cathode processes in near neutral environments. At lower pH value, the adsorption state of the inhibitor was mainly the unprotonated dodecylamine. It was unfavorable to adsord on the metal surface with the positive net charge. The adsorption state of dodecylamine on the metal surface might be attributable to the adsorption of the unprotonated dodecylamine after the reduction of the dodecylamine onium on the cathode site. The ratio of the unprotonated to the protonated of dodecylamine increased with the increase of pH value, which was favorable to the adsorption of the inhibitor. The higher electron cloud density of the unprotonated provided much stronger coordination with the 'd ' orbital of the metal atom and hence the adsorbed inhibitor was more stable and tight on the metal surface.Lauric acid reduced the corrosion rate of N80 carbon steel both in acidic and near neutral environments. The inhibition ability and the minimum effective inhibitor concentration were both pH-dependent. The former decreased and the latter increased with pH value. Lauric acid inhibited mainly the anode reaction in the test pH values. The adsorption state of the inhibitor was not totally consistent with that in the solution. At lower pH value, laurate adsorbed on the metal surface by the electrostatic interaction and changed the charge state of the metal surface from the positive to the electrically neutral. The carboxyl group bonded chemically with the metal in the active-dissolution state. At higher pH values, although the net charge state was favorable to the adsorption of laurate, the adsorption of the inhibitor had no obvious influence on the charge state of the metal surface. Besides, the protective corrosion product film was formed at higher pH value which was favorable to the adsorption of the inhibibitor and hence did not prevent the corrosion effectively.The interaction between the inhibitors and the corrosion products in near neutral environments has relation with the type of the inhibitors and pH value of the solution. At pH 6.9, dodecylamine has no obvious influence on the morphology of FeCO3 crytal, while lauric acid has obvious dissolution-erosion action to FeCO3 crystal. Both inhibitors have some inhibitive role to the formation of FeCO3 crystal and the addition of the inhibitors reduced the adhesion strength of the film on the metal surface. Dodecylamine has certain positive role to the protection ability of the film while lauric acid on the contrary. At pH 7.4, both of dodecylamine and lauric acid contributed to the grain refinement and orderly arangement of FeCO3 crystals. The adhesion strengths of the corrosion product film were enhanced in both cases. Dodecylamine and lauric acid not only inhibited the uniform corrosion, but also reduced the sensitivity of the metal to the localized corrosion and the former was more obvious than the latter.