N80 Steel Containing Of Co <sub> 2 </ Sub> / H <sub> 2 </ Sub> S High-temperature High-pressure Two-phase Medium In The Electrochemical Corrosion Behavior And Corrosion Inhibition Mechanism Study

An Ag/AgCl (saturated KC1) electrode, which was used as an internal reference electrode in the electrochemical research under the conditions of high temperature and high pressure, was developed by means of electrolyzing. In the simulating gas field downhole CO2 and H2S/CO2 corrosion environment under high temperature and high pressure conditions, the mechanisms of corrosion and inhibition on the N80 steel were studied and the effect on the corrosion behavior of corrosion scale was investigated by using AC impedance spectroscopy, potentiodynamic polarization technique, SEM and XRD et. The relation of the mechanical properties of the corrosion scale and corrosion rate of N80 steel was analyzed also. The results show that:In the CO2 corrosion environment, the anodic reactions are controlled by chemical activation step. OH" adsorbs on the surface of N80 steel, and then become into FeOHads and FeOH+ads, Fe2+ is found at last. The reduction of H2CO3 and HCO3-is the most important cathodic reaction. The EIS (Electrochemical Impedance Spectroscopy) curves change with the scale covering the electrode surface. The capacitive arc in high frequency section extends but that contract in low frequency section. The general corrosion occurs.In the H2S/CO2 corrosion environment, the amount of adsorptions increases because of the high adsorption capacity of HS-. The reduction of H2S is the main cathodic reaction. The anodic and cathodic reactions are accelerated to different degree. The mass transport process through sulfide film is checked. It was found that the corrosion behavior of the steel is controlled by mass diffusion. The splits of sulfide film induce the pitting.The partial pressure of CO2 mainly affects the corrosion behavior of the bare steel. The electrochemical activity of N80 steel increases with the partial pressure of CO2 increasing, so the corrosion rate is accelerated. The partial pressure of CO2 has little effect on the N80 steel as covered by integrated corrosion scale. CO2 corrosion is predominant when the environment contains little H2S, but the corrosion rate is accelerated. H2S corrosion becomes the main one with the partial pressure of H2S increasing. Pitting occurs. However, when the partial pressure of H2S increases to 2psi, the tendency of pitting reduces.The derivation of thiazo changes the structure of inner corrosion scale and restrains pitting. The inhibition mechanism is "geometric blocking effect". The inhibition efficiency is very low due to the low adsorption capacity of the inhibitor. Itcannot adsorb on the corrosion scale, so the inhibition efficiency obviously reduces after the corrosion scale covering the surface of N80 steel. Imidazoline, as an anodic inhibitor, decreases the corrosion rate by negative catalytic effect. It can not only adsorb on the surface of N80 steel, but also cooperate with corrosion scale, so it has good inhibitive effect.It can be found from SEM analysis that: CO2 corrosion scale is compact and uniformity, so N80 steel exhibits general corrosion; H2S corrosion scale is made up of big crystals and has pitting caused by scale split. These results coincide with the electrochemical model put forward in the paper.The investigation results on the relations of the mechanical properties of the corrosion scale and corrosion rate show that: The corrosion rate increases with the bounding strengthen of the scale to N80 steel and hardness of scale decreasing.