The Study Of Corrosion Mechanism Of G105 Steel In Various NaCl Solution And Selection Of Corrosion Inhibitor

Fluids of well corrupt drilling tools badly because it belongs to gas, liquid, solid multiphase blending flow, especially O₂ or/and CO₂ exists. In order to reduce drilling tools corrosion, the corrosion mechanism of G105 steel with present of O₂ or/and CO₂ must be studied and inhibitors will be applied.The main research content and innovation of paper are listed as follows:Aimed at the corrosion of G105 drill pipes in aerated drilling fluids, the impact of dissolved oxygen(DO) of 3.5wt%NaCI solution and brine on the corrosion behavior of G105 steel is studied using weight loss method. The tests reveal that the corrosion of G105 steel in brine is considered as oxygen corrosion; the temperature and DO affect the corrosion rate of G105 steel in aerated 3.5wt%NaCl solution; at the same temperature, the corrosion rate increases with the content of DO; at the same aerated condition, the corrosion rate increases when the temperature is elevated from 298K to 338K and reaches the maximum at 338K.The corrosion rate of G105 steel and DO of 3.5wt%NaCI solution takes on linear relationship.When O₂ and CO₂ are present at the same time,the corrosion rate of G105 steel is less than the sum of the corrosion rate when O₂ or CO₂ exists alone.The reason is that the deoxidization of O₂ used a part of H+ or the corrosion production of 3.5wt%NaCl+10%O₂+40%CO₂ is so much to restrain the corrosion rate.The electrochemical impedance spectroscopy (EIS) was used to analyze the electrode of O₂ or/and CO₂ corrosion of G105 steel in various electrolytes, and corresponding thermodynamics equation was used to analyze the possible anodic and cathodic reactions. The results demonstrate that O₂,H+, H2CO3 and HCO3- all could be reduced. But the reduction rate is different in various electrolytes. In 3.5wt%NaCl solution saturated with 10%O₂, the activation controls anodic reaction and the reduction of O₂ is main and diffuse rate controls cathodic reaction rate. In 3.5wt%NaCl solution saturated with 40%CO₂, the main anodic reaction are Fe→Fe2++2e and Fe+HCO3-→FeCO3+H++2e and the most important cathodic reaction are 2H2CO3+2e→2HCO3-+H2 controlled by the activation and 2HCO3-+2e→2CO32-+ H2 controlled by the diffuse rate. In 3.5wt%NaCl solution saturated with 10%O₂ and 40%CO₂, the main anodic reaction are Fe→Fe2++2e and Fe+HCO3-→FeCO3+H++2e. 1/2O₂+2H++2e→H2O or 2HCO3-+2e→2CO32-+ H2 is the main cathodic reaction and the diffuse rate of O₂ or HCO3-controls cathodic reaction rate. The reduction rate of H2O is low and could not influence cathodic exchange current density.Making use of corrosion inhibitors in reason is an efficient method to prevent the metal and its alloy from eroding in environmental medium. Imidazoline inhibitors are selected by weight loss method, and the inhibitory efficiency is above 80% in 3.5wt%NaCl solution saturated with 10%O₂ and 40%CO₂ at 338K. The inhibitory efficiency becomes lower along with the temperature go up.Using the SEM and stereo-microscope to observe the corrosion appearance of metals, the result is that the imidazoline inhibitor protected G105 samples well.Inhibitory mechanism is studied by electrochemistry methods such as polarization curve, EIS and cyclic polarization curve(CV).(1) Results of polarization curve show that the inhibitor is a mix-type inhibitor, which mainly inhibits anodic process. (2) The reaction of forming film has three stages: firstly, the imidazoline inhibitor activately adsorpts on mild steel surface; secondly, the imidazole bond and the rest bond arrive at the equilibrium adsorption while the film of corrosion production becomes thick and the absorption resistance (Rt) becomes great gradually; finally , the frail bond brushs off and the absorption induction disappears. (3) The inhibitory capability comes from covalent adsorption of the N atom and metal. (4) Inhibitor desorption was studied by electrochemistry methods such as polarization curve and CV. When anodic polarization potential exceeded–0.52V electronic current increased rapidly and inhibitor would not control anodic process, so that the anodic desorption potential (Edes) are -0.526V and -0.518V.HSJ-C did not desorpt obvious. (5) After the imidazoline inhibitor was added, the G105 samples were protected well. Because the inhibitory film can lay over activity points on mild steel and inhibitors have competitive adsorption on mild steel surface result in the concentration of Cl- on steel surface fell, so that the inhibitor could prevent corrosion.