| X80 steel has lots of good properties, such as, high strength, high toughness and excellent resistance to stress corrosion performance, which is widely used in transporting gas, oil. Corrosion problem has been received extensive attention, but however, the study on the cathodic process and corrosion in electrochemical thin film is seldom reported. The impact of p H and dissolved oxygen(DO) concentration on corrosion behaviors of X80 steel in simulated red solution was studied by means of potentiodynamic polarization, electrochemical impedance spectroscopy(EIS), electrochemical noise(EN), scanning Kelvin probe and three-dimensional video microscope. The type of cathodic reaction has been achieved and mechanisms of corrosion have been investigated. The differences of corrosion rules between solution and electrochemical thin film have been analyzed. The main research results are as following:The theoretical calculation showed cathodic reaction meets the conditions for the hydrogen reduction reaction and oxygen reduction reaction, driving force of oxygen reduction reaction is greater than the hydrogen reduction. Hydrogen reduction process is verified using hydrogen collection method. Oxygen concentration of simulated red soil solution, immersed in the for 24 h, decreased from 4.29 ppm to 2.56 ppm, which confirmed that oxygen has participated in the cathode reaction. Therefore, cathodic process reacted oxygen and hydrogen reduction process with different proportion. EIS under different potential was employed to study corrosion of X80 steel in simulated red soil solution at p H 4.0, the results showed that reaction was controlled by oxygen diffusion between-0.7 V potential to-0.9 V potential; when potential fell to 1.1V ~ 1.5V, it was mainly controlled by hydrogen reduction.As p H decreases, corrosion current density increases. Cathode process of X80 steel in simulated red soil solution at p H 5.5 is mainly controlled by the oxygen depolarization. X80 was formed a corrosion product film, while EIS is composed by incomplete capacitive arc at high-frequency and complete capacitive arc at low-frequency. As p H decreases the corrosion of X80 steel in simulated red soil solution is controlled by electrochemical activation. The surface of X80 steel generated porous corrosion products with numerous pitting, the low-frequency region of EIS appears inductive arc.Dissolved oxygen(DO) has a great influence on corrosion behavior of X80 steel. With the increase of dissolved oxygen concentration, the cathodic reaction is accelerated. Dissolved oxygen concentration contributes on the composition, density and coverage of corrosion products. Corrosion products were intact, porous,which formed as Fe3O4 and γ- Fe OOH under dissolved oxygen concentration 20.2ppm solution; the corrosion products were cellular, incomplete and composed of Fe3O4 and γ- Fe OOH when dissolved oxygen concentration is 3.6ppm.X80 steel coated flake-like corrosion products on the surface, which are given priority to with Fe2O3, γ- Fe OOH and Fe CO3 when dissolved oxygen concentration is 0.85 ppm.In the bulk solution, the corrosion of X80 steel is more serious, its cathodic process is controlled by hydrogen evolution and oxygen reduction, meanwhile corrosion products generated is porous and easy to be dissolved; with the thin liquid film thickness decreases, the tendency of corrosion fell at the beginning and then increases, the conclusions were consistent with that of EIS. Under deoxidized state, as the thin film thickness decreases, the cathodic process translated from the hybrid control into diffusion control. Corrosion rate is gradually reduced. |
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