Weak Acid Corrosion And The Corrosion Of A3 Steel In Nacl Solution Behavior Research

Carbon and low alloy steels are the most commonly used metal materials and have been widely used in transportation, industry systems, oil and gas pipelines, mechanical appliances, etc. However, steels are susceptible to corrosion in environment containing oil production and corrosion of carbon steels causes significant economic loss. The majority of oil and gas pipelines failures result from corrosion of carbon and low alloy steels. In order to improve the corrosion resistance of steel material and make steel material being used effectively in various applications, domestic and foreign researchers have used a variety of methods to effectively improve its corrosion resistance. So it is very important to studying the corrosion inhibitor in acidic medium.In this paper, A3steels as anode materials were used as the study objects. The Weight-loss measurement, electrochemcal polarization curves, and electrochemical impedance spectroscopy were used to investigate the corrosion behavior and the corrosion mechanism of A3steel in near neutral2.0%NaCl solution. The corrosion resistance and electrochemical behavior of carbon steel in synthetic seawater with/without imidazoline inhibitor under various conditions was investigated using potentiodynamic polarization, electrochemical impedance spectrum, SEM observations and the inhibitive ability of the inhibitor was evaluated by weight-loss method. Furthermore, the inhibition performance, adsorption behavior and the inhibition mechanism of the imidazoline inhibitors were also investigated. The main research results are as follows:In the pH range of4.0-7.0, the corrosion ability of A3steel was strongly dependent on pH value of the solution. The corrosion resistance Rct values significantly and Rct increases with pH, while the Cdl values and the corrosion rates decrease remarkably. The corrosion resistance under neutral conditions is better than that under acidic conditions.The imidazoline derivative shows good inhibition performance for the corrosion of steel in synthetic seawater, the corrosion resistance of steel at higher pH is higher than that at lower pH. The imidazoline derivative has been proved to be a credible inhibitor for the corrosion of steel in synthetic2.0%NaCl solution at different pH. The imidazoline derivative inhibited both anode and cathode corrosion reactions. It cts as a mixed-type corrosion inhibitor, which mainly suppress the anodic processes of the corrosion reactions by forming a protective adsorption film on steel surface.Good inhibition performances at2.0%NaCl solution with different imidazoline inhibitor concentration and during a long time immersion are observed. The corrosion inhibition of the studied imidazoline derivative for steel increases with increasing its concentration. The imidazoline exhibits good inhibition performance for the corrosion of steel at its high concentration. The inhibition efficiency is up to90%at its concentration of250mg/l. The reason of anti-corrosion of inhibitor relied on strong absorption on metal surface which prevented the contact between metal surface and water. After the imidazoline inhibitor was added, the steel samples were protected well.The inhibitive effect of inhibitor is due to the formation of a chemisorbed film on the metallic surface occurred by the formation of coordination bonds between the heteroatom in the inhibitor molecule and Fe on the metallic surface, as it was demonstrated by the XPS and AFM analysis of the tested surfaces. EIS showed that the reason of anti-corrosion of inhibitor relied on strong absorption on metal surface which prevented the contact between metal surface and water. It was discovered that imidazoline derivative follows Langmuir isotherm, and the adsorption equilibrium constant, adsorption energy and Gibbs adsorption free energy were calculated.The growth and decay law of film formed on A3steel are researched by the weight-loss, EIS and weak polarization curve tests. The results show that:during the early stage of immersion, the inhibition performance first increases with time within96h, due to the adsorption of imidazoline derivative. The film of imidazoline inhibitor can form quickly, has long life for192h, and obviously decay in120h, but can be auto-repaired. In96～120h, the corrosion current density of A3steel dropped, the charge transfer resistance is greatly higher, the film is more extend and tighten than at the early stage. The film of imidazoline inhibitor is best in96h. On that condition, an inductance arc appears at low the EIS fig of A3steel during the the longer stage of immersion, which shows the frequency desorption inhibitor on electrode surface. Then the inhibition efficiency reaches the maximum value and tends to decrease slightly. However, at the end of immersion, the compound still retain extremely high inhibition efficiency. The inhibition efficiency at192h is still as high as81.0%.