Corrosion Behavior And Mechanism Of X80 Steel In Cl^- And HCO₃^- Solution Under The Influence Of Alternating Current（AC）

In recent years, ferrous tube of AC corrosion problem is increasingly serious with the development of energy industry. Although the drainage protection and cathodic protection measurements was conducted, a high transmittance damage of pipeline still happen. AC corrosion process is a complex process, the corrosion behavior and mechanism is still in an exploring stage. In this paper, the AC corrosion behavior of X80 steel under AC application was systematically investigated in different pH value and different environment media solution using immersion test, polarization curve, electrochemical impedance spectroscopy, interference potential test, and so on. The main conclusions as follow:(1) In the different pH value of NaCl solution, alternating current(AC) influenced both the corrosion rates and the corrosion behavior of X80 steel. In the absence of AC, the corrosion rates varies as the increase of solution pH, corrosion rate in the acid solution was greater than in the neutral and alkaline solution. When the 50 A/m2 full wave AC was applied, the corrosion rates of X80 steel increased almost 30 times in the neutral and alkaline solution, whereas in the acidic solution corrosion rates increased 20 times compared to that without AC. In addition, without the application of AC, pitting corrosion can be observed on the surface of X80 steel in the acid solution, with the formation of numerous pittings. Meanwhile, in the neutral and alkaline solution, non-uniform general corrosion with limited pittings occurred. After the application of full wave AC, both the non-uniform general corrosion and the pitting effects got strengthened in all the three solution, whereas the oval and flat pittings were observed on the surface of sample in acid solution and in the neutral and alkaline solution the pittings appeared like grooves.(2) In the acid NaCl solution, the corrosion rates after resectively applying the positive and negative cycle of AC remarkably increased compared with that without AC, which rose with the growth of the positive half-wave AC current density and dropped with the increase of negative half-wave AC current density. In the acid NaCl solution, pitting corrosion was observed on the X80 steel surface in the absence of AC. After the positive half-wave AC of 10A/m2～50A/m2 was given, the appearance of sample surface transformed into general corrosion. As the current density of positive half-wave AC increased to 80A/m2～100A/m2, non-uniform general corrosion with limited pittings presented on the sample surface. In contrast, after applying the negative periodical AC with the same current density as the positive cycle, non-uniform corrosion with abundant pittings were shown on the X80 steel surface, and the number of pittings increased with the rose of current density. The application of negative half-wave AC is like the effects of cathodic protection(CP). When the current density is pretty low, the sample is in a state of lacking protection with obvious anodic dissolution and general corrosion is in a dominant position. As the negative half-wave AC current density increased, the effects of CP are strengthened, resulting in the decrease of anodic dissolution. However, at this occasion, the corrosion effects of Cl- behaved more apparently and the pittings were reinforced.(3) In the Acid NaCl solution, the result of the nonlinear ac impedance technology shows that:Under the influence of AC with a constant frequency, the impedance of corrosion decreased as voltage amplitude of sinusoidal AC increased, which means the corrosion tendency increased; After the application of AC with a constant voltage amplitude, the corrosion resistance decreased in low frequency AC while increasing in relatively high frequency AC; After applying AC with a constant voltage amplitude and frequency, when the polarization potential ranges from -1.1 V to -0.7 V, and the double layer compacity increased with the positive shift of polarization potential; in contrast, when the polarization potential varies between-0.7 V and-0.4 V, double layer compacity decreased with the positive shift of polarization potential.(4) In the presence of full wave AC, the influence of solution pH on the corrosion behavior is more heavily than that of AC. Without the AC current, non-uniform general corrosion were observed in the acid and alkaline NaHCO3 solution while general corrosion occurred in neutral solution. After the application of 50A/m2 full wave AC, non-uniform general corrosion still dominated the X80 steel surface with a long strip like pitting in acid solution, while non-uniform general corrosion with limited pittings were observed in neutral solution and pittings are dominant in alkaline environment. But overall the pittings are less obvious than that in NaCl system.(5) In the acid NaHCO3 solution, the influence of positive and negative half-wave AC on the corrosion behavior is similar with that in acid NaCl solution. The corrosion rates increased with the growth of the positive periodical AC current density and dropped with the increase of negative half-wave AC current density. Non-uniform general corrosion took place on the sample surface with or without AC. As the increase of positive half-wave AC density, the roughness of sample surface increased. In the presence of negative periodical AC, non-uniform general corrosion with pittings were observed and the pittings were strengthened as the increase of current density.