Study On CO₂ Corrosion Resistance Of Low Alloy Steel After Surface Nanocrystallization Nitriding

During the empolder and transmission of petroleum and natural gas, The CO₂ corrosion has been a difficult problem to solve, it can erode subterranean pipeline and overground production equipments, as a result of rupturing the pipeline, equipment invaluable, and pipe laying explode.The low alloy steel such as 40Cr, 35CrMo are processed by new technology of surface nanocrystallization and conventional gas nitriding technology. Corrosion weight-loss method, SEM, TEM, XRD and electrochemical techniques were used to study the effect of different treatments on the CO₂ corrosion behavior of the two steels under simulated environment. We hope this research could give some help to solve the CO₂ corrosion problem through studing the effect on the nitriding and corrosion resistance. The result showed that:1. Surface nanocrystallization can improve the microstructure and properties. A nanostructure layer is formed on the surface of 40Cr, 35CrMo steels after ultrasonic surface processing. The grains are greatly refined to nanometer level, the surface roughness is significantly reduced and the surface hardness is also increased greatly.2. Surface nanocrystallization has the option to effectively improve the permeability nitrogen treatment such as increasing the thickness and hardness of nitrogen layer, shortening the nitrogen time.3. The CO₂ corrosion resistance of low alloy steel after nitriding is dramatically improved by surface nanocrystallization because of the lower corrosion rate. One compact corrosion product scale is formed on the 40Cr and 35CrMo steal after nitriding. The scale has small grain size, ordered arrangement and low porosity content. The dense corrosion product scale can effectively prevent the surface of base metal from medium to improve the corrosion resistance.4. Surface nanocrystallization can change the components of corrosion product scale. The surface corrosion product scale of nano-sample is composed of Cr₂O₃ and FeCO₃ compared to FeCO₃ before surface treatment. This compact and completed scale has certain anion selectivity that can effectively prevent steal from corrosive medium to reduce the corrosion rate.