Study Of Welding Property And Corrosion Behaviours For Cr Containing Low Alloy Pipeline Steels

To meet the growing challenges from the CO2 corrosion in pipelines, a new kind of Cr-containing low alloy pipeline steel is developed, and the research on the corrosion of this steel is becoming a popular topic. Recently, the low Cr-containing steel, which shows an excellent performance-price ratio with a better CO2 corrosion resistance, is expected to replace the carbon steel used for pipelines. However, the research of the welding property for low Cr-containing steel is very insufficient, and the corrosion mechanism is still unclear, which have restricted its development and application in the oil and gas industry. Therefore, the mechanical performance and the galvanic corrosion of a welded joint, the corrosion behaviours in CO2 environment with high temperature and high pressure, and the corrosion mechanism of low Cr-containing steel were investigated in this work.The welded joints of the novel 3Cr pipeline steel were fabricated via the gas tungsten arc welding technique using an inhouse 3Cr welding wire and two kinds of commercial wires (H08Cr3MoMnA and TGS-2CML). Mechanical properties of the joints were measured, and microstructure characteristics were observed. The results showed that the inhouse 3Cr welding wire is the best match for the 3Cr steel. Compared with the two selected commercial wires, the inhouse welding wire can provide the most optimal microstructure. The microstructure of weld zone for the inhouse welding wire is the finest and most uniform, and there exist no sharp boundary between the weld zone and heat affected zone. Under selected welding procedure, the joints of the inhouse 3Cr welding wire can provide good mechanical properties, which can easily meet the DNV-OS-F101 standard. Moreover, an electrochemical corrosion system with small cathode (HAZ, WZ) and large anode (BM) is established. This can lead to full protection of WZ and HAZ and also an almost negligible acceleration of the corrosion on BM, which is of particular advantage for the life and safety of the pipeline systems in the petroleum and natural gas industries.The electrochemical characteristics and the development of the corrosion film on the 3Cr steel surface were investigated in situ through immersion tests and EIS technique. A model of the formation of the corrosion film is proposed, which suggests that the formation of the corrosion film on the 3Cr steel surface is basically divided into three stages:dynamic adsorption of FeOH/CrOH, incomplete-coverage film, and integral film. The increasing of film resistance, charge transfer resistance and dielectric constant of corrosion film will inhibit the corrosion of substrate and then reduce the corrosion rate of the substrate.The cathodic reaction mechanism and the prepassivation were investigated using a homemade electrochemical autoclave, and the mechanism of spontaneous pressivation for low Cr-containing steel was proposed. Prepassivation is also a direct criterion for corrosion rate. The addition of chromium to the steel substrate can cause spontaneous prepassivation by simultaneously affecting the cathodic and anodic processes. For the anodic process, the addition of chromium can increase the complexity of the anode reaction, and the chromic hydroxides continually form on the substrate surface. The addition of chromium alters the anodic process by providing 3Cr steel with prepassivation characteristics. For the cathodic process, acidification, caused by the addition of Cr, occurs first. Subsequently, the cathodic reaction begins to change from the direct reduction of HCO3- to that of H+. This is critical because it facilitates the increase in mixed potential. Thus, the OCP enters into the potential range of prepassivation and spontaneous prepassivation occurs.In the absence of interface inhibitors, i.e. benzamide, an interaction effect between the passive layer formation and the inhibitor adsorption was observed on the 3Cr steel surface. When the inhibitor concentration is below 200 mM, the presence of the inhibitor has no significant impact on the passive layer formation of 3Cr steel. However, above a threshold of roughly 600 mM, inhibitors can quickly adsorb on the steel surface and significantly inhibit the formation of Cr(OH)3 passive layer. During 200 mM and 600 mM, the passive layer formation and the inhibitor adsorption occurred simultaneously. This study is very important to the type and concentration selections of corrosion inhibitors for low Cr steel in petroleum industries.