Study On Corrosion Resistance Of X80 Pipeline Steel And Its Welded Joint

X80 pipeline steel has been used widely in the world because of its high strength, high toughness and good weldability. The mainline of the second stage of West-to-East Natural Gas Transmission Project is the longest X80 pipeline steel line in the world. Welding is the main technology to connect the pipeline steel and welded joint is the weak zone of the pipe. Thus, the working life and safety of pipeline depend highly on the success of welding. In addition, corrosion has been the most important reason for the failure of pipeline. The serious economic loss, even the serious catastrophic consequences, can be caused by the burst and leakage of pipeline. Thus, it is necessary to study the corrosion resistance of pipeline steel and its welded joint.In the study, the corrosion behavior and corrosion mechanisms of X80 pipeline steel in high-pH solution and H2S-containing environments were studied. The microstructure and corrosion resistance of different zones of submerged arc welded joint were also studied in different corrosion environments to find the weak zone of corrosion resistance. In addition, the microstructure and corrosion resistance of coarse-grained heat-affected zone (CGHAZ) in different environments were studied by welding thermal simulation to obtain the better heat input. Simultaneously, effects of the peak temperature in the secondary welding thermal cycles on the microstructure evolution and corrosion resistance in CGHAZ were studied by welding thermal simulation.The corrosion behavior of X80 pipeline steel in 0.5M Na2CO3-1M NaHCO3 solution was studied. Results showed that the apparent passive happened on the steel surface from 25℃ to 95℃. The stability of passive film decreased, and the corrosion driving force and corrosion rate increased with the temperature increasing. Pitting can be introduced by adding a certain amount of Cl- in this solution and increasing temperature can inhibit the pitting because of the increasing of OH- concentration. The corrosion product film was formed on the steel surface in the immersion process and its main chemical composition was FeCO3. The higher of the selective anodic polarization potential in the stable passive region was good for the stability of passive film. The potential of stress corrosion of X80 pipeline steel in this solution was higher. Cathodic protection can suppress the anodic dissolution effectively; however, its protection effect was lowered because of the introduction of hydrogen. The anodic protection effect was better because of its promotion effect for the formation or repair of passive film and the inhibiting effect of the crack growth. In addition, the higher the anodic potential was, the better the protection effect was.The corrosion behavior of X80 pipeline steel in distilled water and NACE A solution with saturated H2S at 50℃ was studied. Results showed that the corrosion product film was formed on the steel surface in two solutions. A series of changes of the shape, size, chemical composition and crystal structure of the corrosion products can be seen as the immersion process developed. Mackinawite and cubic FeS were the initial and secondary corrosion products, respectively. In addition, the obvious delamination phenomenon of corrosion product film happened in NACE A solution and the model was established. The corrosion rate of X80 pipeline steel in NACE A solution was higher than that of in distilled water. However, it is difficult for the corrosion products to adhere to the steel surface in the initial immersion process in NACE A solution. The adhesion speed in NACE A solution was accelerated after that the thickness of corrosion products obtained one critical value and it is higher than that of in distilled water. The corrosion tendency and corrosion rate decreased because of the protection of corrosion product film. The galvanic current between sulfide-film-covered electrode and fresh steel electrode in the two corrosion environments was studied. Results showed that the galvanic effect between two electrodes was strong and the fresh steel electrode dissolved preferentially. The galvanic current was large at the initial stage and it increased with the cathode/anode electrode area ratio and the immersion time of sulfide-film-covered electrode. However, the galvanic current decreased to close to zero with the galvanic experiment developing. It can be attributed to the change of fresh steel electrode’s surface state.It means that a new layer of corrosion product was formed on the surface of fresh steel electrode. It is indicated that the sulfide film has higher potential to have a better self repair function and the serious localized corrosion can’t be introduced due to local loss of sulfide in H2S-containing environment.The microstructure and corrosion resistance of submerged arc welded joint of X80 pipeline steel were studied. Results showed that the main microstructure of welded metal is acicular ferrite. The prior austenite grain boundaries can be seen clearly in CGHAZ and the main microstructure is granular bainite. Microstructure of fine-grained heat-affected-zone (FGHAZ) is dominated by polygonal ferrite and some bulk M/A components can be observed around the polygonal ferrite grain boundaries. The position of the highest and lowest micro-hardness of weld joint located in the welded metal (WM) and heat-affected zone (HAZ) close to the base metal (BM), respectively. In 0.5M Na2CO3-1M NaHCO3 solution at 25℃, the passive film can be formed on the surface of BM, WM, CGHAZ and FGHAZ. The passive film stability of BM was similar to that of WM. The corrosion resistance and the passive film stability of CGHAZ were the worst, while those of FGHAZ were the best. A layer of corrosion products was formed on the surfaces of BM, WM, CGHAZ and FGHAZ after immersion for 96 h in 5 wt.% NaCl solution with saturated H2S at 55℃. The chemical composition and crystal structure of corrosion products formed on different samples surface are similar, however, their micro-morphologies are different. The corrosion products formed on BM and FGHAZ surfaces are regular and compact resulting in the better protective effect. However, there are some holes on the corrosion products formed on CGHAZ surface resulting in the worse protective effect. It was found the CGHAZ has the worst corrosion resistance in welded joint, making it be the weak zone of corrosion resistance in welded joint.The CGHAZ welding thermal simulation samples of X80 pipeline steel were prepared by using Gleeble 3500 tester with heat input of 10～55 kJ/cm and peak temperature of 1300℃. Results showed that the microstructure of CGHAZ is dominated by granular bainite. The lath size of granular bainite increased with the increasing of heat input. A certain amount of ferrite can be seen gradually with the heat input increasing. In addition, the M/A component concentration increased firstly and then decreased with the increasing of heat input, and the shape of M/A component gradually changes from acicular to granular. In 0.5M Na2CO3-1M NaHCO3 solution at 25℃, although passive happened on both BM and CGHAZ, the passive film stability and corrosion resistance of CGHAZ were weaker than those of BM. The stability of passive film and corrosion resistance of CGHAZ increased firstly and then decreased, and showing the best corrosion resistance when heat input is 30 kJ/cm. In 5 wt.% NaCl solution with saturated H2S at 55℃, the corrosion resistance of CGHAZ was weaker than that of BM. The corrosion resistance of CGHAZ increased as the heat input increased, which can be attributed to the decrease of holes of the corrosion products. In NS4 solution with continuous 5% CO2/N2 at 25℃, the corrosion resistance of CGHAZ was weaker than that of BM. It can be found some holes around the M/A components regions of BM and CGHAZ samples after immersion for 2 days. This can be attributed to the preferentially dissolution of M/A components or their surrounding area. The quantity and area ratio of pitting holes decreased with the heat input increasing, resulting in the improvement of corrosion resistance. When the heat input of 55 kJ/cm is employed, only a little number of small pitting holes can be observed around some coarse M/A components. Therefore, when considering the corrosion resistance of CGHAZ, the establishment of welding procedure should be based on the consideration of the specific corrosion environment.The effects of secondary welding thermal cycle on the microstructure and corrosion resistance of CGHAZ were studied by welding thermal simulation. The heat inputs in the initial and secondary welding thermal cycles were 30 kJ/cm and 10 kJ/cm respectively. The peak temperature in the initial welding thermal cycle is 1300 ℃. The peak temperatures in the secondary welding thermal cycle are 650,780,950, 1150 and 1300℃, respectively, corresponding to the subcritically CGHAZ (SCGHAZ), intercritically reheated CGHAZ (ICCGHAZ), supercritically CGHAZ (SCCGHAZ), and unaltered CGHAZ (UCGHAZ). Results showed that a series of microstructure gradient was formed in reheated CGHAZ. The M/A constituents in the prior-austenite grain boundaries of the ICCGHAZ were seriously coarsened, resulting in a necklace-shaped microstructure. This phenomenon can be attributed to the partly austenizing and diffusion of carbon atoms during the heat process in the secondary welding thermal cycle. That made the ICCGHAZ become the weakest zone in 0.5M Na2CO3-1M NaHCO3 solution at 25℃ and 5 wt.% NaCl solution with saturated H2S at 55℃. In addition, the corrosion resistance in UCGHAZ is weaker than that of CGHAZ because of the smaller size of the lath of granular bainite and the acicular M/A components. The microstructure in SCGHAZ is similar to CGHAZ due to the tempering-like treatment; however, its corrosion resistance is better than that of CGHAZ. The microstructure in SCCGHAZ was fine compared with CGHAZ and its corrosion resistance was enhanced.