The Research On The Corrosion Failure Mechanism Of Pressure Vessel Materials Under Multi-media Environment

With the development of oil and gas field, the content of H₂S, CO₂ and Cl- in produced water is higher and higher than before. The high content of H₂S, CO₂ and Cl-, high hypersalinity, high temperature, large range of pH value fluctuation is being the inevitable trend in tarim oilfield. There are usually comprehensive corrosion, stress corrosion, pitting corrosion, intergranular corrosion, crevice corrosion and scale corrosion failure forms for equipments under this complicated corrosive medium condition. The chloride ion corrosion should be considered as well as the corrosion of H₂S and CO₂, which make the corrosive behavior of materials and it weld joint complicated. The research on corrosive behavior mostly focused on the single medium at present. The corrosion problem in complicated medium has become the hidden danger of accident of pressure vessel, which became one of the bottlenecks of this kind of oil and gas field development. The failure mechanism of the pressure vessel materials and its welding joints commonly used at home and abroad mostly concentrated in single corrosive medium, such as CO₂, H₂S or Cl-. There is not the unification of the appropriate selection criteria in the aspect of pressure vessel materials. The materials used commonly in pressure vessel, such as low alloy steel, stainless steel and non-ferrous alloy, was chosen as the studying object. The ingredient of corrosive medium was based on the oil and gas field produced water with high content of Cl-, CO₂ and H₂S. The corrosive behavior of 16MnR,2205 and B30 was investigated in complicated medium contained Cl-,CO₂ and H₂S, and the simulation test in laboratory and the coupon test were also done at trarim oil and natural gas pipeline.The comprehensive corrosion and local corrosion of 16MnR steel were firstly studied in this project. The local corrosion results show that the anti sulfide-stress cracking and resistance of sec sensitivity of 16MnR （HIC） is much better than other materials in this series materials. The difference of cracking time of 16MnR （HIC） between the base metal and welding joint is small comparing with 16Mn series of other materials during the Constant load tensile test when the the load was biger than 0.9ReL. The critical stress met the requirement of Sc^ 10 through beam bending test. The stress intensity factor values of 16MnR （HIC） is higher than that of 16Mn series of other materials. The stress corrosion sensitivity index increases with the increasing temperature during the slow strain corrosion process, and its changing becomes gently when the temperature is higher than 40℃. The stress corrosion sensitivity index of 16MnR （HIC） is close to safety zone（close to 25%）. The stress corrosion sensitivity index decreases with increasing pH value in acid medium, and it increases with increasing pH value in alkaline medium. The fracture accompanying second crack has quasi cleavage fracture characteristics. There was corrosion pits and dimple at later cracking area. The comprehensive corrosion rate of 16MnR is smaller than 0.25 mm/a under room temperature condition, and the corrosion surface is uniform and without signs of local corrosion. The corrosion rate at pH=6,8 lower 2-3 times than when pH=4. The corrosion rate increase with the increasing temperature, and the increasing rate decreased when the temperature is higher than 50℃. The corrosion products are FeS1-x and FeCO3 that formed porous corrosion film at the inirial corrosion stage. The iron sulfides deficient S would transform to iron sulfides deficient Fe with the extension of time or rise of temperature. The iron sulfides deficient Fe is monoclinic or hexagonal crystal structure, which make the density of corrosion film increased. Therefore, the corrosion rate decreased owing to the increasing density of corrosion film.The corrosion behavior of 2205 duplex stainless steel was studied in complex medium. The hydrogen embrittlement happened in ferrite area during the local corrosion process. The constant load tensile show that all samples of 2205 failed when the loading was 0.95σs. The fracture has obvious cleavage fracture morphology characteristics. The cleavage fracture mainly happened at the ferrite area. There were some small dimples at austenite area to some extent. The corrosion potential and pitting potential presented dereasing tendency with the increasing temperature, Cl- concentration and partial pressure of CO₂ under the same pH value conditions. The corrosion potential and pitting potential dcreased to different extent with the increasing pH value under some corrosion medium. The strain rate is 10-6/sec for 2205 in slow strain test in complicated medium. The stress of sample in complicated medium was lower than that in air under same strain condition after elastic-plastic deformation. The cracking strain in complicated medium was about 15% lower than that in air. The main fracture has fan-shaped river pattern with second crack, which indicated that embrittlement happened during tensile test in complicated medium. There were crack appeared near loading area after four point bending test for 2205 duplex stainless steel in medium. The micro structure after bending shows that selective corrosion happened in ferrite area. The DL-EPR results indicated that the intergranular corrosion resistance of 2205 was excellent. The reactivation rate of sensitizing treatment sample was small （2.38×10-3）,which demonstrated the excellent intergranular corrosion resistance of 2205. The corrosion rate of 2205 was largest when the Cl- concentration was 0 and room temperature at pH=4. The largest corrosion rate happened at 50℃ when the Cl- concentration was 170g/L. The turning point of corrosion rate happened when the Cl- content was 57-100ppm under above two conditions. The corrosion rate was not more than 0.0005mm/a when the temperature was less 50 ℃ with the Cl- content being 57-100ppm. The corrosion rate increased when the temperature was higher than 50℃, and the corrosion rate was not more than 0.004mm/a. The corrosion rate is higher at 50 ℃ when the Cl- concentration was 0 and at pH=6, and he corrosion rate is smaller than 0.0025mm/a. The corrosion rate increased quickly when the temperature rosed to 80 ℃ and the Cl- concentration rose from 100g/L to 170g/L, and the corrosion rate is not more than 0.004mm/a. The passivation film formed when the 2205 sample was dipped into complicated medium. The unstable pitting nuclei with 3-5 μm in diameter appeared on the surface at pH=4, and the diameter of unstable pitting nuclei is 15-20μm at pH=6. The diameter of unstable pitting nuclei presented increasing tendency with the increasing temperature and Cl- concentration. The corrosion process model was set up on the basis of corrosion appearance chacteristics and the composition of corrosion products.The corrosion behavior of B30 in complex corrosion medium was studied. The polarization curve show that the corrosion potential shifted to positive with the increasing partial pressure of CO₂ and Cl- concentration under the condition of constant partial pressure of H₂S. The corrosion current decreased with the increasing partial pressure of CO₂ and Cl-content. The pitting experiments show that corrosion rate increased with the increasing temperature. There were many corrosion pits with different size on the surface. The corrosion pit center was not completely corroded. Elements analysis results show that the mian elements were Cu, O and S in the corrosion pits. Slow strain test show that the maximum value of stress corrosion sensitivity index is not beyond 25% under all corrosion conditions. The increasing rate of stress corrosion sensitivity index when the temperature rose from room temperature to 50 ℃ is larger than that when the temperature rose from 50 □ to 80 □. The stress corrosion sensitivity index in neutral corrosion medium is lower slightly than that in acid or alcoline corrosion medium at T=50□. The corrosion rates of B30 inreased with the increasing temperatuer and H₂S concentration under different pH value conditions during general corrosion process. The increasing rates of corrosion rate when the H₂S concentration are larger than 1000mg/L are higher than that when the H₂S concentration are lower than 1000mg/L under same pH value condition. The increasing rate of corrosion rate at T>50℃ is obviously larger than that at T<50℃. The corrosion process could be inhibited with the increasing CO₂ partial pressure and Cl- content under the same H₂S content owing to the absorption of several kinds of ions on the surface, which resulted in the inhibition of the corrosion process to some extent. The cuprous sulfide, cuprous chloride and small amount of cuprous oxide formed owing to the sulfur ions existing, which constituted porous corrosion film. Part of CuCl and Cu2S would transform into CuS and CuO along with the corrosion process. The film consisted of CuS and CuO was still porous, which aggravated the corrosion process. The looseness of corrosion film constituted with cuprous oxide is low under the condition of without sulfur ion existing in corrosion medium, which resulted in the inhibition of ions transition process. Therefore, the corrosion rate decreased accordingly.The laboratory simulation test and track sample included 09CrAlMoRE,16MnR in natural gas pipeline,2205 lining container, NK-HITEN610U2 （CF62） steel plate and their welding joint. The results show that the stress corrosion cracking resistance of 20R> 16MnDR、16MnR（HIC）、NK-HITEN610U2、254SMO、NAS354N、2205、316L、0Cr13 was good in complicated corrosion medium. The materials selection guidelines for pressure container in high content of C1-,H₂S,CO₂ medium environment was compiled on the basis of the above research results and the operating medium for pressure vessel equipments in Tarim oilfield.