Analysis of corrosion resistance of 2205 duplex stainless steel coiled tubing
In order to prolong the service life of coiled tubing in acid oil and gas fields containing H2S, CO2 and other corrosive media and prevent accidents, 2205 duplex stainless steel coiled tubing was developed. In order to study its corrosion resistance, intergranular corrosion, hydrogen induced cracking, stress corrosion, pitting corrosion, working condition corrosion and other corrosion tests were carried out. The results show that the developed 2205 duplex stainless steel coiled tubing has good corrosion resistance and is suitable for use in acid oil and gas fields.
Coiled tubing has been widely used in workover, completion and stimulation. With the development of acid oil and gas fields containing a large number of corrosive media such as H2S and CO2, the common carbon steel coiled tubing is prone to uniform corrosion or local corrosion with high corrosion rate caused by CO2 in service, and the coiled tubing will be subjected to tensile stress of self weight and periodic plastic strain in the process of operation, which will cause material performance degradation and plastic reduction under the synergistic effect of H2S medium in the well, It is easy to cause cracking of the pipe body, or the pipe will suddenly break when the stress is far lower than its yield strength. The light causes the whole coiled tubing or the whole well to be scrapped, and the heavy causes the hydrogen sulfide to overflow with the oil and gas, causing great damage to the wellhead equipment and the surrounding ecological environment, seriously restricting the development of acid oil and gas fields [1,2,3,4,5].
At present, many oil and gas fields in China, such as Sichuan, Changqing, Huabei, Xinjiang, Jianghan and other oilfields, contain H2S, CO2 and other corrosive media, so a large number of coiled tubing with strong corrosion resistance are urgently needed to be applied in the above areas. Duplex stainless steel has excellent properties of ferrite and austenite, and has strong corrosion resistance . Therefore, in order to prolong the service life of coiled tubing in acid environment and prevent accidents, 2205 duplex stainless steel coiled tubing was developed. In this study, the corrosion resistance of the product was comprehensively analyzed, in order to provide guidance for the reasonable and safe application of 2205 duplex stainless steel coiled tubing in the oilfield.
Test materials and methods
The test materials are as follows Φ 31.8mm × 3mm 2205 duplex stainless steel coiled pipe, which is made of 2205 duplex stainless steel hot rolled coil after precision forming, welding and heat treatment, its chemical composition is shown in Table 1.
Table.1 chemical composition of 2205 duplex stainless steel coiled tubing
Intergranular corrosion, hydrogen induced cracking, stress corrosion, pitting corrosion and working condition corrosion tests were carried out on the pipes
- (1) Intergranular corrosion test was carried out according to method e of GB/T 4334 standard. The weld and base metal samples of 2205 duplex stainless steel coiled pipe were placed in CuSO4 solution in micro boiling state. After continuous boiling and soaking for 16h, the samples were bent 180 ° in a die with a diameter of 5mm. After bending, the cracks caused by intergranular corrosion on the outer surface of the bending samples were observed under a 10 times magnifying glass.
- (2) According to NACE TM0284 standard, the hydrogen induced cracking (HIC) resistance test of 2205 duplex stainless steel continuous pipe weld and base metal samples was carried out in a solution (0.5% acetic acid saturated with H2S + 5% NaCl aqueous solution). The test time was 96h and the solution temperature was 25 ℃. After the test, the surface of the sample was checked for microcracks and hydrogen bubbles under a 10 times magnifying glass. Cut the sample, observe whether there is crack in the sample section, and calculate the crack rate, which requires the crack sensitivity rate (CSR) ≤ 0.03%; Crack length ratio (CLR) ≤ 5%; Crack thickness ratio (CTR) ≤ 0.5%.
- (3) According to nacetm0177 standard, sulfide stress corrosion test (SSC) was carried out on longitudinal specimens in solution a by four point bending method. The loading stress was 72% of the yield strength 552mpa of 2205 duplex stainless steel coiled tubing, the soaking time was 720h, and the solution temperature was 24 ℃. After the test, check the surface of the sample for cracks or fracture under a 10 times magnifying glass.
- (4) Pitting test was carried out according to astmg48 standard. The weld and base metal samples of 2205 duplex stainless steel coiled pipe were immersed in a solution containing 6% FeCl3 at 22 ℃ for 72 h. The corrosion rate was calculated. The corrosion rate was evaluated according to astma923c. The pitting density, pitting quantity and pitting characteristics were observed under 20 times magnifying glass.
- (5) According to Yb/t5362 standard, the chloride stress corrosion test was carried out. The U-shaped specimens of 2205 duplex stainless steel coiled pipe weld and base metal were put into boiling (143 ℃) 42% MgCl solution for 48 h, and then the cracks or fractures were observed.
- (6) Carry out corrosion test under simulated working conditions, test solution and gas medium are configured according to actual oil and gas field working conditions, and carry out relevant tests according to astmg111 standard. After the test, the average corrosion rate was calculated by weight loss method.
Results and analysis
Intergranular corrosion behavior
In the pipe manufacturing process, due to the unreasonable production process, M23C6, Cr2N and other precipitates are easy to be enriched at the weld and base metal grain boundary, and chromium poor zone is formed along the grain boundary, which reduces the corrosion resistance and toughness of the pipe in acid oil field, and easily leads to brittle cracking of the pipe. In view of the above problems, the intergranular corrosion evaluation was carried out on the pipes, and the 20 mm steel plates were prepared respectively × The 80 mm weld and base metal samples were continuously boiled and soaked in the micro boiling CuSO4 solution for 16 h, and then bent. No obvious cracks were found on the outer surface of the weld and base metal samples. The morphology of the weld and base metal after corrosion is shown in Figure 1. Among them, A1 and A2 are weld samples, A3 and A4 are base metal samples. The test results show that there is no brittle cracking in the weld and base metal of 2205 duplex stainless steel continuous pipe, and the pipe is not sensitive to intergranular corrosion.
Figure.1 specimen morphology after intergranular corrosion test
Hydrogen induced cracking behavior
In the acid oil and gas field containing H2S, a part of hydrogen atoms generated in the corrosion process adsorb on the surface of the pipe and diffuse into the pipe matrix. Hydrogen atoms combine into hydrogen molecules in the pipe matrix, which is easy to cause bubbles or microcracks on the pipe surface [7,8,9]. In order to evaluate the HIC resistance of 2205 duplex stainless steel continuous pipe, hydrogen induced cracking tests were carried out on the weld and base metal. It can be seen from the test that: after 96 h soaking in saturated a solution containing H2S, the weld and base metal samples have no cracks and blisters, and the morphology after HIC test is shown in Figure 2. After calculation, the crack sensitivity rate (CSR) = 0%; Crack length ratio (CLR) = 0%; The crack thickness ratio (CTR) = 0%, which meets the requirements of nacetm0284. It can be concluded that 2205 duplex stainless steel coiled tubing is not easy to produce hydrogen induced cracking due to hydrogen absorption in the corrosive environment of aqueous solution containing H2S.
Figure.2 specimen morphology after HIC test
Sulfide stress corrosion behavior
Under the condition of acid oil and gas field containing wet H2S, the active H atom penetrates into the internal lattice of the metal material, and S is adsorbed on the Fe surface, which increases the brittleness of the pipe and causes the pipe fracture under the synergistic effect of waigara stress and residual stress . Aiming at this phenomenon, the anti sulfide stress corrosion performance evaluation of 2205 duplex stainless steel coiled pipe was carried out. The results show that the tensile stress of 398mpa was applied to the pipe weld and base metal by four point bending method, and the weld and base metal samples did not fracture after soaking in solution a for 720h. The morphology after corrosion is shown in Figure 3. It can be seen that the coiled tubing is not easy to crack, brittle fracture and other phenomena when operating in sour oil and gas fields containing H2S, and has good sulfide stress corrosion resistance.
Fig.3 specimen morphology after SSC in sulfide stress corrosion test
There is a certain amount of CaCl2 in acid oil and gas field, in which Cl – ion has small radius and strong penetration ability, which is easy to penetrate the tiny pores in stainless steel passivation film, reach the metal surface, and interact with the metal to form soluble compounds, leading to the formation of corrosion holes on the surface of pipe [11,12]. The pitting corrosion resistance of 2205 duplex stainless steel continuous pipe was evaluated. The pipe weld and base metal samples were immersed in the solution containing 6% FeCl3 at 22 ℃ for 72 hours. The results showed that no obvious pitting corrosion was found on the pipe surface under 20 times magnifying glass, as shown in Figure 4. The specific measured values are shown in Table 2, in which the corrosion rate is calculated according to formula (1). The test results show that the corrosion rate of all samples is far less than 10mdd, which meets the maximum acceptable corrosion grade specified in astma923c. It can be seen from the evaluation results that 2205 duplex stainless steel coiled tubing has strong pitting corrosion resistance in chloride environment.
In the formula:
R — corrosion rate, MMD;
M – mass of sample before test, Mg;
After M — mass of sample after test, Mg;
S — total surface area of sample, DM2;
T — test time, D.
Figure.4 sample morphology after pitting test
Table.2 pitting test results
Chloride stress corrosion behavior
In chloride environment, the metal surface of stainless steel will produce sliding steps under stress, and the passive film will be destroyed locally, exposing the active “fresh” metal. Cl – interacts with metal to form soluble compound, and finally forms pit and tip crack. With the appearance of soluble substance, anodic polarization occurs, and passive film is formed around the corrosion pit and crack tip; Then, under the action of tensile stress, the stress concentration at the crack tip at the bottom of the etch pit is caused, and the passive film breaks again, forming a new active anode region. The above process is repeated, and finally the deep transgranular crack of stainless steel is formed [13,14].
Because there is a certain amount of Cl – in the working condition of oil and gas field, and the coiled tubing is easy to be subjected to its own tensile stress and other stress loads, it is very important to evaluate the chloride stress of stainless steel coiled tubing. Therefore, the 2205 duplex stainless steel coiled pipe weld and base metal samples are made into U-shaped samples, which are immersed in boiling (143 ℃) 42% MgCl solution for 48h. The test results show that the samples are not cracked, and 2205 duplex stainless steel coiled pipe has good resistance to chloride stress corrosion. The test results are shown in Figure 5.
Figure.5 specimen morphology after chloride stress corrosion test
Corrosion behavior in simulated environment
In well coupon test
In order to verify the corrosion resistance of 2205 duplex stainless steel coiled tubing in acid oil and gas field, the weld, base metal coupon samples of 2205 duplex stainless steel coiled tubing and carbon steel 80s tubing coupon samples commonly used in oil field were put into a typical gas well working environment of an oil field. The test period was 52 days. The working conditions in the well are shown in Table 3 and table 4.
Table.3 typical gas well conditions (gas quality) of an oilfield
Table.4 typical gas well conditions (water quality) of an oilfield
After the test, the corrosion rate of the sample is calculated according to formula (2), and the test results are shown in Figure 6.
In the formula:
G — weight loss, G;
R — material density, g/cm3;
S — surface area of the sample exposed to corrosive medium;
T — Corrosion period, D;
V — average corrosion rate, mm/a
Figure.6 Comparison of corrosion rate results under working conditions
The results show that the corrosion rate of 2205 duplex stainless steel coiled pipe weld is 0.0038 mm/a, and that of base metal is 0.0042 mm/a. the corrosion rate of coiled pipe weld is basically the same as that of base metal, which is significantly better than that of 80s tubing (corrosion rate is 0.0458 mm/a). It can be inferred that 2205 duplex stainless steel coiled tubing is completely suitable for medium and long-term operation in gas wells with coexistence of H2S and CO2.
Laboratory simulation test
Aiming at the working conditions of a gas well in a domestic oil field dominated by CO2 corrosion, the simulated working condition corrosion tests of 2205 duplex stainless steel coiled tubing and carbon steel velocity string were carried out in a high temperature and high pressure kettle. The total pressure was 20MPa, and the test conditions are shown in Table 5. The corrosion rate of 2205 duplex stainless steel continuous pipe weld is 0.00681 mm/A, and that of base metal is 0.00611 mm/A; The corrosion rate of carbon steel and base metal is 1.96009mm/a and 2.01247mm/a respectively. The corrosion rate of 2205 duplex stainless steel coiled tubing is much lower than that of carbon steel, so it is more suitable for long-term service in gas wells dominated by CO2 corrosion.
Table.5 simulated working conditions of gas wells dominated by CO2 corrosion
- (1) The intergranular corrosion test shows that there are no obvious cracks on the outer surface of 2205 duplex stainless steel coiled pipe weld and base metal sample, which proves that the manufacturing process of 2205 duplex stainless steel coiled pipe is reasonable, and there are no obvious M23C6, Cr2N and other intermetallic precipitates at the pipe weld and base metal grain boundary.
- (2) Standard tests such as hydrogen induced cracking test, sulfide stress corrosion test, chloride stress corrosion test and pitting corrosion test were carried out on 2205 duplex stainless steel coiled pipe. No problems such as hydrogen induced cracking and stress corrosion were found. It is proved that 2205 duplex stainless steel coiled pipe has good corrosion resistance under different corrosion conditions.
- (3) According to the actual corrosion conditions of oil and gas wells, the corrosion performance test of 2205 duplex stainless steel coiled pipe was carried out. The results show that the corrosion rate of 2205 duplex stainless steel coiled pipe is low, which is more suitable for operation and application in acid oil and gas fields.
Author: Wang Haitao; Bi Zongyue; Zhao Yong; Liu Yun; Li Hongbin; Zhao Bo; Zhang Ge
Source: China 2205 Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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