Comparison of corrosion properties of 316L and 2205 stainless steel
316L austenitic stainless steel and 2205 duplex stainless steel are widely used in the oil and gas industry because of their good corrosion resistance. However, due to the different composition and structure of the two materials, there are some differences in the corrosion behavior under the same corrosion environment. According to the corrosion characteristics of two kinds of stainless steel, the corrosion behavior of two kinds of materials under the same working condition was studied.
Working condition and chemical composition
See Table 1 for the working conditions selected in the comparative corrosion test and table 2 for the chemical composition of the two stainless steels.
Table 1 Working conditions selected for comparative corrosion test
Table 2 Chemical composition of two kinds of stainless steel
(1) According to GB / T 17897-2016 test method for corrosion of stainless steel by ferric chloride (6% FeCl3 + 0.05 mol / L HCl), the comparative test was carried out for 5 h, 14.5 h and 24 h respectively. A dial depth indicator or focus microscope is used to measure the depth of pitting or crack corrosion.
(2) According to JB / T 7901-2001 “metal materials laboratory uniform corrosion full immersion test method”, carry out comparative test under the environment of deaeration system (see Table 1 for specific conditions). 600 × 800 × 1200 × sandpaper is used to grind the sample step by step to eliminate machining knife marks. After cleaning, degreasing and air drying, the size is measured and weighed. Then the sample is insulated and installed on a special test stand, and put into the medium in the autoclave. After the test, the surface of the sample is removed with distilled water (if necessary, membrane removal solution) to remove corrosion products and anhydrous alcohol After dehydration and drying, weigh with bs124s electronic balance, and calculate the corrosion rate according to formula (1).
- R — corrosion rate, mm / A;
- M — sample mass before test, G;
- M1 — mass of sample after test, G;
- S — total area of sample, cm2;
- T — test time, h;
- D — material density, kg / m3.
(3) According to GB / T 15970.7 corrosion stress corrosion test of metals and alloys Part 7: slow strain rate test, the slow strain rate test shall be carried out under the environment of deaeration system (see Table 1 for specific conditions).
Test methods and results
2205 and 316L stainless steel were used to process corresponding samples respectively, meeting the total surface area of more than 10 cm2. The test was conducted according to method B (6% FeCl3 + 0.05 mol / L HCl) of GB / T 17897-2016 corrosion test method for stainless steel of metals and alloys. The test time was 5 h, 14.5 h and 24 h respectively, and the temperature and actual working conditions were correspondingly increased to 60 ℃. The test results are shown in table 3-5, and the pitting morphology is shown in Figure 1-3.
Table 3 Pitting test results of 316L and 2205 stainless steel immersed for 5 h
Table 4 Pitting test results of 316L and 2205 stainless steel soaked for 14.5h
Table 5 Pitting test results of 316L and 2205 stainless steel soaked for 24 h
Figure 1 Pitting morphology of 316L and 2205 stainless steel after 5 h immersion
Fig. 2 Pitting morphology of 316L and 2205 stainless steel after 14.5h immersion
Figure 3 Pitting morphology of 316L and 2205 stainless steel after 24 h immersion
The average corrosion rate is used to express the pitting degree of different soaking time, as shown in Figure 4. In 5-24 hours, with the extension of time, the pitting degree first increases, then decreases. In terms of pitting degree, the pitting resistance of 316L is worse than that of 2205 stainless steel, and the average pitting rate is about 5 times of 2205 stainless steel, while it is about 6 times after 24 hours, which shows that 316L is not rusty Although the average corrosion rate of steel decreased in the follow-up, the degree of decrease was less than that of 2205 stainless steel, which was mainly due to the addition of anti pitting elements Mo and N in 2205 stainless steel.
Fig. 4 Relationship between pitting rate and time of two kinds of stainless steels
Soaking uniformity test
The specimens of the same size as the pitting test are used to conduct the uniform immersion test in the deoxidizing system environment (see Table 1 for specific conditions) in accordance with the standard of JB / T 7901-2001 metallic materials laboratory uniform corrosion full immersion test method, and the test results are shown in Table 6. The macro morphology of 316L and 2205 stainless steel after 168 h immersion is shown in Fig. 5, and the micro morphology is shown in Fig. 6. It can be seen from table 6 and figure 5 that pitting occurs in both 2205 and 316L. Although pitting is relatively small, the pitting degree of 316L is slightly heavier than that of 2205 stainless steel.
Table 6 Test results of 316L and 2205 stainless steel immersed for 168 H
Stress corrosion test
Fig. 5 Macromorphology of 316L and 2205 stainless steel after 168 h immersion
Two kinds of stainless steel are cut and machined to make 115 mm × 15 mm × 3 mm sample. According to GB / T 15970.7 corrosion stress corrosion test of metals and alloys Part 7: slow strain rate test, the slow strain rate test is carried out under the environment of deaeration system (see Table 1 for specific conditions). The slow stretch rate in air is 3.5 × 10-4 mm / s, and the slow stretch rate in solution is 1×10-4 mm/s。 The test steps are as follows: firstly, deaeration with N2 for 1.5 h, heating up to 60 ℃, adding 0.01 MPa of H2S gas, 1.0 MPa of CO2 gas, stabilizing, and then introducing N2 to achieve a total pressure of 20 MPa. The test results are shown in Table 7, and the stress-strain curve is shown in Figure 7. The results show that the stress corrosion sensitivity of 316L and 2205 stainless steel in this environment is very low, or the effect of time accumulation has not been fully reflected. It can be seen from table 7 that the elongation in air environment is lower than that in solution environment, which is mainly caused by different strain rate, but has little effect on tensile strength.
Table 7 Stress corrosion test results of 316L and 2205 stainless steel
Fig. 6 Microstructure of 2205 and 316L after 168 h immersion
Fig. 7 Stress strain curve of 316L and 2205 stainless steel stress corrosion test
- (1) In the 5-24 h pitting test, the average corrosion rate of 316L is about 5 times that of 2205 stainless steel, and it reaches 6 times after 24 h. The pitting rate of 316L stainless steel and 2205 stainless steel is accelerated.
- (2) The results of 168 h uniform corrosion test show that 2205 stainless steel has better corrosion resistance than 316L stainless steel, and there is slight pitting in both types of stainless steel, and 316L is relatively serious.
- (3) The stress corrosion tests in service show that the two kinds of stainless steels have excellent stress corrosion cracking resistance, but the environmental elements are weakened because of the fast evaluation method slow strain rate test.
- (4) The comprehensive test results show that both 316L and 2205 stainless steel have pitting risk in the selected environment. Therefore, from the perspective of risk control, 2205 stainless steel has better pitting resistance than 316L stainless steel, so it has better corrosion resistance than 316L in three tests. Although both of them show excellent stress corrosion resistance in the rapid stress corrosion evaluation, if the time increases again, once pitting is formed, the sensitivity of stress corrosion will increase.
Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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