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20MnTiB steel is the most widely used high-strength bolt material for steel structure bridges in my country, and its performance is of great significance to the safe operation of bridges.Based on the investigation of the atmospheric environment in Chongqing, this study designed a corrosion solution simulating the humid climate of Chongqing, and carried out stress corrosion tests of high-strength bolts simulating the humid climate of Chongqing.The effects of temperature, pH value and simulated corrosion solution concentration on the stress corrosion behavior of 20MnTiB high-strength bolts were studied.
20MnTiB steel is the most widely used high-strength bolt material for steel structure bridges in my country, and its performance is of great significance to the safe operation of bridges.Li et al. 1 tested the properties of 20MnTiB steel commonly used in grade 10.9 high-strength bolts in the high temperature range of 20~700 ℃, and obtained the stress-strain curve, yield strength, tensile strength, Young’s modulus, and elongation. and expansion coefficient.Zhang et al. 2, Hu et al. 3, etc., through chemical composition testing, mechanical property testing, microstructure testing, macroscopic and microscopic analysis of the thread surface, and the results show that the main reason for the fracture of high-strength bolts is related to thread defects, and the occurrence of thread defects Large stress concentrations, crack tip stress concentrations and open-air corrosion conditions all lead to stress corrosion cracking.
High-strength bolts for steel bridges are usually used for a long time in a humid environment.Factors such as high humidity, high temperature, and the sedimentation and absorption of harmful substances in the environment can easily cause corrosion of steel structures.Corrosion can cause high-strength bolt cross-section loss, resulting in numerous defects and cracks.And these defects and cracks will continue to expand, thereby reducing the life of high-strength bolts and even causing them to break.So far, there are many studies on the effect of environmental corrosion on the stress corrosion performance of materials.Catar et al4 investigated the stress corrosion behavior of magnesium alloys with different aluminum contents in acidic, alkaline and neutral environments by slow strain rate testing (SSRT).Abdel et al.5 studied the electrochemical and stress corrosion cracking behavior of Cu10Ni alloy in 3.5% NaCl solution in the presence of different concentrations of sulfide ions.Aghion et al.6 evaluated the corrosion performance of die-cast magnesium alloy MRI230D in 3.5% NaCl solution by immersion test, salt spray test, potentiodynamic polarization analysis and SSRT.Zhang et al.7 studied the stress corrosion behavior of 9Cr martensitic steel using SSRT and traditional electrochemical testing techniques, and obtained the effect of chloride ions on the static corrosion behavior of martensitic steel at room temperature.Chen et al.8 investigated the stress corrosion behavior and cracking mechanism of X70 steel in simulated sea mud solution containing SRB at different temperatures by SSRT.Liu et al.9 used SSRT to study the effect of temperature and tensile strain rate on the seawater stress corrosion resistance of 00Cr21Ni14Mn5Mo2N austenitic stainless steel.The results show that the temperature in the range of 35~65℃ has no significant effect on the stress corrosion behavior of stainless steel.Lu et al. 10 evaluated the delayed fracture susceptibility of samples with different tensile strength grades by a dead load delayed fracture test and SSRT.It is suggested that the tensile strength of 20MnTiB steel and 35VB steel high-strength bolts should be controlled at 1040-1190MPa.However, most of these studies basically use a simple 3.5% NaCl solution to simulate the corrosive environment, while the actual use environment of high-strength bolts is more complex and has many influencing factors, such as the pH value of the bolt.Ananya et al. 11 studied the effect of environmental parameters and materials in the corrosive medium on corrosion and stress corrosion cracking of duplex stainless steels.Sunada et al. 12 conducted room temperature stress corrosion cracking tests on SUS304 steel in aqueous solutions containing H2SO4 (0-5.5 kmol/m-3) and NaCl (0-4.5 kmol/m-3).The effects of H2SO4 and NaCl on the corrosion types of SUS304 steel were also studied.Merwe et al.13 used SSRT to study the effects of rolling direction, temperature, CO2/CO concentration, gas pressure and corrosion time on the stress corrosion susceptibility of A516 pressure vessel steel.Using NS4 solution as a groundwater simulating solution, Ibrahim et al. 14 investigated the effect of environmental parameters such as bicarbonate ion (HCO) concentration, pH and temperature on stress corrosion cracking of API-X100 pipeline steel after peeling off the coating.Shan et al. 15 studied the variation law of stress corrosion cracking susceptibility of austenitic stainless steel 00Cr18Ni10 with temperature under different temperature conditions (30~250℃) under the condition of black water medium in simulated coal-to-hydrogen plant by SSRT.Han et al.16 characterized the hydrogen embrittlement susceptibility of high-strength bolt samples using a dead-load delayed fracture test and SSRT.Zhao17 studied the effects of pH, SO42-, Cl-1 on the stress corrosion behavior of GH4080A alloy by SSRT.The results show that the lower the pH value, the worse the stress corrosion resistance of the GH4080A alloy.It has obvious stress corrosion sensitivity to Cl-1, and is not sensitive to SO42- ionic medium at room temperature.However, there are few studies on the effect of environmental corrosion on 20MnTiB steel high-strength bolts.
In order to find out the reasons for the failure of high-strength bolts used in bridges, the author has carried out a series of studies.High-strength bolt samples were selected, and the reasons for the failure of these samples were discussed from the perspectives of chemical composition, fracture microscopic morphology, metallographic structure and mechanical properties analysis19, 20.Based on the investigation of the atmospheric environment in Chongqing in recent years, a corrosion scheme simulating the humid climate of Chongqing is designed.Stress corrosion experiments, electrochemical corrosion experiments and corrosion fatigue experiments of high-strength bolts in Chongqing simulated humid climate were carried out.In this study, the effects of temperature, pH value and concentration of simulated corrosion solution on the stress corrosion behavior of 20MnTiB high-strength bolts were investigated through mechanical property tests, fracture macroscopic and microscopic analysis, and surface corrosion products.
Chongqing is located in southwest China, the upper reaches of the Yangtze River, and has a humid subtropical monsoon climate.The annual average temperature is 16-18°C, the annual average relative humidity is mostly 70-80%, the annual sunshine hours are 1000-1400 hours, and the sunshine percentage is only 25-35%.
According to reports related to sunshine and ambient temperature in Chongqing from 2015 to 2018, the daily average temperature in Chongqing is as low as 17°C and as high as 23°C. The highest temperature on the bridge body of Chaotianmen Bridge in Chongqing can reach 50°C °C21,22.Therefore, the temperature levels for the stress corrosion test were set at 25°C and 50°C.
The pH value of the simulated corrosion solution directly determines the amount of H+, but it does not mean that the lower the pH value, the easier corrosion occurs.The effect of pH on the results will vary for different materials and solutions.In order to better study the effect of simulated corrosion solution on the stress corrosion performance of high-strength bolts, the pH values of the stress corrosion experiments were set to 3.5, 5.5 and 7.5 in combination with literature research23 and the pH range of the annual rainwater in Chongqing.2010 to 2018.
The higher the concentration of the simulated corrosion solution, the more ion content in the simulated corrosion solution, and the greater the influence on the material properties.In order to study the effect of the simulated corrosion solution concentration on the stress corrosion of high-strength bolts, the artificial laboratory accelerated corrosion test was realized, and the simulated corrosion solution concentration was set to level 4 without corrosion, which were the original simulated corrosion solution concentration (1×), 20 × original simulated corrosion solution concentration (20 ×) and 200 × original simulated corrosion solution concentration (200 ×).
The environment with the temperature of 25℃, pH value of 5.5, and the concentration of the original simulated corrosion solution is the closest to the actual use conditions of high-strength bolts for bridges.However, in order to speed up the corrosion test process, the experimental conditions with a temperature of 25 °C, a pH of 5.5 and a concentration of 200 × original simulated corrosion solution were set as the reference control group.When the effects of the temperature, concentration or pH value of the simulated corrosion solution on the stress corrosion performance of high-strength bolts were investigated respectively, other factors remained unchanged, which was used as the experimental level of the reference control group.
According to the 2010-2018 atmospheric environment quality briefing issued by the Chongqing Municipal Bureau of Ecology and Environment, and referring to the precipitation components reported in Zhang24 and other literatures reported in Chongqing, a simulated corrosion solution based on increasing the concentration of SO42- was designed.The composition of precipitation in the main urban area of Chongqing in 2017.The composition of the simulated corrosion solution is shown in Table 1:
The simulated corrosion solution is prepared by chemical ion concentration balance method using analytical reagents and distilled water.The pH value of the simulated corrosion solution was adjusted with a precision pH meter, nitric acid solution and sodium hydroxide solution.
In order to simulate the humid climate in Chongqing, the salt spray tester has been specially modified and designed25.As shown in Figure 1, the experimental equipment has two systems: a salt spray system and a lighting system.The salt spray system is the main function of the experimental equipment, which consists of a control part, a spray part and an induction part.The function of the spray part is to pump the salt mist into the test chamber through the air compressor.The induction part is composed of temperature measuring elements, which sense the temperature in the test chamber.The control part is composed of a microcomputer, which connects the spray part and the induction part to control the whole experimental process.The lighting system is installed in a salt spray test chamber to simulate sunlight.The lighting system consists of infrared lamps and a time controller.At the same time, a temperature sensor is installed in the salt spray test chamber to monitor the temperature around the sample in real time.
Stress corrosion samples under constant load were processed in accordance with NACETM0177-2005 (Laboratory Testing of Sulfide Stress Cracking and Stress Corrosion Cracking Resistance of Metals in a H2S Environment).Stress corrosion specimens were first cleaned with acetone and ultrasonic mechanical cleaning to remove oil residues, then dehydrated with alcohol and dried in an oven.Then put the clean samples into the test chamber of the salt spray test device to simulate the corrosion situation in the humid climate environment of Chongqing.According to the standard NACETM0177-2005 and the salt spray test standard GB/T 10,125-2012, the constant load stress corrosion test time in this study is uniformly determined to be 168 h.Tensile tests were carried out on the corrosion samples under different corrosion conditions on the MTS-810 universal tensile testing machine, and their mechanical properties and fracture corrosion morphology were analyzed.
Figure 1 shows the macro- and micro-morphology of the surface corrosion of high-strength bolt stress corrosion specimens under different corrosion conditions.2 and 3 respectively.
Macroscopic morphology of stress corrosion specimens of 20MnTiB high-strength bolts under different simulated corrosion environments: (a) no corrosion; (b) 1 time; (c) 20 ×; (d) 200 ×; (e) pH3.5; (f) pH 7.5; (g) 50°C.
Micromorphology of corrosion products of 20MnTiB high-strength bolts in different simulated corrosion environments (100×): (a) 1 time; (b) 20 ×; (c) 200 ×; (d) pH3.5; (e) pH7 .5; (f) 50°C.
It can be seen from Fig. 2a that the surface of the uncorroded high-strength bolt specimen exhibits bright metallic luster without obvious corrosion.However, under the condition of the original simulated corrosion solution (Fig. 2b), the surface of the sample was partially covered with tan and brown-red corrosion products, and some areas of the surface still showed obvious metallic luster, indicating that only some areas of the sample surface were slightly corroded, and the simulated corrosion solution had no effect on the surface of the sample. Material properties have little effect.However, under the condition of 20 × original simulated corrosion solution concentration (Fig. 2c), the surface of the high-strength bolt specimen has been completely covered by a large amount of tan corrosion products and a small amount of brown-red corrosion.product, no obvious metallic luster was found, and there was a small amount of brown-black corrosion product near the surface of the substrate.And under the condition of 200 × original simulated corrosion solution concentration (Fig. 2d), the surface of the sample is completely covered by brown corrosion products, and brown-black corrosion products appear in some areas.
As the pH decreased to 3.5 (Fig. 2e), the tan-colored corrosion products were the most on the surface of the samples, and some of the corrosion products had been exfoliated.
Figure 2g shows that as the temperature increases to 50 °C, the content of brown-red corrosion products on the surface of the sample decreases sharply, while the bright brown corrosion products cover the surface of the sample in a large area.The corrosion product layer is relatively loose, and some brown-black products are peeled off.
As shown in Figure 3, under different corrosion environments, the corrosion products on the surface of 20MnTiB high-strength bolt stress corrosion specimens are obviously delaminated, and the thickness of the corrosion layer increases with the increase of the concentration of the simulated corrosion solution.Under the condition of the original simulated corrosion solution (Fig. 3a), the corrosion products on the surface of the sample can be divided into two layers: the outermost layer of corrosion products is evenly distributed, but a large number of cracks appear; the inner layer is a loose cluster of corrosion products.Under the condition of 20× original simulated corrosion solution concentration (Fig. 3b), the corrosion layer on the surface of the sample can be divided into three layers: the outermost layer is mainly dispersed cluster corrosion products, which are loose and porous, and have no good protective performance; The middle layer is a uniform corrosion product layer, but there are obvious cracks, and the corrosion ions can pass through the cracks and erode the substrate; the inner layer is a dense corrosion product layer without obvious cracks, which has a good protective effect on the substrate.Under the condition of 200× original simulated corrosion solution concentration (Fig. 3c), the corrosion layer on the surface of the sample can be divided into three layers: the outermost layer is a thin and uniform corrosion product layer; the middle layer is mainly petal-shaped and flake-shaped corrosion The inner layer is a dense corrosion product layer without obvious cracks and holes, which has a good protective effect on the substrate.
It can be seen from Fig. 3d that in the simulated corrosion environment of pH 3.5, there are a large number of flocculent or needle-like corrosion products on the surface of the 20MnTiB high-strength bolt specimen.It is speculated that these corrosion products are mainly γ-FeOOH and a small amount of α-FeOOH interlaced26, and the corrosion layer has obvious cracks.
It can be seen from Fig. 3f that when the temperature increased to 50 °C, no obvious dense inner rust layer was found in the corrosion layer structure, indicating that there were gaps between the corrosion layers at 50 °C, which made the substrate not completely covered by corrosion products. Provides protection against increased substrate corrosion tendency.
The mechanical properties of high-strength bolts under constant load stress corrosion in different corrosive environments are shown in Table 2:
It can be seen from Table 2 that the mechanical properties of the 20MnTiB high-strength bolt specimens still meet the standard requirements after the dry-wet cycle accelerated corrosion test in different simulated corrosion environments, but there is a certain damage compared with the uncorroded ones.sample.At the concentration of the original simulated corrosion solution, the mechanical properties of the sample did not change significantly, but at the 20× or 200× concentration of the simulated solution, the elongation of the sample decreased significantly.The mechanical properties are similar at the concentrations of 20 × and 200 × original simulated corrosion solutions.When the pH value of the simulated corrosion solution dropped to 3.5, the tensile strength and elongation of the samples decreased significantly.When the temperature rises to 50°C, the tensile strength and elongation decrease significantly, and the area shrinkage rate is very close to the standard value.
The fracture morphologies of the 20MnTiB high-strength bolt stress corrosion specimens under different corrosion environments are shown in Figure 4, which are the macro-morphology of the fracture, the fiber zone in the center of the fracture, the micro-morphological lip of the shear interface, and the surface of the sample.
Macroscopic and microscopic fracture morphologies of 20MnTiB high-strength bolt specimens in different simulated corrosion environments (500×): (a) no corrosion; (b) 1 time; (c) 20 ×; (d) 200 ×; (e) pH3.5; (f) pH7.5; (g) 50°C.
It can be seen from Fig. 4 that the fracture of the 20MnTiB high-strength bolt stress corrosion specimen under different simulated corrosion environments presents a typical cup-cone fracture. Compared with the uncorroded specimen (Fig. 4a), the central area of the fiber area crack is relatively small. , the shear lip area is larger.This shows that the mechanical properties of the material are significantly damaged after corrosion.With the increase of the simulated corrosion solution concentration, the pits in the fiber area in the center of the fracture increased, and obvious tear seams appeared.When the concentration increased to 20 times that of the original simulated corrosion solution, obvious corrosion pits appeared at the interface between the shear lip edge and the surface of the sample, and there were a lot of corrosion products on the surface.sample.
It is inferred from Figure 3d that there are obvious cracks in the corrosion layer on the surface of the sample, which does not have a good protective effect on the matrix. In the simulated corrosion solution of pH 3.5 (Figure 4e), the surface of the sample is severely corroded, and the central fiber area is obviously small. , There are a large number of irregular tear seams in the center of the fiber area.With the increase of the pH value of the simulated corrosion solution, the tear zone in the fiber area in the center of the fracture decreases, the pit gradually decreases, and the pit depth also decreases gradually.
When the temperature increased to 50 °C (Fig. 4g), the shear lip area of the fracture of the sample was the largest, the pits in the central fiber area increased significantly, and the pit depth also increased, and the interface between the shear lip edge and the sample surface increased. Corrosion products and pits increased, which confirmed the deepening trend of substrate corrosion reflected in Fig. 3f.
The pH value of the corrosion solution will cause some damage to the mechanical properties of 20MnTiB high-strength bolts, but the effect is not significant.In the corrosion solution of pH 3.5, a large number of flocculent or needle-like corrosion products are distributed on the surface of the sample, and the corrosion layer has obvious cracks, which cannot form a good protection for the substrate.And there are obvious corrosion pits and a large number of corrosion products in the microscopic morphology of the sample fracture.This shows that the ability of the sample to resist deformation by external force is significantly reduced in an acidic environment, and the degree of stress corrosion tendency of the material is significantly increased.
The original simulated corrosion solution had little effect on the mechanical properties of the high-strength bolt samples, but as the concentration of the simulated corrosion solution increased to 20 times that of the original simulated corrosion solution, the mechanical properties of the samples were significantly damaged, and there was obvious corrosion in the fracture microstructure. pits, secondary cracks and a lot of corrosion products.When the simulated corrosion solution concentration was increased from 20 times to 200 times of the original simulated corrosion solution concentration, the effect of the corrosion solution concentration on the mechanical properties of the material was weakened.
When the simulated corrosion temperature is 25℃, the yield strength and tensile strength of the 20MnTiB high-strength bolt specimens do not change much compared with the uncorroded specimens.However, under the simulated corrosion environment temperature of 50 °C, the tensile strength and elongation of the sample decreased significantly, the section shrinkage rate was close to the standard value, the fracture shear lip was the largest, and there were dimples in the central fiber area.Significantly increased, pit depth increased, corrosion products and corrosion pits increased.This shows that the temperature synergistic corrosion environment has a great influence on the mechanical properties of high-strength bolts, which is not obvious at room temperature, but more significant when the temperature reaches 50 °C.
After the indoor accelerated corrosion test simulating the atmospheric environment in Chongqing, the tensile strength, yield strength, elongation and other parameters of the 20MnTiB high-strength bolts were reduced, and obvious stress damage occurred.Since the material is under stress, there will be a significant localized corrosion acceleration phenomenon.And due to the combined effect of stress concentration and corrosion pits, it is easy to cause obvious plastic damage to high-strength bolts, reduce the ability to resist deformation by external forces, and increase the tendency of stress corrosion.
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