# Study on Stress Corrosion Properties of 1Cr17Ni2 Stainless Steel

Authors: Angang Cao; Bo Yin; Wei Li; Guilu Wang
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

1Cr17Ni2 stainless steel is used in aviation, ships, machinery, and other fields. However, stress corrosion will lead to its strength reduction, thus failure. In this paper, we studied the stress corrosion resistance of 1Cr17Ni2 stainless steel bolts and nuts using fasteners’ stress corrosion test method and fasteners’ stress corrosion test method in a simulated environment with the accelerated stress corrosion test method of fasteners. The influence of seawater concentration, temperature, stress, test duration, and other parameters was fully considered during the test. The test results showed that 1Cr17Ni2 stainless steel would appear to stress corrosion crack under a specific corrosive environment, but the occurrence probability was not high. Therefore, in the complex seawater salt fog environment, the stress corrosion of 1Cr17Ni2 stainless steel cannot be ignored, which provides a practical reference for applying 1Cr17Ni2 stainless steel in a complex environment.


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## Body

## 1. Introduction

The atmospheric corrosion of steel has always been of great concern to corrosion researchers and engineers. It is the most widespread corrosion process in natural environments and can cause severe destruction of steel materials, resulting in immense financial loss and severe threats to personal safety. According to an investigation, corrosion costs more than 4 trillion dollars per year, of which almost 50% is caused by atmospheric corrosion.1Cr17Ni2 stainless steel is a widely used martensitic-ferric type stainless steel developed by adding 1.5%∼2.5% Ni based on Cr17 type stainless steel [1–3]. It is characterized by retaining the corrosion resistance of ferritic stainless steel and has a high strength of martensitic stainless steel. The mechanical properties of 1Cr17Ni2 stainless steel are up to 1320 Mpa. The steel has good corrosion resistance, thermal stability, and impact toughness. It is a unique material for manufacturing the shaft, pin, bolt, and other parts under high temperature and corrosive environments. It is widely used in aviation, shipbuilding, machinery, and other fields [4–7].A particular type of membrane disc coupling is in the environment of high temperature and high salt spray for a long time. For the need for anticorrosion, the bolts and nuts are made of 1Cr17Ni2 stainless steel. A period of operation later, the nut at the junction of the membrane disc has been damaged many times for unknown reasons, as shown in Figure1.Figure 1
The phenomenon of nut damage.For the damaged nut, the crack cracked along the axial direction. It is because the nut is subjected to axial tension during operation. The number of cracks in each nut is 1–4, and some cracks have wholly penetrated through the nut, breaking the nut. The fracture condition is shown in Figure2.Figure 2
The condition of nut fracture.Through the establishment of the fault tree, several aspects of nut crack were investigated one by one. The design strength of the bolts and nuts was checked, which met the design requirements. Check the chemical composition, mechanical properties, flaw detection report, and other documents of raw materials provided by the manufacturer, and recheck the chemical composition, mechanical properties, grain size, and other properties of bolts and nuts, and no problems were found. After checking the processing and production documents, no out-of-tolerance of nut size and unqualified thread were found. The use of bolts and nuts was analyzed. The installation was carried out according to the operation and maintenance manual requirements, and no problems were found. The use environment of bolts and nuts is analyzed: the nut with the fault has been in the position where the water vapor is relatively concentrated in the seawater salt mist for a long time, accompanied by high temperature, sunlight exposure, vibration, and other harsh environments. Finally, combined with professional institutions to crack the nut fracture analysis, the fault nut crack is determined to cause stress corrosion [8–16].Previous researchers have carried out many studies on the corrosion resistance of 1Cr17Ni2 stainless steel. Liu studied the effect ofδ-ferrite on the corrosion resistance of 1Cr17Ni2 stainless steel in different solutions by weight loss rate testing, optical microscopy (OM), and scanning electron microscopy (SEM). The results show that δ-ferrite weakens the corrosion resistance of 1Cr17Ni2 stainless steel in FeCl3 solutions, and δ-ferrite is the selective dissolution phase [17]. Yang et al. investigated the atmospheric corrosion behavior and properties degradation law of 1Cr17Ni2 high-strength bolt exposed in marine and industrial atmosphere environments for up to 36 months in virtue of scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), potentiodynamic polarization curve (PPC), electrochemical impedance spectroscopy (EIS), optical microscope (OM), tensile strength test, double shear strength test, and tension fatigue test. The test results showed that high-strength bolts exhibited higher corrosion susceptibility in the marine atmosphere than in the industrial atmosphere [18]. Yang et al. investigated the atmospheric corrosion behavior of 30CrMnSiA high-strength steel exposed in rural, industrial, and marine atmosphere environments in China for 60 months due to the weight loss, X-ray diffractometer (XRD), scanning electron microscope (SEM), and grey relational analysis. The results showed that 30CrMnSiA high-strength steel exhibited the highest corrosion susceptibility and the lowest corrosion susceptibility in the marine and rural environments, respectively [19]. Wu et al. studied the corrosion behavior of Ni-advanced weathering steel and carbon steel and conventional weathering steel in a simulated tropical marine atmosphere by field exposure and indoor simulation tests. Results indicated that the additive Ni in weathering steel improved the resistance of Ni-advanced weathering steel to atmospheric corrosion in a simulated tropical marine environment [20]. Zhang et al. conducted atmospheric stress corrosion-cracking (SCC) tests for a 316 L stainless steel (SS) at an applied stress of 1.1 σ0.2 with a chloride deposition density of approximately 100 μg/cm deposited as MgCl2 from solution. The test results showed that the stress corrosion cracks growth rate decreased with increasing RH and cracked depth [21]. However, there were relatively few studies on the sensitivity of 1Cr17Ni2 stainless steel to stress corrosion in high salt spray seawater. Therefore, it is essential to study the corrosion behavior of 1Cr17Ni2 stainless steel fasteners in high salt spray seawater [22–28].In this study, we studied the stress corrosion resistance of 1Cr17Ni2 stainless steel bolts and nuts using the stress corrosion test method of fasteners and the stress corrosion test method of fasteners in a simulated environment with the accelerated stress corrosion test method of fasteners. The influence of seawater concentration, temperature, stress, test duration, and other parameters was fully considered during the test. It has been verified that 1Cr17Ni2 stainless steel will appear to stress corrosion crack under a specific corrosive environment, but the occurrence probability is not high. Therefore, 1Cr17Ni2 stainless steel is not recommended as a fastener material in high concentration seawater and salt fog environment. It provided a practical reference for applying 1Cr17Ni2 stainless steel in a complex environment.


## 2. First Stress Corrosion Test

### 2.1. Testing Program

#### 2.1.1. Test Device

According to the requirements of stress corrosion test for fasteners in Chinese National Military Standards “GJB 715.7–90 Fastener Test Methods Stress Corrosion,” the dimensions, strength, and material parameters of bolts and nuts are consistent with those of the products [29–33]. The thickness and material of the mounting block are compatible with the parameters of the membrane disc connected by bolts and nuts in the product. Other parameters are designed according to the test requirements above. The drawing of the mounting block is shown in Figure 3.Figure 3
The parameters of the mounting block.


#### 2.1.2. Test Chamber

The test chamber and auxiliary devices are made of plastic and will not affect the test conditions. The alternating immersion device in the test chamber is constructed to generate the alternating immersion cycle specified in the verification scheme. The test chamber is shown in Figure4.Figure 4
Test chamber.


#### 2.1.3. Test Medium

According to the requirements of “GJB 715.7-90, Fastener Test Methods Stress Corrosion,” the salt solution’s mass fraction was 3.5%. The preparation of 3.5% sodium chloride aqueous solution, by mass, dissolved 3.5 ± 0.5 parts of salt in 96.5 amounts of deionized water. The temperature of the salt solution is 24 ± 3°C, and the pH value is maintained in the range of 6.5∼7.5. The pH of the salt solution is checked every 24 h, and the pH is adjusted using analytical pure dilute hydrochloric acid or chemical pure sodium hydroxide.


#### 2.1.4. Installation Requirements

When installing, the bolts, nuts, and mounting blocks are assembled. There should be at least two complete threads between the screw tail of the bolt sample and the nut support surface. Put it into the alternately dry and wet test chamber. The specific bolt number and installation torque are shown in Table1.Table 1
Installation requirements.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)1–4～1–61Cr17Ni230 (2 times rated torque)


#### 2.1.5. Test Requirements

The test process is an alternate immersion cycle test, which means that the test piece is immersed in the salt solution for 10 ± 1 min every hour, followed by drying in the air for 50 ± 1 min. After the parts are dry, visually inspect them to see any signs of fracture. The test time is 15 days, the test parts should be checked at least once every 24 h, and the test bolts and nuts should be recorded. The experimental data observed every day were recorded, and photos were taken regularly.


### 2.2. Results and Discussion

No cracks appeared during the test. After 360 hours of stress corrosion test, the state of the test piece is shown in Figure5. As shown from the figure, after 360 hours of stress corrosion test, bolts and nuts of 1Cr17Ni2 material all showed many corrosion traces, but no stress corrosion cracks were found.Figure 5
The status of test piece after test. (a) The state of the test piece in the combined state after the test. (b) The state of the bolt after the test. (c) The state of the nut after the test.
(a)(b)(c)To further verify the existence of small cracks in the test piece, fluorescent flaw detection was carried out according to the tolerance requirements in China National Standards “HB/Z 61-1988 Penetrant Inspection.” The penetrant with fluorescent dye was infiltrated into the small cracks on the surface of the workpiece. After cleaning, the adsorbent was applied to make the fluorescent liquid in the defect ooze out of character. Yellow-green fluorescent spots or stripes appeared under ultraviolet lamp irradiation [34–36]. The inspection photos are shown in Figure 6. According to the inspection results, although there is a fluorescent solution at the screw threads of bolts and nuts, the fluorescent solution is flaked, which is not cleaned up, and there is no corrosion crack at the screw threads. Therefore, according to the results of fluorescence inspection, no stress corrosion crack appeared in bolts and nuts of 1Cr17Ni2 material.Figure 6
Inspection photos.According to the above results, after 360 hours of stress corrosion test, the bolts and nuts of 1Cr17Ni2 material all showed many corrosion traces, but no stress corrosion cracks appeared. The stress corrosion test for fasteners in Chinese National Military Standards “GJB 715.7-90 Fastener Test Methods Stress Corrosion” was wholly carried out. The environment was relatively single, which was vastly different from the actual operating environment of the product. Therefore, the test was improved, and the second stress corrosion test was designed.


## 2.1. Testing Program

### 2.1.1. Test Device

According to the requirements of stress corrosion test for fasteners in Chinese National Military Standards “GJB 715.7–90 Fastener Test Methods Stress Corrosion,” the dimensions, strength, and material parameters of bolts and nuts are consistent with those of the products [29–33]. The thickness and material of the mounting block are compatible with the parameters of the membrane disc connected by bolts and nuts in the product. Other parameters are designed according to the test requirements above. The drawing of the mounting block is shown in Figure 3.Figure 3
The parameters of the mounting block.


### 2.1.2. Test Chamber

The test chamber and auxiliary devices are made of plastic and will not affect the test conditions. The alternating immersion device in the test chamber is constructed to generate the alternating immersion cycle specified in the verification scheme. The test chamber is shown in Figure4.Figure 4
Test chamber.


### 2.1.3. Test Medium

According to the requirements of “GJB 715.7-90, Fastener Test Methods Stress Corrosion,” the salt solution’s mass fraction was 3.5%. The preparation of 3.5% sodium chloride aqueous solution, by mass, dissolved 3.5 ± 0.5 parts of salt in 96.5 amounts of deionized water. The temperature of the salt solution is 24 ± 3°C, and the pH value is maintained in the range of 6.5∼7.5. The pH of the salt solution is checked every 24 h, and the pH is adjusted using analytical pure dilute hydrochloric acid or chemical pure sodium hydroxide.


### 2.1.4. Installation Requirements

When installing, the bolts, nuts, and mounting blocks are assembled. There should be at least two complete threads between the screw tail of the bolt sample and the nut support surface. Put it into the alternately dry and wet test chamber. The specific bolt number and installation torque are shown in Table1.Table 1
Installation requirements.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)1–4～1–61Cr17Ni230 (2 times rated torque)


### 2.1.5. Test Requirements

The test process is an alternate immersion cycle test, which means that the test piece is immersed in the salt solution for 10 ± 1 min every hour, followed by drying in the air for 50 ± 1 min. After the parts are dry, visually inspect them to see any signs of fracture. The test time is 15 days, the test parts should be checked at least once every 24 h, and the test bolts and nuts should be recorded. The experimental data observed every day were recorded, and photos were taken regularly.


## 2.1.1. Test Device

According to the requirements of stress corrosion test for fasteners in Chinese National Military Standards “GJB 715.7–90 Fastener Test Methods Stress Corrosion,” the dimensions, strength, and material parameters of bolts and nuts are consistent with those of the products [29–33]. The thickness and material of the mounting block are compatible with the parameters of the membrane disc connected by bolts and nuts in the product. Other parameters are designed according to the test requirements above. The drawing of the mounting block is shown in Figure 3.Figure 3
The parameters of the mounting block.


## 2.1.2. Test Chamber

The test chamber and auxiliary devices are made of plastic and will not affect the test conditions. The alternating immersion device in the test chamber is constructed to generate the alternating immersion cycle specified in the verification scheme. The test chamber is shown in Figure4.Figure 4
Test chamber.


## 2.1.3. Test Medium

According to the requirements of “GJB 715.7-90, Fastener Test Methods Stress Corrosion,” the salt solution’s mass fraction was 3.5%. The preparation of 3.5% sodium chloride aqueous solution, by mass, dissolved 3.5 ± 0.5 parts of salt in 96.5 amounts of deionized water. The temperature of the salt solution is 24 ± 3°C, and the pH value is maintained in the range of 6.5∼7.5. The pH of the salt solution is checked every 24 h, and the pH is adjusted using analytical pure dilute hydrochloric acid or chemical pure sodium hydroxide.


## 2.1.4. Installation Requirements

When installing, the bolts, nuts, and mounting blocks are assembled. There should be at least two complete threads between the screw tail of the bolt sample and the nut support surface. Put it into the alternately dry and wet test chamber. The specific bolt number and installation torque are shown in Table1.Table 1
Installation requirements.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)1–4～1–61Cr17Ni230 (2 times rated torque)


## 2.1.5. Test Requirements

The test process is an alternate immersion cycle test, which means that the test piece is immersed in the salt solution for 10 ± 1 min every hour, followed by drying in the air for 50 ± 1 min. After the parts are dry, visually inspect them to see any signs of fracture. The test time is 15 days, the test parts should be checked at least once every 24 h, and the test bolts and nuts should be recorded. The experimental data observed every day were recorded, and photos were taken regularly.


## 2.2. Results and Discussion

No cracks appeared during the test. After 360 hours of stress corrosion test, the state of the test piece is shown in Figure5. As shown from the figure, after 360 hours of stress corrosion test, bolts and nuts of 1Cr17Ni2 material all showed many corrosion traces, but no stress corrosion cracks were found.Figure 5
The status of test piece after test. (a) The state of the test piece in the combined state after the test. (b) The state of the bolt after the test. (c) The state of the nut after the test.
(a)(b)(c)To further verify the existence of small cracks in the test piece, fluorescent flaw detection was carried out according to the tolerance requirements in China National Standards “HB/Z 61-1988 Penetrant Inspection.” The penetrant with fluorescent dye was infiltrated into the small cracks on the surface of the workpiece. After cleaning, the adsorbent was applied to make the fluorescent liquid in the defect ooze out of character. Yellow-green fluorescent spots or stripes appeared under ultraviolet lamp irradiation [34–36]. The inspection photos are shown in Figure 6. According to the inspection results, although there is a fluorescent solution at the screw threads of bolts and nuts, the fluorescent solution is flaked, which is not cleaned up, and there is no corrosion crack at the screw threads. Therefore, according to the results of fluorescence inspection, no stress corrosion crack appeared in bolts and nuts of 1Cr17Ni2 material.Figure 6
Inspection photos.According to the above results, after 360 hours of stress corrosion test, the bolts and nuts of 1Cr17Ni2 material all showed many corrosion traces, but no stress corrosion cracks appeared. The stress corrosion test for fasteners in Chinese National Military Standards “GJB 715.7-90 Fastener Test Methods Stress Corrosion” was wholly carried out. The environment was relatively single, which was vastly different from the actual operating environment of the product. Therefore, the test was improved, and the second stress corrosion test was designed.


## 3. Second Stress Corrosion Test

### 3.1. Testing Program

Because the first stress corrosion test’s test conditions were ideal, the salt solution was sodium chloride aqueous solution, the medium was single, the concentration was moderate, and the temperature was room temperature. In addition, the test time was short, only 360 hours. The second stress corrosion test was designed further to verify the sensitivity of 1Cr17Ni2 material to stress corrosion. Specific improvement measures are as follows:(1)
The salt solution is prepared with seawater, the salt concentration is not less than 20%, and the pH value is kept within the range of 6.0∼7.0(2)
The ambient temperature of the test should be maintained at 35°C or above(3)
Continue the test of 1Cr17Ni2 bolts and nuts after the first test to increase the test time(4)
The 1Cr17Ni2 nut is grooved to increase stress concentration and accelerate stress corrosion(5)
Increase the installation torque of the nut and increase the stress in the test process (according to the size of the thread of the test piece, the limit installation torque is about 40 N·m)The improved test device is shown in Figure7.Figure 7
The test device of the second test.The test and installation requirements not mentioned are the same as the first stress corrosion test. The number, installation torque, and grouping of bolts in this test are shown in Table2.Table 2
Sample grouping and installation torque.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)1–4～1–61Cr17Ni230 (2 times rated torque)6–1～6–31Cr17Ni215 (rated torque)6–4～6–61Cr17Ni230 (2 times rated torque)6–7～6–91Cr17Ni240 (limit installation torque)


### 3.2. Results and Discussion

No cracks appeared during the test. 1Cr17Ni2 material test piece after 360/720 hours accelerated stress corrosion test is shown in Figure8. Rust occurs in six aspects and thread of the bolt head. Yellow rust on the smooth rod should be caused by flow pollution of head rust. Six elements of the nut also appeared with a lot of rust, and internal thread corrosion rust spots can be seen. As the samples numbered 1–1∼1–6 have been tested once, their corrosion is significantly worse than those numbered 6–1∼6–9. Compared with the surface of the test parts with installation torques of 15 N·m, 30 N·m, and 40 N·m, there is no noticeable difference in corrosion conditions. At the same time, no crack defect was found on the surface of each test piece.Figure 8
The status of the test piece after the second stress corrosion test. (a) The state of the bolt after the test. (b) The state of the nut after the test.
(a)(b)To further verify the existence of small cracks in the test piece, fluorescence inspection was carried out on the test piece according to the China National Standards “HB/Z 61-1988 Penetration Inspection” requirements. As shown in Figure9(a), after the fluorescent inspection of bolts of 1Cr17Ni2 material, flecks of fluorescence were shown at the head and thread of bolts, indicating that the rust was attached with fluorescent solution, and no crack defects were found. Many fluorescent displays were found on the nut surface after the fluorescent inspection of the 1Cr17Ni2 material nut; as shown in Figure 9(b), many fluorescent displays were found on the nuts surface. The infiltration of fluorescent solution should cause surface corrosion. No crack defects were found.Figure 9
Fluorescence inspection results of test pieces. (a) The photo of bolt flaw detection. (b) The photo of nut flaw detection.
(a)(b)Based on the above results, two stress corrosion tests have been carried out. No stress corrosion cracks were found in bolts and nuts of 1Cr17Ni2 material. It is proved that the 1Cr17Ni2 material has a specific resistance to stress corrosion in the general environment.


## 3.1. Testing Program

Because the first stress corrosion test’s test conditions were ideal, the salt solution was sodium chloride aqueous solution, the medium was single, the concentration was moderate, and the temperature was room temperature. In addition, the test time was short, only 360 hours. The second stress corrosion test was designed further to verify the sensitivity of 1Cr17Ni2 material to stress corrosion. Specific improvement measures are as follows:(1)
The salt solution is prepared with seawater, the salt concentration is not less than 20%, and the pH value is kept within the range of 6.0∼7.0(2)
The ambient temperature of the test should be maintained at 35°C or above(3)
Continue the test of 1Cr17Ni2 bolts and nuts after the first test to increase the test time(4)
The 1Cr17Ni2 nut is grooved to increase stress concentration and accelerate stress corrosion(5)
Increase the installation torque of the nut and increase the stress in the test process (according to the size of the thread of the test piece, the limit installation torque is about 40 N·m)The improved test device is shown in Figure7.Figure 7
The test device of the second test.The test and installation requirements not mentioned are the same as the first stress corrosion test. The number, installation torque, and grouping of bolts in this test are shown in Table2.Table 2
Sample grouping and installation torque.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)1–4～1–61Cr17Ni230 (2 times rated torque)6–1～6–31Cr17Ni215 (rated torque)6–4～6–61Cr17Ni230 (2 times rated torque)6–7～6–91Cr17Ni240 (limit installation torque)


## 3.2. Results and Discussion

No cracks appeared during the test. 1Cr17Ni2 material test piece after 360/720 hours accelerated stress corrosion test is shown in Figure8. Rust occurs in six aspects and thread of the bolt head. Yellow rust on the smooth rod should be caused by flow pollution of head rust. Six elements of the nut also appeared with a lot of rust, and internal thread corrosion rust spots can be seen. As the samples numbered 1–1∼1–6 have been tested once, their corrosion is significantly worse than those numbered 6–1∼6–9. Compared with the surface of the test parts with installation torques of 15 N·m, 30 N·m, and 40 N·m, there is no noticeable difference in corrosion conditions. At the same time, no crack defect was found on the surface of each test piece.Figure 8
The status of the test piece after the second stress corrosion test. (a) The state of the bolt after the test. (b) The state of the nut after the test.
(a)(b)To further verify the existence of small cracks in the test piece, fluorescence inspection was carried out on the test piece according to the China National Standards “HB/Z 61-1988 Penetration Inspection” requirements. As shown in Figure9(a), after the fluorescent inspection of bolts of 1Cr17Ni2 material, flecks of fluorescence were shown at the head and thread of bolts, indicating that the rust was attached with fluorescent solution, and no crack defects were found. Many fluorescent displays were found on the nut surface after the fluorescent inspection of the 1Cr17Ni2 material nut; as shown in Figure 9(b), many fluorescent displays were found on the nuts surface. The infiltration of fluorescent solution should cause surface corrosion. No crack defects were found.Figure 9
Fluorescence inspection results of test pieces. (a) The photo of bolt flaw detection. (b) The photo of nut flaw detection.
(a)(b)Based on the above results, two stress corrosion tests have been carried out. No stress corrosion cracks were found in bolts and nuts of 1Cr17Ni2 material. It is proved that the 1Cr17Ni2 material has a specific resistance to stress corrosion in the general environment.


## 4. Accelerated Stress Corrosion Test

Due to the short test time and challenging test environment simulation, the first two stress corrosion tests failed to achieve the test results. An accelerated stress corrosion test was designed and carried out further to verify the sensitivity of 1Cr17Ni2 material to stress corrosion.


### 4.1. Testing Program

According to the tolerance requirements in China National Standards, “GB/T 17898–99 Test Method for Stress Corrosion-Cracking Resistance of Stainless Steels in a Boiling Magnesium Chloride Solution,” the test solution was under the configuration of distilled water to analyze pure magnesium chloride stipulated in China National Standard “GB/T 672–2006 Chemical Reagent-Magnesium Chloride Hexahydrate.” The prepared magnesium chloride is a 20% aqueous solution, heated, and its boiling point is adjusted to 155 ± 1°C, and the concentration of magnesium chloride is about 45% [37–41].A container with a total capacity and condensation reflux device was used in the test. The magnesium chloride solution was kept in a slightly boiling heating state during the test process [42–46].The bolts, nuts, and mounting blocks are put into the test container. This time is used as the start time of the test. Every 4 hours, the test pieces are taken out with the fixture washed with distilled water. Use a magnifying glass to observe the crack of the bolts and nuts of the test piece and take photos to record the surface. The number, installation torque, and grouping of bolts in this test are shown in Table3.Table 3
Sample grouping and installation torque.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)2–1～2–31Cr17Ni230 (2 times rated torque)


### 4.2. Results and Discussion

Figure10 shows the surface condition of the test piece after being immersed in boiling magnesium chloride solution for 4 h. It is observed that there is apparent corrosion on the upper part of the No. 1-1 nut. Nut No. 1-2 has an obvious cross-shaped macrocrack. Next to the crack, there is a second crack with apparent rust. Nut No. 1–3 parts are less corroded. Slight yellow rust is on top of nut No. 2-1. Uniform yellow rust is visible on top of No. 2-2 nut and No. 2-3 have no apparent rust. Except for No 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 10
The surface condition of the test piece soaked in boiling magnesium chloride solution for 4 h.Figure11 shows the surface condition of the test piece after being immersed in boiling magnesium chloride solution for 8 h. It can be seen from the observation that the corrosion of the second crack of the nut No. 1-2 is more evident, while no apparent macroscopic crack was observed in other test pieces. The yellow rust on the upper part of the nut No. 2-1 increases.Figure 11
The surface condition of the test piece soaked in boiling magnesium chloride solution for 8 h.Figure12 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 12 h, which is the same as that after washing for 8 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 12
The surface condition of the test piece soaked in boiling magnesium chloride solution for 12 h.Figure13 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 16 h, which is the same as that after washing for 12 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 13
The surface condition of the test piece soaked in boiling magnesium chloride solution for 16 h.Figure14 shows the surface condition of the test piece after it has been immersed in boiling magnesium chloride solution for 32 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces. Corrosion of the test piece No. 2-1 was aggravated. A little yellow rust can be seen on the contact part of the nut and mounting block.Figure 14
The surface condition of the test piece soaked in boiling magnesium chloride solution for 32 h.Figure15 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 72 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces. Corrosion occurred in all the test parts, among which the corrosion of nuts numbered 1-2 was the most serious, and the corrosion of nuts numbered 2-3 was the least.Figure 15
The surface condition of the test piece soaked in boiling magnesium chloride solution for 72 h.According to the above results, only one set of nuts showed stress corrosion cracks after 72 hours of accelerated stress corrosion. It is proved that the 1Cr17Ni2 stainless steel has a specific stress corrosion resistance, and there is a certain probability of stress corrosion crack in the harsh corrosion environment.It can be seen that the stress corrosion crack has little relationship with the installation torques, comparing the two groups of test pieces with installation torques of 15 and 30 N·m, respectively. Stress corrosion will occur in a specific corrosive environment when the stress reaches a certain degree; rather than that, the greater the stress, the more serious the stress corrosion will be.


## 4.1. Testing Program

According to the tolerance requirements in China National Standards, “GB/T 17898–99 Test Method for Stress Corrosion-Cracking Resistance of Stainless Steels in a Boiling Magnesium Chloride Solution,” the test solution was under the configuration of distilled water to analyze pure magnesium chloride stipulated in China National Standard “GB/T 672–2006 Chemical Reagent-Magnesium Chloride Hexahydrate.” The prepared magnesium chloride is a 20% aqueous solution, heated, and its boiling point is adjusted to 155 ± 1°C, and the concentration of magnesium chloride is about 45% [37–41].A container with a total capacity and condensation reflux device was used in the test. The magnesium chloride solution was kept in a slightly boiling heating state during the test process [42–46].The bolts, nuts, and mounting blocks are put into the test container. This time is used as the start time of the test. Every 4 hours, the test pieces are taken out with the fixture washed with distilled water. Use a magnifying glass to observe the crack of the bolts and nuts of the test piece and take photos to record the surface. The number, installation torque, and grouping of bolts in this test are shown in Table3.Table 3
Sample grouping and installation torque.
Serial numberMaterial trademarkInstallation torque (N·m)1–1～1–31Cr17Ni215 (rated torque)2–1～2–31Cr17Ni230 (2 times rated torque)


## 4.2. Results and Discussion

Figure10 shows the surface condition of the test piece after being immersed in boiling magnesium chloride solution for 4 h. It is observed that there is apparent corrosion on the upper part of the No. 1-1 nut. Nut No. 1-2 has an obvious cross-shaped macrocrack. Next to the crack, there is a second crack with apparent rust. Nut No. 1–3 parts are less corroded. Slight yellow rust is on top of nut No. 2-1. Uniform yellow rust is visible on top of No. 2-2 nut and No. 2-3 have no apparent rust. Except for No 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 10
The surface condition of the test piece soaked in boiling magnesium chloride solution for 4 h.Figure11 shows the surface condition of the test piece after being immersed in boiling magnesium chloride solution for 8 h. It can be seen from the observation that the corrosion of the second crack of the nut No. 1-2 is more evident, while no apparent macroscopic crack was observed in other test pieces. The yellow rust on the upper part of the nut No. 2-1 increases.Figure 11
The surface condition of the test piece soaked in boiling magnesium chloride solution for 8 h.Figure12 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 12 h, which is the same as that after washing for 8 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 12
The surface condition of the test piece soaked in boiling magnesium chloride solution for 12 h.Figure13 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 16 h, which is the same as that after washing for 12 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces.Figure 13
The surface condition of the test piece soaked in boiling magnesium chloride solution for 16 h.Figure14 shows the surface condition of the test piece after it has been immersed in boiling magnesium chloride solution for 32 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces. Corrosion of the test piece No. 2-1 was aggravated. A little yellow rust can be seen on the contact part of the nut and mounting block.Figure 14
The surface condition of the test piece soaked in boiling magnesium chloride solution for 32 h.Figure15 shows the surface condition of the test piece after soaking in boiling magnesium chloride solution for 72 h. Except for nut No. 1-2, no obvious macroscopic cracks were observed in other test pieces. Corrosion occurred in all the test parts, among which the corrosion of nuts numbered 1-2 was the most serious, and the corrosion of nuts numbered 2-3 was the least.Figure 15
The surface condition of the test piece soaked in boiling magnesium chloride solution for 72 h.According to the above results, only one set of nuts showed stress corrosion cracks after 72 hours of accelerated stress corrosion. It is proved that the 1Cr17Ni2 stainless steel has a specific stress corrosion resistance, and there is a certain probability of stress corrosion crack in the harsh corrosion environment.It can be seen that the stress corrosion crack has little relationship with the installation torques, comparing the two groups of test pieces with installation torques of 15 and 30 N·m, respectively. Stress corrosion will occur in a specific corrosive environment when the stress reaches a certain degree; rather than that, the greater the stress, the more serious the stress corrosion will be.


## 5. Conclusions

Through three stress corrosion tests on 1Cr17Ni2 stainless steel bolts, nuts, and other test pieces, the following conclusions can be drawn:(1)
1Cr17Ni2 stainless steel material has specific corrosion resistance, but in the wet, alternating salt spray environment, corrosion is faster;(2)
1Cr17Ni2 stainless steel material has a particular stress corrosion resistance; generally, a corrosion environment can be used. However, there is a certain probability of stress corrosion crack occurring in a highly harsh corrosion environment, which leads to component failure.(3)
Excess installation torque of fasteners is not one of the causes of stress corrosion crack, so there can be a particular deviation of mounting torque of pins within the acceptable range.



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*Source: 2435238-2022-04-27.xml*