Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fasteners are a class of mechanical parts that are used for fastening and are widely used, and typically include bolts, nuts, washers, retaining rings, and assemblies. High-strength fasteners refer to fasteners of high specific strength, which usually have excellent corrosion resistance, good high and low temperature resistance, etc. [1, 2, 3], widely used in aerospace, aviation, ship and ground. Weaponry and other defense industries [4, 5, 6]. The reliability of high-strength fasteners directly relates to the reliability of the entire structural system, which in turn affects the overall safety, and has been highly valued by engineering designers. For a long time, designers have paid more attention to the fatigue life, fracture toughness, impact resistance, hydrogen embrittlement mechanism and stress-strain performance of high-strength fasteners [7,8,9,10], for corrosion failure, especially coastal/seashore environment. There are few studies on the environmental adaptability of high-strength fasteners. With the needs of engineering construction in China and the development needs of science and technology, designers have gradually deepened their understanding of the environmental adaptability of high-strength fasteners, and the requirements for protection of high-strength fasteners are getting higher and higher. Materials commonly used as high-strength structural fasteners include alloy structural steels, high-temperature alloys, and titanium alloys. Under normal circumstances, stainless steel, high-temperature alloys and titanium alloys will form a dense oxide film on the surface to prevent corrosion, but the oxide film will be broken in harsh environments such as high temperature, high humidity, high salt and rain. In the case of crevice corrosion, the corrosion resistance of the material will be greatly reduced. In practical engineering applications, sometimes in order to meet special needs, high-strength structural fasteners are required to meet the needs of short-term exposure to marine rain and humidity, and fastener materials have unpredictable corrosion tendency, which leads to technology. risk. Therefore, the corrosion evaluation of high-strength fasteners must be carried out and effective protective measures must be taken to ensure that their environmental adaptability meets the system safety requirements. This paper aims to investigate the short-term corrosion behavior and protection methods of typical high-strength fasteners in Hainan rainwater environment. 

  1. Analysis of Hainan Rainwater Composition
  2. Morphology analysis of high strength fasteners
  3. Tensile force test of fasteners

Experimental method of high strength fasteners

High-strength fasteners made of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V are used as test pieces, and the surfaces of the fasteners are treated with different surface treatments and coatings such as alkyd varnish. The surface treatment methods of fasteners of different materials are shown in Table 1.

Table 1 Surface treatment of high-strength fasteners
Material category Surface treatment Surface coating
30CrMnSiA Without, Fe/Ep.Zn7.c2C, flZnAl12 Without, alkyd varnish
30CrMnSiNi2A Without, Fe/Ep.Zn7.c2C, flZnAl12 Without, alkyd varnish
GH4169 Clean state, Ct.P, pulse anodizing Alkyd varnish, MoS2
Ti-6Al-4V Without, spraying Al, IVD Al MoS2, hexadecyl alcohol

Install bolts (M10), nuts (M10), clamps and washers in the form of Figure 1 to form a combination, apply a torque of 35 N·m to the fasteners; handle fasteners for brush coating The surface of the bolt and nut is coated with alkyd varnish before being applied. After brushing, it is allowed to stand for 24 h and assembled. After forming the assembly, the fastener is applied with a torque of 35 N·m, and the alkyd varnish is applied again. coating. Each test piece is divided into three types: no surface treatment, surface treatment, surface treatment and coating, and numbered according to type. A set of fasteners of the same material in the same state was suspended in a 500 mL beaker. The sample was subjected to constant temperature suspension soaking with a rainwater solution collected from Hainan 1 km away from the sea to maintain the temperature of the rainwater solution (35± 3) °C, sampling period is 4, 24, 48, 96 and 168 h. The composition of the rainwater solution was analyzed before and after the experiment, and the concentration of Cl-, SO42- and the pH value were mainly detected.

img 1 - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fig.1 Schematic of fixed torque fasteners rain-water immersion test

The fastener samples were placed directly at the platform position of Hainan 1 km away from the sea, directly exposed to the tropical marine atmosphere, sampling period was 7, 10 and 20 d, sampled periodically and returned to the laboratory for mechanical properties. .
The tensile force test was carried out on the Hainan rainwater environment immersion torque test and the fasteners before and after the Hainan real atmospheric environment exposure test, and the force-displacement curve was recorded. Comparing the tensile load after the experiment and the tensile load before the experiment, the macroscopic shape analysis of the fracture was carried out after the breaking. The tensile force test is carried out in accordance with “GJB 715.23A-2008 Tensile Strength Test Method for Fasteners”.

2 Results and discussion of high strength fasteners

2.1 Analysis of Hainan Rainwater Composition
The concentrations of Cl- and SO42- and the pH values of the rainwater components before and after the Hainan rainwater immersion test were tested. The experimental results are shown in Table 2. The results show that the high-strength connection of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V materials is immersed. The composition of Hainan rainwater changed little before and after the experiment, indicating that the fastener material had little effect on the rainwater composition, and it was proved that the corrosive ions did not react significantly with the fastener during the immersion experiment.

Table 2 Compositions of rain water before and after immersion of high-strength fasteners
Material category State of rain water pH Cl concentration / mg·L-1 SO42- concentration / mg·L-1
30CrMnSiA Original
After immersion test
7.1
7.1
4.9
5.4
10.4
13.9
30CrMnSiNi2A Original
After immersion test
7.1
7.1
4.9
5.2
10.4
10.0
GH4169 Original
After immersion test
7.1
7.1
4.4
4.3
4.4
4.4
Ti-6Al-4V Original
After immersion test
7.0
7.0
3.9
4.2
4.3
4.9

2.2 Morphology analysis of high strength fasteners
The high-strength joints of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V materials under different surface conditions were compared and analyzed after exposure to different conditions in Hainan rainwater immersion and Hainan rain shower atmosphere.

2.2.1

30CrMnSiA Fasteners From Figure 2, the macroscopic morphology of fasteners of 30CrMnSiA material under different surface conditions after immersion in rainwater for different time can be seen: 4h, fastener screw and head without surface treatment Partial red rust appeared in the part (see Figure 3a); the area of the screw and head red rust of the fasteners without surface treatment increased for 24 h; for 48 h, the surface of the fastener bolts without surface treatment was red rust. Auxiliary nuts for bolt fasteners that have been aggravated without surface treatment, Fe/Ep.Zn7.c2C (electroplated zinc) treatment (see Figure 2b) and flZnAl12 (zinc aluminum dacromet) treatment (see Figure 2d) Fe/Ep.Zn7.c2C treatment) Pitting on the surface. By tracking the changes in the apparent morphology of the fasteners during the experiment, it can be concluded that 30CrMnSiA is highly susceptible to corrosion in the rain environment. At the beginning of the experiment, red rust appears, and Fe/Ep.Zn7.c2C and flZnAl12 are treated on the surface. After that, the corrosion resistance of 30CrMnSiA was improved, but the surface of the bolt after Fe/Ep.Zn7.c2C treatment still showed pitting corrosion after 48 h of experiment. After 168 h, the surface of the bolt after flZnAl12 treatment also showed pitting corrosion. Therefore, the surface of 30CrMnSiA must be treated with a varnish. The bolt fractures after the breaking test were observed. No stress corrosion crack source and characteristic morphology were found, indicating that the fasteners did not undergo stress corrosion during the 168 h rainwater immersion test.

img 2 - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fig.2 Morphologies of 30CrMnSiA fasteners after rain-water immersion test: (a) without surface treatment; (b) Fe/Ep.Zn7.c2C; (c) Fe/Ep.Zn7.c2C+alkyd resin varnish; (d) flZnAl12; (e) flZnAl12+alkyd resin varnish; (f) fracture morphology

img 3 - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fig.3 Morphologies of 30CrMnSiN2A fasteners after rain-water immersion test: (a) without surface treatment; (b) flZnAl12; (c) flZnAl12+alkyd resin varnish; (d) 168 h immersion

By comparing the macroscopic morphology of the samples in the real atmospheric environment exposure experiment in Hainan, it is found that the corrosion morphology of the surface of the material after the exposure test and the immersion experiment is similar. The corrosion of the sample after 10 days of exposure and the surface of the sample immersed for 168 h after rain. Corrosion is equivalent, confirming that the indoor rainwater immersion environment is related to the real marine environment.
2.2.2

30CrMnSiNi2A Fasteners From Figure 3, the macroscopic appearance of high-strength fasteners of 30CrMnSiNi2A material under different surface conditions after immersion in rainwater for different time can be seen: bolts of 30CrMnSiN2A fasteners without surface treatment for 48 h. Red rust on the head (see Figure 3a), rust increases over time. By tracking the change of the apparent morphology of the fastener during the experiment, it can be concluded that for the fastener of 30CrMnSiN2A material, the surface of the bolt without surface treatment appeared red rust after 48 hours of experiment (see Figure 3a); after treatment with flZnAl12 The bolt corrosion resistance is improved (see Figure 3b). After 168 h of experiment, there is no obvious pitting corrosion on the bolt head, but the surface gloss of the sample decreases, and tiny pitting pits appear at the thread, but no red rust appears. The fasteners treated with flZnAl12 and painted did not show corrosion during the experiment.
2.2.3

GH4169 Fasteners Figure 4 shows the macroscopic morphology of high-strength fasteners of GH4169 material in different surface conditions after immersion in Hainan rainwater for 20 days and Hainan atmospheric exposure. GH4169 bolts with cleaning state, Ct.P (passivation treatment) and Ct.P treatment coated with alkyd varnish, pulse anodized and coated with MoS2 GH4169 nut in the rainwater soaking test and Hainan atmospheric exposure experiment throughout the experimental period There is no obvious corrosion phenomenon, indicating that GH4169 fasteners have good corrosion resistance to Hainan rainwater environment.
img 4 - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fig.4 Morphologies of GH4169 fasteners after rain-water immersion test and atmospheric exposure test:(a) without surface treatment; (b) Ct.P; (c) Ct.P+alkyd resin varnish; (d) 20 d exposure

2.2.4

Ti-6Al-4V fasteners can be obtained by tracking the apparent morphology of Ti-6Al-4V fasteners during the experiment. No surface treatment, coating of MoS2+ cetyl alcohol, spraying aluminum + cetyl alcohol, Ti-6Al-4V fasteners with ion-plated aluminum and cetyl alcohol had no obvious changes after 168 h rainwater immersion test and 20-day appearance of Hainan exposure. No obvious signs of corrosion were observed on the fastener surface, indicating Ti-6Al- 4V fasteners have good corrosion resistance to Hainan rainwater environment.
2.3 Tensile force test of fasteners
Tensile force data of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V materials with different surface treatment conditions after immersion in Hainan rainwater for 168 h and Hainan atmospheric environment exposure for 20 d are shown in Fig. 5. The upper limit of the force analysis interval is the mean value of the original sample tensile force × 1.08, and the lower limit of the analysis interval is the mean value × 0.92. If the tensile force exceeds the interval after the immersion test and the exposure test, the abnormal value is determined, and the fastener function is determined to be invalid. The bolt breakage needs to be analyzed. It can be seen from the pull-off force data of Fig. 5 that the tensile breaking force of all fasteners made of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V materials does not exceed the interval of 8% deviation of the average breaking force before the test, which proves The tensile strength test results are higher than the standard allowable values, which can meet the 20-day use requirements in Hainan rain environment.
img 5 - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan

Fig.5 Pulling strength test results of 30CrMnSiA (a), 30CrMnSiN2A (b), GH4169 (c) and Ti-6Al-4V (d) fasteners

Comprehensive analysis of the apparent morphology and tensile force test results show that the 30CrMnSiA and 30CrMnSiNi2A material fasteners without surface treatment have local corrosion of the bolt head during the experimental process, but the mechanical test results can still meet the permissible requirements. The reason may be that the sealing effect of the composite structural member is better, and the corrosive liquid outside does not fully enter the internal bearing bolt portion, thereby having less influence on the mechanical properties of the fastener. Combined with the results of the apparent morphology analysis, the corrosion resistance of the fasteners after surface treatment and surface coating treatment was significantly improved, and the substrate material was well protected. In order to fully meet the requirements of use in Hainan rain environment, 30CrMnSiA and 30CrMnSiNi2A high-strength fasteners should be properly surface treated and surface coated, and fasteners of GH4169 and Ti-6Al-4V materials are sensitive to rain environment corrosion. It is weak and can be used without the mandatory requirements of surface treatment and coating.

Conclusion of high strength fasteners

The corrosion data of high-strength fasteners made of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V were acquired by means of immersion (168 h) test in simulated rain-water of Hainan island and field exposure in marine atmosphere in a side within 1 km from the coastline at Hainan.

  • (1) The tensile strength of 30CrMnSiA, GH4169, 30CrMnSiNi2A and Ti-6Al-4V materials with different surface treatment conditions is immersed for 168 hours in Hainan rainwater and 20 days after exposure to Hainan. .
  • (2) The corrosion tendency of 30CrMnSiA material body in Hainan rainwater environment and Hainan atmospheric environment is obvious. The surface treatment of Fe/Ep.Zn7.c2C and flZnAl12 can improve the corrosion resistance of fasteners to some extent, but still can not avoid the point. When the etch occurs, the surface varnish treatment can well protect the fastener body.
  • (3) The corrosion of 30CrMnSiN2A material body is obvious in Hainan rainwater environment and Hainan atmospheric environment. The surface treatment of flZnAl12 has a good protection effect on fasteners.
  • (4) GH4169 and Ti-6Al-4V material bodies have strong corrosion resistance in Hainan rainwater environment and Hainan atmospheric environment.
  • (5) The exterior of the fastener is well protected with alkyd varnish.

Source: China Fasteners Manufacturer – Yaang Pipe Industry (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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  • DOI:10.2514/3.60460
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study on environmental corrosion and protection of typical high strength fasteners in hainan - Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan
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Study on Environmental Corrosion and Protection of Typical High Strength Fasteners in Hainan
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The corrosion data of high-strength fasteners made of 30CrMnSiA, 30CrMnSiNi2A, GH4169 and Ti-6Al-4V were acquired by means of immersion (168 h) test in simulated rain-water of Hainan island and field exposure in marine atmosphere in a side within 1 km from the coastline at Hainan.
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