Improvement of corrosion protection in the area of ​​frequently disassembled steel fasteners
The purpose is to improve the corrosion protection level of the steel fastener parts that are frequently disassembled. Methods The salt spray test was used to investigate the effect of frequent disassembly on the corrosion of steel fasteners. Develop an improvement plan and verify it by accelerating corrosion tests. As a result, frequent disassembly and assembly will have obvious damage to the protective layer of the fastener zone. The corrosion protection level can be obviously improved by replacing the surface treatment method of the fastener, the protection of the hole wall, the assembly sealing and the spraying corrosion inhibitor. Conclusion The corrosion protection improvement scheme can effectively solve the corrosion problem of the steel fastener parts that are frequently disassembled, and can be promoted and applied in aircraft manufacturing and maintenance.
Through the corrosion inspection of aircraft in service in the marine environment, it is found that the fasteners and surrounding areas are highly susceptible to corrosion, especially steel fasteners that are often disassembled. The specific corrosion part includes both the fastener itself, the fastener hole wall, and the body structure around the fastener hole. Corrosion in the fastener area not only causes the fastener to be disassembled, but also causes the fastener to fail prematurely, which affects the safety of the aircraft. In addition, the local corrosion of the fastener area tends to be a source of corrosion of the body structure, causing extensive corrosion of the body structure. Therefore, it is necessary to take targeted protective measures for the parts of steel fasteners that are frequently disassembled.
1 Impact test on the corrosion performance of the fastener area
1.1 Test method
The single shear butt joint test piece is made by a typical connection form of a certain type of aircraft. The fastener is a navigational standard piece, the material is 30CiMnSiA, the washer and bolt are galvanized and passivated, and the nut is cadmium passivated. The test piece is made of 2A12 aluminum alloy, and the surface is anodized with sulfuric acid and sprayed with epoxy primer and polyurethane cool topcoat. A typical connection test piece is shown in Figure 1.
The fasteners were disassembled in advance, the number of times was 1, 5, 10, 20 times, and then the 90-h neutral salt spray test was carried out.
1.2 Test results
After a 90-h neutral salt spray test (5% mass of NaCl), the corrosion of the fastener area was observed.
The fasteners of the unassembled fasteners are basically intact, the parts of the nuts are exposed to wear and the exposed parts of the bolts become black, tarnish, and the walls of the fasteners maintain a metallic luster. After the salt spray test, the bolts that were disassembled once showed a small amount of red rust on the exposed parts, and the non-exposed parts lost their luster. The area of ​​red rust increased with the number of disassembly and assembly. After the gasket was disassembled for 10 times, the coating was obviously damaged, and white corrosion products appeared. After 20 times of disassembly, the coating disappeared and red rust appeared on the surface. The disassembled nut coatings lose their luster but are not corroded. The wall of the fastener hole after disassembly and assembly is tarnished, and grayish white corrosion product plaques appear on the surface. Corrosion of the fasteners for 1 time and 20 times of disassembly is shown in Figure 2.
Tests have shown that after the fastener is disassembled, corrosion products can penetrate into the fastener holes, causing corrosion of the hole walls and fasteners. The degree of coating damage on the surface of the fastener is proportional to the number of disassembly. The damage of galvanized bolts and gaskets is serious, and the cadmium-plated nuts are basically intact.
1.3 Analysis of corrosion causes
Frequently disassembled steel fastener areas are prone to corrosion. After analysis, there are several reasons.
1) The surface treatment layer of the fastener is poor in corrosion resistance. Upon inspection, it was found that the corrosion occurred mainly by steel fasteners with a diameter of 8 mm or less. The bolts and gaskets of this part are mainly treated by galvanizing in the domestic surface. The galvanized layer has not only poor corrosion resistance in the marine environment, but also easily falls off after disassembly and assembly.
2) The fastener hole wall is not protected. In the manufacture of the aircraft, if the surface of the fastener hole is not subjected to special surface protection, the corrosion medium will directly cause corrosion of the substrate.
3) The surface paint layer is invalid. After the fastening structure is disassembled, the coating at the hole and the joint surface of the fastener is broken. If the re-installation does not take effective supplementary protection measures, the corrosive medium can directly penetrate into the hole of the fastener and the joint surface, resulting in The fastener and the wall of the hole are corroded and extend from the hole of the fastener to the structure of the body.
Therefore, for the corrosion protection of the steel fastener connection area that is frequently disassembled, the work should be carried out from three aspects: improvement of fastener surface treatment, fastener hole wall protection, and improvement of assembly process.
2 corrosion protection improvement program
According to the analysis of the corrosion causes of the frequently disassembled steel fasteners, and with reference to the design and repair experience of aircraft corrosion protection at home and abroad, the corrosion protection improvement plan was formulated.
1) Improve the surface treatment process of fasteners. According to the test results, the corrosion resistance of cadmium plating in the marine environment is obviously better than that of the galvanizing process. Therefore, the gaskets and bolts originally used in the galvanizing passivation process are changed to the cadmium passivation process for surface treatment.
2) Surface protection of the fastener hole wall. For the wall of the aluminum alloy structural fastener without surface treatment, the surface treatment is treated with alodin and then coated with an anti-corrosive primer; for the wall of the steel fastener without surface treatment, a layer of phosphorus is applied. Primer.
3) Wet assembly of fasteners. After the fastener hole wall primer is cured, the fastener is wet-assembled with grease. The use of grease assembly not only serves as a sealing function, but also prevents the medium from penetrating into the gap, and the fastener is easily disassembled for easy field construction.
4) Spray corrosion inhibitor. After the fasteners are assembled, a layer of soft-film water-replacement corrosion inhibitor is sprayed on the surface of the fastening joint. Corrosion inhibitor is an important means of corrosion prevention and control. It has good permeability and water displacement. It can quickly penetrate into the gap of aircraft structure and replace the moisture in the gap and metal surface to the outside, so that the metal is isolated from moisture. At the same time, a protective film is deposited on the metal surface to effectively delay the corrosion of the metal material.
3 accelerated verification test
3.1 Test content
For the steel fastener parts that are frequently disassembled, the typical joints are made by the corrosion protection improvement scheme and the original design scheme, and the improvement scheme is verified by the accelerated corrosion test method.
3.2 Preparation of test pieces
The test piece adopts the typical single shear screw structure of the aircraft. The test piece plate is 2A12 aluminum alloy commonly used for aircraft structure. The fastener adopts 4mm diameter bolt and its matching gasket and nut. The surface treatment technology adopts the cadmium plating passivation process, and the galvanized fasteners are used for comparison verification. The fastener hole wall is coated with a layer of TB06-9 primer. The surface of the fastener holes and fasteners are coated with No. 3 compound calcium sulfonate grease during assembly, and the soft film water replacement type corrosion inhibitor is sprayed at the joint gap of the structure.
3.3 Test methods
Evaluation experiments using an accelerated test environmental spectrum can significantly shorten the research process. In the process of compiling the environmental spectrum, with reference to the accelerated test environment spectrum of the aircraft structure in the marine environment, the accelerated test environment spectrum of a typical aircraft is compiled, including two environmental modules: wet heat exposure and salt spray test. The accelerated test environment spectrum is shown in Figure 3.
In order to simulate the impact of disassembly and assembly on the protective performance of the fastener in the corrosive environment, the fasteners need to be disassembled and disassembled 20 times before the test, and disassembled and disassembled once after each cycle of the damp heat test and the salt spray test. Improve the protective test piece, and spray the soft film water replacement type corrosion inhibitor after reassembly.
3.4 Test results and analysis
Accelerated corrosion test 1. After the cycle, the original design of the fastener hole wall showed white corrosion products, the surface of the fastener was covered by red rust, as shown in Figure 4. The improved test piece was subjected to 12 cycles of accelerated corrosion test, and no corrosion occurred on the fastener hole wall and the fastener surface, as shown in Fig. 5.
4 application in a typical structural part of a certain aircraft
In combination with the overhaul task of a certain type of military aircraft, the protective measures for the detachable flap were improved. After the aircraft was overhauled, it has been in service for 5 years. The rectification parts have been dismantled and dismantled many times without corrosion.
5 Conclusion
1) By adopting comprehensive protection measures for surface protection and assembly sealing, it can effectively solve the corrosion problem of steel fastening joints that are frequently disassembled, and can be promoted and applied in aircraft manufacturing and maintenance.
2) Protective measures such as painting the wall of the fastener hole and supplementing the spray corrosion inhibitor after assembly can be referenced in the design and repair of corrosion protection of other joint structures.
Article source: "Equipment Environmental Engineering"
Sun Yidong Wang Xiaolong(China Special Aircraft Research Institute, Jingmen, Hubei 448035)
Butterfly Valve,Wafer Type Butterfly Valve,Triple Offset Butterfly Valve,Wafer Butterfly Valve
LIANGGONG VALVE GROUP CO.,LTD , https://www.lgvalvegroup.com