At present, the service life of military aircraft is generally more than 20 years, with the increase of military aircraft service time, corrosion will gradually become one of the main forms of structural damage. In particular, military aircraft deployed in coastal areas have been exposed to high humidity, high salt, rain, high stress, high frequency and other environments for a long time, resulting in serious corrosion of metal material structures. For the more hidden body structure corrosion, the corrosion repair space is small and difficult, and it is necessary to decompose or replace large parts when necessary, and the repair cycle is difficult to guarantee, which directly affects the reliability and completeness of the equipment.

 

In order to prolong the service life of military aircraft, ensure the normal combat training and combat tasks of the troops, and predict the hidden danger of corrosion in advance, it is necessary to master the corrosion types and protective repair measures of the commonly used metal materials of military aircraft.

 

 

I. Types and characteristics of corrosion of metal materials of military aircraft

 

1. Corrosion types of metal materials

 

According to the mechanism of corrosion, corrosion can be divided into chemical corrosion and electrochemical corrosion.

 

Chemical corrosion refers to the destruction caused by pure chemical reaction between metal materials and other substances (water, oxygen, sulfur dioxide, kerosene, gasoline, etc.). Usually occurs REDOX reaction, resulting in corrosion products, such as metal materials on military aircraft are damaged by chemical reactions affected by rain or humid air.

 

Electrochemical corrosion refers to the corrosion caused by the electrochemical action of metals in the electrolyte solution, which mainly occurs at the edge of metal contact with different potentials. Electrochemical corrosion is the most serious, the most common corrosion, as long as there are two different kinds of potential metal, two different potential metal contact, electrolyte action of these three conditions will occur electrochemical corrosion, such as titanium alloy and aluminum alloy and other components overlap, different materials skin seam riveting, lap wire and structural joints are very easy to occur electrochemical corrosion.

 

2) Classification by corrosion form

 

According to the form of corrosion can be divided into comprehensive corrosion and local corrosion.

 

Comprehensive corrosion refers to the uniform distribution of corrosion on the surface of the metal material, so that the entire metal material structure becomes thin, affecting the strength and stiffness of the metal structure, and finally failure. Both chemical corrosion and electrochemical corrosion will cause comprehensive corrosion, which will thin the structure of metal materials and reduce the strength and stiffness. Overall corrosion of military aircraft structure usually occurs outside the aircraft structure, easier to detect and find, generally will not cause major accidents, compared with local corrosion its risk is less. Comprehensive corrosion is mostly caused by the long-term interaction between the structure surface and the external corrosion environment without protective coating.

 

Local corrosion is concentrated only in a specific part or area of the metal structure and almost no corrosion occurs in most other areas. According to the relevant data statistics show that the common local corrosion of aircraft is slot corrosion, small hole corrosion, point corrosion, intergranular corrosion, filamentary corrosion, stress corrosion, galvanic corrosion and so on. Compared with full corrosion is more hidden, difficult to detect and find, more dangerous.

 

2. Characteristics of metal corrosion

 

The color characteristics and appearance characteristics of corrosion of common metal materials in military aircraft are shown in Table 1 and Table 2.

 

Table 1 Color characteristics of corrosion of common metal materials

 

 

Table 2 Appearance characteristics of corrosion of common metal materials

 

 

Second, easy to corrode parts and reasons

 

1. Landing gear bay and landing gear

 

The landing gear compartment is a non-sealed compartment, which is often switched on and off during the take-off and landing process and the parking state of the aircraft. The external environment exposed to the landing device is complex, and it is easy to be polluted by external humid air, saline water, sand, soil or other foreign objects, resulting in corrosion of the metal structure. At the same time, there are many load-bearing structures inside the landing gear compartment, which are easy to corrod under the combined action of external forces and corrosive environment

 

2. Bilge area

 

The bilge of the aircraft fuselage is a place prone to corrosion. Corrosive media such as water, oil, organic gas and moisture leaking from other cabins accumulate for a long time, causing the anti-corrosion paint layer of the bilge to fall off, resulting in structural corrosion. In the process of daily maintenance, it is difficult to reach the part visually, and it is easy to miss.

 

3. Skin

 

Under normal circumstances, the primer and top paint on the skin surface are well protected and are not prone to corrosion, but when the protective paint layer is seriously scratched and damaged due to assembly, repair, construction or other reasons, corrosion will quickly occur under the action of external rain, humid air, polluted gas and other corrosive environments, especially at the connection between the skin and the girder. Under the influence of accumulated dirt, moisture and other corrosive media, large-scale corrosion is easy to occur, resulting in more corrosion products, resulting in skin bulges.

 

4. Bearing

 

In the process of manufacturing and assembly, due to the poor quality of bearing installation, clearance adjustment is not in place, resulting in long-term friction between bearings and shafts, coupled with improper daily maintenance, insufficient lubrication, such as the selection of lubricants does not meet the relevant technical requirements, will cause long-term dry grinding heat bearing, in the natural environment in the corrosive medium phase is prone to corrosion.

 

5. The whole machine exhaust hole, drainage hole and heat dissipation hole

 

The vents, drainage holes, heat dissipation holes and the surrounding areas on the aircraft are the accumulation areas of water and water vapor, which provides favorable conditions for corrosion.

 

 

Third, corrosion detection

 

1. Visual inspection

 

Visual inspection is the most commonly used and direct and effective way of corrosion inspection. Visual inspection can improve the accuracy of inspection with the help of tools such as flashlight, reflector, magnifying glass and industrial endoscope. The disadvantage of this method is that it can not detect hidden corrosion, and it is difficult to quantitatively analyze corrosion damage, which is generally used as the first step of corrosion inspection. When corrosion is suspected inside the structure, other non-destructive testing means can be used to confirm and quantitative analysis.

 

2. Tapping method

 

The tapping method is a method to detect corrosion by tapping the inspection part with the tapping rod. The principle is that the corrosion changes the cohesion and strength of the material structure and thus changes the frequency of sound resonance. Therefore, the corrosion of the detected part can be determined by judging the sound of the tapping. Although this method can detect hidden corrosion to a certain extent, the accuracy is not high, and it depends on the experience and level of the inspector.

 

3. Eddy current testing

 

Eddy current detection method uses the principle of electromagnetic induction to detect current changes in coils. Due to the difference in eddy currents between corroded structures and non-corroded structures, this method can detect defects by comparing current changes in coils. The disadvantage is that it is only applicable to conductive structures, and due to edge effect, the detection sensitivity is low near the edge, and it is not applicable to hard magnetic materials.

 

4. X-ray detection

 

X-rays have different penetrations to different materials, forming different negatives, and corrosion on the negatives will mostly show irregular, uneven edges of spots or blocks. X-ray detection is suitable for complex structures and internal defects, and can be in situ inspection. The disadvantage of this method is that the equipment is expensive, the operation is professional, and the radiation is harmful to the human body.

 

5. Ultrasonic detection

 

Ultrasonic detection method is to use the nature of ultrasonic propagation in different media to determine the corrosion of the measured structure, suitable for all materials, all types of damage detection, can accurately locate the size and position of corrosion, but different materials should use different detection frequencies. The disadvantage of this method is that the detector should have rich operating experience and professional ability, and the coupling agent should be added between the test part and the probe.

 

Fourth, corrosion protection

 

1. Adopt anti-corrosion design

 

1) For the upper part of the aircraft structure, the cover mouth frame is used in the form of sealant to prevent rain from entering the fuselage structure, and there are many water leakage holes and oil leakage holes in the fuselage lower structure.

 

2) In order to prevent electrochemical corrosion between metals of different potentials, a layer of anti-corrosion insulation layer is added between different metal components, so that the two can not be directly in contact, so that there is no conditions for electrochemical corrosion.

 

3) Adopt anodic oxide film protection layer and apply corresponding protective paint layer for various complex shapes of rocker arms, conduit joints, compressor blades and structures in high temperature areas.

 

2. Create a protective layer

 

Metal protection mainly refers to the protection of alloy, the most commonly used and most effective method is to establish a protective layer on the surface of the alloy to isolate the electrolyte, so that it does not have electrochemical corrosion conditions. For example, the galvanized layer of steel parts is commonly used as the protection of screws, nuts, and actuating cylinder shells, the cadmium plating layer is commonly used as the protection of springs, screws, nuts, etc., and the chromium plating layer is often used to make the coating of steel parts to withstand friction.

 

3. Perform regular maintenance

 

1) For the dust on the surface of the aircraft structure and other debris such as metal chips left over from the construction process, according to the requirements of the process regulations, clean rags and washing oil can be used to regularly clean and maintain the oil, dust, water and other impurities on the aircraft structure, or non-corrosive gases such as nitrogen can be used to blow away.

 

2) The aircraft structure on the vent hole, drainage holes, corners, grooves, threads and other parts of the dirt, should be regularly wrapped in a clean cloth on the skewer to scrub clean, in the rainy season or more sand can shorten the inspection and maintenance cycle, too much oil can be cleaned with oil washing liquid, and then wipe the oil with a rag or gas dry.

3) When there is slight rust on the surface of the aircraft structure, it can be wiped with a clean rag dipped in kerosene. If the rust is serious, it can be rubbed with sandpaper dipped in No. 2 low temperature grease.

4) The anti-corrosion coating on the surface of the aircraft is thin, the hardness is small, and it is easy to be damaged by collision and friction during use, exposing the metal matrix. The surface of the machine can be evenly smearing with oilcloth or brush, and the crevices, corners, grooves and threads on the machine can be smeated with brush and grease.

 

The military aircraft deployed in the coastal areas are in the environment of high humidity, high salt and fog for many years, coupled with the adverse environmental effects of rain, wind and sand, which makes it difficult to completely avoid the corrosion problem of military aircraft. However, the daily corrosion protection and repair work of aircraft can be increased by being familiar with the corrosion types of metal materials commonly used in military aircraft and the protective and repair measures. The corrosion problem of the general aircraft body structure is controlled within the acceptable range, so as to ensure the reliability and completeness of the equipment, and meet the normal combat training and combat tasks of the troops.

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