The corrosion of materials is associated with the design of materials and has always affected their service life. With the development of high-end equipment in China, the application environment of materials is increasingly harsh, and higher requirements are put forward for the protection of materials. As one of the core contents of national defense construction, military equipment has the characteristics of many kinds, large quantity, long storage time and complex and diverse application environment. In particular, large equipment such as aircraft, ships and nuclear power are usually required to be able to be used in a harsh environment for a long time, and the corrosion of any part will bring safety risks to the entire equipment and affect the operation and service of the equipment.

 

In many natural environments, the ocean is a very harsh corrosive environment, and the protection of ordinary material oxide film for the Marine environment is weak. According to incomplete statistics, Marine corrosion loss accounts for about 1/3 of the total corrosion loss of materials, so the loss caused by Marine corrosion is much higher than that of other environmental corrosion. In view of the effects of mechanical corrosion, electrochemical corrosion and biological corrosion in the Marine environment, its protection is mainly divided into three aspects: material selection and reasonable design, material surface protection, and cathodic protection of impressed current or sacrificial anode.

 

In addition, the aging of materials in the polar plateau region, wind erosion and abrasion, as well as high temperature and pressure in nuclear reactors and irradiation are also worthy of attention.

 

1. Marine atmospheric environment corrosion and protection

 

Marine atmospheric corrosion is mainly caused by thin liquid film in humid atmospheric environment, and corrosion mostly occurs in high temperature and high humidity coastal areas. Especially when the high temperature and high humidity Marine atmosphere contains acidic pollutants or salt particles, the corrosion problem will be further aggravated. The high temperature and high humidity atmospheric environment will cause corrosion of metal substrate, such as magazine plaque corrosion, which often occurs in the high temperature and high humidity Marine atmospheric environment of small weapons. At the same time, it is easy to cause the failure of the protective coating of the equipment, such as coating aging, under film corrosion, bubbling and falling off during the storage of the projectile. High temperature and high humidity atmospheric environment will also cause rubber, plastic and other non-metallic materials deformation, brittleness, cracking, swelling, mildew and other phenomena.

 

Coating surface protection technology is one of the widely used and effective anti-corrosion technologies to protect military equipment. The design and selection of surface protective coating should fully consider the different application environments of military equipment, and the coating protection system with special functions should be studied according to the actual needs. For the surface of the ship easily soaked by seawater, the use of arc spraying zinc, aluminum coating to give the ship's metal surface excellent corrosion resistance to seawater. In response to the adhesion and corrosion of microorganisms in seawater, intelligent coatings with anti-fouling and bactericidal functions are used in the shipbuilding industry. In addition to the above common surface coating technologies, other surface protection technologies, such as amorphous alloy chemical coatings and nanoparticle composite coatings, have also shown great application potential.

 

At present, graphene coatings and self-healing coatings are the focus of Marine anti-corrosion coatings research in recent years. For the study of graphene, the oxidation resistance of the graphene coating prepared by Chen et al. is greatly improved compared with the original Cu/Ni matrix. Research on graphite coatings mainly focuses on organic coatings and inorganic coatings. Prasai et al first invented the method of preparing graphene coatings, that is, using polymethyl methacrylate as the intermediate medium to prepare the required graphene coating on the surface of the matrix, and the corrosion resistance of the material has been greatly improved.

 

In addition, graphene can also be used to modify existing coatings, for example, the Ningbo Institute of Materials of the Chinese Academy of Sciences added graphene to water-based epoxy coatings, compared with pure epoxy resin anticorrosive coatings, its coating performance has been improved. On the other hand, the application of graphene modification in the field of inorganic coatings is gradually increasing, Wan Chunyu et al. found that adding graphene to inorganic anti-corrosion coatings, in the case of coating amount of only 100 to 150mg/dm², the salt spray resistance of the coating can be as high as 1200h, indicating that its anti-corrosion performance has been greatly improved. Shen Haibin et al. added graphene instead of chromium metal to the Dachro coating, and the coating can also have good corrosion resistance, and is also very friendly to the environment.

 

Self-healing anti-corrosion coating is a kind of new intelligent protective coating. When the coating is damaged, the coating recovers its anti-corrosion performance under certain conditions. The existing self-healing coatings are mainly divided into two categories: self-healing and non-self-healing. Self-healing coatings often use embedded film forming substances or corrosion inhibitors to repair damaged coatings.

 

2. Deep sea environment corrosion and protection

 

The dissolved oxygen content, salinity, temperature and biological conditions of seawater in deep sea are different from those in shallow sea, and the corrosion behavior of materials in deep sea is different from that in shallow sea. In terms of dissolved oxygen in seawater, as the depth of the ocean increases, the dissolved oxygen in the ocean gradually decreases, but due to the supplement of ocean currents, the dissolved oxygen in the deep sea area will gradually increase. Secondly, the salinity in the deep sea domain is about 35PSU and does not change with the depth of the deep sea.

With the increase of the depth of the ocean, the temperature of the sea water gradually decreases, which will reduce the reaction speed of the cathode and anode process, and can reduce the diffusion rate of oxygen, and the corrosion of materials in the deep sea environment has slowed down, especially metal materials such as carbon steel. In addition, due to the existence of microorganisms, the corrosion of metal materials is more serious in the shallow sea, and the number of Marine microorganisms in the deep sea field is rare, and the corrosion of materials is mostly caused by anaerobic bacteria, which occurs in the deep sea bottom. By building a series of deep-sea simulation devices, researchers at Northeastern University found that the electrochemical corrosion in the deep-sea environment is more serious, and the performance of the sacrificial anode is reduced, and the galvanic corrosion is intensified.

The main corrosion protection measures for deep-sea equipment include anti-corrosion coating and cathodic protection. Different from the shallow sea anticorrosive coating, the protective properties and service life of the coating in the deep sea environment are mainly related to the permeation behavior of the coating under high water pressure. In foreign countries, systematic experiments and summaries of corrosion problems of various types of equipment in the deep-sea environment are mainly based on epoxy coatings, but no relevant standards have been established for pressure resistant anti-corrosion coatings for deep-sea equipment. For domestic deep-sea equipment anticorrosive coating, the service life is generally not less than 15 years, but there is still a certain gap between its service life and the requirements, and the domestic deep-sea equipment anticorrosive coating mainly includes epoxy resin anticorrosive coating, fluorocarbon anticorrosive coating and silicone resin coating.

 

Compared with the shallow sea, the cathodic protection system in the deep sea environment has changed greatly because of the problem of water pressure. As far as the sacrificial anode protection is concerned, its corrosion resistance decreases in the deep-sea environment. In order to solve the protection problem, foreign researchers have carried out systematic test and research in different sea areas and different depths to develop more suitable sacrificial anode materials. Domestic research on cathodic protection in deep-sea environment mainly focuses on protection mechanism and influencing factors, and obtains a complete big data.

 

3. Extreme cold, plateau corrosion and protection

 

Extreme cold, high altitude areas are harsher than common climates. The plateau environment is mainly characterized by extreme high and low temperature and high irradiation intensity. Low temperature and high wind speed are also polar climate characteristics, such as the Arctic region, the lowest temperature of -60 ℃, the maximum wind speed of 50m/s. In addition, the polar environment also has problems such as fragmentation of ice erosion.

 

The anti-corrosion organic coatings in extremely cold and plateau areas include traditional epoxy coatings, alkyd coatings and polyurethane coatings. Acrylic paint also has good weather resistance, and its gloss retention is better than epoxy paint and alkyd paint, which is suitable for use at lower temperatures. Although the extremely cold and plateau environment is low temperature and dry, the ozone concentration is high, the ultraviolet irradiation is strong, the coating aging is faster, the adhesion is reduced, the color is discolored, the powder is powdered, and the luster is lost. The research focus on the extremely cold environment is to achieve the purpose of anti-icing by reducing the adhesion of the coating surface, which is mainly divided into three categories: sacrificial coating, ice-phobic coating and super-hydrophobic coating.

 

Stress corrosion fracture of metal equipment and components is also one of the key problems in the field of polar and high cold anticorrosion. Stress corrosion generally occurs in the medium of low stress and weak corrosion, the fracture failure occurs suddenly, and the harm is great. The metal components of aircraft, such as door frame, wing SPAR, propeller hub, etc., will suffer serious damage due to stress corrosion fracture.

 

Sand and dust in the high desert can cause serious abrasion problems for military equipment, systems and airborne equipment. For example, dust and debris trapped in turbulent air induced by helicopter rotors can wear down moving parts, metal surfaces and coatings of aircraft. Fine dust particles easily enter the fuselage and destroy the precision metal structure and electronic systems inside the body. The composite coating with elastic polyurethane topcoat can effectively resist abrasion of hard materials such as sand and stone for aircraft aluminum base skin and other metal structural parts which are easily subjected to sand wear. Laser cladding of copper - and nickel-based alloy coatings with high hardness and good wear resistance is used to delay the wear failure of the control ring of gun recoil machine.

 

4. Nuclear radiation corrosion and protection

 

For the development of nuclear power technology, China's research direction is mainly focused on the fuel cladding used in nuclear power equipment, power generation equipment and so on. However, in the case of nuclear reactors, the corrosion caused by high temperature, high pressure and strong irradiation puts forward more stringent requirements for the selection of materials.

 

Zirconium alloy clad fuel systems have been successfully used in light water reactors for more than 40 years, showing good radiation and corrosion resistance. In the harsh environment of light water reactor, high temperature resistance and corrosion resistance of cladding materials and other equipment are required, and the most critical thing is that the materials can also resist radiation damage. According to the existing research results, the replacement materials of zirconium alloy are mainly divided into two categories: one is Fe based alloy materials based on FeCrAl; The other is ceramic materials such as SiC/SiC composites, MAX phase ceramic materials and so on. However, whether it is a new type of alloy material or ceramic material, there are certain problems, as far as Fe base alloy material is concerned, its machining and welding need to be further studied; However, ceramic materials are difficult to be designed as cladding structures because of their inherent characteristics, such as high brittleness and low strength.

 

At the same time, accident tolerant coatings for zirconium alloys are being developed, which can provide the necessary protection under abnormal high temperature or LOCA conditions, and can also improve the material's resistance to high temperature, high pressure, water vapor and other properties in LWRs. The new terpolymer layer structure MAX phase ceramic material has high Young's modulus, low Vickers hardness and shear modulus, easy processing, good thermal and electrical conductivity. The oxidation-induced product of the MAX phase material containing aluminum is Al₂O₃, which is densely covered to form an oxide film due to its coefficient of thermal expansion being the same as before oxidation. Based on the radiation tolerance and structural stability of MAX phase materials, they can be applied to advanced nuclear reaction systems. The higher resistance of MAX phase materials to radiation damage allows current nuclear reactors to operate at higher temperatures.

 

Material protection technology under ordinary environment can no longer meet the needs of mechanical equipment under harsh environmental conditions such as high-speed, high temperature, high pressure and heavy load in the increasingly developing Marine, aviation, aerospace and nuclear energy industries. Special coating technologies such as graphene heavy anti-corrosion coating, anti-icing coating and self-healing intelligent coating developed in recent years have solved the long-term and reliable protection problems of materials in the Marine atmosphere, polar environment and nuclear power environment, and effectively supported the research and development of major equipment in China's Marine, aerospace, shipbuilding, nuclear power and other industries.

 

With the development of high-end machinery and equipment, the environment faced by materials will become more demanding, and the future material surface protection technology will develop in the direction of multiple functional integration to adapt to the complex and changeable environment, as well as ultra-long life