Once the corrosion of aircraft materials occurs, it will not only hinder the performance of the aircraft itself, but also cause great loss to people’s lives and property. Corrosion is a surface phenomenon of the interaction between material and environment. electrochemical corrosion occurs at the solid-liquid interface between solid material and electrolyte solution, and chemical corrosion occurs at the solid-gas interface between solid material and ambient gas environment. Therefore, the systematic and comprehensive analysis of a corrosion problem requires, on the one hand, a detailed analysis of the characteristics of the material itself, that is, the intrinsic factors; on the other hand, a comprehensive analysis of the surrounding environment, as well as the medium in the environment and the state of its existence, the external cause of corrosion.

1. Corrosion resistance of aluminum alloys aluminum chemical property is very active, but because it combines easily with oxygen in air, it forms a dense and stable alumina film (passivation film) on the surface, which can protect the inner metal from further oxidation, therefore, pure aluminum in the atmosphere has a good corrosion resistance. However, the tensile strength of annealed pure aluminum is rather low, only 45mpa, so its application is limited to the non-structural parts in aircraft structure. Aluminum alloys used in aircraft construction can be roughly divided into two types: heat-treatable and non-heat-treatable.

The non-heat-treatable Aluminum Alloy (1) Al-Mn series (3000 series) is commonly used as 3A21 aluminum alloy. Manganese is the main alloy element in the alloy. It has high strength, good plasticity and technological properties. The microstructure of 3A21 alloy at room temperature is a solid solution and (α + Al6Mn) eutectic at grain boundary. The corrosion resistance of the alloy is good because the electrode potential of α solid solution is almost equal to that of AL and MN phases. The disadvantage of this kind of alloy is that it can not be used when the temperature is higher than 100 °C, because it is more sensitive to stress corrosion cracking (SCC) . (2) Al-Mg series alloys (5000 series) have been widely used in the aviation industry due to the fact that the density of magnesium, the main alloy element, is smaller than that of aluminum, as well as its good weldability and anti-seismic properties, commonly used 5A02,5A03,5A06 and other aluminum alloys. The strength of this kind of alloy is generally higher than that of 3A21 al-mn alloy. In practical use, the material is a single-phase solid solution structure, so it has good corrosion resistance. Its corrosion resistance in air and sea water is better than that of 3A21 alloy and equal to that of pure aluminum, while in acid and alkaline medium, its corrosion resistance is slightly worse than that of 3A21 alloy. The higher the magnesium content, the higher the strength. But its magnesium content should not be too high, should be controlled within 8% (mass fraction) . The reason is that when the magnesium content is higher than 8% , if the magnesium content is annealed, the Mg5Al8 phase will precipitate continuously on the grain boundary, which leads to the deterioration of its corrosion resistance (intergranular corrosion and stress corrosion) .

Heat-treatable aluminum alloys, which have high strength and are also called duralumin alloys, are one of the most important structural materials in aviation. In general, the strength of aluminum alloy decreases with the increase of temperature. When the aircraft speed is high, the aerodynamic heating will affect the strength of aluminum alloy. The corrosion resistance of aluminum alloy is weakened, and the common corrosion forms are pitting corrosion, crevice corrosion, intergranular corrosion, denudation corrosion and stress corrosion. Pitting corrosion is one of the most common corrosion forms of aluminum alloy. Aluminum Alloy is prone to crevice corrosion because water and dirt can accumulate in crevice, which causes hydration of coating and aluminum coating and reduces its protective effect. Al-Cu, Al-Cu-mg and Al-Zn-Mg alloys tend to produce intergranular corrosion. The intergranular corrosion is related to heat treatment process. The corrosion tendency of natural aging is lower, the corrosion tendency of artificial aging is higher, and the corrosion tendency of overaging is lower. The intergranular corrosion sensitivity of Al-Cu-Mg hard aluminum alloy is higher than that of T3, so the alloy is usually treated by natural aging except the component which works at high temperature. In aircraft structure, Al-Cu-Mg alloys have the most denudation, and Al-Mg, Al-Mg-si and Al-Zn-Mg alloys also have denudation, but no denudation occurs in the deformed Al-Si system. Al-Cu, Al-Cu-mg duralumin alloys, especially Al-Zn-Mg, Al-Zn-Mg-cu super-duralumin alloys, are prone to stress corrosion.

(1) Al-Cu-Mg and Al-Cu-Mn alloys (2000 series) Al-Cu-Mg alloys are one of the most important series of heat-treatable strengthened aluminum alloys, and the most widely used alloy in aircraft structures is 2024 aluminium alloy, the main strengthening phase of these alloys is CuMgAl2 and Cual2, which are usually used in the state of t 3 and have the characteristics of high fracture toughness and high fatigue crack Propagation Resistance. However, this state of corrosion (intergranular corrosion) performance is not good enough, generally after the use of thin aluminum coating, but also with anodizing treatment, Aladdin chemical treatment to further improve its corrosion resistance. The newest and best alloy in the 2000 series is the 2524 aluminum alloy, which has been successfully used in the B777 aircraft due to its much improved toughness and fatigue resistance compared to the 2024 aluminium alloy. (2) the most important alloy of Al-Zn-Mg-Cu series (7000 series) is 7075 Aluminium Alloy, which has the highest strength under the condition of t 6, but the lowest fracture toughness and poor corrosion resistance, especially to intergranular corrosion and stress corrosion. In order to enhance its corrosion resistance, it is necessary to carry out over-aging treatment, commonly T73 treatment, that is, first of all, solid solution treatment of aluminum alloy, then it is subjected to a two-stage aging process (heating and holding at lower temperatures for a period of time, followed by heating and holding at higher temperatures for a period of time) . After this treatment, although the tensile strength of the material decreased by about 15% , the stress corrosion resistance and intergranular corrosion resistance of the material were greatly improved. 7055 is the aluminum alloy with the highest degree of alloying and the highest strength at present. The T77 treatment process has been successfully studied recently, which makes the alloy maintain high fracture toughness and good resistance to stress corrosion cracking at high strength, the alloy has been successfully applied to the main structure of the B777 airliner. It should be pointed out that the process of solution treatment (quenching) should be strictly controlled before the two-stage aging treatment, otherwise the intergranular corrosion resistance of the material will be seriously affected.

Modern aircraft structures rely heavily on aluminum alloys, which are also used for aircraft skeletons, skins, and stressed parts. In the modern subsonic civil aircraft, the aluminum alloy material is still the aircraft components of the pillar material, and constantly introduced updated aviation aluminum lithium alloy and so on. As a result, the important role of aluminium alloys in aviation will continue for many years. Most of the structural parts of Airbus A380 aircraft are made of new and advanced metals, with aluminium alloys accounting for the largest proportion (61% of the airframe’s structural mass) , particularly in the A380 wings (more than 80% of which are made of Aluminium Alloy) . In order to improve properties, increase strength and damage tolerance, enhance stability and improve corrosion resistance, an innovative aluminum alloy material and process technology has been developed. The main achievements of A380-800 aircraft in aluminum alloy structure include: (1) the introduction of a wide sheet metal material on the fuselage panels, reducing the number of connectors, thus reducing the quality; The use of advanced al-li alloy extrusions on the beams of the main floor is comparable to that of carbon fiber reinforced plastics. The A350, which went into service in 2014, also uses the latest Aluminium Lithium Alloy, which has been reduced in mass due to a reduction in material density, at the same time, the parts of the new material can be repaired by the same technology and method as the existing aluminum alloy parts. The corrosion resistance of titanium alloys is not only rich in titanium resources, but also has low density, high specific strength, high heat resistance and excellent corrosion resistance, therefore, titanium and its alloys are widely used in the fields of aviation, chemical industry, missile, aerospace and ship.

Titanium alloys are widely used in aircraft structures and non-structures (see table 1) . The wide use of titanium alloys in aviation is based on one or more of the following reasons:

 (1) excellent corrosion resistance, without pitting during corrosion. 、

(2) high specific strength. 

(3)High operating temperature.

 (4) weight reduction, density about 40% less than steel.

 (5) reduction of space constraints. 

(6) compatibility with other materials.

Pure titanium has high strength (tensile strength of commercial pure titanium is 550 ~ 700MPa after annealing) , which is about 6 times that of aluminum. Titanium has the advantages of both steel (high strength) and aluminum (light weight) , so the specific strength of titanium is very high in structural materials. The linear expansion coefficient of titanium is small, and the thermal stress produced in high temperature or hot working process is small; the thermal conductivity is bad, only 1/5 of that of iron; the friction Coefficient is big (μ = 0.42) , so it is difficult to cut and grind; the elastic modulus of titanium is low, and the yield strength is high, therefore, titanium and its alloys have great resilience in cold deformation processing, and are not easy to form and straighten; pure titanium has good plasticity, its toughness is more than 2 times of pure iron. Industrial pure titanium can be divided into three brands: TA1, TA2 and TA3 according to its impurity content. The number of grades increased, the impurity content increased, the strength of titanium increased, and the plasticity decreased. To increase the strength of a titanium alloy, alloying elements can be added to the titanium. Alloy elements melt into Α-ti to form α-solid solution and Β-ti to form β-solid solution. Elements such as AL, C, N, O and B raise the temperature of α and β isomorphism transition, which is called α stabilized element, while elements such as FE, MO, MG, CR, MN and V lower the temperature of α isomorphism transition, which is called β stabilized element Tin, zirconium and other elements have no obvious effect on the transition temperature, so they are called neutral elements.

Titanium alloys can be divided into α titanium alloy, β titanium alloy and (α + β) titanium alloy. (α + β) titanium alloys have the advantages of both α and β titanium alloys, good heat resistance and plasticity, and can be strengthened by heat treatment. Therefore, (α + β) titanium alloy is widely used, especially TC4(TI-6AL-4V) . The stability of titanium alloy oxide film is much higher than that of aluminum and stainless steel. When the protective film is damaged by mechanical operation, it can be recovered quickly. Therefore, titanium and titanium alloys have high corrosion resistance in many highly active media.

The corrosion resistance of high strength and high quality alloy structural steel is better than that of carbon steel in humid industrial atmosphere and marine atmosphere. However, in the absence of protective measures, they still have a variety of carbon steel corrosion tendency. Generally in the air and neutral medium can be corrosion-resistant steel called stainless steel, and corrosion-resistant steel can work in a variety of strong corrosive medium called acid-resistant steel. Usually the stainless steel and acid-resistant steel collectively known as stainless acid-resistant steel, referred to as stainless steel. Stainless Steel “Stainless”is only relative, in certain conditions, stainless steel will also corrosion, so there is no absolute “Stainless”stainless steel. According to the internal microstructure, stainless steel can be divided into Martensitic stainless steel, ferritic stainless steel, Austenitic stainless steel and duplex stainless steel. MARTENSITIC stainless steel Martensitic stainless steel is a kind of chromium stainless steel with high carbon content. Its carbon content ranges from 0.1% to 0.9% , and its chromium content ranges from 12% to 18% . This type of steel is characterized by higher carbon content than other types of stainless steel, in addition to the addition of alloy element chromium, sometimes also added a small amount of molybdenum or nickel, such as 1Cr17Ni2,9Cr18mov and so on. The corrosion resistance of Martensitic stainless steel is worse than that of ferritic stainless steel and Austenitic stainless steel, and the higher the carbon content, the worse the corrosion resistance. MARTENSITIC stainless steel is resistant to corrosion in air, seawater and oxidizing media, but not in non oxidizing acids such as sulfuric acid and hydrochloric acid. MARTENSITIC stainless steel are less resistant to local corrosion, such as pitting, intergranular and stress corrosion, and are more sensitive to hydrogen embrittlement. Therefore, they should be avoided in environments where local corrosion may occur.

FERRITIC stainless steel ferritic stainless steel is a kind of iron-based alloy with body-centered cubic crystal structure, such as CR13, CR17 and CR25 ~ 28, which mainly consists of chromium (12% ~ 18%) , ni, MO, Cu, Ti and NB are often added to improve the corrosion resistance. The most striking feature of ferritic stainless steels is their excellent resistance to stress corrosion in aqueous solutions containing Cl-ions, which is much better than Austenitic stainless steel. Common ferritic stainless steel has poor resistance to pitting and crevice corrosion, and its performance can be improved by increasing CR content, for example, when CR content reaches 25% or above, its resistance to pitting and crevice corrosion can be improved. In addition, the pitting and crevice corrosion resistance of ferritic stainless steel can be improved by adding Mo element. The chromium content and nickel content of Austenitic stainless steel Austenitic stainless steel are above 18% and 8% respectively, and they have single phase austenite structure at room temperature. AUSTENITIC stainless steel is the most important and widely used type of stainless steel, which has not only excellent corrosion resistance, but also excellent comprehensive mechanical properties, technological properties and welding properties. Type 18-8 stainless steel (i.e. 17% ~ 19% chromium, 7% ~ 9% nickel) is a common Austenitic stainless steel.

There are both austenite and ferrite phases in the room temperature microstructure of austenitic-ferritic duplex stainless steel. It combines the excellent toughness and weldability of Austenitic stainless steel steel with the high strength and resistance to chloride stress corrosion cracking of ferritic stainless steel. Compared with the pure Austenitic stainless steel, the austenitic-ferrite duplex stainless steel has lower intergranular corrosion sensitivity, that is, it has good anti-intergranular corrosion performance. At low stress level, austenite-ferrite duplex stainless steel shows better resistance to SCC than Austenitic stainless steel, but the resistance to SCC decreases with increasing stress, not even a Austenitic stainless steel. AUSTENITIC-FERRITIC DUPLEX STAINLESS STEEL has higher pitting corrosion resistance. In general, stainless steel corrosion resistance is high. But stainless steel in the medium containing chloride, due to the role of chloride ion, stainless steel passivating film in the weak area, the location of defects and sulfides inclusion or grain boundary carbide place. It is easy to produce crevice corrosion in the tiny crevice where the stainless steel component is connected with other components.

The corrosion resistance of composites mainly includes Resin Matrix corrosion, reinforcement corrosion, interface corrosion, stress corrosion and corrosion fatigue. Non-metallic materials, such as plastic, rubber and so on, can not conduct electricity, generally speaking, corrosion resistance is higher than metal materials, so non-metallic materials are also widely used as a protective layer to improve the corrosion resistance of metal materials. In the aspect of aircraft materials, with the improvement of safety, economy, comfort and environmental protection of civil aircraft, composite materials are widely used. Models such as the Boeing B737 and Airbus A320 already use composites based on epoxy, reinforced with carbon fiber, glass fiber and Kevlar. The main types of fibers used in aircraft composites are carbon fiber and boron fiber, and the main matrix material is epoxy. There are several types of composite structures in aircraft: reinforced lay-ups attached to metal structures, frames composed of wrapped pipe fittings, composite sandwich structures, reinforced or unreinforced skin structures, wound rotating casing or pressure vessel. Composite Structure must meet the flight temperature, humidity, ultraviolet and other media of atmospheric corrosion environment requirements. For the composite components, the leading edge of the wing, the Radome and other parts which are easy to be corroded by rain, the plane is directly impacted by raindrops when flying in rain, which makes the surface of the composite components debonding, cracking and being corroded by rain, etch pits are formed and even the composite material is stripped. For these parts should be effective anti-corrosion coating for surface protection. For composite structures with conductive requirements (such as lightning protection) , lapped wires shall be used, and current shall not be conducted through direct contact between the composite and metal materials (such as aluminum alloys) or through fasteners.

The protection requirements for aircraft composite materials are as follows: 

(1) the materials shall meet the requirements of the corrosion environment of the aircraft structure such as temperature, humidity, ultraviolet radiation and atmosphere; 

(2) titanium alloys with equivalent electric potential shall be preferred;

 (3) when matched, a NON-HYGROSCOPIC, non-corrosive and non-conductive insulating layer shall be installed at the interface.

 (4) an effective anti-corrosive coating shall be used for surface protection at the location susceptible to rain corrosion. 

Source: CAITONG

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