Coating Systems in the Aviation Industry

Paint Systems in the Aerospace Sector Demands and Expectations for Paint Systems in the Aerospace Industry The aerospace industry is categorized into four segments: commercial air transport, military aviation, general aviation, and the space sector. Air transport is growing rapidly due to the increase in air transport network infrastructure and advances in aviation technology, creating significant added value for the global economy. Given its current position, air transport is considered one of the major indicators of global development. The global aviation market is projected to grow at an annual rate of 5.9% through 2030 (1). According to the ESPAS report, given that global growth rates are not expected to exceed 4.4%, the sector's increasing importance becomes evident (2). In the civil aviation sector, Boeing and Airbus, the world's leading companies, maintain market dominance in the passenger aircraft category (3). According to a forecast report published by Airbus, there is a demand for 27,848 new aircraft, with these demands projected to consist of 26,921 passenger aircraft and 927 cargo aircraft. With the lifting of restrictions imposed due to the Covid-19 pandemic in Turkey and globally, passenger traffic growth over the next 20 years is forecast to be 5.1% annually according to Boeing projections (4,5). Increasing demand from end-users in the aerospace sector is directing the paint market for air industry. Today, paint demand in the sector has increased due to environmental factors as well. The aerospace coatings market is projected to grow from USD 1.4 billion in 2021 to USD 2 billion by 2031 (6). Aluminum, aluminum alloys, and composite plastics are widely used surface materials in aircraft paint systems due to their light weight and high strength. Aluminum alloys containing magnesium, nickel, cobalt, copper, and titanium are used in aerospace applications. While the alloying process improves mechanical properties compared to pure aluminum, corrosion resistance decreases. The Al 2024 alloy, which is widely used in space and aerospace industries, is preferred despite being susceptible to corrosion due to its superior physical performance. These surface materials require high corrosion resistance and excellent adhesion to the surface. Aluminum surfaces must undergo certain treatments to achieve high adhesion and increase corrosion resistance. Aluminum easily corrodes under the influence of oxygen or other oxidizers in air, and the aluminum oxide molecules formed on the surface firmly adhere to the surface, creating an impermeable layer. This prevents corrosion of the underlying aluminum layer (7). Aluminum surfaces can be coated homogeneously with aluminum oxide using electrochemical methods (anodizing), and surface treatment can also be performed using chromate-based liquids. However, considering the harmful effects of chromate on human health and the environment, the use of chromium-free, zirconium-based surface treatments that can increase adhesion performance equally on aluminum and composite surfaces has been brought to attention (8). The primary purpose of the paint system applied to the aircraft surface is to minimize the effects of corrosion and wear, enabling the vehicle to withstand harsh conditions. Corrosion in aircraft weakens structural integrity, causing safety problems and high repair costs. Corrosion is a naturally occurring process where chemical and electrochemical reactions between metal materials and their surrounding environment result in the degradation of metallic properties. Reactions occurring between an electronic conductor (electrode) and an ionic conductor (electrolyte) form an electrochemical cell. The basic elements of this cell are the anode, cathode, physical contact element, and electrolyte solution. The fundamental task of paint systems applied to metal surfaces is to create a barrier layer between the metal surface acting as an electrode and electrolytes such as saltwater that come into contact with the surface. If impurities, dirt, scratches, or cracks are present on the metal surface, a potential difference is created on the surface, which is sufficient to initiate the corrosion process. Some areas on the surface act as anodes and others as cathodes, creating corrosion cells. At the anode, metal atoms pass into the electrolyte as ions through oxidation reactions, and the anode corrodes. The electrons released as a result of the oxidation reaction are used in the reduction of hydrogen ions in acidic environments, releasing hydrogen gas, while in neutral or oxygen-dissolved environments, they are used in the reduction of oxygen molecules, forming hydroxyl ions. The ions resulting from these simultaneous reactions combine and precipitate near the metal surface, forming rust (9). Filiform corrosion, which appears as a performance test in AMS 3095 specification, is a type of under-film corrosion and is considered an important problem in aerospace applications. The mechanism of filiform corrosion is the progression of horizontal filaments under the film starting from the weak point of the painted surface. On aluminum surfaces, it occurs at 70-95% relative humidity and 20-40°C temperature. A prerequisite for the initiation of this type of corrosion is exposure of the painted surface to contaminants such as chloride, sulfate, and carbonic acid. Water and oxygen enter through the weak point of the painted surface. The metal surface releases electrons to form metal ions, and these ions react with water to produce hydrogen ions (H+). H+ ions attract chloride and sulfate ions in the environment, triggering osmotic influx of more water into the tip of the filament. In the inactive tail section, water is removed. While oxygen levels are highest at the corrosion initiation point, concentration gradually decreases in the direction of corrosion progression. Metal hydroxides and hydrogen ions form at the tip as a result of corrosion. With the formation of these products, oxygen concentration decreases at the tip and pH value drops. Corrosion continues progressively from this point forward (10). In aerospace primers, strontium chromate-based anticorrosive pigments containing hexavalent chromium are used to provide effective corrosion resistance on aluminum surfaces. Strontium chromate has been included on the hazardous substances list due to the threat it poses to the environment and human health, and has been added to the list of substances causing specific health hazards due to its carcinogenicity. Its use has been restricted in the aerospace sector, and its use outside aerospace is prohibited. For this reason, chromium-free, environmentally friendly primers are being developed (11). In aerospace applications, composite materials have begun to serve as an alternative to aluminum surfaces due to weight and corrosion considerations. In composite panel applications, paints with ease of sanding and surface finishing as well as high surface defect coverage properties have begun to be preferred. Airborne particles and rain droplets cause erosion in high-speed vehicles. Aerospace materials such as aluminum and composites have weak resistance to erosion. This situation can lead to serious deficiencies in aerodynamic and structural aspects of aircraft where aerodynamic configuration is critical. Erosion is more frequently observed in specific areas of the aircraft surface such as the radome, wing leading edge, and horizontal stabilizers at the rear of the fuselage. Particle erosion is a dynamic process where solid particles remove material from the surface they impact. Due to the low particle content at altitudes where aircraft operate and the higher destructive effect of rain droplets, rain erosion is given greater emphasis in aerospace applications. In rain erosion, a supersonic shock wave referred to as the "water hammer effect" is created within the droplet at impact due to the incompressibility of liquids. Rain droplets striking a solid surface at high speed move by doubling their velocity parallel to the surface (horizontal jet) and strike the indentations and protrusions (surface defects) in the coating surface, abrading the coating film. For this reason, water causes more damage compared to solid particles. In developing coatings resistant to rain erosion, the absorption of the generated shock wave depends on the material's elasticity. As surface roughness on the coating decreases, the destructive effect of the water jet also decreases. High adhesion strength of coatings is a factor in preventing erosion (12,13). The technical specifications that paints for civil aviation and military aviation sectors must possess are defined by standards. For civil aviation aircraft exterior fuselage paint, the "AMS 3095 Paint: High Gloss Exterior for Airlines" standard defined by the Society of Automotive Engineers is accepted. This standard contains the properties and performance parameters of paints applied to Al 2024 alloy. For military aviation, similar standards containing comparable provisions are prepared by defense industry organizations of respective countries. Aircraft paint systems must be resistant to hydraulic fluids with reactive phosphate ester structure, oils, and fuels. During flight, aircraft reach temperatures of -50°C and lower, and at high altitudes are exposed to higher levels of ultraviolet (UV) radiation. To extend the service life of aircraft and increase flight safety, paint systems with high elasticity at low temperatures and high UV radiation resistance must be used. The paint system must have excellent abrasion resistance against exposure to dust, rain, or snow mixed with rain. Additionally, the painted surface must be smooth to minimize friction during flight. An important point here is that as crosslink density increases, elasticity and abrasion resistance properties decrease (8). The thickness of applied paint directly affects the quality of aircraft and components due to weight considerations, and is also a sensitive parameter for energy costs. For this reason, all performance characteristics required of the paint must be provided at low film thicknesses. Aircraft paint systems use epoxy primers with good adhesion to aluminum surfaces, high corrosion resistance, and amine-cured low crosslink density. Low crosslink density provides high elasticity and facilitates the removal of coatings from aircraft surfaces during the paint stripping phase in hangars. Two-component polyurethane coatings applied over the primer must have high gloss, image clarity, and high exterior durability. These coatings must have high crosslink density to provide resistance to chemical materials. Aircraft paint systems contain high amounts of solvent. Reducing Volatile Organic Compound (VOC) emissions is an important issue being emphasized due to health risks and environmental pollution. Additionally, studies related to water-based systems have gained momentum to comply with VOC limits specified in aerospace specifications (8). Kanat Boyacılık Tic. and San. A.Ş. has established a specialized unit for the defense and aerospace sector, specializing in civil and military aerospace paints and meeting sector demands. With the product range to be developed, it aims to support the national economy and contribute to localization and nationalization policies.

References

(1) https://www.embroker.com/blog/aerospace-market-growth-strategies/ (2) https://espas.secure.europarl.europa.eu/orbis/sites/default/files/generated/document/ en/The%20Global%20Economy%20in%202030.pdf (3) https://thinktech.stm.com.tr/tr/sivil-havacilik-sektor-degerlendirmesi- (4) Çizmecioğlu, M. (2013). Türkiyede sivil havacılık ve hava yolu ulaşımı üzerine bir araştırma (Master's thesis, Fen Bilimleri Enstitüsü). (5) S. H. G. M (2021). Sivil Havacılık Genel Müdürlüğü Faaliyet Raporu 2021. TC Ulaştırma ve Altyapı Bakanlığı, Ankara. (6) https://www.factmr.com/report/aerospace-coatings-market (7) Tunçgenç, M. (2004). Genel Boya Bilgileri. Akzo Nobel Kemipol (8) Goldschmidt, A., & Streitberger, H. J. (2003). BASF handbook on basics of coating technology. William Andrew. (9) Davis, J. R. (Ed.). (2000). Corrosion: Understanding the basics. Asm International. (10) Delplancke, J. L., Berger, S., Lefebvre, X., Maetens, D., Pourbaix, A., & Heymans, N. (2001). Filiform corrosion: interactions between electrochemistry and mechanical properties of the paints. Progress in organic coatings, 43(1-3), 64-74. (11) Jones, F. N., Nichols, M. E., & Pappas, S. P. (2017). Organic coatings: science and technology. John Wiley & Sons. (12) Elhadi Ibrahim, M., & Medraj, M. (2019). Water droplet erosion of wind turbine blades: Mechanics, testing, modeling and future perspectives. Materials, 13(1), 157. (13) Valaker, E. A., Armada, S., & Wilson, S. (2015). Droplet erosion protection coatings for offshore wind turbine blades. Energy Procedia, 80, 263-275. Begüm Özyağcı Tokgöz Research and Development Chemist Kanat Boyacılık