Requests and Expectations of the Aviation Sector in Paint Systems
Aviation industry is categorized in four sections as commercial air transportation, military aviation, general aviation, and space sector. Air transportation provides added value
to the world economy by growing rapidly with the increase in the number of air transportation networks and the developments in aviation technology.
Air transportation is considered as one of the significant indicators of global development
considering present situation. The global aviation market is projected to grow by 5.9% each year until 2030 (1).
The increasing importance of the sector becomes evident considering that the global growth rate will not exceed 4.4% according to the ESPAS report (2). Boeing and Airbus companies, which are the world leaders in the civil aviation sector, possess the market domination in the passenger aircraft category (3).
According to the forecast report published by Airbus, there are 27,848 new aircraft
demands, and it is predicted that these demands will be 26,921 passenger and 927 cargo aircraft.
The increase in passenger traffic for the next 20 years is predicted to be 5.1% per year according to Boeing forecasts with the removal of the restrictions imposed due to the Covid-19 pandemic in our country and in the world (4-5).
The increasing demand of end users in the aviation industry is driving the paint market for the aerospace industry. Today, the need for paint in the sector has increased with the effect of environmental factors.
It is estimated that the aviation coating market will reach the level of 2 billion USD from 1.4 billion USD from 2021 to 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 aviation. Compared to pure aluminum, the mechanical properties are improved by the alloying process, while the corrosion resistance decreases.
Al 2024 alloy, which is widely used in the aerospace industry, is preferred due to its superior physical performance, although it is an alloy that is susceptible to corrosion. High corrosion resistance and excellent adhesion to the surface are required for these surface materials.
To achieve high adhesion on aluminum surfaces and increase corrosion resistance, the
surfaces must be subjected to some processes. Aluminum is oxidized easily with the effect of oxygen or other oxidizers in the air, and aluminum oxide molecules formed on the surface are firmly attached to the surface and form an impermeable layer. Thus, corrosion of the underlying aluminum layer is prevented (7).
Aluminum surfaces can be coated homogeneously with aluminum oxide (anodization)
using electrochemical method, and surface treatment is performed with chromate-based liquids. However, considering the harmful effects of chromate on human health and the environment, zirconium-based surface treatments which can increase adhesion performance on aluminum and composite surfaces at the same rate become a current
issue (8).
The main purpose of the paint system applied to the surface of the aircraft is to minimize the effects of corrosion and wear and to ensure that the vehicle it is applied to withstand harsh conditions.
The presence of corrosion in aircraft can weaken structural integrity, thus causing safety issues and high repair costs. Corrosion is a natural occurring process that causes deterioration of metallic properties of materials because of chemical and electrochemical
reactions with their environment.
Reactions between an electronic conductor (electrode) and an ionic conductor (electrolyte) form an electrochemical cell. Anode, cathode, physical contact element and electrolyte solution are basic elements of this cell.
The main purpose of paint systems applied to metal surfaces is to form a barrier layer between the metal surface acting as an electrode and electrolytes such as salt water in contact with the surface.
Impurities, dirt, scratches, or cracks on the metal surface create potential difference on the surface and this is sufficient to initiate the corrosion process. Some areas on the surface act as anodes and some areas as cathodes, forming corrosion cells. Metal atoms pass into the electrolyte as ions with the oxidation reaction at the anode and the anode is corroded.
Electrons that are released with oxidation reaction are used in the reduction of hydrogen ions in acidic environments and releases hydrogen gas. In the presence of neutral or dissolved oxygen, released electrons with oxidation reaction are used in the reduction of oxygen molecule to form hydroxyl ions.
The ions formed with these simultaneous reactions coprecipitate in the region close to the metal surface and form rust (9).
Filiform corrosion, which is included as a performance test in the AMS 3095 specification, is a type of crevice corrosion and is considered as an important problem in aviation. The
mechanism of filiform corrosion is in the form of filaments which grow under the coating initiating from defects of the painted surface.
Filiform corrosion takes place on aluminum surfaces at 70 to 95% relative humidity and at temperatures ranging from 20 and 400C. Exposure of the painted surface to pollutants such as chloride, sulfate and carbonic acid is a prerequisite for the initiation of filiform corrosion.
Water and oxygen penetrate from defects of the painted surface. The metal surface releases electrons to form metal ions, and these ions react with water to form hydrogen ions (H+). The hydrogen ion attracts ions such as chloride, sulfate which induces more water intake to the head of filament by osmotic pressure.
Water is removed from the inactive tail part of the filament. Concentration of oxygen gradually decreases in the direction of the corrosion progress, while the oxygen level is highest at the starting point of the corrosion.
Metal hydroxide and hydrogen ions are formed at the head of the filament. Therefore, oxygen concentration and pH value are decreased at the head. The continuity of filament
growth is sustained through this mechanism towards the direction of the filament head (10).
Strontium chromate-based anticorrosive pigments containing +6 valent chromium are used in aviation primers to provide effective corrosion resistance on aluminum surfaces. Strontium chromate has been included in the list of hazardous substances due to threat it poses to the environment and human health and has entered the list of substances that
pose special health hazard due to its carcinogenicity.
The use of strontium chromate is restricted in the aviation industry and prohibited outside of aviation. Therefore, chromiumfree, and environmentally friendly primers are being developed (11).
Composite materials have started to be an alternative to aluminum surfaces due to effects such as weight and corrosion in aviation. In composite panel applications, paints which have high ability to cover surface defects and enable sanding, repair, paste and polish have started to be preferred.
Factors such as solid particles in the air and raindrops cause erosion in high-speed vehicles. Aerospace surface materials such as aluminum and composites have inadequate resistance
to erosion.
This situation can lead to serious problems in terms of corrosion and structural aspects in aircraft where aerodynamic structure is critical. Erosion is more common in certain areas of the aircraft surface, such as the radome, wing leading edge and tailplane.
Particle erosion is a dynamic process in which impingement of a solid particle to the surface causes the loss of material. Due to less number of particles in higher altitudes and destructive effect of raindrops, rain erosion is considered more important in aviation.
The liquids cannot be compressed so a supersonic shock wave called “Water Hammer Effect” occurs inside the droplet with the impact. When raindrops impact on a solid surface
at high speed, move parallel to the surface (horizontal jet) by multiplying its speed and erode the coating film by striking surface defects.
Therefore, water causes more damage than solid particles. The ability of the shock wave to be damped depends on the elasticity of the material in the development of a coating that is resistant to rain erosion.
As the roughness of the surface decreases, the destructive effect of the water jet decreases. The high adhesion strength of the coating is effective in preventing erosion (12,13).
Technical qualifications of the coating are defined by the standards in civil and military aviation industry. “AMS 3095 Paint: High Gloss Exterior for Airlines” standard which is defined by Automotive Engineers Association (Society of Automotive Engineers International, SAE International) is accepted for civil aviation exterior system.
The standard includes properties and performance parameters of the coating applied
on the Al 2024 alloy. Standards containing similar requirements for military aviation are prepared by the organizations of the countries that direct the defense industry.
The aircraft coating system must be resistant to hydraulic fluids, oils, and fuels, which are reactive chemicals in the phosphate ester structure. The aircraft reaches temperatures of
-50°C and below during flight and exposed to more ultraviolet (UV) radiation at higher altitudes.
Coating systems with high elasticity at low temperatures and high UV radiation resistance
should be preferred to extend the life of the aircraft and to increase flight safety. The paint system must have excellent wear resistance to exposures such as dust, rain, or rain mixed snow.
In addition, the painted surface must be free from roughness to minimize friction during flight. The important point is elasticity and abrasion resistance reduce as the crosslink density increases (8).
While thickness of the applied paint directly affects the quality of the aircraft and its
components due to weight and it is also a sensitive parameter for the energy cost. For this reason, all the performance parameters should be provided at low film thicknesses.
Epoxy primers that have high adhesion on aluminum surfaces, have high corrosion resistance and are cured with amine to form low crosslink density, are used in aircraft paint
systems. The low crosslink density provides high elasticity and facilitates the removal of the coating from the aircraft surface during the paint removal process.
Two-component polyurethane coatings applied on the primer should have high gloss, image clarity and high external strength. These coatings must have high crosslink density to be resistant to chemical materials.
Aircraft paint systems contain high amounts of solvents. Reducing the Volatile Organic Compound (VOC) emission is a significant issue due to health risks and environmental pollution. To ensure the VOC limits specified in specifications, studies on water-based systems have gained importance (8).
Kanat Paints & Coatings specializes in the field of civil and military aviation paints by establishing a special unit in the aerospace and defense department and meets the demands from the sector. The company aims to contribute to the localization and nationalization policy by supporting the national economy with the product range to be developed.
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
R&D Chemist
Kanat Boya
