Aero engines and other parts of aircraft are designed on the principle of a very high power-to-weight ratio, and hence each of the parts is expected to ensure the highest performance for a profitable long-term operation of the asset.
Corrosive deterioration of the critical surfaces of the aircraft can come in the way of ensuring reliable and safe operational performance. If the corrosion is not detected and prevented in time, it can endanger human lives.
The coating systems needed for corrosion protection of these exposed surfaces of the aircraft must be selected carefully to match the challenges of the hostile environment in which the aircraft operates. The coating must be applied with great care to ensure a productive longevity of the aircraft. Aircraft maintenance engineers and designers need to minimize the metal losses by looking for optimum solutions.
Introduction to Aircraft Corrosion
The major contributors to the corrosive deterioration of the aircraft are hostile environmental factors and climatic conditions. Airplanes operating in polluted industrial environments as well as in saline coastal climate conditions are susceptible to severe corrosion of the exposed surfaces. Water vapor, either alone or in combination with acidic constituents—such as salts present in a marine environment or fumes and pollutant particles from industries—can initiate electrochemical reactions on the metallic surfaces of the aircraft, leading to the corrosive deterioration of surfaces.
Aircraft manufacturers design airplanes to prevent corrosion through the careful selection of structural materials and coatings, as well as the provision of water drainages, moisture sealants, and chemicals such as corrosion inhibitors. A planned corrosion-control program is necessary for the complete service life of the airplane. For more on this subject, be sure to read Corrosion Knowledge Management versus Corrosion Management: An Essential Tool for Assets Integrity Management.)
Common Types of Aircraft Corrosion
On an aircraft surface, corrosion can appear in various forms. The most common is crevice corrosion, occurring wherever moisture is entrenched in between two proximate surfaces. Under certain conditions, it can gradually form pitting or exfoliation corrosion.
Crevice corrosion can accelerate quickly and results from environmental catalysts present at the metallic surfaces. It can take the form of oxygen differential cell corrosion. The entrapped moisture in the crevice contains less oxygen when it is in a crevice than when it is on an open surface. The lower oxygen content creates an anode at the surface, and the open-air metal surface forms a cathode. Contaminants create acidic ions such as chloride ions, which migrate to the anode to create a corrosive condition. An effective sealing of joints is the best way to eliminate this form of corrosion.
Other dangerous forms include stress corrosion cracking (SCC) and exfoliation corrosion. Both forms occur rapidly and become destructive by following the grain boundaries of alloys of aluminum. Exfoliation occurs in different planes, with the leaf-shaped separation of the grain structure. Both of these corrosion forms affect the structural integrity, which in turn affects the load-carrying capability and other mechanical parameters. These forms need to be controlled at the design stage by keeping stresses low and selecting a suitable grain structure of the metal alloys. Pitting corrosion can often lead to fatigue failures in critical areas.
General corrosion consumes material at a uniform rate. Initially the risk is low, but it can affect the structure over a period of time if unattended. Complete maintenance is needed, as well as the repair coating of affected surfaces.
Galvanic corrosion takes place due to the electrical connection of two metals of different galvanic electrode potentials within the area of an electrolyte. This can take place at the joint of an aluminum-bronze bushing used in an aluminum alloy or even inside the aluminum alloy particles because of potential difference between aluminum and copper layers. (Related reading: The Corrosion Properties of Aluminum and Its Alloys.)
Techniques of Corrosion Control
Proper planning at the design verification and review stage for effective corrosion control must be based on the comprehensive consideration of engineering performance requirements and operational environments. (Learn more about the importance of a good upfront design in How to Control Corrosion by Improving Design.) The requirements need to be matched by a prudent material selection, coating and surface finish selection, moisture drainage, use of joint sealants, selection of corrosion-inhibiting chemicals, access for inspection and maintenance, effective corrosion prevention and corrective action plans in service, and the consideration of regulatory issues.
Engineering material selection
The selection of appropriate metal alloys is essential for ensuring long-term corrosion prevention and control. Aluminum is the automatic choice for most of the aircraft structures and components/parts. Along with aluminum alloys, steel and its alloys are also used in aircraft, both being highly prone to attacks of corrosion.
Corrosion prevention in this case involves cleaning the surfaces and providing a surface finish through layers of coatings. For the surfaces of aluminum alloys, the coating of a corrosion-inhibiting primer is the first coat. For the low-alloy steel components, the surface coating consists of a thin cadmium plating and a chemical corrosion inhibitor primer.
Stainless steel components are also cadmium-plated and primed as in the case of alloy steel components, as they may be connected to aluminum parts. A primer is used on titanium components too. Corrosion-inhibitor primers are required to be hydraulic fluid-resistant (Skydrol® for example) polyurethanes and epoxies.
The moisture from a pressurized fuselage is required to be drained by drain holes with valves. Fluids are also required to flow in the direction of drain holes through a system of drain paths.
Crevice corrosion at joints is minimized by sealing the gaps on the joint surfaces with a polysulfide sealant. The sealant is used in skin-to-stringer as well as skin-to-shear types of joints in the fuselage, areas of skin doublers, chord-to-skin joints for the wing and wheel structure, etc. In cases of severe proneness to corrosion, such as electrical bonds and antenna peripheries, fillet seals are used.
As the aircraft structures are built from different metals, they can suffer deterioration of the surface due to metals reacting to the environment. The electrochemical process of corrosion can begin even during the process of manufacturing and temporary storage. Even from the moment the airplane is delivered by the seller, it must be thoroughly inspected and preserved as well as protected from the corrosive environment in which it has to operate.
This protection needs to be considered right at the design and component material selection stage. By adding certain constituents to metallic substances, we can enhance the corrosion resistance of the system. Surface coatings used for the aircraft and the components will also protect the aircraft.
The airframe, made up of metals as well as non-metallic composites, also require regular inspection for monitoring of aircraft from the perspective of corrosion. Even though the airframe may not become corroded directly, the metallic accessories and components attached to the airframe could be subjected to deterioration.
Aircraft Areas Prone to Corrosion Damage
Corrosion is the surface deterioration of metal due to electrochemical or chemical reactions occurring on the surfaces. These kinds of reactions can occur internally too. Water present in the environment can be deposited on the surface and the metal particles will combine with the oxygen dissolved in the water, with impurities acting as an electrolyte. Acidic particles present in the environment can cause chemical corrosion by reacting directly with the metal at a rapid rate.
Inspections must be performed regularly to detect and recognize the signs of corrosion on the aircraft's surfaces. Inspection checklists are comprehensive as they cover every part and surface. Based on the peculiarity of the aircraft model, some special areas need to be critically examined.
For reciprocating engines as well as jet engines, the following areas need detailed examination:
- The exhaust area
- Exhaust pipes and nozzles
- Rivet heads and crevices nearby
These are affected because exhaust emissions lead to corrosive deposits. The deposits could be trapped deep inside and are not easily cleaned.
Bilge and areas in vicinity
These may have sumps of dirty water, floating oils, used hydraulic fluids and debris, and used oils that necessarily contain water settling slowly at the bottom, which sets corrosion in motion. Inspection and regular maintenance of the coatings in this area must be undertaken. When planes operated from ships are repaired, the corrosion applicable to marine equipment should also be considered.
The space under galleys and lavatories, as well as waste disposal openings at the exteriors of the aircraft, need to be carefully inspected, as the nature of waste products and chemicals in lavatories could lead to corrosion. The coating may need to be repaired frequently.
The landing gear, as well as the wheel well area, suffers due to water, gravel, salt, chemicals, mud, dust and debris of various kinds. Surface coatings on these parts can’t be made foolproof. A thin preservative coating may only be masking a dangerous onset of corrosion.
During braking, heat generation can cause the cracking of anticorrosion preservative layers (if they are used on the surface). Different trouble spots must be inspected for entrapped water, rust, and abrasive wear. Typical debris traps and moisture traps—such as ribs, stiffener crevices, and lower skin areas—must be inspected. If the low-point drains are partially clogged by debris, water can accumulate and cause corrosive deterioration.
Cooling vents and the front surface areas of the engine get rubbed due to airborne sand, salt particles, and gritty dust from runways, as well as snow and rain, causing the erosion of protective coatings and the sporadic deposition of salt. Inspecting technicians should be able to highlight these occurrences as and when they are noticed.
In the case of helicopters, gearboxes and rotor heads—along with other bare surfaces—need to be inspected for evidence of corrosive failure. Additionally, all types of cables are prone to internal corrosion and external corrosion.
Battery vents openings and the compartments
The battery vent openings and compartments are known as the most corrosion-prone spaces of aircraft. Gases and fumes emanating from the heated electrolyte will naturally spread to the nearest cavities and coating holidays, and attack all the unprotected metal surfaces as well as crevices. Acids may get deposited in small quantities.
Surface finishing and anodizing
Electrolytic cleaning or chemical cleaning of surfaces is resorted to before the surface finishing for corrosion protection. After polishing and buffing, the anodizing process is started. The sheet of aluminum alloy is connected as the positive pole in the bath of electrolytes (comprising chromic acid), which produces an aluminum oxide layer on the metallic surface. This process effectively increases the thickness of the protective aluminum oxide layer. This layer can be partially restored during annual maintenance servicing, through suitable surface treatments. The anodized layer provides effective resistance to corrosion. In addition, this film provides a strong bond for paint. After the anodizing process, aluminum surfaces are immediately primed and coated by approved paint.
Metallizing and high temperature coatings
The aircraft engines continuously face severe challenges as the load-carrying parts are operated at higher and higher temperatures with enhanced velocities for an improved performance. Corrosion and other forms of metal loss reduce the component efficiency. Thermal sprays and cold sprays of various coatings help to minimize corrosion and other forms of metal loss. Barrier coating on components such as turbine blades has proved effective.
High technology coating processes, such as high velocity oxygen fuel spraying (HVOF) and plasma spray, are used for critical parts of the aircraft engines and other components. New barrier coatings for high temperature applications are being tried out.
Aircraft engine components from ceramic matrix composites are being developed by GE Aviation in collaboration with Turbocoating SPA of Parma, Italy. Coating manufacturers are offering platinum aluminides, based on proprietary vapor deposition processes, for aluminizing the components and surfaces used at extreme high temperatures. Ceramic coatings have superior properties when compared with organic coatings, in terms of better high-temperature corrosion resistance as well as erosion resistance.
Turbine blades of jet engines are made from nickel-based alloys and sometimes also use ceramic materials similar to zirconia. These blades are coated with a ceramic barrier coating for heat and corrosion protection.
Ceramic coatings offer excellent protection against exposure to high temperature cycles and salty environments. They combine good adhesion, thermal stability, hardness, and flexibility. Proprietary ceramic composite coating systems with corrosion protective organic topcoat sealers are being developed.
Thermally-fused ceramic composite with an aluminum-rich organic top coat has the advantage of oxidation resistance even at high temperatures. Proprietary ceramic coatings specifically formulated for combustion chamber and valves, sea wind exhausts, and expansion chambers are used in large aircraft.
Requirements of Top Coatings and Primers
Coatings used in military aircraft have to meet the stringent requirements of military standards. U.S. military standards for aviation coatings are referred to as MIL-PRF-85285E. Commercial aircraft coatings in the U.S. generally comply with AMS 3095 - SAE Standards (aerospace material specifications). Aircraft manufacturers also specify certification standards for the coatings to be applied. Boeing's requirements are prescribed in the standard BMS-10-83K.
Urethane primer is formulated as a high-performance corrosion inhibitor for all types of aircraft. It has good intercoat adhesion. Some of the newer formulations are offered as chrome-free urethane primers, meeting the stringent regulatory requirements. These are free from hexavalent chromium. These may also help to prepare a substrate for top coatings; for good adhesion requirements working as sanding surfacers.
Urethane top coatings
Urethane consists of two parts, comprising the base and certain binders with catalysts. Its advantages are:
Urethane may contain isocyanides as activators; in which case, more care is needed in spraying. Certain proprietary formulations of urethane are exclusively formulated for interior coating with a soft texture.
Advanced polyurethane primer as a chrome hazard-free formulation ensures very good corrosion resistance, along with a long pot life, making it suitable for coating a large aircraft. The film has higher flexibility and aviation hydraulic fluid resistance (Skydrol resistance).
Polyurethane top coatings
Polyurethane coatings are the most preferred coating in the aviation industry, due to advantages such as:
- High chemical resistance
- Natural ultraviolet ray (sunlight) resistance
- High abrasion resistance
- High stain resistance
- Directness of image (DOI) with high gloss retention
This is generally preferred by corporate and commercial aircraft builders, as it also provides a wet look. Compared to epoxies, polyurethane has:
- Faster cure time
- Higher susceptibility to moisture during curing
- Very high gloss retention
- A very high abrasion resistance
Acrylic urethane coatings
Acrylic urethane dries quickly to form a hard surface after coat application. It is less resistant to harsh chemicals and needs additional UV inhibitors for protection against the UV rays of sunlight. However, it provides a rugged and durable coating, as well as effective chip resistance, making equipment look new even in extremely harsh environments.
Recent epoxy primers are also formulated as chrome hazard-free primers, having superior phosphate ester hydraulic fluid resistance (Skydrol resistance), as well as high corrosion resistance. These provide long wet edge, which is advantageous in coating a large aircraft.
Epoxy top coatings
Epoxies produce hard, tough, and chemical-resistant layers when applied on a suitably primed substrate. They are better compared to polyurethanes in water resistance as well as adhesion. They are less susceptible to moisture during curing. Skin irritation can occur while handling epoxies, but they do not contain isocyanine.
Solvent-free epoxies can be applied in thick films, but they have high moisture susceptibility during curing. These solvent-free epoxies have a short pot life and low color stability. Unlike solvent-based epoxies, which may shrink up to 70%, the solvent-free types do not suffer from film shrinkage. As they are softer than urethane, they have better chipping resistance. They are highly preferred for steel tube frames. Epoxy coatings are also used for coating propeller blades, providing abrasion resistance and corrosion protection.
Zinc chromate is a pigment used in different formulations for corrosion resistance, which is added to primers of alkyds, polyurethanes, and epoxies. However, it is restricted by regulators as a carcinogenic substance.
Photo-luminescent coating kits
Proprietary photo-luminescent coating kits have been developed for military use, which makes the tips of propeller blades and helicopter rotors visible in low visibility conditions and night operation. The coating system improves visibility for the safety of people in hangers, repair stations, helipads and airstrips.
A polyester-urethane combination coating has multiple advantages of high solids with multiple components, generally used as top gloss finishing coating in both military as well as commercial aviation. (Find the answer to the question: What do I need to consider when it comes to applying high solids coatings?) Individually, polyester coatings generally provide an exotic look with super high gloss needed for interiors, with scratch resistance and a glass-like clear film.
Advanced aviation mica coatings
On account of their exotic appearance, micas are now used widely for aircraft liveries. These are easily repaired and have the additional advantage of flexibility and adjustability of properties for different environmental conditions.
After the aircraft frame and components are manufactured, they are essentially protected by coating with layers of corrosion-resistant alloys, ceramics and paints.
After applying the top coat, the job is only half done, because the coating effectiveness needs to be monitored regularly and corrective and preventive actions are needed at regular intervals.
- The critical actions must include moisture prevention for all surfaces.
- Corrosive salt deposits need to be removed by a thorough cleaning (by rinse-clean processes).
- Delaminated or chipped surfaces need to be repaired early, after stripping, sanding and primer application if needed.
The selection of an appropriate coating, based on the operational conditions of aircraft and the substrate compatibility, is important for ensuring the economic service life of the aircraft.