As aircraft are subjected to harsh climates, high pressures, and high altitudes, they are often made with durable materials and blanketed in protective coatings. One of the most common forms of wear that aircraft face is corrosive deterioration, which if not detected or prevented ahead of time, may endanger aircrew and passengers alike.
Aircraft coatings utilized for the corrosion protection of exposed surfaces must be selected to match the challenges of the environments in which aircraft operate. More than that, the coating must be applied precisely to ensure the longevity of the aircraft. Typically, aircraft maintenance engineers are tasked with finding sealants and coatings that offer maximum protection.
Corrosive deterioration is sourced primarily from hostile environmental factors and extreme climatic conditions. For example, aircraft that traverse saline coastal climate conditions experience severe surface corrosion. Similarly, water vapor in combination with acidic constituents like salts often found in marine environments can result in electrochemical reactions on the metallic surfaces of aircraft.
Aircraft manufacturers are responsible for designing airplanes with structural materials, protective coatings, water drainages, moisture sealants, and corrosion inhibitors to ensure their optimal functionality. With this in mind, corrosion-control programs are necessary for the service life of aircraft. There are a few common types of corrosion, all of which appear in various forms.
Crevice corrosion is the first basic form, occurring in locations where moisture is present between two surfaces in close proximity to each other. Over time, it can form pitting or exfoliation corrosion. It is important to note that crevice corrosion can take the form of oxygen differential cell corrosion, which can be prevented with the effective sealing of joints.
Another dangerous form of corrosion includes stress corrosion cracking (SCC). Occurring at a rapid rate and affecting the structural integrity of aircraft, SCC can affect load-carrying capabilities which can lead to fatigue failures in critical components. Next, galvanic corrosion materializes as a result of the electrical connection of two metals with different galvanic electrode potential within the area of an electrolyte.
When it comes to understanding the metrics for coatings, primers, and sealants, one must be aware of the specifications they must meet in order to comply with general aviation requirements. In the case of military aircraft and commercial aircraft, there are certain standards manufacturers and engineers must adhere to. The following sections will cover a few of the most commonly used aviation coatings for corrosion prevention.
Urethane primers are formulated as high-performance corrosion inhibitors with excellent intercoat adhesion. Newer formulations offer chrome-free options, meeting strict regulatory requirements. In addition to chrome-free options, they are also free from hexavalent chromium, which is a toxic, carcinogenic element. When paired with urethane top coatings, they provide high durability, chemical and abrasion resistance, as well as a high gloss finish.
This type of primer contains a chrome hazard-free formulation that offers enhanced corrosion resistance. It also provides higher flexibility and aviation hydraulic fluid resistance. Polyurethane top coatings have major advantages as well, providing natural UV resistance, high abrasion and stain resistance, faster cure times, and high gloss retention.
One of the biggest benefits of using acrylic based coatings is their rapid drying time that offers a hard surface finish and chip resistance. However, it is less resistant to harsh chemicals and necessitates UV inhibitors.
Formulated as chrome-free primers with great phosphate ester hydraulic fluid resistance and high corrosion resistance, epoxy primers provide a long wet edge, which is ideal for coating large aircraft. Epoxy top coating produces durable, chemical-resistant layers when they are applied to primed substrates. It is important to note that they may cause skin irritation if handled without care. There is also a solvent-free epoxy option that offers chip resistance for propeller blades, providing increased abrasion resistance and corrosion protection.
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