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Hexavalent chromium has served as the primary means of corrosion protection in the aircraft industry since and allowed for the distinctive bare-metal finishes of the World War II era. Generally referred to as chrome, chromate, or chromium, hexavalent chromium is found in the surface preparation processes and primers of traditional aircraft paint systems.
While chromates serve this purpose very effectively, questions have been raised regarding safety and possible environmental hazards. They also come with stringent regulatory requirements. Chromates are known cancer-causing agents, or carcinogens. According to the Occupational Safety and Health Administration OSHA , health impairments from continued workplace exposure could include increased risk of lung cancer and irritation or damage to the nose, throat, lungs, eyes and skin.
A chrome-free paint process is environmentally conscious. The EPA also takes a keen interest in contaminants leaving a facility through waste water and exhaust vents.
Paint processes that use chromates must capture waste streams and treat them at onsite pretreatment facilities. All contaminated materials, from waste water and exhaust filters to masking materials, must be properly disposed of as hazardous waste. He is a strong advocate for chrome-free paint systems. Although hazardous materials like chromium and lead could be present in trace amounts, the levels are low enough to meet the tolerances for landfill disposal.
Traditional chromate paint systems use an acid etch to remove impurities, a chromate conversion coating to seal and protect the aluminum and its alloys, a solvent wipe and a zinc chromate primer as a second corrosion resistant barrier. Chrome-free paint systems function in much the same way. PreKote serves as an adhesion promoter, creating a stronger bond between the primer and substrate than traditional pretreatments. However green color element is not a paint.
Its called Zinc Chromate primer. Zinc is anodic to Aluminium thus susceptible to corrosion and hence save aircraft structure from galvanic corrosion. Zinc chromate is not really a cover coat to prevent corrosion, it is a coating to better cause paint adherence. Aluminum quickly oxidizes in air and requires nothing more, because aluminum oxide itself is a sufficient protective barrier. It is an alloy of aluminum with copper, manganese and magnesium, to provide the physical properties needed in aviation; with a thin surface of pure aluminum, to protect the alloy.
The alloy provides the strength, flexibility and fatigue resistance needed. A common misconception is that planes use the same materials as ships and skyscrapers, namely steel. Planes are not made out of steel or iron, although modern aircraft do contain some trace elements of both. These materials, while far stronger, are not suitable for flight as they stress at high temperatures thanks to air resistance and as they are too heavy.
Current generation aircraft like the Boeing and Airbus A are built from a new type of material called composites. This material mix has many advantages, such as being lighter than aluminum, and has a simple structure that does not fatigue or corrode to elements. For example, Boeing constructed the fuselage of the as multiple barrel pieces, as opposed to many smaller aluminum panels that need to be connected later.
This more robust frame from composites allows better pressurization and wider openings for windows. In the future, the aviation industry is looking at materials like polymers and ceramics that are even lighter and have incredible tensile strength.
We may also see aircraft constructed entirely out of composite materials as the US Air Force did with the B-2 stealth bomber and do away with the elements of the past.
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