Aircraft wheels are critical parts that endure harsh conditions in operation (e.g., heat load from braking, mechanical stress from landing/braking, and corrosion damage from moisture). Anodizing and use of a primer/topcoat are required to protect aluminum aircraft wheels. However, anodizing fails to remain intact in extreme operating conditions, resulting in corrosion damage. Furthermore, legacy anodizing schemes are not environmentally friendly, using sulfuric acid baths, Cr (VI) sealers, and chromated (Cr (VI)) primers. The overall objective of this project is to validate plasma electrolytic oxidation (PEO) coating as an environmentally friendly alternative to anodizing for superior protection of aircraft aluminum wheels.

The PEO process is eco-friendly (Cr free) and transforms the metal surface into a durable nanocrystalline oxide layer with exceptional adhesion. For this effort, PEO process will be optimized for application on Al-2XXX, meeting or exceeding anodizing performance in terms of thermal cycling, paint adhesion, and corrosion resistance. PEO will be scaled-up to accommodate wheels.

PEO process is an environmentally friendly alternative to anodizing. Unlike anodizing, PEO process is free of hazardous Cr and acids. PEO coating is expected to be compatible with current paint schemes and nondestructive inspection (NDI) workflow. PEO coating could enable Cr-free primers or even a paint-free system (avoiding strip-NDI-paint cycles).

PEO coatings offer superior protection. When assets are better protected, there will be less waste created by service, overhaul, and replacement. PEO offers superior protection compared to anodizing.

• PEO Creates a Unique Structure - Unlike anodizing, PEO coating is conformal, dense, and crack-free around corners, where damage frequently initiates.
• PEO is Corrosion-Resistant - PEO coating is equivalent or better than anodizing in American Society for Testing and Materials (ASTM) B117 (neutral salt fog) and ASTM G85 Annex 4 conditions (acidic salt fog with SO₂ gas).
• PEO is Highly Resistant to Thermal Cycling - PEO survives 1000s of hours on piston crowns in engine testing and 100s of heat-quench cycles from 400°C into water, with no coating damage.

PEO coating technology offers compelling cost savings:

• Reduced Risk Profile – The superior performance of PEO is likely to allow complete elimination of Cr in paint stack-ups while meeting or exceeding performance requirements. Elimination of Cr would reduce the need for engineering controls and personal protective equipment for de-painting personnel, with corresponding cost savings.
• Improved Durability/Asset Protection – Increased durability of PEO coating is expected to significantly reduce scrap rates of wheels and eliminate the cost of re-anodizing wheels due to coating failure.
• Potential Elimination of De-paint/Re-paint - If PEO enables a paint-free option compatible with NDI, this may eliminate the need for stripping/re-painting of assets.