Chromating of Aircraft Parts

Chromating, also known as chromate conversion coating, is a chemical surface treatment used on aircraft parts to improve corrosion resistance, enhance paint adhesion, and provide a protective passive layer on metal surfaces. It is commonly applied to aluminum, magnesium, cadmium, and zinc alloys used in aerospace structures.

How Chromating Works

1. Surface Preparation – The metal part is cleaned to remove dirt, grease, and oxidation.
2. Chromate Bath Immersion – The part is immersed in or sprayed with a chromate-containing solution, typically chromic acid or chromium salts.
3. Chemical Reaction – The chromate reacts with the metal surface to form a thin, gel-like protective film.
4. Drying & Curing – The part is rinsed and dried, allowing the chromate layer to harden.

Types of Chromate Conversion Coatings

1. Hexavalent Chromate (Traditional Alodine/Iridite)

• Highly effective corrosion resistance.
• Recognizable by its yellow or iridescent finish.
• Contains toxic Cr⁶⁺ (hexavalent chromium), which is being phased out due to environmental regulations.

2. Trivalent Chromate (Eco-Friendly Alternative)

• Less toxic, compliant with environmental regulations (RoHS, REACH).
• Usually clear or slightly blue in color.
• Provides slightly less corrosion resistance compared to hexavalent chromate but is still widely used.

Aerospace Applications of Chromating

✔ Aircraft Fuselage & Structural Components – Prevents corrosion on aluminum airframes. ✔ Landing Gear & Fasteners – Enhances corrosion resistance and adhesion for paint. ✔ Electrical Components – Provides corrosion protection while maintaining electrical conductivity. ✔ Magnesium & Zinc Components – Protects against oxidation and environmental exposure.

Benefits of Chromate Conversion Coatings

✅ Corrosion Protection – Forms a passive layer that prevents oxidation and extends component lifespan. ✅ Improves Paint Adhesion – Creates an excellent bonding surface for primers and paints. ✅ Maintains Electrical Conductivity – Class 3 chromate coatings allow grounding in avionics and electrical components. ✅ Lightweight Protection – Does not add significant weight to aircraft structures.

Relevant Standards

• MIL-DTL-5541 – Military specification for chromate coatings on aluminum.
• AMS 2473 & AMS 2474 – Aerospace Material Standards for chromate treatments.
• RoHS & REACH Compliance – Environmental regulations phasing out hexavalent chromium.