Chromate Conversion
Understanding Chromate Conversion
Bare aluminum rapidly forms a native oxide layer (2 to 4 nm) that provides initial corrosion resistance but is too thin for demanding environments (salt spray, humidity, industrial atmospheres). Anodizing builds a thick, hard oxide (5 to 75 μm) that is excellent for wear resistance but is electrically insulating, with surface resistance exceeding 10 MΩ. For RF applications where the housing surface must carry ground currents, interface with EMI gaskets, or mate with waveguide flanges, an insulating coating is unacceptable. Chromate conversion fills this gap: it produces a corrosion-resistant film that is thin enough to remain conductive.
The process involves immersing clean, deoxidized aluminum in a chromic acid solution (hexavalent Cr6+ like Alodine 1200S, or trivalent Cr3+ like SurTec 650) for 1 to 5 minutes at room temperature. The chemical reaction converts the aluminum surface into a mixed aluminum-chromium oxide gel that dries to a hard, adherent film. Class 1A (gold/iridescent, maximum protection) provides 168+ hours of salt spray resistance and contact resistance below 2.5 mΩ on a 1 square inch contact. Class 3 (clear/colorless) provides lighter protection for subsequent painting. For RF hardware, Class 1A is standard on external housing surfaces, flange faces, and ground planes. Internal waveguide surfaces that carry microwave current in the skin depth are left bare or gold-plated, because even the thin chromate film can add 0.01 to 0.05 dB of loss at the surface.
Skin Depth and Coating Impact
δ = √(2ρ / (ωμ)) [m]
At 10 GHz:
δAl ≈ 0.84 μm (comparable to chromate thickness)
Contact Resistance (MIL-DTL-5541):
Rc < 2.5 mΩ on 1 in² [Class 1A]
Where ρ = resistivity (2.65 × 10-8 Ω·m for Al), ω = angular frequency, μ = permeability. Chromate coating on external surfaces has negligible RF impact; on internal waveguide surfaces at 10+ GHz, it adds 0.01 to 0.05 dB loss per interface.
Aluminum Surface Treatment Comparison
| Treatment | Thickness | Contact R | Salt Spray | RF Use |
|---|---|---|---|---|
| Bare aluminum | 2 to 4 nm (native) | < 1 mΩ | < 24 hrs | Internal waveguide |
| Chromate (Class 1A) | 0.25 to 1.0 μm | < 2.5 mΩ | 168+ hrs | External housings, flanges |
| Anodize (Type II) | 5 to 25 μm | > 10 MΩ | 336+ hrs | Non-RF surfaces only |
| Anodize (Type III) | 25 to 75 μm | > 100 MΩ | 1,000+ hrs | Wear surfaces only |
| Gold plate | 0.5 to 5 μm | < 0.5 mΩ | Excellent | RF contact surfaces |
Frequently Asked Questions
Why is chromate preferred over anodize for RF waveguide housings?
Anodize produces a thick insulating oxide (5 to 75 μm, >10 MΩ) that blocks RF ground currents and EMI gasket contact. Chromate conversion is much thinner (0.25 to 1.0 μm) and remains conductive (<2.5 mΩ). For waveguide housings, EMI enclosures, and connector mounting surfaces requiring both corrosion protection and electrical continuity, chromate is the standard. Internal waveguide surfaces are left bare or gold-plated.
What is the difference between hex and trivalent chromate?
Hexavalent (Cr6+, Alodine 1200S) produces gold/iridescent finishes with self-healing properties but is carcinogenic and restricted by REACH/RoHS. Trivalent (Cr3+, SurTec 650) produces clear to light blue coatings meeting MIL-DTL-5541 Type II with matching salt spray and contact resistance but no self-healing. Most defense programs now accept trivalent alternatives.
How does chromate conversion affect RF performance?
At microwave frequencies, skin depth in aluminum is 0.5 to 2 μm (comparable to chromate thickness). On internal waveguide surfaces, the coating adds 0.01 to 0.05 dB loss per interface. On external housings and flanges, impact is negligible because current flows through the conductive chromium oxide matrix. Best practice: chromate-coat external surfaces, leave internal RF surfaces bare or gold-plate them.