Chemical Film
Understanding Chemical Film
Bare aluminum rapidly forms a native oxide layer of 2 to 5 nm in air, which provides some corrosion resistance but is too thin and inconsistent for military and aerospace environments. Chemical film treatment replaces this native oxide with a controlled chromate conversion layer that is chemically bonded to the aluminum surface. The process involves immersing or brushing the degreased and deoxidized aluminum part in an acidic chromate solution for 1 to 5 minutes at room temperature, producing a gold (Class 1A) or clear (Class 3) coating that air-cures within 24 hours.
For RF engineers, the crucial property is that chemical film preserves electrical conductivity. Unlike anodize, which creates a thick insulating oxide (25 to 75 μm of Al2O3 with resistance exceeding 10 MΩ), chemical film maintains contact resistance below 5 mΩ at bolted chassis joints. This is essential for maintaining EMI shielding effectiveness: a chassis seam with even 100 mΩ of contact resistance creates a slot antenna that radiates at frequencies where the seam length approaches λ/2. Engineering drawings for RF assemblies routinely specify "chem film per MIL-DTL-5541 Class 3 on all mating and grounding surfaces" while allowing anodize on non-electrical wear surfaces.
Contact Resistance and Shielding Impact
Rc = ρ / (2a) [Ω]
Slot Radiation from Chassis Seam:
SEloss ≈ 20·log10(λ / (2Lslot)) [dB reduction in SE]
Skin Depth of Aluminum:
δAl = (ρ / (π f μ0))½ = 0.83 μm at 10 GHz
Where ρ = resistivity (2.65 × 10-8 Ω·m for Al), a = contact spot radius, Lslot = seam gap length. Chemical film thickness (0.25 to 1.0 μm) is comparable to one skin depth, so RF current passes through with negligible additional loss.
Chemical Film Classes and Properties
| Property | Class 1A (Hex-Cr) | Class 3 (Tri-Cr) | Anodize Type III |
|---|---|---|---|
| Thickness | 0.5 to 1.0 μm | 0.25 to 0.5 μm | 25 to 75 μm |
| Contact Resistance | < 5 mΩ | < 3 mΩ | > 10 MΩ |
| Salt Spray (ASTM B117) | 672 hrs | 336 hrs | 1,000+ hrs |
| RoHS/REACH | Non-compliant (Cr6+) | Compliant (Cr3+) | Compliant |
| RF Ground Use | Yes | Yes (preferred) | No (insulator) |
Frequently Asked Questions
Why is chemical film preferred over anodizing for RF grounding surfaces?
Hard anodize (Type III) creates a 25 to 75 μm aluminum oxide layer that is an excellent insulator, with contact resistance exceeding 10 MΩ. This blocks RF ground current at chassis joints, degrading shielding and creating ground loops. Chemical film (Class 3) produces a 0.25 to 1.0 μm coating with contact resistance below 5 mΩ, preserving EMI ground continuity. RF drawings specify chem film on all mating/grounding surfaces while allowing anodize only on non-electrical wear surfaces.
What is the difference between Class 1A and Class 3 chemical film?
Class 1A uses hexavalent chromium for a gold coating (0.5 to 1.0 μm) with maximum corrosion protection (672 hours salt spray). Class 3 uses trivalent chromium (RoHS/REACH-compliant) for a clear/light-blue coating (0.25 to 0.5 μm) with 336 hours salt spray. Class 3 has 20 to 30% lower contact resistance, making it preferred for RF joints. Most defense programs are transitioning to Class 3 to eliminate hexavalent chromium.
How does chemical film affect RF surface resistivity?
The chromate layer (0.25 to 1.0 μm) is comparable to one skin depth of aluminum at 10 GHz (0.83 μm), so RF surface current passes through with negligible additional loss. Measurements show less than 0.01 dB extra insertion loss per waveguide inch versus bare aluminum. In contrast, anodize (25+ μm of insulating oxide) completely blocks surface current and is unsuitable for any RF-carrying surface.