Guides / Finishing

When the Part Has to Carry Current, Anodize Is Wrong

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

An engineer specifies black anodize on an enclosure, the enclosure goes into a rack, and the ground bond fails. Anodizing is aluminum oxide, and aluminum oxide is a good insulator. That is the point of it for most applications and a problem for this one.

The two finishes side by side

	Chem film (MIL-DTL-5541)	Type II anodize
Thickness	Well under 1 um	5 to 25 um
Electrical conductivity	Conductive, passes ground bonds	Insulating
Effect on dimensions	Negligible	About half the thickness per surface
Corrosion protection	Good	Better
Wear resistance	Very low	Moderate
Appearance	Clear to gold iridescent	Clear or dyed any colour
Paint adhesion	Excellent, standard primer base	Good
Cost	Lower	Higher

Where chem film is the right answer

Anything that has to ground. Enclosure faces, chassis, rack mounts, RF shielding, busbar contact areas. If a ground bond or an EMI seal touches the surface, that surface cannot be anodized.

Tight tolerance features you do not want to mask. Because the film is a fraction of a micron, it does not meaningfully change a fit. On a part with a lot of close tolerance features, chem film avoids the entire dimensional growth problem rather than managing it.

Under paint or powder coat. Chem film is the standard primer base for aluminum. It gives the coating something to key into and stops corrosion creeping under a chip.

Low quantity parts where anodizing minimums hurt. Anodizing has a batch minimum charge. On ten small parts that minimum can exceed the machining cost. Chem film is usually cheaper at low volume.

Copper bearing alloys. 2024 and 7075 both need protection and both anodize unevenly. Where appearance does not matter, chem film is a cheaper and more predictable way to get corrosion resistance onto them.

Where it is the wrong answer

Anything that slides, rubs, gets handled a lot or has to look good. Chem film wears off. It is also not a decorative finish: Class 1A comes out gold to brown iridescent and Class 3 is close to clear, and neither can be dyed a colour.

For wear surfaces, go to Type III hardcoat. For anything visible, Type II.

Classes, and why they matter

MIL-DTL-5541 has two classes worth knowing:

• Class 1A is the thicker one, for maximum corrosion protection and as a paint base. Usually visibly gold.
• Class 3 is the thinner one, specified where low electrical resistance is the priority. Nearly colourless.

If the part grounds, ask for Class 3. If it is being painted or just needs to survive, ask for Class 1A.

There is also a hexavalent chromium question. Traditional chem film used hexavalent chrome, which is restricted under RoHS and REACH. Trivalent chrome alternatives (often marked TCP) are now standard and meet the same specification. If your product ships into the EU, write TRIVALENT CHROMIUM ONLY on the drawing and avoid a compliance problem later.

Writing it up

CHEMICAL CONVERSION COAT PER MIL-DTL-5541 CLASS 3,
TRIVALENT CHROMIUM. ALL SURFACES.

Or, for the mixed case that most enclosures actually need:

ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 15 um NOMINAL.
MASK GROUND PAD FACE 'A'. CHEM FILM MASKED AREA
PER MIL-DTL-5541 CLASS 3 AFTER ANODIZING.

The second version costs more and it is the one that passes the ground bond test.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• Chromate conversion coating, MIL-DTL-5541
• RoHS restrictions on hexavalent chromium
• Anodizing

Frequently asked questions

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