Chromate Conversion Coating (Chem Film / Alodine) Explained
Chromate conversion coating — commonly called chem film, Alodine or Iridite — is a thin chemical coating for aluminum that adds corrosion resistance while staying electrically conductive. It's specified by MIL-DTL-5541, comes clear or gold, and is often used as a paint primer or where a part must be both protected and grounded.
See our parts finishing service and aluminum machining guide, or compare with anodizing.
Key takeaways
- Chromate conversion coating (chem film / Alodine / Iridite) is a very thin conversion layer on aluminum — it barely changes dimensions.
- Unique benefit: it's corrosion-resistant AND electrically conductive (unlike anodizing, which insulates) — ideal for grounding and RF/EMI parts.
- Specified by MIL-DTL-5541: Type I (hexavalent chromium, gold) and Type II (trivalent, RoHS-friendly, clear); classes for corrosion (1A) or conductivity (3).
- Excellent paint/primer base; much cheaper and faster than anodizing but less durable/abrasion-resistant.
- Get chem-film-finished parts with your machining — request a quote.
What is chromate conversion coating?
Chromate conversion coating is applied by immersing or brushing a chemical solution onto aluminum, which reacts with the surface to form a thin protective film (typically well under 1 µm). Because it's a conversion of the surface rather than a deposited layer, it adds essentially no measurable thickness — important for tight-tolerance parts. The brand names Alodine and Iridite are the same process; chem film is the generic aerospace term.
Why use it — corrosion resistance + conductivity
The stand-out property is that chem film protects aluminum from corrosion while remaining electrically conductive. Anodizing, by contrast, forms an insulating oxide. That makes chromate conversion coating the go-to for electrical enclosures, grounding points, RF/EMI housings and any part that must be both corrosion-protected and electrically bonded. It's also an excellent primer base, improving paint adhesion.
MIL-DTL-5541 types and classes
| Designation | Meaning |
|---|---|
| Type I | Hexavalent chromium — gold/yellow, highest corrosion resistance |
| Type II | Trivalent chromium — clear, RoHS/REACH-friendly (no hexavalent Cr) |
| Class 1A | Maximum corrosion protection (and paint base) |
| Class 3 | Low electrical resistance (conductivity) with corrosion protection |
Chem film vs anodizing
Both protect aluminum, but they're different tools. Chem film is thin, cheap, fast, conductive and a great paint base, but soft and less durable. Anodizing is thicker, harder, more wear- and corrosion-resistant, dyeable to colors — but electrically insulating and more expensive. Choose chem film when you need conductivity, low cost, or a primer; choose anodizing when you need hardness, wear resistance or color.
Applications
- Electronics and RF/EMI enclosures, grounding and bonding surfaces
- Aerospace aluminum parts (as a paint primer under MIL specs)
- Low-cost corrosion protection for aluminum brackets and housings
Frequently asked questions
What is chem film / Alodine?
What is the difference between chem film and anodizing?
Is chromate conversion coating conductive?
What is the difference between Type I and Type II chem film?
Does chem film change part dimensions?
Sources & further reading: ASTM International · MatWeb — aluminum properties.
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