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Chem film (chromate conversion)
Chem film, also called chromate conversion coating or by trade names such as Alodine and Iridite, is a chemical dip that converts the aluminum surface into a protective film under 0.00004 in (1 µm) thick. It adds corrosion resistance, improves paint adhesion and stays electrically conductive. It is specified to MIL-DTL-5541, Type I (hexavalent) or Type II (trivalent), Class 1A or Class 3.
- Applies to
- Aluminum alloys, wrought and die cast, including A380
- Adds
- 3 days
- Cost
- Low
Colors
- Gold / yellow iridescent (Type I)
- Clear to light blue iridescent (Type II)
Specs
| Thickness | Under 0.00004 in (1 µm); no practical dimensional change |
|---|---|
| Class 1A | Maximum corrosion protection, painted or unpainted |
| Class 3 | Low electrical contact resistance for grounding and EMI |
| Salt spray | 168 h ASTM B117, unpainted, per MIL-DTL-5541 |
| Color | Type I: gold to iridescent yellow; Type II: clear to light iridescent |
| Handling | Film is soft when fresh; allow about 24 h before heavy handling |
Call out on the drawing
- MIL-DTL-5541 Type I (hexavalent chromium) or Type II (non-hexavalent)
- MIL-DTL-5541 Class 1A or Class 3
- MIL-DTL-81706 (qualified conversion coating chemicals)
- Trade names: Alodine (Bonderite), Iridite, SurTec 650
Questions
Class 1A vs Class 3 chem film?
Class 1A is for maximum corrosion protection and as a paint base. Class 3 is a thinner film for low electrical resistance where grounding or EMI shielding matters, with somewhat less corrosion protection. Specify Class 3 only on parts that must conduct.
Is chem film RoHS compliant?
Type I contains hexavalent chromium and is restricted under RoHS and REACH. Type II trivalent coatings are RoHS compliant and are the default for commercial products; many military programs now call Type II as well.
Chem film vs anodize?
Chem film is thinner, cheaper, faster and conductive, with moderate corrosion protection. Anodize is thicker, harder, insulating, more corrosion and wear resistant, and comes in colors. Use chem film for grounding and paint prep; use anodize for wear, appearance and harsher exposure.
Does chem film change dimensions?
No meaningful change. The film is under 0.00004 in (1 µm), so threads and tight fits need no allowance.
Other finishes on these processes
Price it with this finish.
The builder adds lot charges, per-part cost and lead time for the finish you pick.