Chem Film (Chromate Conversion Coating)
Also: chromate conversion coating, Alodine, Iridite, MIL-DTL-5541, chemical film, chem-film, yellow chromate, clear chromate, trivalent chromate, TCP, SurTec 650, Bonderite
Chem film (MIL-DTL-5541, Alodine) is a chromate conversion coating on aluminum: sub-micron, conductive, corrosion resistant, and the standard paint base. Clear or gold.
44 shops tagged Chem film among 3,387 finishing and heat treating companies in the Noramark directory
Chem film is a chemical conversion coating on aluminum. The part is cleaned, deoxidized and dipped, sprayed or brushed with a chromate solution for one to five minutes; the aluminum surface reacts to form a thin, gel-like chromium-aluminum oxide film that dries in place. The film is 0.25-1 µm thick, invisible on the dimensions, and it does two things well: it slows corrosion on bare aluminum, and it gives paint, primer and adhesives a surface to bond to. It also stays electrically conductive, which anodize does not.
MIL-DTL-5541 is the spec everyone quotes. Type I is the original hexavalent chromate (Alodine 1200, Iridite 14-2), which gives the familiar gold-to-brown iridescent colour and the best corrosion resistance. Type II is the hexavalent-free chemistry (trivalent chromium process, TCP; Alodine 5200, SurTec 650), clear to light blue-violet, required for RoHS and ELV and now the default in most new work. Class 1A is the thicker film for maximum corrosion protection and as a paint base; Class 3 is a thinner film that trades a little protection for low electrical contact resistance, the choice for grounding surfaces, EMI gaskets and bus bars.
Chem film is cheap, fast and undemanding of the part, but it is soft: it scratches with a fingernail, wears off with handling and offers no abrasion resistance. Bare Class 1A chem film on 2024 passes 168 hours of salt spray on test panels; that is a bench figure, and in real outdoor service chem film is a primer, not a finish. Think of it as the aluminum equivalent of a phosphate: what you put under paint, or what you use where anodize would break a ground.
At a glance
| Typical tolerances | Film 0.25-1 µm (0.00001-0.00004"), below any drawing tolerance; no masking is needed for fits, threads or bores. The pre-treatment matters more: the deoxidize step can etch 1-3 µm from the surface and dull a machined finish, and a heavy etch on a 2xxx or 7xxx alloy can leave smut that must be removed. |
|---|---|
| Size limits | Tank dipping to 2.5-3 m (8-10 ft) is common; larger panels and assemblies are sprayed or brushed with pen or swab kits (touch-up per MIL-DTL-5541 is allowed and common in aerospace repair). Small parts go in baskets. Assemblies with trapped volumes are brush-applied. |
| Surface finish | Follows the incoming surface after a slight etch. Type I Class 1A: gold to tan iridescent, darker on copper-bearing alloys; Class 3: light gold to nearly clear. Type II: clear to faintly blue or iridescent, harder to see, sometimes dyed for inspection. No hardness change; the film is soft and easily marked. Contact resistance under about 5 milliohm per square inch on Class 3 per the usual MIL-DTL-81706 test. Salt spray on bare Class 1A: 168 h on 2024-T3 panels; in service, use under paint. |
| Lead time | 1-3 business days, frequently same day; it is the fastest aluminum finish there is. Add time for RoHS documentation on Type II or for contact-resistance testing on Class 3. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
All aluminum alloys, wrought and cast, including 6061, 5052, 2024, 7075 and die-cast A380; high-copper and high-silicon alloys need a deoxidize step and may come out darker. Also applied by related chemistries to magnesium, zinc, cadmium and zinc-plated steel, each under its own spec. Not applicable to steel, stainless, titanium or plastics.
What drives the cost
- Lot size: minimum lot charges dominate; large baskets are pennies per part
- Type II (trivalent) chemistry, which costs more per litre and needs tighter control than Type I
- Cleaning and deoxidizing of heavily machined, cast or oxidized parts
- Brush or pen application on large assemblies and for touch-up
- Racking of cosmetic parts instead of basket handling
- Testing and certs: salt spray coupons, contact resistance on Class 3, MIL-DTL-5541 conformance
When to use it
- The paint or powder base on any aluminum part (Class 1A)
- Electrical grounding surfaces, EMI shielding contacts and RF enclosures that must stay conductive (Class 3)
- Corrosion protection on aluminum parts that cannot be anodized: assemblies, tight-tolerance fits, castings
- Aerospace and defense aluminum hardware where MIL-DTL-5541 is called out on the drawing
- A cheap, fast protective finish on internal or non-cosmetic aluminum parts
When not to
- Any surface that will be handled, abraded or worn: chem film scratches off; use anodize
- Cosmetic parts: the colour is uneven and fades, and Type II is nearly invisible
- Outdoor exposure without paint over it; it is a primer, not a topcoat
- Steel, stainless or plated parts: use phosphate, zinc or passivation there
- Where hexavalent chromium is banned (RoHS, ELV, REACH markets) and Type I is specified; switch to Type II
Design tips
- Specify "Chemical conversion coat per MIL-DTL-5541, Type II, Class 3" (or Type I, Class 1A); the class and type are the whole spec.
- Class 1A under paint and for corrosion; Class 3 where a ground, an EMI gasket or a bus bar touches the part.
- Chem film after all machining, deburring and blasting; it is the last step before paint.
- No masking is needed for fits, but mask any surface that must stay bright aluminum (contact pads for welding, for example).
- Choose Type II for anything sold into RoHS or ELV markets, and say so on the drawing; Type I is still allowed on many military drawings.
- If the film must be visible for inspection, ask for Type I gold or a dyed Type II; a clear Type II is hard to verify by eye.
- On castings and 2xxx or 7xxx alloys, allow for a darker, more mottled colour and a light etch of the surface.
Frequently asked questions
- What is the difference between chem film and anodizing?
- Chem film is a sub-micron chemical conversion film: soft, conductive, fast, cheap, and mostly a paint base. Anodizing is a 8-75 µm grown oxide: hard, insulating, dyeable and a finish in itself. Use chem film for grounds and under paint; anodize for wear, colour and bare exposure.
- Is chem film conductive?
- Yes. Class 3 chem film is specified for low electrical contact resistance and is the standard treatment on grounding surfaces, EMI gaskets and RF enclosures. Class 1A is also conductive but with higher resistance. Anodize is an insulator.
- Is Alodine the same as chem film?
- Alodine is a Henkel trade name for chromate conversion products (Alodine 1200 is hexavalent Type I; Alodine 5200 is Type II). Iridite, SurTec 650 and Bonderite are other brands. "Chem film" and "chromate conversion coating" mean the process; MIL-DTL-5541 is the spec.
- How thick is chem film?
- About 0.25-1 µm (10-40 millionths of an inch), so thin that no dimension on a drawing changes and no masking is needed for fits or threads. It is the thinnest aluminum finish in common use.
- Is chem film RoHS compliant?
- Type II (trivalent chromium) is; Type I (hexavalent) is not. Specify MIL-DTL-5541 Type II for anything sold into RoHS, ELV or REACH-controlled markets, and confirm the shop's chemistry on the cert.
