Chemical Conversion Coating
Anodizing & conversion coatings · Chemical bath
Chem film is a thin chemical conversion coating for aluminum that improves corrosion resistance while retaining useful electrical conductivity.
What is Chem film?
Aerospace and electronics drawings commonly specify MIL-DTL-5541 Class 1A or Class 3.
How it works
A cleaned and deoxidized aluminum part is sprayed with or immersed in a qualified conversion solution. The solution reacts with the aluminum rather than depositing a separate metal layer, producing a hydrated conversion film about 0.1–1 µm thick. Hexavalent-chromium systems provide strong self-healing corrosion performance; qualified trivalent-chromium systems reduce regulated substances. Class 1A emphasizes corrosion protection and paint adhesion, while Class 3 limits electrical contact resistance.
Why specify it
- Adds essentially no dimensional change (typically well under 1 µm)
- Class 3 preserves low-resistance grounding surfaces
- Class 1A improves paint and primer adhesion
- MIL-DTL-5541 qualification supports aerospace procurement
Where it falls short
- Softer and less wear-resistant than anodize
- Scratching can expose bare aluminum
- Hexavalent-chromium chemistries are tightly regulated
- Color and iridescence vary by alloy and chemistry
Chem film at a glance
Chem film is a middle-of-the-range finish; its best score is corrosion protection (60 of 100), at a budget cost. It adds little lubricity. Surface hardness is not rated: the finish leaves the part's own surface.
Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.
Chem film key numbers
Typical values from the governing specifications and shop practice; bars are this finish against the highest value among anodizing & conversion coatings finishes.
- Thickness
- 0.1–1 µm; Class 3 is at the thin end and Class 1A toward about 1 µm. Acceptance is by performance, not thickness.
- Corrosion (salt spray)
- 168 h per ASTM B117 per MIL-DTL-5541 — Class 1A qualification panels are evaluated after 168 hours; this is not a universal service-life prediction.
- Electrical
- Conductive
- Colours
- clear
- gold
- iridescent
- Appearance
- Clear to pale blue, gold, or iridescent depending on the qualified chemistry.
Specifications for Chem film
The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard, and US Government documents are free to download there.
| Standard | Type / Class | Thickness | Requirement / test | Status |
|---|---|---|---|---|
Chemical Conversion Coatings on Aluminum and Aluminum Alloys · DoD | Type I Class 1A | — | Hexavalent-chromium conversion coating for maximum corrosion protection and paint adhesion. · ASTM B117, 5% salt spray, 168 h; wet-tape adhesion when painted. | Active |
| Type I Class 3 | — | Thin hexavalent-chromium conversion coating for low electrical contact resistance. · ASTM B117, 5% salt spray, 168 h; contact resistance not over 5000 µΩ/in² as supplied and 10000 µΩ/in² after exposure when tested at 200 psi. | ||
| Type II Class 1A | — | Non-hexavalent conversion coating for maximum corrosion protection and paint adhesion. · ASTM B117, 5% salt spray, 168 h; wet-tape adhesion when painted. | ||
| Type II Class 3 | — | Thin non-hexavalent conversion coating for corrosion protection with low electrical contact resistance. · ASTM B117, 5% salt spray, 168 h; contact resistance requirements apply when specified. |
Test and related methods: ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus), MIL-DTL-81706 (Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys).
Drawing call-out
CHEMICAL CONVERSION COAT PER MIL-DTL-5541, TYPE II, CLASS 3, CLEAR; MASK THREADED CONTACT STUDSPattern: CHEMICAL CONVERSION COAT PER MIL-DTL-5541 TYPE <I|II>, CLASS <1A|3>, [COLOR], [MASKING]
Which materials take Chem film?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | The primary substrate covered by MIL-DTL-5541.
| |
| With care | Only aluminum DMLS/SLM parts; steel and titanium builds cannot be chem-filmed. | |
| No | Brass is not processed under MIL-DTL-5541. | |
| No | Copper alloys use different conversion or plating processes. | |
| No | Fused-filament polymers are incompatible with this process. | |
| No | Use a magnesium-specific conversion process. | |
| No | Polymer powder-bed prints are incompatible with this process. | |
| No | Organic polymers cannot take this inorganic metal-finishing process. | |
| No | Photopolymer prints are incompatible with this process. | |
| No | Polymer powder-bed prints are incompatible with this process. | |
| No | Stainless steel uses passivation, not aluminum chem film. | |
| No | MIL-DTL-5541 chem film is an aluminum conversion process. | |
| No | Titanium uses a different conversion or anodic treatment. | |
| No | Zinc alloys are not covered by MIL-DTL-5541. |
How Chem film is applied, and what it does to the part
- 1Clean / degrease
- 2Deoxidize / desmut
- 3Rinse
- 4Apply conversion solution
- 5Dry
| Dimensions | The 0.1–1 µm film is normally negligible for machined tolerances, but contact areas must meet the specified Class 3 resistance. |
|---|---|
| Build-up per side | 0–1 µm — The reacted film is essentially negligible on ordinary machined tolerances. |
| Masking | Usually needed. Mask bonding areas only when bare metal is required; protect trapped cavities from retained solution. |
| Electrical | Conductive |
| Temperature limit | 80 °C (176 °F) continuous |
| Appearance | Clear to pale blue, gold, or iridescent depending on the qualified chemistry. |
| Added lead time | 2–4 working days on top of the bare part |
Design rules for Chem film
What to decide on the drawing before the part goes to the finisher.
- State Type I or Type II and Class 1A or Class 3.Chemistry and electrical performance cannot be inferred from the words chem film.
- Identify electrical bonding faces.Class 3 is selected where low contact resistance matters.
- Provide drain paths in blind cavities.Retained chemistry can stain or continue attacking the part.
- Specify any required topcoat separately.Conversion coating alone is not a paint system.
Chem film versus the alternatives
Finishes engineers weigh against Chem film, and when each wins.
| Finish | Corrosion | Hardness | Cosmetic | Cost | Thickness | Choose it when |
|---|---|---|---|---|---|---|
Chem film this page | 60 | — | 35 | $ | 0.1–1.5 µm | — |
| 71 | 54 | 88 | $$ | 5–25 µm | when greater wear resistance and decorative color are required |
The radar overlays Chem film with Anodizing Type II, its most common alternative — chosen when greater wear resistance and decorative color are required. Scores in the table are 0–100 across every finish in the catalog.
Cost, lead time and compliance
Relative cost
Chemical Conversion Coating is a budget-cost finish ($): cheaper than about 85 % of the finishes FabDigit quotes.
Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates cross-checked against US job-shop pricing. The exact figure appears on the instant quote once the part is uploaded.
Lead time and compliance
| Added lead time | 2–4 working days |
|---|---|
| RoHS | Conditional |
| Notes | MIL-DTL-5541 Type II is hexavalent-chromium-free; Type I is not normally RoHS compliant. |
What Chem film is used for
- Aerospace structural frames
- Electronic chassis grounding
- Satellite components
- Thermal-management heat sinks
Chem film — frequently asked
Is chem film conductive?
Yes, especially MIL-DTL-5541 Class 3, which is intended for low electrical contact resistance. It is less insulating than a 5–25 µm anodic oxide but is not identical to bare-metal conductivity.
How thick is Alodine chem film?
A typical conversion film is about 0.1–1 µm. MIL-DTL-5541 primarily controls performance, so drawings usually specify type and class rather than a thickness.
What is the difference between Class 1A and Class 3 chem film?
Class 1A prioritizes maximum corrosion protection and paint adhesion. Class 3 is used for electrical bonding and must meet lower contact-resistance limits.
Can chem film replace anodizing?
It can replace anodize where conductivity and negligible buildup matter. It cannot match the wear resistance of Type II or 25–75 µm Type III anodize.
Is MIL-DTL-5541 chem film RoHS compliant?
Type II qualified systems contain no hexavalent chromium and are the usual RoHS choice. Type I systems use hexavalent chromium and require regulatory review.
Other anodizing & conversion coatings finishes
Boric-Sulfuric Acid Anodizing Type ICBoric-sulfuric acid anodizing is a thin aerospace aluminum anodize recognized as MIL-A-8625 Type IC.CorrosionHardnessCost$$$
Chromic Acid Anodizing Type IType I and low-voltage Type IB chromic-acid anodizing form thin, corrosion-resistant oxide on aluminum with less fatigue penalty than thick sulfuric anodize.CorrosionHardnessCost$$$
Hard Anodizing Type IIIType III hard anodizing forms a thick, dense aluminum-oxide layer for sliding, abrasive, and high-load service.CorrosionHardnessCost$$$
Hard Anodizing Type III With PTFEPTFE hard anodize combines a 25–75 µm Type III oxide with a fluoropolymer impregnation or post-treatment to reduce friction.CorrosionHardnessCost$$$
Magnesium Chemical Conversion CoatingMagnesium conversion coating reacts with magnesium alloys to improve corrosion resistance and paint adhesion with minimal buildup.CorrosionHardnessCost$$$
Nitriding and QPQNitriding diffuses nitrogen into steel to create a hard wear-resistant case without depositing a conventional coating.CorrosionHardnessCost$$$$
Plasma Electrolytic OxidationPlasma electrolytic oxidation, also called micro-arc oxidation, creates a thick ceramic oxide on aluminum or magnesium using high-voltage micro-discharges.CorrosionHardnessCost$$$$
Sherardized Zinc CoatingSherardizing is a thermal-diffusion zinc treatment for steel fasteners and small components, standardized by ASTM A1059.CorrosionHardnessCost$$$Compare every anodizing & conversion coatings finish side by side →