Guide
Chem film vs anodize: which aluminum finish should you specify?
Chem film is a thin chemical conversion coating that forms when a clean aluminum surface reacts with a conversion solution, with no electric current applied. Anodizing, by contrast, is an electrolytic process that grows a much heavier aluminum oxide out of the surface. On a drawing, chem film is usually called out to MIL-DTL-5541 (trade names such as Alodine and Iridite refer to it), and anodizing to MIL-A-8625 or its successor MIL-PRF-8625. The choice turns on four things: whether the surface must conduct, how much wear it sees, whether a coating build-up can be tolerated, and which alloy the part is made from.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for mechanical engineers and sourcing managers who have to put a finish note on an aluminum part: a machined enclosure, a die-cast housing, a heat sink or a bracket. The requirements quoted here come from the two military specifications that most commercial drawings still borrow, MIL-DTL-5541F for conversion coatings and MIL-A-8625F for anodic coatings, read in their published text. Where a point is practice rather than specification, it is labeled as typical guidance. It also covers die castings, selective chem film on grounding pads and the hexavalent chromium question.
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What is the difference between chem film and anodize?
Chem film is a very thin conversion layer formed by immersion, spray or brush without current, while anodize is a thicker oxide grown electrolytically with the part as the anode. That difference in build is why chem film stays electrically conductive and nearly dimension-neutral, and anodize is insulating, harder and more protective.
MIL-DTL-5541F describes its coatings as the product of a chemical reaction between the conversion material and the aluminum surface, applied “nonelectrolytically” after machining, forming, welding and heat treatment are finished. It sorts them two ways. The type says what is in the chemistry: Type I contains hexavalent chromium and Type II contains none. The class says what the film is for: Class 1A gives maximum corrosion protection, painted or unpainted, and Class 3 gives corrosion protection where low electrical resistance is needed. Per MIL-DTL-5541F paragraph 6.1.2, the two classes can use the same qualified chemical, since one material can be listed on QPL-81706 for both, and differ mainly in immersion time, so Class 3 is simply the thinner film.
Each type and class combination, and how to write it on a drawing, is decoded in the MIL-DTL-5541 type and class guide.
MIL-A-8625F covers electrolytically formed anodic coatings. Types I and IB are chromic acid coatings, IC and IIB are their non-chromate alternatives, Type II is conventional sulfuric acid anodizing and Type III is hard anodize; Class 1 is undyed and Class 2 dyed. The film is then sealed to close its pores.
The two specifications also connect. Per MIL-DTL-5541F paragraph 6.1, conversion coatings are used to repair damaged anodic coatings, and MIL-A-8625F paragraph 6.10.1.1 suggests treating tight-tolerance Type II surfaces that were lapped or honed after anodizing with a conversion coating from the same qualified products list. Many parts carry both, each on its own faces.
| Property | Chem film (MIL-DTL-5541F) | Anodize (MIL-A-8625F) |
|---|---|---|
| Process | Chemical reaction, no current; immersion, spray or brush (3.3) | Electrolytic; part is the anode in an acid bath (1.1, 3.4) |
| Designations | Type I (with Cr(VI)) or Type II (no Cr(VI)); Class 1A or Class 3 (1.2) | Types I, IB, IC, II, IIB, III; Class 1 undyed or Class 2 dyed (1.2) |
| Default if not stated | Type I if no type is given (3.1); Class 1A recommended if no class is given (6.5) | No default type; the drawing must name one |
| Salt spray requirement | 168 hours, ASTM B117, both classes (3.6.1, 4.4.1) | 336 hours, ASTM B117, Types I, IB, IC, II, IIB (3.7.1.2, 4.5.3) |
| Electrical behavior | Class 3 intended for low-resistance contacts (6.1.2) | Insulating oxide; contact faces must be masked |
| Wear | Abrasion resistance relatively low (6.7) | Type III is the wear coating (6.1.3) |
| Dimensional change | No thickness requirement in the specification; treated as negligible in practice | About half the coating thickness per surface (6.10.1) |
| Color | Clear to iridescent yellow, brown, gray or blue (6.8) | Natural or dyed (Class 2) |
| Paint base | Improves paint adhesion; wet tape test required (3.6.2) | Can be painted; usually specified for its own properties |
Which one conducts electricity for grounding and bonding?
Chem film does, and Class 3 is the version written for it; anodize does not, because the oxide it grows is an electrical insulator. If a face has to carry a ground, an EMI gasket or a bonding strap, that face gets chem film or stays bare.
MIL-DTL-5541F paragraph 6.1.2 places Class 3 where lower-resistance contacts are needed relative to both Class 1A and MIL-A-8625 anodic coatings. The resistance limits sit in the qualification specification: per MIL-DTL-5541F paragraph 6.1.2.1, Class 3 materials are qualified under MIL-DTL-81706 to no more than 5 milliohms per square inch as supplied and 10 milliohms per square inch after 168 hours of salt spray, measured at a nominal electrode pressure of 200 psi. For production parts, the specification leaves the test method and limits to the contract (paragraph 3.7), so if resistance matters, the drawing or purchase order has to say so.
This is where selective finishing comes in. A cast or machined enclosure is often anodized or powder coated on its cosmetic exterior while the mating flange, gasket land or ground boss must stay conductive. The usual route is to mask those features during anodizing, then apply Class 3 chem film to them. Draw the conductive areas as a hatched zone with a local note, rather than describing them in words, so the masking and the inspection both work from the same picture.
How do chem film and anodize compare on corrosion and wear?
Sealed anodize is the more protective and far more wear-resistant finish, and chem film is a light protective film that is at its best under paint. The specifications show the gap directly: they ask for twice the salt-spray exposure for anodize.
Per MIL-DTL-5541F paragraph 3.6.1, Class 1A and Class 3 test panels must survive 168 hours of neutral salt spray to ASTM B117 with only a few small isolated pits. Per MIL-A-8625F paragraph 4.5.3, Type I, IB, IC, II and IIB anodic coatings are exposed for 336 hours under a similar pit-count criterion. These are process-control tests on flat panels, not a prediction for your part, and MIL-DTL-5541F notes that Class 3 is more susceptible to corrosion than Class 1A because it is thinner.
On wear, the gap is wider. MIL-DTL-5541F paragraph 6.7 states plainly that conversion coatings have relatively low abrasion resistance: they survive moderate handling but come off under severe wear. Type III hard anodize is the finish written for wear and abrasion. Chem film also loses protection with heat. Per MIL-DTL-5541F paragraph 6.14, unpainted conversion coatings begin to lose corrosion resistance at 140 °F (60 °C) or higher, during drying, later fabrication or service, which matters for parts near motors, heaters or a cure oven.
For painted parts the ranking changes: chem film is a standard pretreatment under primer and paint, and MIL-DTL-5541F requires a wet tape adhesion test for it.
Do chem film or anodize change part dimensions?
Anodize does, and the specification tells you by how much; chem film is thin enough that it is normally ignored in tolerance stacks. That makes chem film the safer choice for threads, bores and press fits that cannot be reworked.
Per MIL-A-8625F paragraph 6.10.1, each coated surface grows by about half the anodic coating thickness, and the specification recommends showing both a machining dimension and a coated dimension on the drawing. Threads are more sensitive, because the growth acts on both flanks: per MIL-A-8625F paragraph 6.10.2, the pitch diameter grows by a multiple of the surface growth. For the arithmetic on hard coat, threads and fits, the anodizing vs powder coating vs zinc plating guide works through the numbers.
MIL-DTL-5541F sets no thickness requirement at all; it controls the film by performance tests and by the qualified chemical. As typical industry guidance, a Class 1A or Class 3 film is treated as having no meaningful effect on machined tolerances, which is why it is the common choice for tapped holes and precision bores on aluminum. Even so, write on the drawing whether dimensions apply before or after finishing, so there is no argument at incoming inspection. For turned parts held to a tight tolerance, any anodic build-up has to be planned into the machining dimension.
Which finish works on die-cast aluminum?
Chem film works on die castings and is a common base for powder coat or paint, while anodizing high-silicon die-casting alloys gives a darker, less uniform film that the specification restricts for hard coat. On a typical ADC12 or A380 housing, chem film is the easier finish to get right.
Per MIL-A-8625F paragraph 3.4.3, Type III hard coat is not applied to alloys with a nominal silicon content above 8.0 percent without the buyer’s approval, and Type I chromic acid coatings carry their own copper and silicon limits in paragraph 3.4.1. Common high-pressure die-casting alloys such as A380 and ADC12 sit above that silicon level. The North American Die Casting Association rates 380 and 383 mid-scale (3 of 5, where 1 is most desirable) for anodized appearance but 4 of 5 for anodized protection (NADCA, Table 4.3). The ADC12 vs A380 guide covers the alloy side.
Chem film has its own casting caveat. MIL-DTL-5541F paragraph 6.8 notes that color may be less uniform on castings and welds than on wrought alloys, because of surface impurities and oxidation, and paragraph 4.2.2 addresses cleaning castings whose preparation would damage wrought test panels. In practice, release agent and machining fluid must come off completely, and the drawing should say whether color variation is acceptable on non-cosmetic faces. The die casting and CNC milling pages describe the partner-foundry casting and lead-plant machining steps that come before the finish.
What do Type I and Type II mean for RoHS?
Type I chem film contains hexavalent chromium and Type II contains none, so a part going into electrical or electronic equipment sold in the EU is normally specified as Type II. The trap is that MIL-DTL-5541F defaults to Type I when the drawing does not name a type.
That default is in paragraph 3.1 of the specification, which also forbids substituting one type for the other unless the contract allows it. The EU restriction sits in Annex II of the RoHS Directive as amended by Delegated Directive (EU) 2015/863, which lists hexavalent chromium among the restricted substances in homogeneous materials (EUR-Lex, (EU) 2015/863). A conversion film is a homogeneous material in its own right, so a bare “chem film per MIL-DTL-5541” note can put a hexavalent coating on a part that otherwise meets the directive.
Anodizing raises the same question in a different place. The chromic acid Types I and IB use a hexavalent bath, and MIL-A-8625F lists a sodium or potassium bichromate (dichromate) seal among the sealing options, so name Type II or IIB and a non-chromate seal when RoHS applies. Type II trivalent-chromium and chromium-free conversion chemistries are the common replacements, and their color can be faint or clear. This page explains the directive; it does not assess any product, so ask for a written declaration covering the specific coating on your part.
How should the finish be written on the drawing?
Name the specification, the type and the class, and show on the drawing which faces get which finish. A note that leaves out the type or class hands the decision to the finisher, and the specification’s defaults may not be what you want.
The examples below are generic wording to adapt, not any supplier’s standard. Check each against the current revision of the specification you cite, which can be confirmed in the Department of Defense ASSIST database.
- CHEMICAL CONVERSION COAT PER MIL-DTL-5541, TYPE II, CLASS 1A. Unpainted or painted parts where corrosion protection comes first and no hexavalent chromium is allowed.
- CHEMICAL CONVERSION COAT PER MIL-DTL-5541, TYPE II, CLASS 3. ELECTRICAL RESISTANCE PER [TEST METHOD AND LIMIT]. Chassis, enclosures and brackets that must bond or ground.
- ANODIZE PER MIL-PRF-8625, TYPE II, CLASS 2, BLACK, NON-CHROMATE SEAL. MASK AREAS A. AFTER ANODIZE, CHEM FILM PER MIL-DTL-5541, TYPE II, CLASS 3, AREAS A. Cosmetic anodized exterior with conductive ground pads.
- CHEMICAL CONVERSION COAT PER MIL-DTL-5541, TYPE II, CLASS 1A, THEN POWDER COAT [PRODUCT, COLOR]. MASK GROUND BOSS B. Die-cast housings where color and a paint base matter.
When the part is made overseas, add three lines to the RFQ: whether the conversion material must be listed on QPL-81706 (MIL-DTL-5541F paragraph 3.1 ties the specification to it), how coating presence and Class 3 resistance will be checked, and whether a non-QPL trivalent or chromium-free equivalent is acceptable. The RFQ preparation guide covers the rest of the package. The surface finishing page lists the finishes our partner plants apply, anodizing on aluminum among them, and the quote names where each step is done; raise any chem film callout in the RFQ so the finishing route is confirmed before quoting, and send the drawing for a quote with the note written as you intend it.
Frequently asked
Is Alodine the same as chem film?
Alodine is a trade name for one family of conversion chemicals; chem film is the generic term. On a drawing, cite MIL-DTL-5541 with a type and class rather than a brand, so any qualified chemistry can meet it.
Can a part be anodized after chem film?
Not directly: the conversion film has to be stripped during the anodizing pretreatment. The normal order is the reverse, anodize first with masked areas, then chem film the masked or reworked areas, which MIL-DTL-5541F describes as a repair use.
What happens if the drawing just says “chem film”?
Under MIL-DTL-5541F the finisher may default to Type I, which contains hexavalent chromium, and the specification recommends Class 1A when no class is given. If you need Type II or conductivity, write it.
Is clear chem film acceptable if you cannot see it?
Yes, many Type II coatings are nearly colorless. MIL-DTL-5541F points to a spot test for Type I and to the chemical maker’s method for Type II, so agree on the check with the finisher.
Should a die-cast housing be anodized or chem filmed?
In most cases chem film, often under powder coat, because high-silicon casting alloys anodize dark and unevenly. Anodize only if the alloy and the appearance requirement have been agreed first.
Sources
- MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, US Department of Defense, 11 July 2006 (paragraphs 1.2, 3.1, 3.3, 3.6, 3.7, 4.4.1, 6.1, 6.5, 6.7, 6.8, 6.12, 6.14)
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993, with Amendment 1 of 2003 (paragraphs 1.2, 3.4, 3.7.1.2, 3.8, 4.5.3, 6.10)
- Commission Delegated Directive (EU) 2015/863 amending Annex II to Directive 2011/65/EU, European Commission, 2015
- Aluminum Alloys, including Table 4.3, NADCA Design, North American Die Casting Association, accessed September 2026