Chemical Conversion Coating to MIL-DTL-5541: Engineer's Reference
MIL-DTL-5541 chem film comes in two types, Type I with hexavalent chromium and Type II without, and two classes, 1A for maximum corrosion protection and 3 for low electrical resistance. The coating must be made with a material qualified to MIL-DTL-81706, and the drawing should state type and class because the spec defaults to Type I.
Updated
Two documents: the process spec and the materials spec
Chem film is governed by a pair of specs. MIL-DTL-5541 covers the coating as applied to parts: types, classes, cleaning, application, process control and acceptance. MIL-DTL-81706 covers the chemicals: it qualifies proprietary conversion products, which are then listed on the qualified products list (QPL-81706). A finisher conforms to 5541 by running an 81706 qualified product within its listed type, class, form and method.
| Spec | What it covers | Status as checked October 2026 |
|---|---|---|
| MIL-DTL-5541F | Conversion coatings on aluminum: Types I and II, Classes 1A and 3 | Active, revision F dated 11 July 2006; supersedes MIL-C-5541E |
| MIL-DTL-81706B w/Amendment 1 | Conversion materials: types, classes, Forms I to VI, Methods A to D, qualification tests | Active; Notice 1 of 9 April 2019 validated it for use |
| QPL-81706 | Qualified products by type, class, form and method | Maintained in the DLA qualified products database |
| MIL-C-5541 (revision E and earlier) | Earlier designation of the coating spec | Superseded by MIL-DTL-5541F |
Paragraph 3.1 of MIL-DTL-5541F says that if no type is specified, Type I is used, and that Type I and Type II may not be substituted for each other unless the contract or order allows it. If the class is not specified, the notes recommend Class 1A. A drawing note that says only "chem film per MIL-DTL-5541" therefore produces hexavalent Class 1A. For how chem film compares with anodize and passivation, see our surface finishing overview.
Type I, Type II and trivalent chromium (TCP)
Type I materials contain hexavalent chromium; Type II materials contain none. MIL-DTL-81706B describes Type II as formulated from salts, catalysts, activators and accelerators with no hexavalent chromium, and it does not prescribe the chemistry. Qualification is by performance.
A common Type II family is trivalent chromium, often called TCP. As one example, as of October 2026 SurTec describes its SurTec 650 as a chromium(III) passivation based on a NAVAIR patent, QPL listed and meeting MIL-DTL-81706B and MIL-DTL-5541F. We name it only to show what a Type II product listing looks like, not as a recommendation. Ask your finisher which QPL product it runs and confirm the listing covers the class and application method used on your parts.
Practical differences buyers notice:
- Appearance. Qualified coatings range from clear or colorless to iridescent yellow, brown, gray or blue, and color varies with alloy. Clear Type II coatings are hard to inspect visually. For Type I, MIL-DTL-5541F points to the ASTM B449 spot test to confirm the coating is present; for Type II it defers to the manufacturer's method.
- Drying temperature. MIL-DTL-81706B limits Type I coatings to 140 °F surface temperature during drying or curing and Type II to 250 °F or the supplier's limit, whichever is lower. MIL-DTL-5541F adds that unpainted conversion coatings start losing corrosion resistance at 140 °F or higher, during drying, later fabrication or service.
- Coating weight. Class 1A Type I material must produce at least 40 mg/ft2 in qualification; Class 1A Type II at least 10 mg/ft2. A thinner film is not automatically a worse one, but it is one reason legacy qualification data on Type I does not transfer to Type II automatically.
Before switching an existing drawing from Type I to Type II, check paint system qualification, electrical bonding data and any prime or program finish specification that still names Type I.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Class 1A versus Class 3, and contact resistance
Class 1A is for maximum corrosion protection, painted or unpainted. Class 3 is a thinner film for corrosion protection where low electrical resistance is required. MIL-DTL-5541F notes that the same conversion material can be listed for both classes because thickness is controlled by immersion time, and that Class 3 coatings, being thinner, are more susceptible to corrosion.
Contact resistance is where most purchasing mistakes happen. MIL-DTL-5541F paragraph 3.7 says electrical resistance testing of Class 3 coatings is performed only as specified in the contract or order, which must give the test method, frequency and required values. The qualification values come from MIL-DTL-81706B: at 200 psi electrode pressure, not more than 5,000 microhms per square inch as applied and 10,000 microhms per square inch after salt spray, with individual readings up to 20 percent over the limit allowed if the average conforms.
So if your bonding or EMI design depends on a resistance value, put the value, the test method and the sampling on the drawing or PO. Otherwise the finisher is not required to measure it. The spec also warns that panel surface roughness and electrode flatness can affect readings more than coating thickness does: rough panels read lower because the film fractures under the electrode, and an electrode that is not flat reads higher because the real contact area is smaller.
Salt spray, paint adhesion and process control
Process control panels for MIL-DTL-5541F see 168 hours of 5 percent neutral salt spray to ASTM B117, inclined 6 degrees plus or minus 2 from vertical, after drying 24 hours at 60 to 100 °F. Acceptance is no more than 5 isolated pits per panel and no more than 15 across all five panels, none larger than 0.031 in, ignoring 0.25 in at edges, markings and holding points. Loss of color is not a rejection.
| Test | Class | Requirement |
|---|---|---|
| Salt spray, process control (5541F) | 1A and 3 | 168 h, pit limits above |
| Salt spray, material qualification (81706B) | 1A | 336 h on 2024-T3 and 7075-T6; 168 h for Form VI, Method D pens |
| Salt spray, material qualification (81706B) | 3 | 168 h on 6061-T6 |
| Wet tape paint adhesion | 1A and 3 | No intercoat separation, 4A or better by ASTM D3359 Method A, over 0.6 to 0.9 mil epoxy primer |
| Contact resistance | 3 | Only as specified in the contract or order |
Process control tests run at least monthly with no more than 35 days between tests, and solution analysis at least weekly. When a process control test fails, production stops, and all traceable, retrievable work since the last passing test is rejected unless shown to conform. Painted parts and parts built into assemblies are not considered retrievable. Lot acceptance is visual, with a default lot of one day's production sampled to ASQ Z1.4 level II.
Option 2 test panels are 2024-T3 for Class 1A and 6061-T6 for Class 3. The notes point out that 2024-T3 is more susceptible to salt spray failure because of its copper, so a finisher passing on 6061 is not proof of performance on a 2024 part. The notes also say conversion coating in an unheated facility in cold weather is not recommended, since many materials react poorly at low temperature.
MIL-DTL-81706 forms, methods and touch-up pens
MIL-DTL-81706B classifies materials by form and application method, and its part number encodes all four choices. The spec's own example, M81706-1-A-2-B, decodes as Type I (hexavalent), Class 1A, Form II (powder), Method B (brush-on or wipe-on).
| Form | Description | Method | Description |
|---|---|---|---|
| I | Concentrated liquid | A | Spray |
| II | Powder | B | Brush-on or wipe-on |
| III | Premixed liquid for touch-up brushing | C | Immersion |
| IV | Premixed thixotropic liquid | D | Applicator pen or presaturated device |
| V | Premeasured thixotropic powder | ||
| VI | Premixed liquid in a self-contained applicator |
Touch-up pens are Form VI, Method D. Under MIL-DTL-5541F, touch-up of mechanical damage or contact marks is allowed only if the contract or order permits it, must use QPL material approved for the applicable type, class, form and method, and is limited to 5 percent of the item's surface area; above that, the acquisition activity must approve. As of October 2026, a Boeing Distribution product sheet lists Henkel's Bonderite M-CR 871 Aero pen as MIL-DTL-81706 Type 2 and M-CR 1132 Aero as Type 1, both Class 1A and 3, with a two year shelf life from manufacture. Check the date code on receipt, because MIL-DTL-81706B only requires materials to have at least a 12 month storage life and pens past their date are a common receiving finding.
Chem film is also the standard repair for damaged anodize and the specified post treatment for IVD aluminum, covered on our anodizing and plating and cadmium alternatives pages.
Hexavalent chromium rules: DFARS 252.223-7008
On DoD contracts that include it, DFARS 252.223-7008 (January 2023) prohibits delivering an item that contains hexavalent chromium above 0.1 percent by weight in any homogeneous material, or that requires hexavalent chromium materials to be removed or reapplied during sustainment, unless the contracting officer specifies otherwise. The contractor must flow the clause to subcontracts for supplies, maintenance and repair, and construction materials, including commercial items.
- The clause defines homogeneous material as material of uniform composition that cannot be mechanically separated, and states that it does not include conversion coatings that chemically modify the substrate.
- The sustainment wording may matter for a design whose repair instructions call for Type I touch-up in service.
- Hexavalent chromium produced as a by-product of manufacturing is excluded.
- DFARS 223.7304 exempts legacy systems already containing hexavalent chromium and later sustainment contracts for systems where its use was previously approved.
- Where use is needed, DFARS 223.7305 requires authorization at a level no lower than a general or flag officer or a Senior Executive Service member at the Program Executive Office or equivalent level, in coordination with the component Corrosion Control and Prevention Executive.
How the conversion coating carve-out applies to a specific Type I part is a contract interpretation question for your contracting officer, not for the finisher. What a buyer can do is make the type explicit on the drawing, ask whether the contract carries 252.223-7008, and keep the answer in the order file. Defense buyers can read more on our defense contractors page.
What to call out on the drawing or PO
- Spec and revision: MIL-DTL-5541F, Type I or Type II, Class 1A or 3
- Application method if it must be restricted (immersion versus spray or brush)
- For Class 3: resistance limit, test method and test frequency, or a statement that no test is required
- Whether touch-up of damage and contact marks is permitted, and with which form
- Colorless coating, if required
- Paint system to be applied afterward, so adhesion panels match production
- Masked areas, and which surfaces get Class 3 versus Class 1A on mixed parts
- Whether DFARS 252.223-7008 applies to the contract
- A cert per lot naming the spec, type, class, QPL product and lot
Two compatibility notes from the spec are worth adding to your finish review: excessively thick conversion coatings can cause paint blistering, and compatibility problems have been reported between conversion coatings and some chemical agent resistant coatings (CARC), which are covered on our paint and primer systems page. Sequencing and approved processor requirements are on our special processes page. Back to the coatings library.
Get chem film sourced with the right type and class
Send the drawing and finish callout through the special process request form, and tell us whether the contract carries DFARS 252.223-7008 and whether Class 3 resistance testing is required. We source the work from qualified finishers, flow down the type, class, test and touch-up requirements, check the certs before parts ship, and a person replies within one business day. Every order ships with a certificate of conformance. Controlled drawings follow our controlled programs process.
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Questions
What does MIL-DTL-5541 default to if no type is given?
Type I, the hexavalent chromium coating. Paragraph 3.1 of MIL-DTL-5541F says Type I is used when no material type is specified, and that Type I and Type II cannot be substituted for each other unless the contract or order allows it. If no class is given, the notes recommend Class 1A. State both type and class on every drawing to avoid getting a coating you did not intend.
What contact resistance does Class 3 chem film have to meet?
Only what your contract or order specifies. MIL-DTL-5541F leaves the method, frequency and values to the buyer. The material qualification limits in MIL-DTL-81706B are 5,000 microhms per square inch as applied and 10,000 after salt spray, measured at 200 psi electrode pressure. If grounding or EMI performance depends on the value, put the limit and the test method on the drawing or purchase order.
Is TCP the same as Type II chem film?
TCP, a trivalent chromium conversion chemistry, is one kind of Type II material, but Type II is defined only as containing no hexavalent chromium. Any product qualified to MIL-DTL-81706 Type II and listed on QPL-81706 for your class, form and method meets the requirement. Ask the finisher to name the QPL product it uses, and confirm paint and bonding qualification before changing an existing Type I callout.
Can I use a chem film touch-up pen on delivered parts?
Only if the contract or order permits touch-up. MIL-DTL-5541F limits touch-up to mechanically damaged areas and contact marks, with QPL material approved for the right type, class, form and method, and to no more than 5 percent of the item's surface area. Pens are MIL-DTL-81706 Form VI, Method D. Check that the pen type matches the drawing, Type I or Type II, and that it is in date.
Does DFARS 252.223-7008 ban Type I chem film?
Not automatically. The clause bars hexavalent chromium above 0.1 percent by weight in any homogeneous material, but its definition of homogeneous material excludes conversion coatings that chemically modify the substrate. It also bars items that need hexavalent chromium removed or reapplied in sustainment, and legacy systems are exempt. Whether a Type I callout is acceptable on your contract is a question for the contracting officer.
How long is chem film tested in salt spray?
For process control under MIL-DTL-5541F, 168 hours of 5 percent neutral salt spray per ASTM B117, with no more than 5 pits per panel and 15 across five panels, none over 0.031 inch. Qualification of Class 1A materials under MIL-DTL-81706B uses 336 hours on 2024-T3 and 7075-T6, except pens at 168 hours. Class 3 materials qualify at 168 hours on 6061-T6.
Related
Sources
- EverySpec: MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys (11 July 2006)
- EverySpec: MIL-DTL-81706B with Amendment 1, Chemical Conversion Materials for Coating Aluminum (2 May 2006)
- EverySpec: MIL-DTL-81706B Notice 1, validation (9 April 2019)
- Acquisition.gov: DFARS 252.223-7008 Prohibition of Hexavalent Chromium (January 2023)
- Acquisition.gov: DFARS 223.7304 Exceptions
- Acquisition.gov: DFARS 223.7305 Authorization and approval
- Boeing Distribution: Henkel Touch-N-Prep coating pens product sheet
- SurTec: SurTec 650 trivalent chromium passivation for aluminium
- HDIAC Technical Inquiry 2018-3809: common protective coatings for aircraft
- EverySpec: MIL-DTL-83488D, Coating, Aluminum, High Purity (1 April 1999)
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