How to choose an aluminum conversion coating: Cr6, Cr3 or none

Choose hexavalent chromate only where a specification still requires it, trivalent chromium for conductive or paint-base parts, and zirconium-based films for painted commercial parts. This holds for aluminum parts before paint or as a bare finish. High-copper 2024 is harder to protect without chromate than 6061 or 7075 (NASA, 2017), so test any substitute on the actual alloy.

• MIL-DTL-5541F defines Type I as compositions containing hexavalent chromium and Type II as compositions containing none, and Type I applies when no type is named (MIL-DTL-5541F, 2006).

• Zirconium conversion coatings are typically 10 to 80 nm thick, considerably thinner than earlier conversion coatings (Kraš, 2024).

• Chromium trioxide sits on the EU REACH Authorisation List with a sunset date of 21 September 2017 (EU REACH Annex XIV, 2026).

• The EU RoHS Directive limits hexavalent chromium to 0.1 % by weight in homogeneous materials of electrical and electronic equipment (EU RoHS, 2026).

This guide covers chemical conversion coatings on aluminum and aluminum alloys, as a bare finish or under paint. It does not cover anodizing, conversion coatings on zinc, steel or magnesium, or parts whose drawing already names a qualified product, where the drawing governs.

Which conditions decide the aluminum conversion coating?

Five conditions decide the aluminum conversion coating: the specification on the drawing, the alloy family, bare or painted service, the need for electrical conductivity, and the hexavalent chromium rules of the market. The specification comes first. MIL-DTL-5541F divides coatings into Type I, compositions containing hexavalent chromium, and Type II, compositions containing no hexavalent chromium, and states that if no material type is specified, Type I shall be used (MIL-DTL-5541F, 2006). It does not permit substituting one type for the other unless the contract or order allows it (MIL-DTL-5541F, 2006).

The class sets the job of the film. Class 1A is for maximum protection against corrosion, painted or unpainted; Class 3 is for protection against corrosion where low electrical resistance is required (MIL-DTL-5541F, 2006). Class 3 coatings are thinner and therefore more susceptible to corrosion than Class 1A coatings (MIL-DTL-5541F, 2006).

The alloy matters because copper weakens every chemistry. MIL-DTL-5541F notes that, due to high copper content, 2024-T3 test panels are more susceptible to salt spray failure than 6061-T6 panels (MIL-DTL-5541F, 2006). For commercial work, ASTM B449-93(2022) covers rinsed and nonrinsed chromate conversion coatings on aluminum and is listed as active by ASTM as of 2026-10-13 (ASTM International, 2022).

Which aluminum conversion coating fits which alloy and service?

The aluminum conversion coating that fits a part follows from its alloy family, its service and its market. Hexavalent chromate stays the default only where a specification or a bare high-copper alloy demands it; trivalent chromium covers bare and conductive parts; zirconium-based films cover painted 5xxx and 6xxx parts.

The decision table below sets out which aluminum conversion coating chemistry to specify for each set of conditions.

One row is easy to miss: a drawing that says only "MIL-DTL-5541 Class 1A" calls for hexavalent chromate by default, so a shop that wants to run trivalent chromium needs Type II named on the drawing or order (MIL-DTL-5541F, 2006). The 2024 row matters next, because a coating that passes on 6061 can fail on 2024 (NASA, 2017). The EU row covers coating in the EU: a REACH authorisation governs placing chromium trioxide on the market and using it (EU REACH, 2026). For coated parts imported into the EU, REACH Article 69(2) instead has ECHA consider, after the sunset date, whether the use of the substance in articles poses a risk that is not adequately controlled (EU REACH, 2026), so check imported parts against RoHS, adopted restrictions or sector rules. Treat every row as a starting point for qualification on the production alloy, temper and paint system: ESTCP found it critical that any new non-chromate material be tested to failure against the chromated control (ESTCP, 2013).

Can trivalent chromium replace hexavalent chromate on 2024 or 7075 aluminum?

The NASA results suggest trivalent chromium can replace hexavalent chromate on 7075 aluminum once it is qualified on the alloy and temper, with results that varied by product, but on 2024 aluminum it replaces chromate only where testing on the actual alloy and paint system shows it passes. In a NASA and ESA program, four hexavalent chromium free pretreatments were salt spray tested to ASTM B117 on 2024-T3, 2024-T8, 6061-T6, 7075-T6 and 7075-T73; in general they did not perform well on 2024-T3 and 2024-T8, and performed better on 6061-T6, 7075-T6 and 7075-T73 (NASA, 2017). The same report attributes the spread to copper: corrosion resistance decreases with increasing copper content, which makes 2024 difficult to protect (NASA, 2017).

Paint changes the picture. An ESTCP demonstration found that the trivalent chromium process (TCP) gives both painted and unpainted corrosion protection and electrical conductivity in corrosive environments, and that TCP performed well in high corrosion environment testing (ESTCP, 2013). TCP also showed good paint adhesion and painted corrosion performance under both chromated and non-chromated primers (ESTCP, 2013).

Hexavalent chromate keeps one property the alternatives lack: soluble Cr(VI) in the film can migrate to a defect and seal it, a self-healing ability not replicated by alternative coatings (Kraš, 2024). The same thesis reports that chromium remains the top choice in the aerospace industry because of its more stringent requirements (Kraš, 2024).

When does a zirconium or titanium conversion coating work on aluminum?

A zirconium or titanium conversion coating works on aluminum when the film is a base for paint or powder rather than the only corrosion barrier, and when the line controls its bath closely. Zirconium conversion coatings are typically 10 to 80 nm thick on all substrates, considerably thinner than earlier technologies, which could present a disadvantage (Kraš, 2024). Deposition follows a local pH rise at cathodic sites, and on aluminum alloys the intermetallic particles attract preferential deposition, which leads to inconsistent thickness (Kraš, 2024).

The same thesis reports that zirconium conversion coatings have matched standard phosphate conversion coatings only on aluminum surfaces, and that zirconium and titanium coatings perform generally below chromate conversion coatings because they lack its self-healing protection (Kraš, 2024). The process gains are real: the literature it reviews cites cost reductions of up to 30 % from faster conversion, room-temperature operation, fewer stages and virtually no sludge (Kraš, 2024).

These findings suggest zirconium films for painted 5xxx and 6xxx parts in commercial and architectural work, and not for bare parts or high-copper aerospace alloys. Their thinness also limits the techniques that can measure them (Kraš, 2024), so acceptance likely rests on painted-panel tests rather than on film measurement alone.

Which aluminum conversion coating works best under powder coat or paint?

Under powder coat or liquid paint on architectural aluminum, an approved chrome-free conversion coating performs on a par with chromate, while on military parts the conversion coating must pass with the production paint system. QUALICOAT, the quality label for architectural powder coating on aluminum, allows three pretreatments: chromate conversion coating, alternative conversion coating and anodic pretreatment (QUALICOAT UK & Ireland, 2025). Its UK and Ireland guidance states that most licensed applicators have changed to processes completely free of chromium trioxide, and that its working group's statistical analysis found no significant difference in corrosion performance between the alternatives (QUALICOAT UK & Ireland, 2025).

The QUALICOAT test matrix shows what a painted pretreatment must survive: acetic acid salt spray to ISO 9227 for 1000 hours (2000 hours for the Seaside endorsement), humidity to ISO 6270 for 1000 hours and filiform corrosion to ISO 4623-2 for 1000 hours (QUALICOAT UK & Ireland, 2025). Ask for results on your powder and alloy, because a pass belongs to the coating system, not to the pretreatment alone.

For military parts, MIL-DTL-5541F requires an adhesion rating of 4A or better under ASTM D3359 Procedure A with the production paint system (MIL-DTL-5541F, 2006). It also warns that excessively thick conversion coatings may cause paint adhesion problems such as blistering (MIL-DTL-5541F, 2006).

Which hexavalent chromium rules limit the aluminum conversion coating choice?

Hexavalent chromium rules limit the aluminum conversion coating choice differently by jurisdiction, so check the market and the date before specifying Type I. In the EU, chromium trioxide is listed in REACH Annex XIV with a sunset date of 21 September 2017 (EU REACH, 2026), and REACH Article 56 bars using an Annex XIV substance unless the use is authorised or exempted (EU REACH, 2026). A proposed EU restriction on certain chromium(VI) substances was at the opinion stage: SEAC adopted its draft opinion on 11 June 2026 and the consultation on it closed on 17 August 2026 (ADCR, 2026). Check ECHA's registry of restriction intentions for the current step; the restriction itself is covered in a separate article.

The EU RoHS Directive limits hexavalent chromium in electrical and electronic equipment to 0.1 % by weight in homogeneous materials (EU RoHS, 2026). For US defense deliverables, DFARS 252.223-7008 (JAN 2023) prohibits more than 0.1 % hexavalent chromium by weight in any homogeneous material, and any deliverable that requires removal or reapplication of hexavalent chromium materials during sustainment, unless the contracting officer specifies otherwise; its definition of homogeneous material excludes conversion coatings that chemically modify the substrate (DFARS, 2023). In US workplaces running Type I baths, OSHA limits airborne chromium (VI) to 5 micrograms per cubic meter as an 8-hour time-weighted average (US OSHA, 29 CFR 1910.1026, 2019).

What should you ask a supplier about a chrome-free aluminum conversion coating?

Ask a supplier of a chrome-free aluminum conversion coating six questions before you approve it, and record the answers in the process specification.

1. Which qualification does it hold? For military work, the product must be qualified to MIL-DTL-81706 and listed on QPL-81706 for the selected type, class, form and application method (MIL-DTL-5541F, 2006).

2. Which alloys and tempers were tested, against which control? ESTCP found it critical to test any new non-chromate material to failure against the chromated control (ESTCP, 2013).

3. Under which primers, paints or powders was it tested, and with what adhesion and filiform results?

4. How do you verify a colorless film is present? Type I films can be checked with the spot test in ASTM B449, while Type II films are verified per the manufacturer's recommendation (MIL-DTL-5541F, 2006).

5. What bath limits apply? Many conversion materials do not react sufficiently with aluminum at low temperatures (MIL-DTL-5541F, 2006), and QPL-81706 products from different suppliers cannot be mixed (MIL-DTL-5541F, 2006).

6. What do the SDS and the wastewater permit require? Trivalent chromium processes still contain chromium, so users must balance total chromium wastewater requirements against performance (ESTCP, 2013).

When should you call a specialist about an aluminum conversion coating change?

Call a coatings or corrosion specialist about an aluminum conversion coating change when a Type I to Type II switch touches a qualified part, a 2024 alloy, an electrical bond or a contract clause. In those cases a passing salt spray panel does not prove the part will survive. MIL-DTL-5541F notes that unpainted conversion coatings begin losing corrosion resistance at 140 °F (60 °C) or higher during drying, fabrication or service (MIL-DTL-5541F, 2006). It also notes that surface roughness and electrode flatness can affect Class 3 resistance values more than coating thickness does (MIL-DTL-5541F, 2006), so a resistance limit without a test method on the drawing invites disputes. A specialist can set the test matrix, the acceptance values and the drawing note together. Working a change like this from your own TDS, SDS and site history is the kind of case Lubinpla structures into one cited document for your review.

FAQ

When in the process should an aluminum conversion coating be applied?

Unless the contract or order names another method, MIL-DTL-5541F requires the coating to be applied by spray, brush or immersion after all heat treatments and mechanical operations such as forming, machining, brazing and welding are complete (MIL-DTL-5541F, 2006). Assemblies with nonaluminum parts that the process could attack shall not be coated as assemblies unless those parts are masked (MIL-DTL-5541F, 2006).

Can a damaged aluminum conversion coating be touched up?

Yes, if the contract or order allows it. MIL-DTL-5541F limits touch-up to an area not greater than 5 percent of the total item surface, using QPL-81706 material for the same type and class (MIL-DTL-5541F, 2006). Larger areas need specific approval from the acquisition activity.

Does a trivalent chromium conversion coating remove all chromium concerns?

No. Trivalent chromium processes still contain chromium, so total chromium wastewater limits apply (ESTCP, 2013), and the US EPA has raised concerns about possible reoxidation of Cr(III) to Cr(VI), as reported in a 2024 doctoral thesis (Kraš, 2024). Confirm the current SDS and local discharge rules before switching.

Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-13.

Sources

1. U.S. Department of Defense. MIL-DTL-5541F Chemical Conversion Coatings on Aluminum and Aluminum Alloys. Revision F, 2006. https://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-DTL/download.php?spec=MIL-DTL-5541F.010200.PDF

2. ASTM International. ASTM B449-93(2022) Standard Specification for Chromates on Aluminum. 2022. https://store.astm.org/b0449-93r22.html

3. Greene, B. (NASA). NASA and ESA Collaboration on Hexavalent Chrome Free Coatings. 2017. https://ntrs.nasa.gov/api/citations/20170004876/downloads/20170004876.pdf

4. SERDP and ESTCP. Demonstration and Validation of Non-Chromate Aluminum Pretreatments, WP-200025. 2013. https://serdp-estcp.mil/projects/details/62eb9394-6723-4b57-a567-a5842e01244c

5. Kraš, A. Deposition and Protection Mechanisms of Zirconium Conversion Coatings on Aluminium Alloy, Steel and Zinc Substrates. Doctoral dissertation, Jožef Stefan International Postgraduate School. 2024. https://www.mps.si/en/repository/academic-work/deposition-and-protection-mechanisms-of-zirconium-conversion-coatings-on-aluminium-alloy-steel-and-zinc-substrates/download/

6. QUALICOAT UK & Ireland. Guidance Note No. 5, The Conversion Coating Processes. 2025. https://www.qualicoatuki.org/s/05_the_conversion_coating_processes_250407.pdf

7. European Union. Regulation (EC) No 1907/2006 (REACH), consolidated version of 22 June 2026. 2026. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:02006R1907-20260622

8. ADCR Consortium. ECHA launches CrVI consultation on draft SEAC opinion. 2026. https://www.adcr-consortium.eu/echa-launches-crvi-consultation-on-draft-seac-opinion

9. European Union. Directive 2011/65/EU (RoHS), consolidated version of 1 July 2026. 2026. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:02011L0065-20260701

10. U.S. Department of Defense. DFARS 252.223-7008 Prohibition of Hexavalent Chromium (JAN 2023). 2023. https://www.ecfr.gov/current/title-48/chapter-2/subchapter-H/part-252/subpart-252.2/section-252.223-7008

11. U.S. OSHA. 29 CFR 1910.1026 Chromium (VI). Amended 2019. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1026

12. SurTec. SurTec 650 Global Benchmark Technology in the Aerospace Industry, sales sheet. 2022. https://www.SurTec.com/ecomaXL/files/2022_SalesSheet_SurTec650Aerospace_EN.pdf?download=1

Written by the Lubinpla team from the sources listed and checked against them before publication.