# Trivalent chromium conversion coatings

Supplied chromium(III) conversion-coating formulations, product-form distinctions, measurement bases, and qualification limits.

Reviewed: 2026-10-30

Tags: chemicals, conversion-coatings, aluminum, chromium

## Identity and scope

SurTec **650 concentrate** and **650 RTU liquid** both report **pH 2.9–4.0 at 20 °C**, while **650 RTU Gel** has UV-active dyes and reports about pH 3–4. The shared pH range leaves concentration and viscosity unresolved. MIL-DTL-81706 covers the supplied conversion material. MIL-DTL-5541 covers the reacted aluminum coating. [MIL-DTL-81706B Amendment 1, sections 1.1, 3.2, and 6.1.3](https://quicksearch.dla.mil/WMX/Default.aspx?token=601530)

CHEMEON lists TCP-HF for conversion coating and anodic sealing. Conversion of bare aluminum and sealing existing anodic oxide are separate uses. [CHEMEON TCP-HF product description](https://chemeon.com/chemeon-tcp-hf/)

The examples below concern named aluminum products. They do not establish a common formulation or performance for zinc, zinc-nickel, magnesium, every aluminum alloy, or every chromium(III) passivation product. See the broader [Surface-finishing processes](../../processes/finishing/index.md).

## Supplied products and physical-property basis

The following German-language manufacturer sheets are hosted by Surtec Suisse. The 650 and RTU sheets identify distribution in Switzerland and Liechtenstein; the Gel sheet names SurTec Deutschland. Their internal document dates, rather than their upload-directory dates, identify the versions.

| Exact product and dated sheet | Supplied form and distinguishing feature | Product-property table at 20 °C |
| --- | --- | --- |
| SurTec 650 chromitAL TCP, April 26, 2023 | Liquid concentrate based on chromium(III) salts | Density 1.005 g/ml, range 1.00–1.02; product pH 2.9–4.0. [Sheet, pages 1 and 7](https://www.surtec.ch/wp-content/uploads/2025/10/650D-CH.pdf?download=1) |
| SurTec 650 RTU chromitAL TCP Fertiglösung, March 25, 2021 | Ready-to-use liquid with adjusted viscosity | Density 1.003 g/ml, range 1.00–1.01; product pH 2.9–4.0. [Sheet, pages 1–3](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu_chromital-tcp-fertigloesung.pdf?download=1) |
| SurTec 650 RTU Gel Konversionslösung, March 29, 2021 | Ready-to-use product containing UV-active dyes | Approximate product pH 3–4; no density or numerical viscosity appears in this table. [Sheet, pages 1 and 3](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu-gel_konversionsloesung.pdf?download=1) |

The RTU table retains the manufacturer's pH column label "Konz." despite the product being ready for use. These are product-characterization values, not working-bath targets. The RTU sheet notes possible slight pH increase during early storage. [RTU sheet, page 2](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu_chromital-tcp-fertigloesung.pdf?download=1)

**Synthesis.** Density has mass-per-volume units; specific gravity is a ratio that needs reference conditions. Similar densities do not establish equal chromium content, viscosity, active solids, or interchangeability. Product pH does not measure total acidity or quantify chromium.

The Gel sheet's UV illustration identifies fluorescent areas as residual passivation solution associated with insufficient rinsing. Fluorescence therefore indicates residue in that example, rather than demonstrating protective-film coverage or corrosion performance. [Gel sheet, page 2 and photograph caption](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu-gel_konversionsloesung.pdf?download=1)

## Composition and absence claims

SurTec's current global product description calls 650 a chromium(III)-containing passivation and describes it as chromium(VI)-free. Those statements distinguish oxidation states; they do not mean chromium-free. The page provides no lot-specific analytical result or detection limit. [SurTec 650 product page](https://www.surtec.com/en/products-services/surtec-650/)

The regional sheets identify chromium(III) salts without salt identities or ingredient mass fractions. A chemical name alone does not supply total chromium concentration, dissolved-species distribution, or film composition. No complete formulation or quantitative Cr(VI) absence assay is established by this evidence.

An SDS also has a different purpose from a formulation disclosure. OSHA's public SDS table specifies disclosure of health-hazard ingredients under stated criteria, with provisions for ranges and trade secrets. **Synthesis.** An incomplete ingredient list cannot establish a complete formula or the absence of every unlisted substance. This US disclosure framework does not replace the regional SDS for a European product. [OSHA Appendix D, Table D.1, section 3](https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200AppD)

**Synthesis.** A working solution can acquire contamination absent from the supplied product. Neither a concentrate specification nor a chromium(VI)-free product description establishes the composition of a used bath, a removed residue, or a current production film. No hazard-free, exposure-free, or universal environmental-compliance conclusion follows.

## Film quantities and performance

SurTec 650's regional sheet distinguishes coating mass before subsequent coating, 50–120 mg/m², from its corrosion-protection column, above 110 mg/m². It states that coating mass depends strongly on aluminum roughness. Its gravimetric method excludes high-silicon, sometimes porous aluminum castings; its XRF method requires product-specific calibration. [650 sheet, pages 1, 3, and 6](https://www.surtec.ch/wp-content/uploads/2025/10/650D-CH.pdf?download=1)

**Derived units.** 50–120 mg/m² equals 0.050–0.120 g/m². This is film mass per area, not liquid concentration. It cannot become a film thickness without film-density and structural assumptions, which these comparisons do not establish.

The RTU sheet claims 336 hours in a DIN 50021 SS salt-spray test and cites MIL 5541 and MIL-DTL-81706. It does not identify the tested alloy, specimen count, pit endpoint, or a complete test report alongside that statement. The duration is therefore a bounded manufacturer claim, not a general corrosion lifetime. [RTU sheet, page 1](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu_chromital-tcp-fertigloesung.pdf?download=1)

MIL-DTL-81706B Amendment 1 names a one-square-inch upper contact electrode and nominal pressure of 200 psi. Its parenthetical "25 square mm" conflicts with the stated square inch; that parenthetical is not adopted here. [Specification, sections 3.7 and 4.5.5](https://quicksearch.dla.mil/WMX/Default.aspx?token=601530)

For comparison, MIL-DTL-5541F section 3.7 leaves Class 3 electrical acceptance values and testing details to acquisition requirements. Its explanatory section 6.1.2.1 relates qualification resistance to nominal electrode pressure of 200 psi and warns that roughness and electrode flatness affect readings. **Synthesis.** A resistance number without contact geometry, pressure, sample condition, and method is not a universal electrical property. [MIL-DTL-5541F, sections 3.7 and 6.1.2.1](https://quicksearch.dla.mil/WMX/Default.aspx?token=602203)

SurTec's Gel sheet prints a resistance claim below 5,000 µΩ per square inch with reference to MIL-DTL-81706B. This reference preserves that wording; it does not reinterpret it as bulk resistivity, sheet resistance in Ω/square, or conductivity of the supplied liquid. [Gel sheet, page 1](https://www.surtec.ch/wp-content/uploads/2025/03/surtec-650-rtu-gel_konversionsloesung.pdf?download=1)

## Specification identity and qualification limits

MIL-DTL-81706B Amendment 1 distinguishes Type I compositions containing hexavalent chromium from Type II compositions containing none. Class 1A concerns maximum corrosion protection, painted or unpainted; Class 3 concerns corrosion protection where low electrical resistance is required. Form and application method are separate classification dimensions. **Synthesis.** Type II alone does not identify a particular chromium(III) formulation or establish that all Type II materials contain chromium. [Specification, sections 1.2 and 3.2](https://quicksearch.dla.mil/WMX/Default.aspx?token=601530)

ASSIST lists both MIL-DTL-5541 and MIL-DTL-81706 as active. Its histories show 5541 Revision F of July 11, 2006, and 81706 Revision B with Amendment 1 of May 2, 2006, each with Validation Notice 2 dated February 7, 2024. These are publisher records read on the review date, not an inference from older product literature. [5541 record](https://quicksearch.dla.mil/qaDocDetails.aspx?ident_number=5499), [81706 record](https://quicksearch.dla.mil/qaDocDetails.aspx?ident_number=32592)

The publisher records QPL-81706's transformation to the electronic Qualified Products Database in 2008. The public QPD government-parts page reports a September 21, 2026 dataset update and distinguishes type, class, form, and method combinations. That page alone does not verify an exact manufacturer's product entry. [QPL transformation record](https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=95728), [QPD government-parts page](https://qpldocs.dla.mil/search/parts.aspx?qpl=1604)

SurTec's global pages describe 650 and 650 RTU as QPL-listed and cite MIL-DTL-81706-B and MIL-DTL-5541-F. This research does not verify individual QPD entries, manufacturing sites, forms, or methods for those products. A manufacturer listing claim cannot establish customer approval or an applicator's qualification. [650 page](https://www.surtec.com/en/products-services/surtec-650/), [650 RTU page](https://www.surtec.com/en/products-services/surtec-650-rtu/)

## Source coverage and limits

Reviewed October 30, 2026 PDT. Coverage includes all pages, tables, qualifications, and images of the three regional sheets above, and the public 5541F and 81706B Amendment 1 documents. The regional sheet dates remain 2021 and 2023; no current worldwide formulation revision is established.

Matching manufacturer SDS documents were not verified for these regional examples. The global SurTec TDS downloads redirected to login; CHEMEON's TCP-HF TDS and SDS downloads redirected to restricted access. CHEMEON therefore supplies only a product-role example here, without ingredient or physical-property comparisons. [SurTec download page](https://www.surtec.com/en/products-services/surtec-650/), [CHEMEON document index](https://chemeon.com/downloads/)

No current-lot assay, complete film chemistry, common cross-product corrosion dataset, or customer capability follows from these sources. Paid ASTM, DIN, and ISO criteria have not been reproduced or treated as independently reviewed. Manufacturing formulas and operating recipes are outside this reference.