# Hard anodizing

Hardcoat identity, Type III requirements, alloy and geometry limits, sealing tradeoffs, and dimensional growth assumptions.

Reviewed: 2026-10-29

Tags: processes, finishing, aluminum, anodizing

[Compare surface-finishing options](../surface-finishing.md) before specifying the complete route.

For assumed **50.8 µm total oxide** and **50% outward growth**, **25.4 µm per wall** gives **50.8 µm of bore closure**. The growth fraction is illustrative. MIL-PRF-8625's default Type III **0.002 in** nominal and **±20%** variation at that thickness govern oxide thickness. Verify alloy-specific growth and the actual bore. [MIL-PRF-8625, section 3.7.2.1](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035)


## Identity and relationship to sulfuric anodizing

Hard anodizing forms integral aluminum oxide. IHAA distinguishes alloy-dependent coating properties from secondary treatments such as dyeing and solid-film lubricant impregnation. [IHAA process reference](https://www.ihanodizing.com/anodizing-process/)

MIL-PRF-8625F Amendment 2 distinguishes conventional sulfuric Type II from hard anodic Type III. Type III specifies a coating result rather than one electrolyte. [Sections 1.2 and 3.4.3](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035).

Anoplate describes sulfuric acid as typical for hardcoat, with a harder, denser, thicker, more abrasion-resistant result than its conventional Type II finish. **Synthesis:** electrolyte family and coating requirements answer different questions. A sulfuric bath alone does not establish Type III conformance, and a Type III designation does not supply a universal bath recipe. The [sulfuric acid anodizing reference](sulfuric-acid-anodizing.md) covers conventional finishes and coloring in more detail. [Anoplate hardcoat reference](https://www.anoplate.com/finishes/hardcoat-anodize).

## Thickness, coating mass, and wear

The quantities below describe different properties and acceptance bases.

| Quantity | Source-specific meaning |
| --- | --- |
| Total oxide thickness | AAC's 2006 fact sheet describes approximately 0.5 to 3 mils, or 12.7 to 76.2 µm, as typical industry practice, dependent on alloy and application. Special processes may reach 9 mils, or 228.6 µm. These descriptions are not one processor's guaranteed range. [AAC fact sheet, page 1](https://cdn.ymaws.com/anodizing.site-ym.com/resource/resmgr/Docs/Hardcoat-Fact_Sheet.pdf#page=1). |
| Military thickness requirement | Unless otherwise specified, 0.002 inch nominal. Default variation is ±20% through 0.002 inch, then ±0.0004 inch above it. [MIL-PRF-8625, section 3.7.2.1](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035). |
| Coating mass per area | AAC's guideline describes weighing before and after coating removal. Its unsealed Type III minimum is 4320 mg/ft² per 0.001 inch of oxide. Military substitution of mass for thickness requires the procuring activity's choice. [MIL section 3.7.2.1.1](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035). [AAC guideline, section 3.2](https://www.anodizing.org/wp-content/uploads/2024/05/Guideline-Hardcoat-Anodizing.pdf#page=2). |
| Military abrasion requirement | Unsealed coatings have maximum wear indices of 3.5 mg/1000 cycles for alloys with at least 2% copper, otherwise 1.5. Section 4.5.5 specifies CS-17 wheels, 1000 g load, 70 rpm, and 10,000 cycles. [Sections 3.7.2.2 and 4.5.5](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035). |

One mil equals 0.001 inch or 25.4 µm. **Arithmetic synthesis:** 0.002 inch equals 50.8 µm. A ±20% interval around that nominal is 40.64 to 60.96 µm. These are thickness values, not finished-part tolerances.

**Quantity synthesis:** mass per area and thickness are not interchangeable without an applicable relationship. The guideline's mass threshold scales to 8640 mg/ft² at 0.002 inch. This is threshold arithmetic, not a measured coating density or proof of lot acceptance.

AAC explicitly says hardness results cannot prove wear resistance. Its guideline discusses Knoop and Vickers microindentation under ASTM E384 and the influence of measurement errors on thin oxide layers. It separates abrasion, corrosion, electrical breakdown, and adhesion tests by application. **Synthesis:** a hardness number needs its method and test conditions. It does not independently establish sliding performance, corrosion life, or electrical insulation capability. No universal hardness value or conversion to steel hardness is adopted here. [AAC guideline, sections 2, 3, and 3.9](https://www.anodizing.org/wp-content/uploads/2024/05/Guideline-Hardcoat-Anodizing.pdf).

MIL's nonmandatory notes warn that greater thickness need not improve abrasion resistance. [Section 6.17](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035).

## Sealing, dyeing, and service purpose

MIL prohibits sealing for maximum wear resistance. Sealing is specified for exterior non-maintained corrosion applications that permit reduced abrasion resistance. [Section 3.8.2](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035).

Anoplate likewise associates best abrasion performance with unsealed hardcoat and best corrosion performance with sealed hardcoat. It describes PTFE treatment separately. Its Class 1 finish is undyed, with natural color dependent on alloy and thickness. Black is its most common Class 2 dyed finish. Other dyed colors can be less uniform or decorative because the underlying oxide is dark. These are processor observations, not guaranteed colors for every alloy. [Anoplate appearance and sealing discussion](https://www.anoplate.com/finishes/hardcoat-anodize).

**Synthesis:** color, sealing, lubrication, and wear acceptance are separate attributes. A dark appearance does not establish dyeing, sealing, or a passed abrasion test. A sealed hardcoat and an unsealed wear coating have different performance priorities. A salt-spray result has no direct conversion into years of outdoor service.

## Alloy, temper, and geometry limits

MIL's default Type III exclusions are nominal copper above 5% or silicon above 8%, subject to contractual exceptions. Higher silicon requires procuring-activity approval. Heat-treatable alloys require heat-treated temper before anodizing. Its nonmandatory design notes describe corner voids and small-hole coverage ranging from none to full thickness. [Sections 3.4.3, 6.10.1.1, and 6.10.3](https://quicksearch.dla.mil/WMX/Default.aspx?token=5769035).

AAC identifies alloy, temper, edge and corner radii, contact locations, oxide thickness, and final dimensional tolerances as distinct considerations. It cautions that alloys suitable for machining are not necessarily suitable for hardcoat. Its examples include 2xxx alloy rings and a 6061-T6 compressor scroll. These demonstrate particular applications, not a compatibility guarantee for every part in either family. [AAC fact sheet, pages 1 to 2](https://cdn.ymaws.com/anodizing.site-ym.com/resource/resmgr/Docs/Hardcoat-Fact_Sheet.pdf).

**Synthesis:** alloy designation alone cannot establish uniformity in a deep bore, thread, corner, or rack-contact region. A general thickness range does not establish coverage at each feature. Electrical contact and masking locations also affect which surfaces receive the finish. Anoplate describes masking to retain electrically conductive surfaces or protect close dimensions. [Anoplate masking discussion](https://www.anoplate.com/finishes/hardcoat-anodize).

## Dimensional growth and fit

AAC describes approximately half the hardcoat thickness as outward buildup and half as penetration into the original aluminum. Total thickness includes both. Its opening-diameter example assumes coating on opposing walls. [AAC fact sheet, page 1](https://cdn.ymaws.com/anodizing.site-ym.com/resource/resmgr/Docs/Hardcoat-Fact_Sheet.pdf#page=1).

**Dimensional synthesis:** let `t` be total coating thickness and `f` the outward fraction. With equal coating on opposing surfaces and no pretreatment removal, an external diameter increases by `2 × f × t`; a bore diameter decreases by that amount. For assumed `t = 50.8 µm` and `f = 0.5`, outward growth is 25.4 µm per wall and bore closure is 50.8 µm.

Full, uniform bore coverage is an assumption. Confirm local coverage, alloy-specific growth, pretreatment removal and any masking or post-finish machining on the actual bore. Thread allowance also depends on flank geometry.

## Specification scope and source limits

DLA's active record identifies MIL-PRF-8625F Amendment 2, dated 23 November 2020, for non-architectural applications. The complete 20-page official text was inspected. [DLA document record](https://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=7074).

ISO 10074:2021, edition 4, covers hard anodic oxidation requirements and test methods. Its public scope excludes plasma electrolytic oxidation, micro-arc oxidation, and related spark processes. SAE's public record identifies AMS2469L, revised 16 May 2024, as a hard-anodic-coating specification. Only these publishers' public scope and edition information was inspected, not their purchased full texts. Neither scope establishes clause-by-clause equivalence with the military specification. [ISO record](https://www.iso.org/standard/80119.html), [SAE record](https://saemobilus.sae.org/standards/ams2469l-hard-anodic-coating-aluminum-aluminum-alloys).

Sources were reviewed on 29 October 2026 PDT. AAC's fact sheet is copyright 2006. Its three-page application guideline shows no verified issue date and cites older standards editions. Its recommendations are not substituted for current contractual requirements. Anoplate and IHAA show no verified technical revision date. Their general process descriptions do not establish customer capability, supplier approval, or actual part conformance.

Coverage excludes operating recipes, universal settings, fatigue qualification, food or medical approval, specific coating-lubricant compatibility, and service-life prediction. Generic property descriptions do not establish an application's acceptance criteria or the tested condition of a particular lot.

Related subjects include [Surface-finishing processes](index.md), [anodizing dyeing](anodizing-dyeing.md), and [hot-water anodizing sealing](hot-water-anodizing-sealing.md).