Sulfuric acid anodizing

Anoplate gives roughly 33% outward growth on its sulfuric-anodizing page and 40% in its brochure. At an assumed 15 µm total oxide and equal growth on opposing surfaces, the estimates imply about 10 µm or 12 µm of diameter change. Confirm alloy, pretreatment and actual growth for the specified finish. Sulfuric page Anodize Coatings brochure, page 1

Identity and oxide formation

MIL-PRF-8625F Amendment 2 distinguishes conventional Type II, thin sulfuric Type IIB and hard anodic Type III. Specify the applicable route and finish requirements. MIL-PRF-8625, sections 1.2 and 3.4

Anodizing converts surface aluminum into aluminum oxide. Part of the resulting coating occupies metal that has been consumed, and part grows beyond the original surface. Anoplate's conversion and dimensional explanation.

Barrier-layer oxides and porous anodic oxides are distinct oxide categories in the public abstract of Jude Mary Runge's 2018 ASM Handbook chapter. Conventional sulfuric films are porous before sealing, which permits dye uptake. ASM abstract, Anoplate sulfuric anodizing.

At a finer structural scale, a porous film can itself have a barrier layer at the pore bottoms. Iwai, Kikuchi, and Suzuki distinguish that bottom layer from the overlying porous structure in their 2021 open-access study. Their experiments concern alkaline anodizing of high-purity aluminum, with acidic anodizing discussed for comparison. Synthesis: the structural distinction explains the terminology, but their measured pore dimensions and performance do not characterize a production Type II coating. Study, introduction, Figure 2, and methods.

Clear, colored, and hardcoat finishes

The following names describe different aspects of a finish.

Sealing and coloring are separate treatments. Anoplate describes sealing as helping retain dye, while prolonged direct sunlight can still bleach dyed films. Synthesis: a color name alone establishes neither outdoor durability nor a test result. Sulfuric anodizing reference.

For hardcoat, Anoplate associates maximum abrasion resistance with an unsealed coating and improved corrosion resistance with sealing. Synthesis: a sealed decorative Type II finish and a wear-oriented hardcoat have different purposes, even when both use sulfuric acid. Neither the acid name nor a black appearance makes the finishes interchangeable. Hardcoat reference.

Alloy, temper, and surface preparation

SAF reports that alloy differences can produce different anodized colors even between commonly anodized 6063 extrusions and 5005 sheet. Its example also distinguishes the anodizing rates of 5005-H14 and 5005-H34. An anodizing-quality material designation does not eliminate variation between metal lots. These are appearance and processing observations, not guaranteed color outcomes for every part of those grades. SAF appearance and color variation.

AAC places mechanical finishing before chemical finishing and anodizing. The oxide follows existing texture rather than concealing defects. Chemical etching removes metal and changes appearance. Synthesis: alloy, temper, product form, initial texture, and pretreatment are separate applicability conditions. A shared dye name cannot establish a matched appearance across different material conditions. AAC bulletin, pages 3 to 4.

Coating thickness and finished dimensions

Coating thickness and outward growth are different lengths. Anoplate's sulfuric page describes films of 0.0001 to 0.001 inch, equivalent to 2.54 to 25.4 µm, with 67% penetration and 33% outward growth. Its undated brochure instead gives roughly 60% penetration and 40% growth, explicitly dependent on alloy. These manufacturer descriptions are estimates, not universal tolerances. Sulfuric page, Anodize Coatings brochure, page 1.

Dimensional synthesis: let t be the coating thickness and f its outward-growth fraction. If two opposing faces receive equal coatings, with no pretreatment metal removal, an external dimension increases by 2 × f × t. A bore diameter decreases by the same amount. For an assumed t = 15 µm, the two cited estimates give approximately 10 µm or 12 µm of diameter change. This arithmetic is illustrative, not a measured process allowance. One mil is 0.001 inch, or 25.4 µm.

Anoplate separately describes approximately half-inward, half-outward growth for Type III. That hardcoat estimate is not the Type II estimate above. Hardcoat dimensional reference.

MIL-PRF-8625F Amendment 2 requires Type II coating weight of at least 1000 mg/ft² before dyeing or sealing. Mass per area is not dimensional tolerance. Section 3.7.1.1 and Table I.

Synthesis: final dimensions depend on pretreatment removal, coating growth, thickness distribution, and which faces receive coating. The cited ranges do not establish fit, thread allowance, masking extent, or a processor's achievable tolerance.

Specifications, tests, and conformance

DLA lists MIL-PRF-8625F Amendment 2, dated 23 November 2020, as active for non-architectural applications, superseding MIL-A-8625F Amendment 1. AAC's architectural Class I and Class II designations describe finish categories that can both be clear or colored. They are not the military dye classes. DLA record, AAC bulletin, pages 6 to 7.

ASTM B244-09(2021) describes nondestructive eddy-current coating-thickness measurement. Its public scope and significance text requires the calibration reference and measured article to have the same base-metal conductivity. A thickness reading alone does not establish corrosion resistance or seal quality. ASTM B244 public summary.

ASTM B680-24 describes destructive seal-quality measurement through coating mass loss. Its public scope excludes hard coatings, coatings sealed only in dichromate solutions, and coatings treated to become hydrophobic. It is not a universal seal test for every anodized finish. ASTM B680 public summary.

MIL-PRF-8625 separates process-control tests from lot conformance inspection. Paint adhesion and light-fastness tests have specified applicability conditions. Sections 3.7.1, 4.3, and 4.4.

Evidence synthesis: a process description identifies a coating family. A supplier's qualification or approval statement applies only to its stated scope. A specified test defines an examination and acceptance basis. Actual conformance concerns the applicable part or lot and its inspection evidence. None of these alone proves the other three.

Source basis and coverage limits

This reference was reviewed on 28 October 2026 PDT. The AAC bulletin states an initial release of February 2013 and leaves its review field blank. Anoplate's cited pages and brochure show no technical revision date. Their historical MIL-A terminology is retained as source context, not substituted for the identified MIL-PRF edition. The ASM citation supports only its public abstract. The ASTM citations support public summaries, not the full purchased methods.

The legacy AAC pages titled "Current Anodizing Processes" and "Anodic Coating Specifications" returned HTTP 403 during research. They supply no sole-source technical claim here. The accessible AAC bulletin supports the architectural and coloring distinctions above.

Coverage excludes bath recipes, operating settings, specific processor approvals, fatigue qualification, service-life predictions, and part-specific acceptance. It does not establish alloy-specific coating uniformity, permitted substitutions, or dimensional compensation for a particular geometry.

Related subjects include Surface-finishing processes, hard anodizing, anodizing dyeing, and hot-water anodizing sealing.