Type 2 Class 1 Anodizing: MIL-PRF-8625 Sealing and Testing

Type II Class 1 anodizing is the clear, undyed sulfuric acid anodic coating defined by the U.S. military performance specification MIL-PRF-8625. “Type II” identifies the sulfuric acid electrochemical process, and “Class 1” means no dye is added, so the finish stays translucent and the aluminum’s natural surface shows through. The specification is maintained by the Department of Defense, but the finish is used well beyond military work: consumer electronics, medical devices, automotive trim, and architectural hardware all rely on it for corrosion resistance, moderate surface hardness, and a clean appearance.1EverySpec. MIL-A-8625F – Anodic Coatings For Aluminum And Aluminum Alloys

What the Specification Actually Requires

MIL-PRF-8625 was originally published as MIL-A-8625, and the current version is Revision F Amendment 2. The “A” to “PRF” change moved it from a military specification to a performance specification, meaning it tells the anodizer what results the coating must achieve rather than dictating exactly how to run the bath.2ASSIST-QuickSearch. MIL-PRF-8625 Document Details Most shops and purchase orders still say “MIL-A-8625” out of habit. Both point to the same requirements.

The spec covers six coating types and two classes. Type II is the conventional sulfuric acid process. Class 1 leaves the oxide undyed. Class 2 covers dyed coatings, where color is absorbed into the porous oxide before sealing. A callout of “MIL-PRF-8625 Type II Class 1” therefore specifies a clear sulfuric acid anodize that meets the performance requirements in the spec.

Rather than setting thickness directly, the spec requires a minimum coating weight of 1,000 mg/ft² for Type II, measured before sealing.3CVG Strategy. MIL-PRF-8625F w/Amendment 2 That corresponds to a typical thickness of 0.0002 to 0.0007 inches (about 0.2 to 0.7 mils). Heavier coatings are possible by extending process time, but pushing a sulfuric acid bath past about 0.001 inches generally moves into Type III hardcoat territory.

The oxide layer is hard, landing around 40–60 Rockwell C on the coating itself. It resists light scratching and everyday handling, though it isn’t as wear-resistant as a Type III hardcoat. Sealed Type II coatings must survive extended salt spray exposure per ASTM B117 without pitting. The oxide is also an electrical insulator, so any surface that needs to carry current has to be masked. Thermal stability is high; the coating won’t peel or degrade until temperatures approach the melting range of the base alloy.

Dimensional Growth to Plan For

An anodic coating grows partly into the base metal and partly above the original surface. The general rule is that roughly half the total coating thickness grows inward and half grows outward. For a 0.0005-inch coating, that puts about 0.00025 inches of buildup above the original surface per side. Bores, mating surfaces, and press-fit features need this factored into the tolerance stack before anodizing, not after.

Type II vs. Type III: How to Choose

The most common decision point is whether to specify Type II or Type III. Both use sulfuric acid, but the process conditions and the results differ substantially.

  • Thickness. Type II runs 0.0002 to 0.0007 inches. Type III starts around 0.001 inches and can exceed 0.004 inches.
  • Hardness. Type II reaches roughly 40–60 Rockwell C on the oxide. Type III pushes into 60–70 Rockwell C.
  • Appearance. Type II Class 1 is clear and can look cosmetically clean, especially on 6061. Type III tends to darken to a gray or olive tone that worsens with thickness. If appearance matters, Type II is almost always the better choice.
  • Dimensional change. Type III’s thicker coating means more buildup and more material to account for in tolerances. Precision parts sometimes need grinding after Type III.
  • Cost. Type III is more expensive. It runs at lower temperatures and higher energy, takes longer, and often requires post-process machining.

Choose Type II when you need corrosion protection, a clean look, and moderate wear resistance. Choose Type III when the part will see heavy sliding contact, abrasive environments, or when the coating itself needs to serve as a bearing surface. Many aerospace and defense programs specify Type II Class 1 for the majority of structural components and reserve Type III for wear-critical interfaces like valve bodies and actuator bores.

Alloy and Surface Preparation Drive the Appearance

Because Class 1 is transparent, everything underneath shows through, including the effects of alloying elements on the oxide color.

The 6000-series alloys (6061 and 6063 in particular) are the standard for clear anodizing. Their primary alloying elements, magnesium and silicon, stay soluble during oxide growth and produce a uniform, nearly colorless coating. If cosmetic consistency is the priority, 6061-T6 is the safest choice.

The 2000-series alloys contain significant copper, which shifts the oxide toward a yellowish or brownish hue. The 7000-series (7075, 7050) anodizes well structurally but tends to produce a slightly matte, grayish finish compared to 6000-series parts. Cast alloys with high silicon content often come out distinctly gray or dark, and the surface can look uneven. None of these are failures. The coating still meets spec. The visual result just may not match what the designer expected.

Surface condition before anodizing matters as much as alloy choice. Scratches, tool marks, and machining lines remain visible through the clear coating. The process amplifies imperfections rather than hiding them. If the part needs to look uniform, mechanical preparation happens before it goes to the anodizer: bead blasting with glass media produces a matte satin texture, and polishing or a chemical bright dip creates a bright, reflective surface. Once the oxide grows, the texture is locked in.

How to Specify It on a Work Order

Getting the paperwork right prevents most of the expensive problems in anodizing. The shop needs specific information from the engineering drawing or purchase order:

  • Alloy and temper. “6061-T6” or “7075-T73,” not just “aluminum.” The alloy affects process parameters and the expected finish appearance.
  • Specification callout. “MIL-PRF-8625, Type II, Class 1” is the current language. “MIL-A-8625” is understood but outdated.
  • Masking requirements. Identify every surface that must remain bare: threaded holes that need conductivity, mating surfaces for ground paths, press-fit bores where buildup would change the interference. Use coordinates or marked-up drawings, not vague notes.
  • Surface preparation. Specify bead blast, bright dip, matte etch, or “as machined” to control the final appearance.
  • Sealing method. If the application or customer spec requires a particular seal, such as nickel acetate for aerospace, call it out explicitly.
  • Thickness or coating weight. The spec minimum is 1,000 mg/ft² for Type II. If the design needs a heavier coating, state the requirement.3CVG Strategy. MIL-PRF-8625F w/Amendment 2

Incomplete documentation is where most anodizing problems start. If the shop has to guess the alloy, they may run the wrong deoxidizer. Missing masking instructions can produce coated surfaces that should have stayed conductive, or bare spots where you needed protection. Getting the paperwork right costs nothing. Stripping and re-anodizing a batch costs time and often damages the parts, because stripping requires a caustic or acid bath that removes base material and can push precision components out of tolerance.

Sealing Options and the Hexavalent Chromium Boundary

Freshly formed anodic oxide is full of microscopic pores. For Class 1 clear coatings, the parts skip the dye tank and go straight to sealing, which closes the pores and locks in corrosion resistance. The common methods are:

  • Hot water seal. Immersion in deionized water above 95°C. Simple and traditional.
  • Nickel acetate seal. A hot solution near 95°C at roughly 5 g/L. The nickel deposits into the pores and gives excellent corrosion resistance. Widely used in aerospace.
  • Mid-temperature seal. Proprietary chemistries running 40°C to 70°C, reducing energy and cycle time.
  • Cold nickel fluoride seal. Room-temperature process with good results, but the fluoride chemistry adds waste-treatment complexity.

Dichromate sealing, using hexavalent chromium, was once common and delivers outstanding corrosion performance, but it is increasingly restricted under REACH and similar rules. A sulfuric acid anodize sealed with anything other than dichromate is generally compliant with RoHS and REACH. If a customer or regulatory environment restricts hexavalent chromium, confirm the sealing method with the anodizer before processing. The broader industry trend is the same: Type II sulfuric acid anodizing is increasingly replacing Type I chromic acid anodizing, and many aerospace primes have approved thin-film sulfuric processes as direct Type I substitutes.4US EPA. Thin-Film Sulfuric Acid Anodizing as a Replacement for Chromic Acid Anodizing

Quality Testing and Acceptance

After processing, the coating has to be verified before parts ship. MIL-PRF-8625 defines several acceptance tests, and most aerospace customers require documented evidence of compliance.

Coating Weight

The primary acceptance criterion for Type II is coating weight, not thickness. The spec requires a minimum of 1,000 mg/ft², measured by stripping a test coupon in a phosphoric-chromic acid solution and recording the weight loss.3CVG Strategy. MIL-PRF-8625F w/Amendment 2 The test is destructive and is performed on process-control coupons run alongside the production parts, not on the parts themselves.

Thickness Measurement

For non-destructive verification on actual parts, eddy current instruments are the standard tool. ASTM B244 covers the use of eddy current gauges for measuring nonconductive coatings on nonmagnetic metals and is the go-to method for anodize thickness on aluminum. These gauges work well on flat surfaces and accessible features. Deep bores and tight corners may require specialized micro-probes.

Corrosion Resistance

Salt spray testing per ASTM B117 is the standard corrosion validation. Test panels are exposed to a 5% sodium chloride mist in a sealed chamber for a specified number of hours, then examined for pitting. The required duration depends on the coating type and class specified in the contract. The test is typically performed on representative coupons from each processing lot.

Seal Quality

The seal quality check confirms the pores were properly closed. A common method is the dye-stain test: a sealed coupon is immersed in a dye solution, and if the pores are adequately sealed, the dye is not absorbed. Poorly sealed parts pick up the dye, indicating the corrosion protection is compromised.

What Happens When Parts Fail Inspection

On defense contracts, the government has the right to reject any supplies that do not conform to contract requirements, including coating specifications.5Acquisition.GOV. 48 CFR 46.407 – Nonconforming Supplies or Services The contracting officer can reject outright or, in limited situations, accept nonconforming parts at a reduced price if the deviation is minor and in the government’s interest. Repeated delivery of nonconforming parts is documented in the contractor’s performance record and can affect future contract awards.6Acquisition.GOV. FAR 52.246-2 – Inspection of Supplies-Fixed-Price

In commercial work, the consequences follow the same pattern: rejected lots, stripping and re-processing costs, and schedule delays. Because stripping removes base material, re-anodized parts may fall out of dimensional tolerance. For precision components, a failed coating run often means scrapping the parts entirely.