MIL-A-8625 Type II Anodize: How to Specify a Production-Ready Finish for CNC Machined Aluminum (Class 1 / Class 2) — JLYPT
A drawing note that says “anodize per MIL-A-8625” can either lead to smooth production—or to mismatched color, fit issues, and avoidable rework. The difference is almost always in the details: type, class, thickness, sealing, masking, and inspection expectations. This guide is written for engineers, buyers, and program managers who need MIL-A-8625 Type II anodize on precision CNC machined aluminum parts and want the finish to be repeatable from prototype to production. It’s also designed to help you request quotes faster, reduce back-and-forth, and lock in a finish that holds up in real assembly conditions. If you already have a print and want to route it to a finishing-capable CNC supplier, start here: https://www.jlypt.com/custom-aluminum-anodizing-services/
H2: MIL-A-8625 Type II anodize — What It Is (in Practical Manufacturing Terms)
MIL-A-8625 Type II anodize is a sulfuric acid anodizing process used to produce a controlled anodic aluminum oxide coating. It is widely specified because it’s versatile: it supports both functional corrosion protection and attractive dyed colors, and it can be applied to many wrought aluminum alloys commonly used for CNC machining. Type II is frequently selected when you need:
- A predictable anodic film thickness without the high build typical of hardcoat
- Good corrosion resistance when properly sealed
- Cosmetic flexibility (especially Class 2 dyed coatings)
- A finish that can scale from prototype to production lots with documented controls In short: Type II is often the best “default” when a program needs a robust, professional anodized finish without the heavier dimensional impact of hardcoat.
Understanding “Type” and “Class” Under MIL-A-8625 (So Your RFQ Is Unambiguous)
When people say “MIL-A-8625 anodize,” they may be mixing several different coatings. In the MIL framework, Type and Class are not interchangeable.
Table 1 — Type vs Class (Fast Reference)
| Parameter | What it controls | Options you’ll see | Why it matters |
|---|---|---|---|
| Type | Process family / bath chemistry & outcome | Type I, Type II, Type III | Drives thickness range, hardness tendency, appearance |
| Class | Electrical behavior / dye use | Class 1 (non-dyed), Class 2 (dyed) | Determines whether a dye step is required; impacts color + cosmetics |
Table 2 — MIL-A-8625 Type II Class 1 vs Class 2 (Buyer-Relevant Differences)
| Item | Type II Class 1 (Non-dyed) | Type II Class 2 (Dyed) | Selection guidance |
|---|---|---|---|
| Appearance | Natural/clear to slightly matte | Color required (black, blue, red, etc.) | Use Class 2 for brand/color requirements |
| Typical use | Corrosion protection, base for paint, non-cosmetic internals | Cosmetic housings, panels, knobs, visible brackets | Pick based on aesthetic + identification needs |
| Color variability risk | Low (no dye) | Medium (alloy + prep + dye control) | Control alloy + surface finish for tight matching |
| Handling marks visibility | Moderate | Can be more visible on dark dye | Define cosmetic zones + rack contact strategy |
| Lead-time sensitivity | Lower | Higher (color approval, matching, more checks) | Plan for first-article color approval when needed |
| If your print calls out MIL-A-8625 Type II anodize but doesn’t state class, you’re leaving room for interpretation. If color matters, specify Class 2 and state the color requirement clearly. |
Why Engineers Choose MIL-A-8625 Type II anodize for CNC Parts (and When They Shouldn’t)
Type II is chosen because it balances cosmetics, corrosion resistance, and manufacturability. But it is not universal.
Table 3 — Fit-for-Purpose Guide
| Requirement | Is MIL-A-8625 Type II anodize a strong fit? | Notes |
|---|---|---|
| Cosmetic dyed finish | Yes | Class 2 with controlled pretreatment and sealing |
| Tight tolerance assemblies | Often yes | Manage film growth and masking on critical features |
| Moderate wear contact | Sometimes | Consider lubrication, surface texture, sealing; for heavy wear consider hardcoat |
| Electrical conductivity required | No (by default) | Anodic coatings are insulating; plan masking for grounding pads |
| Maximum abrasion resistance | Not ideal | Type III hardcoat typically used for wear components |
| High cosmetic consistency across alloys | Depends | Best on consistent alloy selection (e.g., 6061) and stable surface prep |
| A reliable supplier doesn’t just “run anodize.” They help you choose a finish that will actually survive your assembly stack-up and use environment. |
The Real Variables That Control Type II Results (Color, Thickness, and Corrosion)
A professional MIL-A-8625 Type II anodize outcome is built on process discipline. Four variables dominate the result:
- Alloy and metallurgical condition (wrought vs cast; copper content; silicon)
- Surface preparation (cleaning, etching, desmutting; optional brightening)
- Anodizing parameters (current density, bath temperature, time)
- Post-anodize steps (dye control for Class 2; sealing method and quality)
Table 4 — Process Controls and What They Affect
| Control point | What you adjust | What it affects | How defects show up |
|---|---|---|---|
| Cleaning | Chemistry + time + agitation | Dye uniformity, sealing performance | Patchy dye, “holidays,” smears |
| Etch | Caustic strength + time | Matte level, edge definition | Over-etch, dimension loss, rounded edges |
| Desmut | Acid selection for alloy | Removes intermetallic residues | Bronze/gray hue, streaking, uneven color |
| Anodizing electrical control | Current density, temperature | Film thickness, porosity, burn risk | Thin areas, burn marks, poor sealing |
| Dye bath control (Class 2) | Concentration, pH, temperature | Shade consistency and depth | Purple cast, weak black, banding |
| Sealing | Time + temperature + chemistry | Corrosion resistance, dye lock | Dye bleed, chalking, early corrosion |
Film Thickness Planning for MIL-A-8625 Type II anodize (Dimensional Reality, Not Guesswork)
Thickness is where many anodizing problems become assembly problems. Type II coatings are thinner than hardcoat, but they still change dimensions—especially on close fits, threads, and precision bores.
Practical dimensional concept
Anodizing forms an oxide that is partly “growth outward” and partly “conversion inward.” In production planning, you should treat anodize thickness as a dimensional factor that must be managed through:
- machining allowances,
- masking of functional surfaces,
- and “final after finish” dimension requirements.
Table 5 — Thickness Strategy by Feature Type
| Feature | Risk under Type II | Recommended approach |
|---|---|---|
| Precision bores | Size shift; potential tight assembly | Mask, or machine with allowance; verify after finish |
| Threads | Fit class can tighten | Prefer masking; alternative: oversize then chase if allowed |
| Datum pads / sealing lands | Flatness and contact behavior can change | Mask if metal-to-metal contact needed |
| Sliding interfaces | Friction change; wear | Confirm use case; consider alternate finish if heavy wear |
| Cosmetic faces | Visual texture telegraphs | Set Ra targets and define cosmetic zone handling |
Table 6 — Example Drawing Notes (Editable Templates)
| Use case | Example note language (customize to your program) |
|---|---|
| Natural finish | “Anodize per MIL-A-8625 Type II Class 1, sealed. Mask datum surfaces A/B as indicated.” |
| Dyed black | “Anodize per MIL-A-8625 Type II Class 2, dye black, sealed. Cosmetic surfaces: faces X/Y; rack marks not permitted on cosmetic faces.” |
| Tight bores | “MIL-A-8625 Type II anodize. Mask bores Ø… and threads M… / 1/4-20 UNC as indicated. Final dimensions apply after anodize.” |
| Color control | “MIL-A-8625 Type II Class 2 dyed color: black. Match to approved control sample. Supplier to maintain process records per lot.” |
| (Your internal drawing standards may require different phrasing; the point is to remove ambiguity so suppliers can quote correctly.) |
H2: Alloy Selection for MIL-A-8625 Type II anodize (Color Matching Starts Here)
Not all aluminum alloys anodize the same—especially for Class 2 dyed colors. Copper and silicon content can influence shade, uniformity, and how “deep” the black appears.
Table 7 — Alloy Behavior Under MIL-A-8625 Type II anodize
| Alloy | Typical anodize behavior | Risk level for cosmetic Class 2 | Recommendation |
|---|---|---|---|
| 6061-T6 | Predictable and widely used | Low | Best baseline for cosmetic dyed finishes |
| 5052 | Uniform oxide, good dye uptake | Low–Medium | Good for formed parts; manage denting |
| 7075-T6 | Can shift toward dark gray, more variability | Medium–High | Use with a defined color standard; expect shade variation |
| 2024 | Challenging due to copper content | High | Avoid for cosmetic dyed requirements when possible |
| Cast aluminum | Porosity and silicon drive uneven appearance | High | Use only if cosmetics are not critical; consider alternate finish |
| If you need strict color matching across multiple part numbers, align alloys across the family whenever possible. |
Surface Preparation Options (Matte, Satin, Bright) and Their Impact on Class 2 Dye
A dyed finish is not just “dye.” The perceived color is strongly influenced by how the surface reflects light. Two parts can be dyed with the same recipe and still look different if the surface texture differs.
Table 8 — Surface Prep Choices and When to Use Them
| Surface prep route | Visual result | Pros | Cons | Best for |
|---|---|---|---|---|
| Standard clean + light etch | Satin/matte | Hides minor machining marks; consistent | Can soften sharp details | General CNC housings, brackets |
| Minimal etch | “Sharper” machined look | Preserves crisp edges, less dimension loss | Shows tool marks more | Precision aesthetic parts |
| Brightening/polish route | Higher gloss | Premium cosmetics possible | More sensitive to scratches and handling | Display-grade components |
| Bead blast + anodize (if specified) | Uniform matte, low glare | Excellent glare reduction | Adds a controlled roughness; needs process discipline | Optics mounts, instrument parts |
| If you care about consistent appearance, specify the surface texture target or describe the intended look (matte vs satin vs gloss) alongside the MIL-A-8625 Type II anodize callout. |
Sealing Methods and Performance (Corrosion Resistance, Dye Lock, and “Rub-Off” Prevention)
Sealing is not a minor detail. Under Type II, especially Class 2 dyed coatings, sealing influences:
- corrosion resistance,
- dye fastness,
- and how the part looks after repeated handling.
Table 9 — Sealing Selection Guide for MIL-A-8625 Type II anodize
| Sealing method | Strengths | Trade-offs | Common use cases |
|---|---|---|---|
| Hot deionized water sealing | Good pore closure; straightforward | Requires water quality control; can slightly affect shade | General-purpose Class 1 and Class 2 |
| Nickel acetate sealing | Strong corrosion performance; good dye retention | More chemistry control; process discipline required | Premium dyed finishes, outdoor exposure |
| Mid-temperature sealing | Balanced energy + quality | Needs stable control | Production programs with repeatable parameters |
| If a program has field exposure, sweat/handling, or cleaning chemical contact, specify the sealing expectation early; it affects both quoting and production controls. |
H2: Masking, Racking, and Contact Points (How to Avoid Ugly Surprises)
Every anodized part must be electrically connected during anodizing. That means there will be rack contact areas. A production-capable anodize plan is one that decides, on purpose, where those contacts go.
Table 10 — Masking & Racking Plan (What to Decide Before You Run Production)
| Item | Decision to make | Why it matters | What you should provide |
|---|---|---|---|
| Cosmetic surfaces | Identify “A-surfaces” | Determines rack contact placement | Mark on drawing or provide screenshots |
| Threads | Mask vs chase | Fit class reliability | Thread spec + functional requirement |
| Precision bores | Mask vs allowance | Assembly yield | Bore tolerance and mating part info |
| Grounding pads | Mask required | Anodize is insulating | Contact locations and acceptable area |
| Fluid sealing faces | Mask strongly recommended | Prevent micro-leaks / compression issues | Gasket type + load path |
| A well-defined masking plan is one of the fastest ways to increase yield and shorten lead times on repeat orders. |
Inspection and Test Methods for MIL-A-8625 Type II anodize (What “Good” Looks Like on Paper)
A strong anodize supplier should be prepared to support inspection requirements that match your risk profile: prototypes, qualification builds, or stable serial production.
Table 11 — Typical QC Elements (Per Lot or Per Requirement)
| Requirement | Typical method | Output you can request |
|---|---|---|
| Coating thickness | Eddy current measurement | Thickness report with sampling locations |
| Visual appearance | Controlled lighting + reference | Cosmetic acceptance record |
| Color consistency (Class 2) | Colorimeter (ΔE reporting) | ΔE results vs control sample (when required) |
| Dimensional checks | CMM / gauges after finish | Post-finish dimension report |
| Traceability | Lot tracking | CoC with lot identification |
| Corrosion qualification (when required) | Standardized salt spray testing | Qualification report (program-specific) |
| If you need consistent receiving inspection, specify your sampling plan and reporting format—then keep it constant across purchase orders. |
Common Defects Under MIL-A-8625 Type II anodize (Root Causes and Prevention)
Knowing what can go wrong helps you prevent it—especially if you are scaling from first prototypes to hundreds or thousands of pieces.
Table 12 — Defect Library (Symptom → Likely Cause → Practical Fix)
| Symptom | Likely cause | Prevention / corrective action |
|---|---|---|
| Color shade drift between lots | Alloy variation; surface prep drift; dye bath instability | Lock alloy; standardize prep route; control sample approval |
| Streaks or “waterfall” marks | Poor rinsing; drag-in contamination; uneven agitation | Improve rinsing sequence; DI water; better agitation |
| Light edges or thin corners | Current density distribution; sharp edges | Add edge break; optimize racking orientation |
| Burn marks | Excess current density; poor contact | Improve racking contact; adjust electrical parameters |
| Powdery rub-off on dark dye | Incomplete sealing | Verify sealing process; adjust time/temperature |
| Patchy dye uptake | Oil/coolant residue; incomplete cleaning | Strengthen degrease; control machining fluids and cleaning |
| Thread fit too tight | Unmasked threads; thickness not planned | Mask threads; revise allowance; verify with GO/NO-GO |
CNC Machining Decisions That Directly Affect Type II Anodize Quality
Anodizing is transparent to what the metal already looks like. Many “anodize issues” are actually machining and handling issues that become visible after the coating step.
Table 13 — CNC-to-Anodize Integration Checklist
| Upstream factor | Why it matters for MIL-A-8625 Type II anodize | Best practice |
|---|---|---|
| Surface roughness (Ra) | Controls reflectivity and dye appearance | Define Ra on cosmetic faces; keep consistent toolpaths |
| Tool marks & chatter | Telegraphed through anodize | Use finishing passes; stable fixturing |
| Burrs | Trap chemicals; cause edge defects | Deburr consistently; avoid smeared edges |
| Coolant residues | Interfere with dye uniformity | Use compatible fluids; clean thoroughly pre-finish |
| Part handling | Scratches become permanent | Protect A-surfaces; use trays/film and controlled packing |
| If you want consistent Class 2 cosmetics, treat machining + handling as part of the anodizing process—not as separate worlds. |
H2: Documentation Package You Can Request (to Make Receiving Inspection Easier)
For programs that require traceability or quality documentation, define the deliverables in the PO so the supplier can quote them correctly.
Table 14 — Documentation Options (Pick What You Need)
| Document / record | What it proves | When to request |
|---|---|---|
| Certificate of Conformance (CoC) | Compliance to MIL-A-8625 Type II anodize callout | Standard production |
| Lot traceability record | Link to process lot and inspection | Regulated or high-risk programs |
| Thickness report | Objective coating thickness evidence | Tight fits, high reliability |
| First Article Inspection (FAI) | Baseline for production | New parts or new supplier |
| Control sample approval | Color match reference for Class 2 | Color-critical programs |
Three Case Studies (How MIL-A-8625 Type II anodize Performs in Real Programs)
Below are three scenarios that mirror typical RFQs for CNC + finishing suppliers. The details are written in a way you can reuse as a template for your own requirements.
Case Study 1 — Aerospace Electronics Bracket (Class 1, Sealed, Fit-Controlled)
Application goal: corrosion protection with predictable assembly dimensions; no dye required
Part profile: CNC machined bracket with multiple bores, datum faces, and fastener interfaces
Key risks: dimension shift on bores; contact mark location; edge thinness near sharp corners
Chosen spec direction: MIL-A-8625 Type II anodize, Class 1, sealed with a defined masking plan for datum pads and fit bores
Execution notes:
- Datum pads masked to preserve electrical/assembly contact behavior where required
- Edge breaks added to improve coating continuity and reduce “light edges”
- Post-finish thickness sampling location defined on non-critical faces to avoid measurement ambiguity
Outcome: stable assembly yield without chasing tolerances after finishing; corrosion resistance improved with sealing and consistent cleaning controls.
Case Study 2 — Instrument Enclosure (Class 2 Black, Cosmetic Control + Batch Matching)
Application goal: consistent black appearance for a customer-facing enclosure; repeat orders expected
Part profile: 6061-T6 housing with large flat surfaces and visible corners
Key risks: shade drift between lots; streaks on flats; rack marks on cosmetic surfaces
Chosen spec direction: MIL-A-8625 Type II anodize, Class 2 dyed black, sealed with cosmetic zones and an approved control sample
Execution notes:
- Cosmetic “A-surfaces” defined; rack contacts forced to hidden interior surfaces
- Surface finish target aligned across machining cells to keep reflectivity consistent
- Color checked against a reference standard for each lot release (program-specific acceptance)
Outcome: repeatable black tone across production lots; fewer cosmetic rejects due to early definition of A-surfaces and handling discipline.
Case Study 3 — Medical Device Subassembly (Class 2 Color ID + Masked Threads)
Application goal: dyed anodize for visual identification and light corrosion protection; threaded interfaces must assemble smoothly
Part profile: precision CNC subassembly with fine threads, sealing surfaces, and mixed geometry
Key risks: thread fit changes; dye bleed; handling marks on small parts
Chosen spec direction: MIL-A-8625 Type II anodize, Class 2 dyed color, sealed with thread masking and defined packaging
Execution notes:
- Threads masked to maintain fit class and reduce assembly torque variability
- Sealing method selected for dye lock to prevent rub-off during cleanroom handling
- Parts packed to protect cosmetic faces and avoid part-to-part contact scuffing
Outcome: reliable assembly torque and consistent color identification; improved receiving inspection due to documented masking and lot traceability.
RFQ Checklist (Send This to Get a Fast, Accurate Quote)
When you request MIL-A-8625 Type II anodize, include the items below. It reduces clarification cycles and helps you get a firm lead time.
Table 15 — RFQ Inputs That Drive Correct Quoting
| RFQ input | Why it matters | Example |
|---|---|---|
| Alloy + temper | Controls cosmetic outcome and dye response | 6061-T6 |
| Type + Class | Eliminates interpretation | Type II Class 2 |
| Color | Drives dye step and inspection | Black / clear / other |
| Sealing requirement | Drives corrosion and dye lock | Nickel acetate sealed |
| Thickness expectation | Drives time and dimensional planning | Target thickness range |
| Masking areas | Protects fits and functions | Threads, bores, grounding pads |
| Cosmetic zones | Determines racking and handling | “A-surfaces” defined |
| Inspection deliverables | Sets documentation and checks | Thickness report, CoC |
| Quantity + schedule | Impacts fixture strategy and lot planning | Prototypes vs production |
| For immediate routing to JLYPT’s anodizing capability and inquiry path: | ||
| https://www.jlypt.com/custom-aluminum-anodizing-services/ |
Internal Linking Suggestions (Conversion-Focused)
Place internal links in the top third of the page and again near the RFQ section:
- JLYPT anodizing services: https://www.jlypt.com/custom-aluminum-anodizing-services/
- JLYPT home page: https://www.jlypt.com/
- CNC machining overview (if applicable): https://www.jlypt.com/cnc-machining-services/
- Contact/RFQ (if applicable): https://www.jlypt.com/contact/
External Reference Links (Non-Competitor, Educational, “DoFollow” Candidates)
These are standards/library style resources—not competing service providers:
- U.S. DoD document search portal (for spec verification): https://quicksearch.dla.mil/qsSearch.aspx
- ISO standards catalog (general anodizing-related standards browsing): https://www.iso.org/standards.html
- ASTM standards catalog (test method browsing): https://www.astm.org/standards.html
- NIST (measurement and metrology references): https://www.nist.gov/
(If you want, I can tailor this list to the exact verification and test methods your customers ask for most—without linking to any competing machine shops or finishers.)
