If the drawing says “anodize per MIL-A-8625,” that’s not one process. It’s a family of processes, and picking the wrong type can mean a part that looks right but fails in service — or a part that meets the corrosion spec but no longer fits in the assembly because the coating grew the OD past tolerance.
Quick Summary: MIL-A-8625 Type II (sulfuric acid anodizing) adds a thin oxide layer (0.0002”–0.001”) that provides corrosion resistance and accepts dye. Type III (hardcoat) builds a much thicker layer (0.001”–0.003”) that adds wear resistance and hardness but grows into the base material and outward roughly 50/50 — meaning it changes your dimensions. If the drawing doesn’t specify which type, ask before you anodize. Getting it wrong is expensive.
Type II vs Type III at a Glance
| Type II (Sulfuric Acid) | Type III (Hardcoat) | |
|---|---|---|
| Coating thickness | 0.0002”–0.001” | 0.001”–0.003” |
| Hardness | Moderate | 60–70 HRC equivalent |
| Dimensional growth per side | ~0.00025” (on 0.0005” coating) | ~0.001” (on 0.002” coating) |
| Color options | Dyeable (black, red, blue, gold, OD green) | Dark gray to near-black only and non-dyed |
| Corrosion resistance | Good | Excellent |
| Wear resistance | Low | High |
| Fatigue life impact | Minimal | Can reduce 25–40% |
| Typical aerospace use | General corrosion protection, cosmetic | Wear surfaces, hard-use components |
What’s the Difference Between Type II and Type III?
Both processes grow an aluminum oxide layer on the part surface using an electrolytic bath. The difference is thickness, hardness, and how much they move your dimensions.
Type II — Conventional Sulfuric Acid Anodizing
Coating thickness: 0.0002”–0.001” (typically 0.0003”–0.0005” for most aerospace work)
Hardness: Moderate (harder than raw aluminum, softer than hardcoat)
Dimensional growth: Minimal — roughly half the coating thickness grows outward, half penetrates into the base metal. On a 0.0005” coating, you’re looking at ~0.00025” per side of dimensional change. Most tolerance bands absorb this without issue.
Color: Can be dyed (black, red, blue, gold, OD green — common in mil/aero). Undyed finish is typically clear to pale gold.
Corrosion resistance: Good. Passes salt spray requirements for most aerospace applications.
Type III — Hardcoat Anodizing
Coating thickness: 0.001”–0.003” (0.002” is the most common aerospace call-out)
Hardness: 60–70 Rockwell C equivalent — hard enough to serve as a wear surface
Dimensional growth: Significant. The same 50/50 rule applies, but on a 0.002” coating, that’s ~0.001” per side of outward growth. On a bore with ±0.0005” tolerance, that’s enough to push you out of spec if you didn’t account for it before machining.
Color: Dark gray to near-black depending on alloy and thickness. Cannot be dyed to bright colors.
Corrosion resistance: Excellent. Also provides wear resistance, abrasion resistance, and some electrical insulation.
Why Does This Matter for Machined Parts?
Because anodizing changes your dimensions, and the magnitude depends on which type you’re running.
If the machinist cuts a bore to the nominal dimension on the drawing and the part gets Type III hardcoat afterward, the bore shrinks by roughly 0.001” per side (0.002” on the diameter). If the bore tolerance was ±0.001”, the part is now out of spec on the tight side. That’s a reject.
The fix is straightforward but has to be planned before machining: oversize the bore (or undersize the shaft) by the expected coating buildup. For Type II, the adjustment is small enough that many shops absorb it within standard tolerance bands. For Type III, the adjustment is mandatory — you have to machine knowing the coating is coming.
eMachineShop’s overview of the aluminum anodizing process covers the general chemistry and color options well. For aerospace work, the additional detail that matters is the dimensional impact and how to spec it correctly on the drawing.
How Should the Drawing Call Out Anodizing?
A complete anodizing callout for aerospace parts should include:
- The specification: MIL-A-8625, followed by the type (II or III)
- The class: Class 1 (undyed) or Class 2 (dyed, with color specified)
- The coating thickness: either a specific range or “per specification” (which defaults to the ranges above)
- Masking requirements: which surfaces should NOT be anodized (threads, press-fit surfaces, electrical contact points, bearing seats)
- Dimensional note: whether the drawing dimensions are before or after anodizing — this determines whether the machinist needs to adjust for coating buildup
That last point causes the most confusion. If the drawing dimensions are “after anodizing” (the finished, coated part has to meet the tolerance), the machinist must pre-compensate. If the dimensions are “before anodizing” (the part has to meet tolerance in the machined state, before coating), no adjustment is needed but the final coated dimensions will be slightly different from what’s on the drawing.
When the drawing is silent on this, ask the customer. Don’t guess — the cost of re-machining or re-anodizing is always higher than the cost of a clarifying email.
Which Alloys Anodize Well and Which Don’t?
Not all aluminum alloys anodize equally. This matters because the alloy affects coating uniformity, color consistency, and corrosion performance.
Standard aerospace alloys (5052, 6061, 6063, 7075) anodize predictably with consistent color and thickness. No surprises.
2024 is the problem child. It anodizes, but the high copper content (3.8–4.9%) produces a yellowish tint that won’t take dark dye colors evenly. We’ve had customers spec “black anodize per MIL-A-8625 Type II Class 2” on 2024 parts and then reject them because the finish came out streaky olive-brown instead of true black. That’s not a process failure — it’s a material limitation. If the drawing calls 2024 and requires uniform black, flag it before the parts go to the anodizer. Switching to 6061 or 7075 solves it if the mechanical properties allow.
Cast and high-silicon alloys produce mottled, inconsistent finishes. The silicon disrupts the oxide layer. If the part is a casting, set expectations with the customer upfront — the anodize will be functional but it won’t be cosmetically uniform.
Does Anodizing Affect Fatigue Life?
Yes. Type III hardcoat anodizing can reduce fatigue life of aluminum parts by 25–40% depending on the alloy and coating thickness. The thick, hard oxide layer is brittle and can initiate fatigue cracks under cyclic loading, documented in MIL-A-8625 itself and in aerospace materials handbooks.
For fatigue-critical parts, the engineer has to weigh the wear/corrosion benefit of hardcoat against the fatigue debit. Common mitigations include shot peening before anodizing (to put the surface in compression) and limiting hardcoat thickness to the minimum required for the application.
This isn’t something most anodizing shops will flag proactively — they anodize to the spec they receive. It’s a design engineering decision, and the drawing should reflect it.
Frequently Asked Questions
Can I anodize only part of a surface? Yes — masking is standard practice. Any surface that shouldn’t be coated (threads, press-fit diameters, electrical contacts, bearing seats) gets masked before the part goes in the bath. Masking adds labor cost per part. Specify masked areas clearly on the drawing.
Does anodizing affect conductivity? Type II and Type III both create an electrically insulating layer. If the part needs electrical continuity at a contact point (grounding lugs, connector shells), that area must be masked or the coating chemically removed after anodizing.
What’s the typical turnaround for anodizing after machining? Most aerospace anodizing houses run 3–7 business days depending on volume and masking complexity. Rush is available but adds cost. If lead time is critical, confirm anodizing turnaround before committing to a delivery date.
Written by James Wright, CEO of Accurate Machine & Tool.
Sources
[1] eMachineShop, “Unlocking the Secrets of Aluminum Anodizing,” emachineshop.com/aluminum-anodizing-process
[2] MIL-A-8625F — Anodic Coatings for Aluminum and Aluminum Alloys
[3] Products Finishing — general anodizing process reference, pfonline.com