Anodizing, conversion & passivation · finish comparison
Anodizing Type II vs Hard Anodizing Type III
Anodizing Type II and Hard Anodizing Type III are both anodizing & conversion coatings finishes; hard anodize is the harder surface (350–550 HV vs 200–400 HV).
Anodizing Type II
Anodizing & conversion coatings · $$ · instant quote
Type II sulfuric anodizing converts aluminum into a porous aluminum-oxide coating that can be clear or dyed.
Hard Anodizing Type III
Anodizing & conversion coatings · $$$ · instant quote
Type III hard anodizing forms a thick, dense aluminum-oxide layer for sliding, abrasive, and high-load service.
When to choose which
Choose Type II anodize
Choose Type II anodize when color and lower buildup matter more than wear.
- Provides a hard 200–400 HV oxide
- Supports clear and many dyed colors
- Sealed coatings meet a 336-hour ASTM B117 requirement under MIL-A-8625
Choose Hard anodize
Choose Hard anodize when severe wear resistance is required.
- Typical hardness is 350–550 HV
- Common 50 µm coating provides strong abrasive-wear resistance
- Sealed specimens are evaluated for 336 hours ASTM B117
What the numbers say
- Hard anodize is the harder surface (350–550 HV vs 200–400 HV).
- Type II anodize is the lower-cost option ($$).
- Type II anodize is the thinner coating (5–25 µm), so it disturbs tight fits less.
- Hard anodize tolerates more heat (200 °C continuous).
Buyer's decision notes
Choose Type II for a colored or clear general-purpose aluminum finish. Choose Type III when the functional surface needs a hard anodic layer for wear, abrasion or a controlled engineering interface and the part can accommodate the process and final sizing plan.
- Same substrate, different intent
- Both are electrolytic coatings for aluminum, but the Type number is a performance route rather than a color choice. Type III should be tied to the wear surface and operating duty; it is not automatically the better finish for every aluminum part.
- Final size
- Specify coating thickness and the finished dimension independently. Anodizing penetrates and grows the aluminum, so nominal coating thickness cannot be copied directly into a machining offset or used as the finished buildup on a diameter.
- Sealing trade-off
- Sealing supports corrosion performance but can change wear and friction behavior. State sealed or unsealed where a sliding, bearing or abrasion surface is the reason for choosing Type III, and keep lubricant or PTFE additions as separate approved requirements.
- Inspection
- Define where thickness is measured and which surfaces are significant. A qualification coupon or salt-spray result verifies a specified process condition; it does not prove wear life for the part geometry or predict corrosion life in service.
Put this on the RFQ
- Call out MIL-PRF-8625 Type, Class, color, seal condition and thickness on significant surfaces.
- Identify bores, threads, datum faces, press fits and electrical contacts that need masking or final machining.
- For Type III wear surfaces, provide the mating material, motion, lubrication, target final size and any post-anodize grinding or lapping requirement.
Primary references
- DLA ASSIST — MIL-PRF-8625 — Active specification defining conventional sulfuric Type II and hard anodic Type III coatings, classes and verification requirements.
FabDigit resources: Type II anodize datasheet · Hard anodize datasheet · surface finish selection guide
Scores side by side
Each score is 0–100 against every finish in this catalog. Corrosion is derived from salt-spray test duration and surface hardness from the finish layer's Vickers hardness where a measured figure exists; a finish that leaves the base metal's own surface is not rated for hardness. Cosmetic, lubricity and cost are FabDigit's engineering assessment. Scores and typical figures are screening data, not matched side-by-side tests or service-life rankings; ASTM B117 is a controlled salt-fog method, not a field-life prediction.
| Score | Type II anodize | Hard anodize |
|---|---|---|
| Corrosion resistance | 71 Very good | 71 Very good |
| Surface hardness / wear | 54 Fair | 61 Good |
| Cosmetic quality | 88 Excellent | 50 Fair |
| Lubricity | 18 Low | 25 Low |
| Cost | 24 $$ | 44 $$$ |
Key numbers
| Property | Type II anodize | Hard anodize |
|---|---|---|
| Thickness | 5–25 µm | 25–75 µm |
| Surface hardness | 200–400 HV | 350–550 HV |
| Salt-spray test duration | 336 h | 336 h |
| Max service temperature | 80 °C | 200 °C |
| Electrical | Insulating | Insulating |
| Colours | clear, black, red, blue, gold, custom dye | natural gray, black |
| Relative cost | Standard ($$) | Premium ($$$) |
| Added lead time | 3–5 working days | 4–7 working days |
| RoHS | Compliant | Compliant |
| Instant quote | Yes | Yes |
Bold marks the side that is ahead on a row where more (or less, for cost and lead time) is better. Ranges are the typical specification window; the drawing call-out fixes the class.
Which materials take each finish
| Material family | Type II anodize | Hard anodize |
|---|---|---|
| Aluminum | Yes | Yes |
| DMLS / SLM | With care | With care |
Specifications and drawing call-outs
Type II anodize
- MIL-A-8625 — Anodic Coatings for Aluminum and Aluminum Alloys
- AMS 2471 — Anodic Treatment of Aluminum Alloys, Sulfuric Acid Process, Undyed Coating
- AMS 2472 — Anodic Treatment of Aluminum Alloys, Sulfuric Acid Process, Dyed Coatings
- ISO 7599 — Anodizing of Aluminium and Its Alloys — Method for Specifying Decorative and Protective Anodic Oxidation Coatings on Aluminium
ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 0.0005 IN THICK; MASK ALL THREADS AND ELECTRICAL CONTACTS
Hard anodize
- MIL-A-8625 — Anodic Coatings for Aluminum and Aluminum Alloys
- AMS 2469 — Hard Anodic Coating on Aluminum and Aluminum Alloys
- ISO 10074 — Anodizing of Aluminium and Its Alloys — Specification for Hard Anodic Oxidation Coatings on Aluminium and Its Alloys
HARD ANODIZE PER MIL-A-8625 TYPE III CLASS 1, 0.0020 IN TOTAL COATING THICKNESS, UNSEALED; MASK THREADS AND BEARING FITS
Type II anodize vs Hard anodize — frequently asked
What is the difference between Type II anodize and Hard anodize?
Anodizing Type II and Hard Anodizing Type III are both anodizing & conversion coatings finishes; hard anodize is the harder surface (350–550 HV vs 200–400 HV). Type II anodize is the lower-cost option ($$). Type II anodize is the thinner coating (5–25 µm), so it disturbs tight fits less.
When should I use Type II anodize instead of Hard anodize?
Choose Type II anodize when color and lower buildup matter more than wear. Conversely: choose Hard anodize when severe wear resistance is required.
Is Type II anodize or Hard anodize cheaper?
Type II anodize is the lower-cost finish ($$ against $$$ for Hard anodize) among the finishes FabDigit quotes. Both are priced per part with a lot minimum; the exact figure appears on the instant quote once the part is uploaded.
Do Type II anodize and Hard anodize protect equally against corrosion?
On paper, yes: both are rated 336 h in neutral salt spray (ASTM B117 or the qualification hours of the governing specification). In service the difference comes from sealing, thickness class and the substrate alloy rather than from the choice between the two.