Type II vs. Type III Anodizing (2026): Hardness, Color, and When to Specify Hard Coat

Type II vs. Type III anodizing is a MIL-PRF-8625F w/Amendment 2 decision between a 1.78 to 25.4 μm sulfuric acid coating for corrosion and cosmetics, and a 12.7 to 114.3 μm hard coat with a 50 μm mandatory default nominal thickness for wear surfaces on 6061-T6 and related aluminum alloys.

Specifying the wrong Type scraps precision aluminum parts. A Type II coating at 200 to 300 HV shears off under press-fit steel bearings, and a Type III at 0.001 in outward growth per side closes a 0.5000 in bore by 0.002 in before the part leaves the tank.

The Six-Gate Hard Coat Test below runs six pass or fail checks (wear load, dimensional headroom, color, alloy, thermal cycle, cost) that resolve the decision before you commit to the drawing. This guide covers Vickers hardness, the 50/50 growth rule, alloy compatibility, MIL-PRF-8625F callout format, and the 2 to 4× cost delta.

1. Type II vs. Type III at a Glance

Type II is a 1.78 to 25.4 μm sulfuric acid coating per MIL-PRF-8625F w/Amendment 2 (23 Nov 2020), dyeable in Class 2 and rated to 336 hours salt spray when sealed. Type III grows 12.7 to 114.3 μm at 400 to 500 HV, natural dark bronze or dyed black, priced 2 to 4× higher for wear surfaces. Every row in the table below cites MIL-PRF-8625F or an EPOC line measurement.

MIL-PRF-8625F designation	Sulfuric acid, conventional	Heavy dense coating	§1.2.1
Intended use per spec	Corrosion protection or paint base under severe service	Wear and abrasion resistant surfaces with improved corrosion protection	§1.2.1
Coating thickness, typical range	1.78 to 25.4 μm (0.00007 to 0.0010 in)	12.7 to 114.3 μm (0.0005 to 0.0045 in)	Table IV
Mandatory default nominal thickness	Not fixed	50 μm (0.002 in) unless otherwise specified	§3.4.3; §3.7.2.1
Vickers hardness on 6061-T6	200 to 300 HV	400 to 500 HV	EPOC line; industry data
Salt spray per ASTM B117-26	336 h, undyed and sealed	336 h, sealed when required	§4.5.3; Table II note 2
Color options	Class 2 dyed in black, blue, red, gold, green; Class 1 clear	Class 1 natural dark bronze or gray; Class 2 dyed black	§1.2.2; §3.5; §3.6
Outward growth per side	~0.0002 in per surface	~0.001 in per surface (50/50 rule)	§6.10.1
Sealing requirement	Completely sealed unless otherwise specified	Unsealed for maximum wear; sealed when corrosion resistance is required	§3.8.1; §3.8.2
Cost multiplier vs Type II	1× baseline	2 to 4×	Industry pricing

On a rotating aluminum shaft where a hardened steel bearing rides on the aluminum surface, Type III at 25 μm or thicker holds the wear surface because MIL-PRF-8625F §1.2.1 designates Type III specifically for wear and abrasion resistant surfaces, while Type II carries no wear acceptance criterion. When the aluminum sees no metal-on-metal contact, Type II at 8 to 12 μm delivers equal corrosion protection at roughly a quarter of the Type III cost. See the complete aluminum anodizing specification guide for the full four-Type overview.

According to Dewey Wu: “On the EPOC line we hold Type III at 25 to 75 μm because 75 μm is the practical upper limit for precision aluminum parts. Coatings above 75 μm shift dimensions more than 0.0015 in per side, which fails most tight-tolerance fits unless the drawing calls out a pre-anodize offset.”

2. The Six-Gate Hard Coat Test

The Six-Gate Hard Coat Test runs six pass or fail checks that resolve Type II vs. Type III on a drawing in under two minutes. Any single gate that fails routes the part back to Type II, or to an alternative finish covered in the anodize vs. alternative finishes guide.

2.1 Gate 1: Wear Load

Metal-on-metal contact, sliding seals, or press-fit steel components against the aluminum pass Gate 1. Type III at 400 to 500 HV carries the wear surface where Type II shears off. On the EPOC line, a Type II coating on a 6061-T6 shaft fails within 200 press-fit cycles against a hardened 52100 bearing race; the same shaft with Type III at 50 μm survives past 100,000 cycles in field data. Fail Gate 1 when the aluminum sees no metal contact and no abrasive media, in which case a Type II housing serves consumer electronics enclosures without a Type III cost penalty.

2.2 Gate 2: Dimensional Headroom

Type III grows outward at approximately 0.001 in per side, so a 0.5000 in bore closes to 0.4980 in after a 0.002 in coating. Pass Gate 2 when the drawing tolerance accepts a pre-anodize offset of 0.001 in per surface, or the feature is masked before the tank. Fatigue-critical fillets, threaded holes without oversized H-limit taps, and bearing bores at final ID without offset fail Gate 2 and route to Type IB or Type IC per §1.2.1 for corrosion protection without dimensional impact. See the 6061 machining preparation for anodize guide for the offset math.

2.3 Gate 3: Color Requirement

Type III is Class 1 natural gray to dark bronze on 6061, or Class 2 dyed black. Pass Gate 3 for dark bronze, natural gray, matte black, or dark charcoal. Fail Gate 3 for apple white, ferrari red, bright blue, or any light or saturated hue: the coating matrix on 6061-T6 already reads dark gray and organic dye deposits at less than 30 percent light transmission. Route light-color requirements to Type II Class 2 dyed finishes; see how thickness drives color consistency and corrosion resistance for ΔE data by Class.

2.4 Gate 4: Alloy Chemistry

Pass Gate 4 on 6061-T6, 6082-T6, and most 5000-series wrought alloys, which form dense uniform coatings at 400 to 500 HV with predictable color. 7075-T6 passes with a caveat: high zinc produces a bronze-yellow shift and roughly 15 percent faster growth than 6061, so batch it separately. Fail Gate 4 on 2024-T3, 2024-T351, and 2000-series with 4 to 5 percent copper (Cu dissolves preferentially in the sulfuric bath and produces voids), and on cast alloys with silicon above 5 percent (A380, A356) which produce mottled coverage. See the 6061-T6 aluminum properties reference.

2.5 Gate 5: Thermal or Chemical Cycle

Aluminum oxide is amphoteric and dissolves in strong acids and strong bases. Pass Gate 5 when the part sees pH 4 to 9 and temperatures below 200 °C. Fail Gate 5 when the part enters high-pH cleaning cycles above pH 10 (Steris washers, autoclaves under alkaline detergents), which strip standard sealed Type III within 10 to 20 cycles per §3.8.2. Reusable medical instruments route to sterilization-resistant proprietary sealing chemistries.

2.6 Gate 6: Cost Ceiling

Type III runs 2 to 4× the Type II unit price. Type III adds 60 to 90 minutes of tank time per batch versus 15 to 20 minutes for Type II, plus refrigeration to 0 to 5 °C and heavy-duty racking to carry up to 40 amps per square foot. Fail Gate 6 when Type II delivers the corrosion and cosmetic result at 336 hours salt spray per §4.5.3 and no wear or dielectric requirement forces the upgrade.

3. Hardness on the Vickers Scale

Type II measures 200 to 300 HV0.05 on 6061-T6; Type III measures 400 to 500 HV0.05 (EPOC records 420 to 480 HV0.05 per ISO 6507-1). Rockwell C readings on either coating are unreliable because the HRC indenter at 150 kgf punches through the oxide into the aluminum substrate. Values above 700 HV in some vendor whitepapers reflect HV0.01 load or 5000-series alloys with denser structure.

MIL-PRF-8625F does not accept Type III on Vickers alone. §4.3.3.2.4 requires Taber abrasion testing on 4 in × 4 in × 0.063 in panels under CS-17 wheels; wear resistance passes when measured mass loss meets the specification limit. Specify hardness as a process-control indicator, not an acceptance criterion.

4. Coating Thickness and the 50/50 Rule

Aluminum oxide grows approximately 50 percent into the substrate and 50 percent outward from the original surface per MIL-PRF-8625F §6.10.1. A 0.002 in (50 μm) Type III coating adds 0.001 in per side and closes a bore diameter by 0.002 in before the part leaves the tank.

4.1 Type II vs. Type III Growth in Practice

Type II at 5 to 25 μm adds 2.5 to 12.5 μm per side and fits inside most IT7 features without a pre-anodize offset. Type III at 50 μm nominal adds 25 μm (0.001 in) per side and turns a slip fit into an interference fit on any bore or shaft held to ±0.0005 in, so a Type III drawing without an offset scraps the batch. See the thickness effect on color consistency and salt spray life for the corresponding color and corrosion effect.

4.2 Complete Math for a Precision Bearing Bore

A bearing bore drawn to 0.5000 in ±0.0005 in final ID with a Type III callout at 0.002 in (50 μm) nominal requires a pre-anodize offset.

Machined bore at 0.5000 in nominal (no offset):

• Post-anodize outward growth per surface: 0.001 in
• Total diameter reduction: 2 × 0.001 in = 0.002 in
• Final bore ID after anodize: 0.4980 in
• Result: fails tolerance band by 0.0015 in, scrap

Machined bore at 0.5020 in nominal (pre-anodize offset applied):

• Post-anodize outward growth per surface: 0.001 in
• Total diameter reduction: 0.002 in
• Final bore ID after anodize: 0.5000 in
• Result: within ±0.0005 in tolerance, bearing seats

The drawing carries both the machining dimension and the coated dimension per §6.10.1.

4.3 Pre-Anodize Offset Table

Bore ID	Type III	0.002 in (50 μm)	0.001 in	−0.002 in ID	+0.002 in ID
Shaft OD	Type III	0.002 in (50 μm)	0.001 in	+0.002 in OD	−0.002 in OD
Plate, coated one side	Type III	0.002 in (50 μm)	0.001 in	+0.001 in thickness	−0.001 in thickness
Plate, coated both sides	Type III	0.002 in (50 μm)	0.001 in	+0.002 in thickness	−0.002 in thickness
Slot width	Type III	0.002 in (50 μm)	0.001 in	−0.002 in width	+0.002 in width
Bore ID	Type II	12 μm	6 μm	−12 μm ID	+12 μm ID
Bore ID	Type II	5 μm	2.5 μm	−5 μm ID	+5 μm ID

4.4 Threads and the 4× Pitch Diameter Rule

Threaded features grow at approximately four times the outward coating rate because coating deposits perpendicular to each flank. A 0.001 in outward growth changes pitch diameter by roughly 0.004 in, which rejects a standard H2 or H3 Go gauge. Mask threaded holes with silicone plugs before anodize when the threads carry no wear load; cut threads oversize to H5 or H7 tap standard when the engagement itself must hard-anodize for service life.

5. Color and Alloy Constraints on Type III

5.1 The Apple White Myth

Type II at 5 to 25 μm holds an open pore structure that fills with organic dye and passes visible light through to the aluminum below, so Class 2 accepts every color. Type III at 25 to 75 μm builds denser cells with alloying elements and sulfur locked into the matrix, producing an intrinsic gray-brown that no dye overrides. Apple white, Ferrari red, and bright blue on Type III muddy out at ΔE 12 or higher on CIELAB.

5.2 Black Anodize: Type II Dyed vs. Type III Natural

Type II Class 2 black on 6061-T6 delivers a deep uniform matte black at ΔE ≤ 2 for consumer housings, medical device covers, and decorative aerospace trim at 8 to 15 μm, sealed per §3.8.1.2. Type III Class 1 unsealed reads dark gray to near-black on 6061-T6 or bronze-black on 7075-T6; Class 2 dyed Type III adds organic dye to the wear-resistant matrix at 400 to 500 HV0.05, suited to hydraulic valve spools, aluminum piston rods, and aerospace landing gear links. Sealed Type III black loses wear performance per §3.8.2.

A drawing that writes only “black anodize” without Type or Class routes to two different processes; §6.2.2 permits the shop to supply Type I, IB, IC, II, or IIB. EPOC quotes Type II Class 2 black by default when the RFQ is ambiguous, then requotes at Type III Class 2 at 2 to 4× the price when a sliding wear surface surfaces later.

5.3 Alloy Compatibility Summary

6061-T6 / 6082-T6	Pass (reference)	400 to 500 HV, dark gray, ±5 μm across batch
5052-H32 / 5083	Pass	Denser coating, slightly darker natural color
7075-T6	Pass with caveat	Bronze-yellow shift, ~15% faster growth; batch separately
2024-T3 / 2024-T351	Fail (route to Type IB or IC)	Cu voids, 250 to 350 HV, pits <100h salt spray
A380 / A356 (>5% Si)	Fail	Non-uniform, mottled, wear performance drops

Non-fatigue-critical 2024 that needs wear resistance routes to electroless nickel per ASTM B733-22 covered in the choice between hard anodizing and electroless nickel plating.

6. Drawing Callout Format for MIL-PRF-8625F

Per §6.2, every anodize callout carries three fields: specification number and date, Type, and Class. Type III adds nominal thickness and seal condition per §6.2(j) and (l). Drawings that omit Type or Class default per §6.2.2 to any Type I, IB, IC, II, or IIB the supplier can produce.

6.1 Three Callout Templates

Type II Class 2 cosmetic finish on a consumer housing:

ANODIZE PER MIL-PRF-8625F w/AMENDMENT 2, 23 NOV 2020, TYPE II, CLASS 2, BLACK.

Type III Class 1 unsealed wear surface on a hydraulic valve body:

HARD ANODIZE PER MIL-PRF-8625F w/AMENDMENT 2, 23 NOV 2020, TYPE III, CLASS 1, 0.002 in NOMINAL, UNSEALED.

Type III Class 2 black wear surface on an aluminum piston, sealed for corrosion tradeoff:

HARD ANODIZE PER MIL-PRF-8625F w/AMENDMENT 2, 23 NOV 2020, TYPE III, CLASS 2, BLACK, 0.002 in NOMINAL, SEALED PER 3.8.2.

6.2 MIL-PRF-8625F vs. Legacy MIL-A-8625F

MIL-PRF-8625F w/Amendment 2 supersedes MIL-A-8625F w/Amendment 1. The PRF designation identifies the document as a performance specification and reflects the administrative redesignation from the former MIL-A nomenclature. Both name the same six Types (I, IB, IC, II, IIB, III) and two Classes (1, 2).

According to Dewey Wu: “Roughly 40 percent of aerospace and defense drawings still specify MIL-A-8625F Type III. We process to MIL-PRF-8625F w/Amendment 2 as the active specification and flag the legacy callout in the FAI report for the customer’s document control team.”

7. Cost and Lead Time

Type III runs 2 to 4× the Type II unit price and 3 to 6× the tank time per batch. Refrigeration to 0 to 5 °C alone often exceeds the anodize current cost; Type III at 24 to 40 amps per square foot demands bolted titanium racking versus Type II spring clips at 12 to 15 amps per square foot. Type II turns around in 3 to 5 business days on the EPOC schedule; Type III adds 2 to 4 business days for longer tank time, dedicated batches to avoid alloy mixing, and Taber or thickness verification per §4.3.3.2.

8. FAQ

What is the difference between hard anodizing and standard anodizing per MIL-PRF-8625F?

Hard anodizing means Type III per MIL-PRF-8625F w/Amendment 2 at a mandatory default 50 μm (0.002 in) nominal thickness, formed in sulfuric acid at 0 to 5 °C and 25 to 100 V DC for wear and abrasion resistant surfaces. Standard anodizing means Type II sulfuric acid at 1.78 to 25.4 μm, formed at room temperature and 12 to 24 V DC for corrosion protection and cosmetic dye acceptance. Type III measures 400 to 500 HV0.05 versus Type II at 200 to 300 HV0.05 on 6061-T6, at 2 to 4× the unit cost.

Can Type III anodizing be dyed apple white, red, or bright blue?

No. Type III on 6061-T6 reads dark gray to bronze in the tank before any dye deposits, and organic dye cannot lighten a matrix that already absorbs more than 70 percent of visible light. Only natural gray-bronze, natural charcoal, and Class 2 dyed black hold on Type III. Route apple white, ferrari red, bright blue, and any saturated hue to Type II Class 2 on a non-wear surface at ΔE ≤ 2.

How much do dimensions change with hard anodizing on a bore or shaft?

A 50 μm Type III coating grows approximately 25 μm (0.001 in) per side outward per §6.10.1. A bore machined to 0.5000 in nominal ID closes to 0.4980 in; a shaft at 0.5000 in OD grows to 0.5020 in. Every Type III drawing at IT7 or tighter carries a pre-anodize offset of 0.001 in per surface. Type II at 12 μm nominal changes diameter by roughly 12 μm total and fits inside most IT8 tolerances without offset.

Why does Type III cost 2 to 4× more than Type II?

Type III requires bath refrigeration to 0 to 5 °C, rectifier voltage up to 100 V DC, tank time of 60 to 90 minutes per batch versus 15 to 20 minutes for Type II, and bolted titanium racking to carry 24 to 40 amps per square foot without arcing. It also demands dedicated batches to avoid mixing 6061 with 7075 or 2024, which reduces throughput, and adds Taber abrasion verification per §4.3.3.2.4.

Which aluminum alloys can be hard anodized to full Type III performance?

6061-T6, 6082-T6, 5052-H32, and 5083 form dense uniform Type III at 400 to 500 HV0.05 on the EPOC line. 7075-T6 passes with a bronze-yellow color shift and roughly 15 percent faster growth; batch it separately. 2024-T3, 2024-T351, and other 2000-series with 4 to 5 percent copper fail because copper dissolves into the bath and produces voids. Cast alloys with silicon above 5 percent (A380, A356) fail because silicon particles do not anodize.

When should a Type III coating be left unsealed?

Type III shall not be sealed when maximum wear resistance is the main function per §3.8.2, because unsealed Type III holds full Taber abrasion performance at CS-17. Route sealed Type III only to exterior non-maintained applications requiring corrosion resistance where reduced wear is acceptable, sealed in boiling deionized water, 5 percent sodium dichromate, or nickel or cobalt acetate. State the seal condition explicitly on every Type III drawing.

9. Get the Callout Checklist

Download the EPOC Type II vs. Type III Anodize Specification Checklist (PDF) for a single-page reference: the Six-Gate Hard Coat Test decision flow, three MIL-PRF-8625F callout templates, the pre-anodize offset table, and Taber abrasion acceptance. Print it for the engineering room wall or attach it to the RFQ package.

Related Resources

• Related Article: Aluminum Anodizing Guide. Pillar page covering all six MIL-PRF-8625F Types for CNC aluminum parts.
• Related Article: Anodize Thickness, Color, and Corrosion Resistance for 6061 Parts. Salt spray, ΔE, and ASTM B244 thickness verification.
• Related Article: Anodize vs. Electroless Nickel, E-Coat, and Powder Coat. Where to route alloys that fail Gate 4.
• Related Capability: Aluminum CNC Machining for Anodize-Ready Parts. Machining offsets for Type III features at IT7.
• Related Standards Reference: 6061-T6 Aluminum Properties for Anodize. Alloy chemistry and anodize behavior data.

Specify the Type, Class, thickness, and seal condition on the drawing before quoting anodize.

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