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Hard Anodizing Type III

Anodizing & conversion coatings · Electrolytic bath

Type III hard anodizing forms a thick, dense aluminum-oxide layer for sliding, abrasive, and high-load service.

CNC machiningSheet metalCastingPremium cost $$$Insulating
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Hard Anodizing Type III on a machined part

What is Hard anodize?

Defense, aerospace, automation, and hydraulic drawings commonly specify MIL-A-8625 Type III at 0.002 in nominal thickness.

How it works

Aluminum is anodized in chilled sulfuric acid under higher current density and voltage than Type II. Low bath temperature slows chemical dissolution so a dense oxide can reach 25–75 µm. A common nominal requirement is 0.002 in (50.8 µm), although MIL-A-8625 allows thickness to be specified for the application. Approximately half grows outward and half consumes the substrate. Sealing improves corrosion resistance but can reduce abrasion performance, so the drawing must state whether sealing is required.

Why specify it

  • Typical hardness is 350–550 HV
  • Common 50 µm coating provides strong abrasive-wear resistance
  • Sealed specimens are evaluated for 336 hours ASTM B117
  • Integral oxide cannot peel like a poorly bonded plated layer

Where it falls short

  • Brittle oxide can craze at sharp edges or under substrate deformation
  • Adds about 12.5–37.5 µm per treated surface
  • Dark gray color varies strongly with alloy and thickness
  • Not suitable for reliable electrical contact

Hard anodize at a glance

Hard anodize is strongest on corrosion protection (71 of 100), at a premium cost.

Corrosion protectionVery good71 / 100
Surface hardnessGood61 / 100
Cosmetic qualityFair50 / 100
LubricityLow25 / 100
Relative costPremium44 / 100
CorrosionHardnessCosmeticLubricityEconomy

Each score is 0–100 against every finish in this catalog. Corrosion comes from salt-spray life (ASTM B117, log scale) and hardness from the finish layer's Vickers or pencil hardness; cosmetic, lubricity and cost are FabDigit's engineering assessment. Economy on the radar is the inverse of cost.

Hard anodize key numbers

Typical values from the governing specifications and shop practice; bars are this finish against the highest value among anodizing & conversion coatings finishes.

Thickness25–75 µm0.98–3 mil
Hardness350–550 HV
Salt spray336 h
Max service temp.200 °C392 °F
Thickness
25–75 µm; 50.8 µm (0.002 in) is a common nominal drawing requirement.
Hardness
350–550 HV — Typical Type III microhardness on suitable wrought alloys; 2000/7000 series and high-silicon castings run lower. Marketing claims of 60–70 HRC confuse superficial hardness with Vickers.
Corrosion (salt spray)
336 h per ASTM B117, sealed, per MIL-A-8625 — Unsealed Type III chosen for maximum abrasion is not represented by the sealed corrosion figure.
Electrical
Insulating
Colours
  • natural gray
  • black
Appearance
Matte gray to charcoal; black dye is possible but shade varies with alloy and thickness.

Where Hard anodize sits among all finishes

Surface hardness (Vickers, log scale) against corrosion protection (salt-spray hours, log scale) for every coating FabDigit offers, one cloud per finish group. Hard anodize is highlighted; hover a dot for its name or open the full map.

02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finisheshard, low corrosion protectionhard and protectiveprotective, softsoft, little protectionType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Type I anodize — corrosion 71, hardness 49PTFE hard anodize — corrosion 71, hardness 61Nitriding — corrosion 55, hardness 73PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERType II anodizePowder coatZinc plateZinc-nickelHard chromeDLCTiN PVDTin plateCadmium plateE-coatCerakoteWet paintHot-dip zincNitridingHVOF coatingHard anodize — corrosion 71, hardness 61Hard anodize
02550751000255075100Corrosion protection (0–100)Surface hardness (HV, log scale, 0–100)Plating — 19 finishesHard coatings — 17 finishesAnodize & conversion — 8 finishesPaint & powder — 7 finishesType II anodize — corrosion 71, hardness 54BSA anodize — corrosion 71, hardness 52Type I anodize — corrosion 71, hardness 49PTFE hard anodize — corrosion 71, hardness 61Nitriding — corrosion 55, hardness 73PEO — corrosion 70, hardness 74Sherardizing — corrosion 83, hardness 56Brass plate — corrosion 35, hardness 44Bronze plate — corrosion 40, hardness 49Cadmium plate — corrosion 70, hardness 26Copper plate — corrosion 32, hardness 39Decorative chrome — corrosion 64, hardness 71Electroless copper — corrosion 20, hardness 41Electroless nickel — corrosion 70, hardness 68Nickel plate — corrosion 51, hardness 55Gold plate — corrosion 68, hardness 39Hard chrome — corrosion 40, hardness 73Hot-dip zinc — corrosion 90, hardness 42Nickel-PTFE — corrosion 72, hardness 57Silver plate — corrosion 55, hardness 39Solder plate — corrosion 42, hardness 9Tin plate — corrosion 34, hardness 0Zinc plate — corrosion 60, hardness 30Zinc-nickel — corrosion 86, hardness 60Cerakote — corrosion 83, hardness 27E-coat — corrosion 83, hardness 15Epoxy coat — corrosion 83, hardness 15Powder coat — corrosion 83, hardness 14Wet paint — corrosion 59, hardness 10AlCrN PVD — corrosion 58, hardness 93AlTiN PVD — corrosion 52, hardness 94CrN PVD — corrosion 62, hardness 83DLC — corrosion 55, hardness 93MoS2 PVD — corrosion 20, hardness 26TiAlN PVD — corrosion 50, hardness 93TiCN PVD — corrosion 40, hardness 92TiB2 PVD — corrosion 40, hardness 94TiN PVD — corrosion 45, hardness 87WC/C PVD — corrosion 48, hardness 77ZrN PVD — corrosion 52, hardness 90CVD alumina — corrosion 70, hardness 85CVD TiCN — corrosion 38, hardness 92CVD TiN — corrosion 45, hardness 87Multilayer CVD — corrosion 62, hardness 88HVOF coating — corrosion 78, hardness 77Plasma spray — corrosion 60, hardness 68Plastic chrome — corrosion 72, hardness 71Electroless plastic — corrosion 58, hardness 39Clear coat — corrosion 20, hardness 12Soft-touch — corrosion 20, hardness 5PLATINGHARD COATINGSANODIZE & CONVERSIONPAINT & POWDERPowder coatZinc plateHard anodize — corrosion 71, hardness 61Hard anodize
Anodize & conversionPlatingPaint & powderHard coatings

Specifications for Hard anodize

The documents a drawing cites to define this finish, with the Type / Class rows that apply. Each links to a page explaining the standard, and US Government documents are free to download there.

StandardType / ClassThicknessRequirement / testStatus
MIL-A-8625
Anodic Coatings for Aluminum and Aluminum Alloys · DoD
Type III Class 113–114 µmHard anodize, non-dyed; thickness must be stated, with 50.8 µm nominal when omitted. · When weight substitutes for thickness: at least 4320 mg/ft² per 0.001 in of unsealed coating (0.465 g/dm² per 25.4 µm). · Thickness or coating weight; abrasion resistance; corrosion test, ASTM B117, 336 h, only when sealed corrosion resistance is specified.Supersedednow MIL-PRF-8625
Type III Class 213–114 µmHard anodize, dyed; thickness must be stated, with 50.8 µm nominal when omitted. · When weight substitutes for thickness: at least 4320 mg/ft² per 0.001 in of unsealed coating (0.465 g/dm² per 25.4 µm). · Thickness or coating weight; abrasion resistance; corrosion test, ASTM B117, 336 h, only when sealed corrosion resistance is specified; light fastness when specified.
AMS 2469
Hard Anodic Coating on Aluminum and Aluminum Alloys · SAE · alternative
Standard thickness38–64 µmDefault hard anodic coating thickness unless another thickness is specified. · Thickness, coating weight, and abrasion resistance.Active
Specified thickness—Purchaser-defined thickness replacing the standard nominal value. · Thickness and applicable performance tests.
ISO 10074
Anodizing of Aluminium and Its Alloys — Specification for Hard Anodic Oxidation Coatings on Aluminium and Its Alloys · ISO · alternative
Class 1—All wrought alloys except those assigned to Class 2. · At 50 ± 5 µm, minimum surface density 1100 mg/dm². · Thickness, surface density, wear, and hardness.Active
Class 2(a)—2000-series alloys and other alloys containing more than 5% copper. · At 50 ± 5 µm, minimum surface density 950 mg/dm². · Thickness, surface density, wear, and hardness.
Class 2(b)—5000-series alloys with at least 2% magnesium and 7000-series alloys. · At 50 ± 5 µm, minimum surface density 950 mg/dm². · Thickness, surface density, wear, and hardness.
Class 3(a)—Casting alloys with less than 2% copper and/or 8% silicon. · At 50 ± 5 µm, minimum surface density 950 mg/dm². · Thickness, surface density, wear, and hardness.
Class 3(b)—Other casting alloys. · By agreement. · Requirements agreed between purchaser and anodizer.

Test and related methods: ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus).

Drawing call-out

HARD ANODIZE PER MIL-A-8625 TYPE III CLASS 1, 0.0020 IN TOTAL COATING THICKNESS, UNSEALED; MASK THREADS AND BEARING FITS

Pattern: ANODIZE PER MIL-A-8625 TYPE <I|IB|IC|II|IIB|III>, CLASS <1|2>, [COLOR], [THICKNESS FOR TYPE III], [SEAL], [MASKING]

Which materials take Hard anodize?

Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library.

Material familyVerdictNotes
Aluminum
YesBest on anodizing-quality wrought aluminum.
  • 6061 is a common stable choice
  • 2000 and 7000 series require tighter process control
  • High-silicon castings produce softer, darker, less uniform oxide
DMLS / SLM
With careOnly aluminum DMLS/SLM parts can be processed; steel and titanium builds cannot.
Brass
NoBrass is not anodized by this aluminum process.
Copper Alloy
NoCopper alloys are not anodized by this aluminum process.
FDM
NoFused-filament polymers are incompatible with this process.
Magnesium Alloy
NoMIL-A-8625 Type III is an aluminum process.
MJF
NoPolymer powder-bed prints are incompatible with this process.
Polymer
NoOrganic polymers cannot take this inorganic metal-finishing process.
SLA
NoPhotopolymer prints are incompatible with this process.
SLS
NoPolymer powder-bed prints are incompatible with this process.
Stainless Steel
NoStainless steel is not anodized by MIL-A-8625 processes.
Steel
NoThis is an aluminum anodic process; steel does not form this oxide.
Titanium
NoTitanium uses a different anodizing chemistry.
Zinc Alloy
NoZinc alloys are not anodized by this aluminum process.

How Hard anodize is applied, and what it does to the part

  1. 1Clean / degrease
  2. 2Etch / desmut
  3. 3Rack and mask
  4. 4Hard anodize in chilled sulfuric acid
  5. 5Dye if specified
  6. 6Seal if specified
  7. 7Inspect
DimensionsA 50 µm coating usually adds about 25 µm per side, or about 50 µm across a coated diameter.
Build-up per side13–38 µm — Approximately half of the oxide grows outward.
MaskingUsually needed. Mask threads, bearing fits, sealing lands, electrical contacts, and bores not dimensioned for buildup.
ElectricalInsulating
Temperature limit200 °C (392 °F) continuous
AppearanceMatte gray to charcoal; black dye is possible but shade varies with alloy and thickness.
Added lead time4–7 working days on top of the bare part

Design rules for Hard anodize

What to decide on the drawing before the part goes to the finisher.

  1. State thickness and sealed or unsealed condition.Wear, corrosion, and dimensions change materially with both choices.
  2. Allow about 50% outward growth.A 50 µm coating adds about 25 µm to each treated surface.
  3. Use generous radii on coated edges.The thick ceramic oxide is vulnerable to edge burning and chipping.
  4. Keep press fits and threads masked unless explicitly compensated.25–75 µm total coating can close precision clearances.

Hard anodize versus the alternatives

Finishes engineers weigh against Hard anodize, and when each wins.

FinishCorrosionHardnessCosmeticCostThicknessChoose it when
Hard anodize
this page
716150$$$25–75 µm—
Anodizing Type II
Hard anodize vs Anodizing Type II →
715488$$5–25 µmwhen color and lower buildup matter more than wear
Hard Anodizing Type III With PTFE
Hard anodize vs Hard Anodizing Type III With PTFE →
716145$$$25–75 µmwhen lower dry friction is required
CorrosionHardnessCosmeticLubricityEconomy
Hard anodizeAnodizing Type II

The radar overlays Hard anodize with Anodizing Type II, its most common alternative — chosen when color and lower buildup matter more than wear. Scores in the table are 0–100 across every finish in the catalog.

Cost, lead time and compliance

Relative cost

Hard Anodizing Type III is a premium-cost finish ($$$): cheaper than about 51 % of the finishes FabDigit quotes.

Finishing is priced per part with a lot minimum, from FabDigit's own China-side rates cross-checked against US job-shop pricing. The exact figure appears on the instant quote once the part is uploaded.

Lead time and compliance

Added lead time4–7 working days
RoHSCompliant
NotesThe oxide process is generally RoHS compliant; verify dyes, seals, and any PTFE-free topcoat separately.

What Hard anodize is used for

  • Hydraulic cylinder bores
  • High-wear racing components
  • Military equipment housings
  • Automation wear plates

Hard anodize — frequently asked

How thick is Type III hard anodize?

The normal engineering range is 25–75 µm. A nominal 0.002 in, or 50.8 µm, is a common MIL-A-8625 callout.

Does hard anodize add thickness?

Yes; about half grows outward. A 50 µm oxide generally adds about 25 µm per surface and 50 µm across a fully coated bore or shaft diameter.

How hard is hard anodized aluminum?

A typical Type III oxide measures about 350–550 HV on suitable alloys. The substrate remains much softer, so thin walls can still dent under concentrated load.

Should Type III anodize be sealed?

Seal it when corrosion and stain resistance are primary. Leave it unsealed when the drawing prioritizes maximum abrasion resistance, because sealing can soften or alter the pore structure.

Can 7075 be hard anodized?

Yes, but 7075 usually finishes darker and process control is more demanding than on 6061. Fatigue-sensitive aerospace parts require engineering review because thick anodize can reduce fatigue strength.