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Finishes / Hard anodizing (Type III)

Hard anodizing services for aluminum parts

Hard anodizing, or Type III anodize, grows a thick aluminum oxide layer out of the part in a cold sulfuric acid bath at high current. The result is a dense, hard surface that resists wear and galling far better than bare aluminum or Type II anodize.

It is specified on pistons, slides, gears, valve bodies, manifolds and any aluminum surface that rubs or sees abrasive wear. Because the oxide grows both into and out of the part, the drawing must say whether dimensions apply before or after anodizing.

Send an RFQ for this finishWhat to put on the drawing
  1. Aluminum substratebase
  2. Oxide grown into the partabout 25 µm
  3. Oxide built above the surfaceabout 25 µm
  4. Seal (optional on Type III)in pores

A 50 µm hard anodize grows about 25 µm above the machined surface, so a bore shrinks by about 50 µm on diameter.

Section view, not to scale. Thickness values are typical, not promised.

Typical values

Typical ranges, not promised. The spec on your drawing and the process control of the shop set the real numbers.

Thickness
25 to 75 µm, 50 µm is the most common callout
Growth
About 50% outward and 50% into the part, per surface
Color
Gray to dark bronze undyed, depending on alloy and thickness. Black dye is common
Electrical
Insulating. Mask any surface that must conduct
Sealing
Optional. Sealing helps corrosion but lowers wear resistance
Edges
Sharp edges can burn or chip. A radius of about 0.5 mm is a common minimum
Fatigue
Thick hard anodize can lower fatigue strength. Check with the part designer on cyclic parts

Specs buyers call out

Put the spec and its type, class or grade on the drawing. Shops quote against it, and approvals narrow the list.

MIL-PRF-8625 Type III
Hard anodic coatings on aluminumClass 1 undyed, Class 2 dyed. The most common callout
AMS 2469
Hard anodic coating treatment, aerospaceProcess and thickness control for aerospace parts
ISO 10074
Hard anodic oxidation coatings on aluminumInternational equivalent used on European drawings
AMS 2482
Hard anodic coating with PTFEFor low friction sliding surfaces

Base materials

Name the alloy and hardness on the RFQ. Preparation changes with the base metal, and some shops only run one family.

Suits

  • 6061 and 6082
  • 7075 and other 7000 series
  • 5000 series wrought alloys

Needs care

  • 2024 and other high copper alloys (thinner, softer layer)
  • Cast alloys like A356 (uneven color)
  • Mixed alloys in one assembly (color mismatch)

Not typical

  • High silicon die castings such as A380 and ADC12
  • Steel, stainless or titanium inserts left in the part
  • Parts with trapped crevices that hold acid

When to specify it, and when to look elsewhere

A good fit for

  • Aluminum surfaces that slide, rub or see abrasive wear
  • Pistons, slides, gears, cylinders and valve parts
  • A hard surface where electroless nickel would be too costly or too thin
  • Parts that need electrical insulation on the surface

Look elsewhere for

  • Cosmetic parts with bright colors: Type II dyes more evenly
  • High silicon die castings: the layer comes out thin and blotchy
  • Parts that must conduct electricity everywhere: use chem film
  • Thin walled parts with tight bores and no allowance for growth

What to put on the drawing and RFQ

Six lines that decide whether quotes compare. Missing items come back as questions and add days.

  • Spec and classMIL-PRF-8625 Type III Class 1 (undyed) or Class 2 (dyed), or your own spec.
  • ThicknessNominal with tolerance, for example 50 µm ±10 µm. Note that it is total thickness, not build-up.
  • DimensionsState whether dimensions apply before or after anodize. Machine bores oversize by the growth.
  • SealSealed or unsealed. Wear surfaces are usually left unsealed.
  • MaskingThreads, ground contacts and press fits. Mark each on the drawing.
  • Alloy and temperName the alloy. Copper and silicon content change color and thickness.

What moves the price

Shops price by load, lot and labor more than by the chemistry itself.

  • Thicker layers take longer at high current and cold bath temperature
  • Masking threads and contacts is hand work per part
  • Each part is racked with an electrical contact point
  • Dyed Class 2 adds a dye step and color checks

Plating and coating cost: what drives the price per part

Questions buyers ask

About hard anodizing (type iii) specifically. For how matching works, see the home page.

What is the difference between Type II and Type III anodizing?

Type II is thinner, about 5 to 25 µm, dyes evenly and is used for color and corrosion. Type III is thicker and much harder, made for wear. Type III is run colder at higher current and costs more.

How much does hard anodize change the size of a part?

About half the coating thickness builds outward on each surface. A 50 µm coating adds about 25 µm per surface, so a shaft grows about 50 µm on diameter and a bore shrinks by the same amount.

Can hard anodize be dyed?

Yes, mostly black. Other colors look darker and less even than on Type II because the oxide itself is gray to bronze.

Which aluminum alloys hard anodize best?

6061, 6082 and 7075 give consistent results. 2024 and other high copper alloys give a thinner, softer layer. High silicon die castings like A380 are poor candidates.

Should a hard anodized wear surface be sealed?

Usually not. Sealing improves corrosion resistance but softens the surface. Many drawings leave wear surfaces unsealed or call for a PTFE treatment instead.

Get quotes for hard anodizing (type iii)

Send one RFQ with the drawing, base material, spec and lot size. We match it with up to 3 shops that run this finish on a line that fits your part, and reply within 24 business hours.

Send my RFQ

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