Types of Anodizing

Type II Anodizing

“Standard” anodizing, more commonly known as Type II anodizing, is based off of military specification MIL-A-8625.

It is the standard specification worldwide for commercially anodized aluminum for a wide range of industries, from medical and aerospace to military and defense. Standard anodizing utilizes a sulfuric acid solution.  Sulfuric acid is the most widely used solution to produce an anodized coating.

Coatings of moderate thickness 1.8 μm to 25 μm (0.00007″ to 0.001″) are known as Type II by MIL-A-8625.

Type III Anodizing

Type III anodizing, more commonly known as “Hardcoat” anodizing produces coatings thicker than 25 μm (0.001″).

Thick coatings require more process control, and are produced in a refrigerated tank near the freezing point of water with higher voltages than the thinner coatings. Hard anodizing can be made between 13 and 150 μm (0.0005″ to 0.006″) thick.

Anodizing thickness increases wear resistance, corrosion resistance, ability to retain lubricants and PTFE coatings, and electrical and thermal insulation. Type III Anodize should not be dyed or sealed to maintain its wear resistance. Sealing will reduce this greatly. 

Hardcoat is a highly abrasion resistant, non-conductive aluminum oxide (Al2O3-xH2O) coating that makes an aluminum surface harder than tool steel due to greater thickness and weight than conventional anodic coatings. Anodic coatings form an excellent base for dry film lubricants, Teflon, paint, and adhesives.

Note: You cannot hardcoat over hardcoat, anodize over anodize, build up hardcoat over anodize, chromic over hardcoat, or just add another 0.0005″ to the surface.

Properties of Hardcoat

Hardness

65 to 70 Rockwell C, 850 to 900 DPH, harder than hard chrome plate

Color

Dark Gray to Black

Coating Thickness

0.002″ average, Hard anodizing can be made between 13 and 150 μm (0.0005″ to 0.006″) thick.

Dielectric

Hardcoat is non-conductive and will withstand 800 volts per 0.001″ thickness.

Machining

Hardcoat can be ground, lapped, honed or polished.

Dyeing

Hardcoat should not be dyed.

Sealing

Hardcoat may be Dichromate sealed.

Resistivity

between 106 to 1012 Ohm-cm

Coating Thickness vs. Corner Radius

Nominal Coating Thickness, inch

Radius on edge & inside corner

0.001

~ 1/32 inch

0.002

~ 1/16 inch

0.003

~ 3/32 inch

0.004