how does hard anodized work?

[Murnax]

Ok, I know how to anodize aluminum (in theory, never actually done it), but how do you hard anodize aluminum. I've been reading about it and apparently hard anodized aluminum can get into the 70s on the rockwell scale. Thats a pretty tough material. I have a set of calphalon cookware that is hard anodized aluminum, and I used to think that it was hard throughout, but now I'm learning that the hardness is just a surface effect like anodized colors. So how is hard anodizing done, and can it be done to any other metals besides aluminum?

 

[Abadir]

The following procedure is for anodizing aluminum parts. Anodizing will protect the aluminum parts by making the surface much harder than natural aluminum. Aluminum oxide is grown out of the surface during anodizing and is extreemly hard. The porous nature of the anodized layer allows the product to be dyed any color that is required. The method I describe is type II anodizing (room temp) and gives an anodized layer of .0002" to .0009". Type III (hard coat) anodizing is done at a much lower temperature and higher current level. It gives an anodized layer up to .002 but is normaly at .0005" (Mil spec) in most anodizing shops. Type III anodized surfaces can typically only be dyed black or dark green.

This text is from this page:
http://www.focuser.com/atm/anodize/anodize99.html

You can also try this page:
http://www.anodizing.org/what_is_anodizing.html

 

[Murnax]

Thanks!