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How hard is hard?

2002

Hi,

I am trying to find out is there a table which will show the hardness of anodised finish's. e.g., Hard @ whatever thickness being equal to a specific hardness e.g., 70 rockwell C. and the same for soft or natural anodised..? We have never been asked to show a hardness before but this customer need's to know. I don't mean to be vague but can someone please help.

Many thanks,

Jim McGeady
- Portsmouth, UK

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2002

Good question - everything is relative. As I understand it, hard anodising refers more to the process than the finished hardness. The anodised surface will have a hardness dependent on the type of alloy being anodised. Generally high strength alloys such as 2000 and 7000 alloys can give micro-hardnesses ranging from 250Hv to 350, but 6000 alloys can give 400-500 (or 45-48Rc). However, care should be taken as the hardness of good aluminium oxide can be between 940 and 1100Hv 965-790Rc), but the determined hardness may be only a reflection of the crushability of the alloy substrate. I believe microhardnesses of up to 1400Hv have been claimed from mixed electrolytes, but I have no experience of such systems. As a rule of thumb, hard anodising is considered to be achieved when the hardness exceeds about 350Hv. I think the British Standards specify hardnesses and resistance to abrasions for anodised coatings, but offhand do not know their exact numbers.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK