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Is Hard Anodize or PTFE coating preferable?





August 21, 2012

Q. Hi, I have a part in aluminum that functions as a latch and engages/ disengages a stainless steel roller. Now there was a great deal of wear on this aluminum part on the contact region. So I tried hard anodizing the part to 50 microns. After the process, the part was not smooth and looked very rough. Is this how a hard anodized part generally looks like? When I carried out some tests on the part, I found that the part was still wearing out as bad as earlier. Could it be because the hard anodizing was not done properly? I measured and found out that the coating thickness was only around 25 - 30 microns. Would the wear be less if I coat with PTFE instead?

Praveen Paul
- Bangalore, Karnataka, India



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August 21, 2012

A. Hi Praveen. I'm sure you already know the following answer, and just want someone to say it out loud for you, so I will --

Sorry, but you simply can't estimate the suitability of a coating to an application by doing a poor imitation of it and trying to extrapolate what might have happened had you done it right instead of it being too rough, and too thin, and too who knows what.

"Do you think my crème brulé would have been famous if I hadn't mistaken the salt canister for the sugar canister, and the yogurt for the heavy cream?" :-)

Please find a capable hard anodizing shop and have samples properly anodized. If they test well, then you might consider trying to move the process in house. If they don't, then you might want to look at an alternate coating like PTFE. Best of luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey



September 6, 2012

Q. Can I redo the hard anodizing on an already hard anodized part if it has not been done properly? This could be a silly question.

Praveen Paul
- Bangalore, Karnataka, India



September 6, 2012

A. Hi Praveen. Not a silly question at all. The anodizing can be removed by dissolving in caustic, but it is better/safer to remove it with a chromic-phosphoric acid mix because that dissolves the anodizing without attacking the aluminum like caustic can. Search the site for "strip anodizing" or "chromic-phosphoric" to find concentrations, operating conditions, and notes.

But the thing is, anodizing is not an additive coating like painting or plating; it's a conversion coating whereby aluminum from the part is converted to aluminum oxide. A 0.002" thick anodized coating consumes about 0.001" of aluminum. So you can have dimensional tolerance issues when you strip anodized coatings. Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


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