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Hard coat anodize coloration problem

We are having some trouble with a white frosty look to my black hard coat anodize. The material is 60601-t6 aluminum. We are asking our coater to hold a thickness of .001" plus or minus .0002". We also are asking for a nice black finish. When I got my parts back one side was nice, but the other side has a nasty white splotchy frost look over the black. I can tell you that the frosty side was bead blasted before coating, and the good side wasn't. Does bead blast cause this to happen? I was pretty sure I had blasted parts before and they came back looking fine.... Please help......

Jeff Frazee
cnc coordinator / asst. foreman - Anderson, IN United States of America
August 13, 2008

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First of two simultaneous responses --

1. If the beads were not designated "aluminum only" then you have pounded iron metal into the Kleenex-like soft aluminum.

2. If the bead blasting was done manually, then a pattern, not seen by the naked eye prior to anodizing, will be magnified with the 0.001 build-up of anodizing.

3. What is the apparent thickness on the back side? If his agitation pattern was not uniform, then the back side may be splotchy anodized due to inadequate air agitation, or too much air hitting the part.

In either case, none of the above should have happened, can be corrected, and future work should be made acceptable.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

August 15, 2008

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Second of two simultaneous responses --

Dear Jeff

My thought about why you received this result on the blasted side only is that the media used may have contained a metal other than aluminum. My experience with chemical process auditing confirms many places do not dedicate blast cabinets to a metal (aluminum) only or change media. When the blasting was done previously, it may have been with new or clean media and this time it wasn't. If this happened it could be the cause. This is a possibility I am not saying it is absolute.

Drew Nosti, CEF
Anodize USA

Ladson, South Carolina

August 16, 2008

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First of two simultaneous responses -- August 20, 2008

A picture would be nice, but a couple of thoughts come to mind.

#1. The "splotchyness" sounds like a case of uneven bead blasting to me. This can be solved by using better bead blasting techniques, and/or a longer etch by you coater.

#2. You may not be getting a decent black due to the fact (in my experience) a bead/sand blasted surface will not anodize at the same rate as a smooth surface. I would wager that if you have 1 mil of anodizing on the smooth surface, that you coating on the bead blasted surface is probably 50% of that. It simply may not be thick enough to get a decent black (depending on the dyestuff used) Your coater will need to calibrate his gauge to the blasted surface in order to get accurate measurements.

Marc Green
anodizer - Boise, Idaho

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Second of two simultaneous responses -- August 20, 2008

Gentlemen,
Thank you for your response! We are currently having the critical holes masked, then stripping the anodize off, then etching the parts, and having them returned here to tumble them to get a uniform surface finish. After that, we will send them back to get recoated. We tried this on 2 parts and it worked well....... This was a very costly lesson to learn, as you probably know.... Thanks again for the help!
And also, a special thanks to finishing.com for making all this sharing of information possible........

Jeff Frazee
- Anderson, IN, United States of America