Metal finishing Q&As since 1989

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Hardcoat Anodize Rinsing Bubbles

I'm trying to track down what causes "bubbling" during final rinsing of freshly hardcoat anodized parts. We've noted it ourselves in our shop and some of the operators have taken it upon themselves to extend the rinse time until the bubbling action ceases. I've talked with other anodizers that have noted the same condition, reportedly most noticeable on 2xxx alloy parts, but they limit the final rinse time to 5 minutes maximum regardless as to whether or not the bubbling has ceased. This leads to 3 questions: (1) what's the reaction occurring that creates the bubbles? (2)is it deleterious to the coating? and (3)what should the final rinse time be?

Milt Stevenson, Jr.
- Syracuse, New York
2005

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First of two simultaneous responses --

Good question, Milt. I run a hardcoat facility, and obviously have seen these bubbles too. It's obviously some type of gas, perhaps either hydrogen, or oxygen escaping from the pores of the anodic coating. Interesting question, I'm not real sure. The reason I'd expect to see this magnified on 2000 series would be the fact that your pores are going to be bigger on the Cu alloys, therefore, trapping more of whatever gas is in the pores.. I don't think its harmful to the coating at all. As far as rinse times, I would recommend a max 5 min in the first rinse (small addition of HNO3 is benificial), followed by a 30 sec to 1 min immersion in a second (cleaner) DI rinse, followed by a final dip/spray rinse in at least 9 meg water. A good counterflow system will help save on water usage.
I'd be interested to hear what other folks think the bubbles are.

Marc Green
anodizer - Boise, Idaho
2005

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Second of two simultaneous responses --

Milt

I have seen the phenomenon you are speaking about at a couple of shops. It occurs most if not all the time on the 2 mil hard anodize we run. Most of the time we do not see it as we have air-agitated rinses. The out-gassing is not as noticeable if your rinse tanks get sour.

1] What is it- probably O2 as it is produced at the anode.

2] Is it harmful-probably not if I'm right on #1

3] We have always employed 2 each 5 minute rinses on hard anodizing. You could perform a dye test on samples rinsed at various times- no HNO3 pre-dip.

Willie Alexander
- Colorado Springs, Colorado
2005