Metal finishing Q&As since 1989

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Anodize tank cathodes gassing

April 9, 2008

We are having an issue in 1 of our 2 anodizing tanks.

I know you like lots of details, so here goes:

We run 2 anodizing tanks Ë they are both approximately 1100 gallons.

In one of the tanks, our Tank #7, we are seeing gassing at the aluminum cathodes, indicative of attack. These tanks are approximately 10 years old & this is a new issue. It happened a few months ago & we ended up neutralizing & dumping the tank & remaking the solution & the problem went away Ë now, a few months later, it is back again.

We have checked for stray current & found none.
We have analyzed for chromates & nitrates in the bath & found none.

These are high volume production tanks. Operating parameters:
- aluminum @ 6-8 g/l (controlled with Ecotec APU unit)
- sulfuric @ 220 g/l Ë we know this is a little high, BUT, we have reason for running at this level & have done so for 30+ years.

When we transferred the problem tank to another tank for neutralization, before dumping, we inserted a cathode & still observed the problem.

Both anodize tanks are the same, except, the problem tank has a MAE, proprietary additive, for higher temp hardcoating. We have been using this additive for 20+ years.

Before I go & dump the tank again, anyone have any ideas?

Thanks,

David Kraft
plating shop employee - Long Island City, New York

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Do you mean to say that the cathodes ae gassing when there is no rectifier power to the tank? If that's the case, then the two possibilities are a) some sort of stray current effect, and b) a bath contaminant, chloride the most likely culprit. If gassing when in use, that's of course normal, and may only be worse in one tank than the other. Reducing surface tension can make the amount of gassing appear to be much less. The materials normally used to reduce spray in chrome plating work well. I don't believe the MAE additive is part of the problem unless incorrectly formulated. Jeffrey Holmes, CEF Spartanburg, South Carolina April 10, 2008 David Did you recently replace the cathodes? Did the cathodes show signs of attack when you last dumped? What contaminate was found in your solution to require dumping? When do you observe the gassing- under power, immediately after, always? Willie Alexander - Colorado Springs, Colorado April 10, 2008 Hi David. 220 gm/L will eat aluminum much faster than 180 gm/L. 220 gm/L with low dissolved aluminum will eat aluminum even faster. Titanium racks left hanging in the tank while other racks are being loaded will set-up a galvanic voltage which will further eat aluminum. All of the above is accelerated with chloride contamination. Robert H Probert Robert H Probert Technical Services Garner, North Carolina April 10, 2008

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April 14, 2008

Thank you everybody fo your help - we have not as of yet found the problem, but here is some additional info:

- both anodize tanks are polypro
- gassing is visible in the morning before tank is started up
- When we transferred the problem tank to another tank for neutralization, before dumping, we inserted a cathode & still observed the problem ** gassing cathode was insulated electrically from the tank
- we have no source for chlorides in the plant
- re: stray current - when we transferred the suspect solution to another tank, the gassing continues. If ALL the tanks are exposed to stray current, how come the 2nd anodize tank is un-effacted.

Thanks again

David A. Kraft
- Long Island City, New York

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Dave,

I see you've carried this inquiry further after our initial discussion. I concur with the discussion thus far with the exception of "we have no source for chlorides." All water sources, even our pristine clean Upstate NY water that New York City pilfers downstate, has chorides in it. Also, doesn't take much chloride as they only accumulate in the electrolyte - only loss is thru drag out.

Milt Stevenson, Jr.
Syracuse, New York
April 15, 2008

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David

Continuing Milt's thoughts on the chloride contamination, you might investigate your sulfuric acid for chlorides. Do NOT rely on the certification. Were both the good and bad tanks made up with the same lot# electrolyte?

Willie Alexander
- Colorado Springs, Colorado
April 17, 2008

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While we don't have any source for chlorides in our plant we understand there is small amount naturally in water.

Both tanks - the 1 with gassing & the 1 without - are made from the same batch of sulfuric.

One new note - Monday early am I look at tank before all power is on & I see virtually no gassing. Tuesday am & the rest of the week each early am I see pretty severe gassing. This morning, Saturday, most equipment is off I see almost no gassing.... just a little. I then went & turned off ALL power, even the main to plant & checked again & still see the little bit - I waited another 10 minutes with all power off & checked again with flashlight & still see the little bit. My guess is Monday am, early when I get in, there will be almost no gassing & then Tuesday am & for the rest of the week we will be back to lots of gassing. Sounds like stray current to me but I haven't located any.

Any more input would be greatly appreciated.

David A. Kraft
- Long Island City, New York
April 19, 2008

Hmmmm...sounds like you get gassing after running a while, and then it gradually subsides when you're shut down. Is your temperature getting higher than you think? Don't guess about chloride - have it checked. It doesn't take much. Jeffrey Holmes, CEF Spartanburg, South Carolina April 21, 2008 I would place half of the cathodes on a PVC pipe, still in the tank and see if those anodes gassed on Monday, relative to the ones still on the buss bar. That should give you a decent idea of where the problem might be. Per the earlier replies, I would do a chloride check. Wet chemistry should be adequate-- IE: the old silver nitrate test. James Watts - Navarre, Florida April 21, 2008

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We recenly had a similar experience and have solved it. Our situation is identical you yours. Do not rule out any potential sources of chloride and fluoride. Check you incoming sulfuric, your propritary MAE additive, and your city water.

You are looking for extremely high levels of contaminants.

Scott Turner
- Green Bay, Wisconsin, USA
April 23, 2008

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I echo Mr. Watts suggestion to run chloride tests on everything that goes into the bath. I would use a gravimetric method - if the samples are sent out to a run of the mill environmental lab, they are apt to try to titrate them. Some of the matrices you describe will yield quite meaningless results, analyzed that way.

Another potential source of chloride is caustic soda [affil links] . Diaphragm cell grade can contain a lot.

Dave Wichern
Consultant - The Bronx, New York
May 10, 2008

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May 13, 2008

Just wanted to update all that have offered help Ë we seem to have figured out the problem.

It appears my Tank #7 anodize which had the gassing problem was getting dragout dripped in it from chromated parts passing over the tank. My Tank #1 anodize which did not have the gassing issue was in a different area where the chromated work never passed over the tank.

Analysis showed fluoride levels in #7 30+ times higher than that of #1. We had only adequate rinsing after the chromate tank, &, in the last 8 months or so, our chromate production has increased over 300%.

To resolve the problem we have:
- begun leaving old aluminum racks in the Tank #7 anodize overnight so that the fluoride will be used up quicker attacking these racks.
- We have also setup another clean rinse for after chromating, with spray sparger, before the hoist carrying finished work goes over the Tank #7 Anodize.

Since these changes have been made we have already seen a drastic reduction in fluorides in the Tank #7 Anodize & the gassing is getting better (lesser).

Thank you everyone for the input.

David A. Kraft
- Long Island City, New York