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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Loss of amperage and glowing parts
Q. A bit of an issue has been stumping us and I thought I'd see if anyone out here can help.
I'm the Director of Quality for a bulk anodizer (sulfuric acid, Type II) here in California. Lately we have noticed our rectifiers behaving oddly - about 30 minutes into a load (well after full amperage, typically 1500 - 2000 amps, had been reached) amperage would fall significantly while voltage climbed slightly (1 or 2 volts). Attempts to increase the amperage would fail. Originally this was just happening with one rectifier, although we have now started to observe this behavior in our second rectifier as well. Furthermore, along with the drop in amperage, today I witnessed arcing electricity under the solution -- flashes and sparks of light illuminated the whole tank. At one point either a part or a piece of a rack (the solution is agitated so I wasn't able to see clearly) started glowing like a light bulb and stayed illuminated for 8 to 12 seconds. Upon inspecting the parts after the load was done we discovered, unsurprisingly, that the parts were massively over-etched, including some pieces that had been eaten in-half.
We have cleaned all connections from the rectifier to the tank, had the rectifier inspected for any problems, changed the acid, and tried varying the load -- so far with no solution to the problem. To further complicate the problem, the issue does not appear reliably (sometimes it will happen in back-to-back loads, sometimes weeks will go by in between occurrences) and does not seem to be tied to any specific part or load size; in fact, some loads that trigger this have been re-cleaned and put back in the tank and run successfully.
Currently we are sending our titanium racks and baskets through our cleaning line on the off chance that this was caused by anodizing the titanium; but from what I've read in other letters that doesn't seem to have a high chance for success.
Any suggestions?
Thanks,
Jim GorsichCompton, California, USA
2003
A. From a key word in your inquiry- "bulk anodizer", do you regularly disassemble your racks and have the contact surfaces sanded and cleaned? Check for burned out bolts and nuts. You may not notice it but I bet there is also an increase of activity of your chiller due to increase in heat generation in the solution. It is most probably a case of poor rack contacts.
Ciraco Dipaloloco- Toronto, Ontario, Canada
2003
A. The power supply is not the problem. A VOM multimeter ⇦this on eBay or Amazon [affil links] reading between the busbars will prove this. I think that the problem is with the age of the rack. The rack fingers gradually erode and get thinner. Also they sometimes lose there springy-ness or elasticity due to use. A weak contact with good agitation is begging for arcing. We had a trashed nitric HF bath that we stripped the Ti racks on a routine basis (off line). It only took a few seconds for a light blue color or maybe 30 sec for a dark blue. This tank rarely was dumped. We did occasionally add HF to keep it active. Also, consider the human factor. Does it happen to only one or two operators or all of them equally? I would re-finger or remake the racks on the worst offending line and see if the problem does not go away. It is cheaper than explaining trashed parts to a customer.
James Watts- Navarre, Florida
2003
2003
Q. Thanks for the answers guys. It does appear to be caused by the titanium needing to be cleaned; we don't stock HF but we soaked the titanium in Nitric and then sanded them down afterward (I read in the archives that anodized titanium, which is what I presume we're cleaning off, is fairly brittle and that sanding or scrapping can remove it) and we haven't had a repeat of the problem since. I have a second question for you though (if you wouldn't mind answering it) - other than HF, is there another chemical way to clean the titanium? Or are manual methods the best if you don't want to deal with HF?
Again, thanks.
Jim GorsichCompton, California, USA
A. Several things attack TiO, but not much removes it besides mechanical and fluorides. Have you considered the use of Ammonium bifluoride to get the fluoride ion? This is a large crystal, kind of like broken glass, that can be added to your nitric tank. It requires minimal safety. Less than your caustic cleaner. I have picked it up with my bare hands (not very smart). The resulting nitric containing fluoride is only slightly more hazardous than just the nitric; rubber gloves ⇦this on eBay or Amazon [affil links] , goggles ⇦this on eBay or Amazon [affil links] and apron ⇦this on eBay or Amazon [affil links] take care of it very nicely. Actual skin contact is little more than the nitric, i.e., get it off now and get clean clothes on. We provided sweat suits for the UH-OHs which fortunately were never used. Any acid contact should be monitored. There is always the remote possibility that the individual did not properly clean up. Many are too proud to admit that they did a boo-boo.
James Watts- Navarre, Florida
2003
2003
Thanks for the tip, I'll look into getting a hold of some Ammonium bifluoride and try it out.
Thanks again,
Jim GorsichCompton, California, USA
A. Use Turco Etch to clean the Ti; it's an alkaline etch without involving HF.
L Jin- Connecticut, USA
October 31, 2018
November 2018
Thanks, L !
If anyone knows of a generic description for such etches please let us know; we avoid brandnames to the extent possible (why?)
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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