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How to identify and probably remove galvanized coating?

I have a 700+ gallon vessel that I believe was used in a commercial air conditioning system. It seems to have had a desiccant inside. It is approximately 42" dia. x 72" high, dome top and rounded bottom. Manway at the top and a few ports. It seems light gray and shows little rust even though it was in an abandoned building for years, exposed to direct rain spray.

I have a feeling it is galvanized. How do I know for sure?

I want to re-purpose the vessel for use in biodiesel production. Waste vegetable oil is slightly acidic and will attack galvanized coatings. How do I remove such a galvanized coating?

I have plenty of citric acid ⇦ this on eBay or Amazon [affil links] around, as well as some Phosphoric/Glycolic Blends. I also have some spare Nitric, though no HF in it. I've read chelants will remove galvanized coatings. After removal, should I bring the pH up to at least 8 to neutralize? I typically use Triethanol Amine (TEA-85) or maybe some MEA. I also have plenty of rust inhibitor to coat the interior after cleaning.

Am I on the right track? Ideas?

Todd Turner
Industrial Cleaning, Chemical blender - El Dorado, Ark., USA
August 15, 2009

Hot 10% caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] solution will do it, with no risk of attack of the substrate. Bill Reynolds [deceased] consultant metallurgist - Ballarat, Victoria, Australia We sadly relate the news that Bill passed away on Jan. 29, 2010. August 20, 2009 Try letter 38853 -spot test for zinc(and cadmium too).Hope it helps and good luck! Goran Budija - Zagreb,Croatia August 20, 2009

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Thanks for the help. How hot is hot? 100 degrees F? 140 degrees F? 180 degrees F? I've been wanting to get a tank cleaning spin nozzle for quite some time... sounds like I have my excuse!

Todd Turner
- El Dorado, Ark., USA
August 23, 2009

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The hotter the stripping solution, the faster it will work. I can't quote a relationship between temperature, contact time, and thickness of zinc removed but would more-or-less expect something like an hour assuming you maintain hot solution and have enough contact time to heat the tank itelf. Since you're looking at a one-off clean-up rather than setting up a production line for de-galvanizing a stream of components, I guess speed of reaction isn't critical. Good luck with getting approval for the spin nozzle!

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
August 26, 2009