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Ted Mooney, P.E. RET
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for Metal Finishing 1989-2025
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Trapped acid bleeds out from galvanizing
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Q. What's the best way to pickle a steel with double plate. Most times after pickling I get acid blow-out from those double plates causing black spots in the galvanized steel
Ismail jagar- Nigeria
March 7, 2025
A. Hi Ismail,
Please attach a pic to email to mooney@finishing.com. We have a general idea of the problem you are describing but details usually matter 🙂
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. This problem is primarily caused by the use of a welded joint unsuitable for galvanizing. But that's not helpful if you already have the problem. Usually the first liquid that the steel is immersed into is what gets trapped in this small cavity. In good galvanizing plants that's a degreaser, which could be acidic or alkaline. For that reason some call it "caustic blowout". If there's no degreaser in use in the plant (there really should be!), then acid is the first liquid in there. It stays through the rest of the process, steaming off at the zinc (450 °C) and oxidizing, leaving mostly iron hydroxide. Next time the steel gets wet, that comes out with the water as a brown stain. At the zinc, the trapped water (acid, degreaser whatever) can explosively turn to steam, sometimes destroying the weld, even cracking the steel.
A simple trick to minimise this problem is to immerse the item in clean water. That water fills the void, minimising the entry of chemicals.
Geoff Crowley, galvanizing consultant
Crithwood Ltd.
Bathgate, Scotland, UK
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