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Thick black foam in passivation tank ?

My name is Mike Nelson I am the Special Processes Manager of Machine Shop. I have a passivation line set up to ASTM A967 nitric 1. This afternoon I found a thick black foam floating on top of my passivation tank. The tank tested good 24% nitric acid 2.5% sodium dichromate. Has anyone seen this before? If so can you tell me what it is or what caused it? Thank you for any help you can provide

Mike Nelson
Special Processes Manager - Chattanooga Tennessee
January 14, 2011

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