Metal finishing Q&As since 1989
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Trying to lower the pH of Chem film causes smut
Q. We normally use nitric acid to bring down the pH of the chem film bath. The problem we encounter is a smutty surface. Is it because we are adding too much nitric at one time? We are adding 500 to 1000 ml each time we need to bring down the pH to under two. Our tank size is about 500 gallons. We are using Alodine 600 from Henkel. Some old platers suggest we use acetic acid [on eBay or Amazon] instead. Will it mess up the tank chemistry or effect the part? Thank you for your help.
Vorakarn Mutsalklisanprocess chemist - Downey, California
June 29, 2012
A. Do not use acetic!! If smutty, sounds like pH is already too low -- so how are you measuring pH, are you sure? High sulphate from deox drag-in can also smut. Dip out 5 gallons and determine how much nitric is needed to get the pH you want, then run a panel in the 5 gallon bucket and be sure you are there before you do further damage to the production tank. If not, send a sample off to your chromate supplier. (I do not charge for "homework" if you want to talk).
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
July 5, 2012
A. Good day Vorakarn.
My Alodine is 1550 U.S. gals., and 800 mls HNO3 will reduce pH by 0.07. Perhaps you have a bath which is loaded with metallics? Have you filtered a small lab sample and done a pH adjust?
Eric Bogner
- Toronto, Canada.
July 9, 2012
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