Summary for aluminium content in chromadizing tank Discussion
The thread on aluminium content in chromadizing tank asks how to analyse aluminium in a chrome tank used for chromate conversion coating. Scott Merritt says that, based on his understanding of chromadizing as treatment with chromic acid, the bath can be tested by taking a 20 mL sample, diluting it to 100 mL with DI water, and titrating with 0.5N NaOH to pH 3.2 to get "A," then continuing to pH 4.8 to get "B." He says aluminium content is calculated as (B-A) x 0.430. In a follow-up, he apologizes for omitting the chromium factors and adds that free acid is A x 0.334 and total acid is B x 0.334. The original question came from Nirbhay Goswami, who wanted a practical analysis method for a pretreatment manufacturing unit.
Q. Sir I work in a pretreatment manufacturing unit. Please tell me how to analyse the aluminium content in chrome tank.
Nirbhay Goswami
– Rudrapur, Uttrakhand, India
October 10, 2012
Ed. note: the word “chromadizing” isn’t used much in the USA to our knowledge, but we believe it means chromate conversion coating of aluminum.
A. I’m not familiar with chromadizing but from what I can gather it is just treating the metal with chromic acid. If so, take a 20 mL sample of the bath and dilute it to 100 mL with DI water, then titrate with .5N NaOH
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to a pH of 3.2. Call this “A”. Continue titrating to a pH of 4.8. Call this “B”. To get your aluminum content it is
(B-A)x .430
Scott Merritt
– Eastman, Georgia, USA
October 12, 2012
A. My apologies, I forgot to include the factors for free and total chromium.
To determine free acid you multiply “A” x 0.334
To determine total acid multiply “B” x 0.334
Scott Merritt
– Eastman, Georgia, USA
October 30, 2012