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Dichromate seal goes pale gradually despite good concentration

After sulfuric Anodising, our parts are sealed in a Sodium Dichromate seal.
This consists of 40-60 g/l Sodium Dichromate dissolved in demin water at a pH of 5.6 - 6.0
at a temperature of 96 - 100 C.
This works fine for 8 or 9 weeks, but then the parts gradually become paler in colour even though the bath concentration is at the top end.
Has anybody else experienced this? Although this is probably cosmetic, if the parts are too pale, they will be rejected by our inspection, and our customers.

Michael Channing
employee - Hampshire, England
2007

Yes, I have seen this, in less than 8 to 9 weeks. I titrated the second rinse following the anodizing and found high sulfuric acid. Too much sulphate drag-in kills the seal.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007

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