Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

Purpose of sodium dichromate in QQ-P-35C passivation solution

I am interested in determining the specific purpose of sodium dichromate in QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil] Type II passivation solutions. I have been told that it is used to "retard" the etching process, however, I have not been able to confirm this. Can anyone either help me or point me in the right direction?

I think it has something to do with a desire to provide secondary passivation - mainly for parts with tight openings to leave a residue that contains Cr 6+: similar to the effect obtained by dichromate treatment specified as an option in the same QQ-P-35. Interestingly, most major aerospace primes have their own specs for passivation, they are using different concentrations of HNO3 with or without additions of Cr6+ (as a Na or K chromate/dichromate) that do not follow material/solution combination of QQ-P-35. Even more interesting, only Boeing in BAC 5625 correctly specifies to use dichromate treatment only on parts with tight openings: for all other parts this treatment is simply useless.

We are doing and testing passivation per many different specs on different materials, my observation is that all alloys that are passivated (per QQ-P-35) with addition of Cr6+ may be passivated as good without it, but some alloys that are to be passivated without Cr6+ if passivated with it MAY show excessive surface oxidation (dark grey discoloration or even be attacked with a noticeable metal removal).

I am student college. now, I am looking for sodium chromate (tetrahedrate) about reaction, Flow diagram, etc. I have been reading Faith and Keyes book Industrial Chemicals [on on Amazonaffil links], but it's very simple. I would like You helping me to solve it.

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