Question about olive drab chromating
I am evaluating OD chromates on zinc (from zinc sulfate bath), and am finding a generally low chromate bath tolerance for dissolved zinc, and am at a general loss as to what to attempt to control (pH, chromate %, dissolved zinc) in my tube/wire application. Immersion times are ~15 seconds.
By low tolerance, I mean that we see a fairly quick loss of color on the plated product, with marginal SST affects. The chromate coating also serves as an adhesion barrier for paint, so the marginal reduction in SST to white rust in minimized.
How long are OD chromate baths 'good' for, in general, and what should I attempt to control to extend bath life, while maintaining a good conversion coating?
Every company uses a slightly different formulation, so there will be some difference between the critical elements of each bath.
I worked with a chromate that was used for both chloride Zn and Cd. One metal was a poison to the other and the zinc required a less aggressive chromate than the Cad did. Your vendor should have a separate set of operating conditions for Zn. There was a terrible tendancy to add excessive brightener to maintain a 100% bright range which made chromating on a panel nearly impossible. Are you using a very dilute acid rinse prior to the chromate. Might help. 15 seconds seems to be a rather short immersion time. If the chromate is slowed down some, it might be less sensitive. Try some different combinations in the lab. The lab folks at your vendor should be able to help if your salesman has not been much help. If push comes to shove, consider using a different vendor. I have had good luck with two vendors and not so good with another.
Good Luck, Jim
- Navarre, Florida
If you are using a proprietary product, I would say the most important factor in chromate life is dragin of other processing solutions. Do you drag in a lot in your tubes?
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