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Originally Posted by hdway
if the black chromate gets contaminated so esily how can you get any amount of items chromated with out replacing the bath
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Ya gotta realize that all chromates are "consumables". ie, is has a life span. The active ingredients eventually get used up, and parts won't chromate properly any more. Any extraneous contamination that uses up the active ingredients just shortens the life.
I'm no chemist, but I think I have a handle on this aspect. Chromates have 2 main ingedients:
- chromium - which imparts the corrosion resistance
- a "colorant" - some chemical which imparts the desired color (if any)
In the case of black chromate, the colorant is a silver salt (contains either silver and/or silver oxide). When silver tarnises, it turns black, and that's what gives our parts the black color. There's only so much silver in the chromate solution to begin with. As you use it, the remaining silver decreases, until there's not enough any more.
Any contamination that attaches to the free silver, essentially uses it up, and it precipitates out to the bottom of the tank. The zinc electrolyte has billions (maybe more) free zinc molecules which attach readily to the silver. Even small amounts of it will severely use up the free silver. 1cc is enough to completely waste a gallon of black chromate (don't ask how I know).
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What about the particals of white in the bottom of the chromate are they suppose to br there?
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No. That's the precipitated silver/zinc.
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When my chromate settled overnight it cleared up.
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Yes, it will clear up, but it's still missing the silver that's precipitated out, so doesn't work properly anymore
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Will the sulphuric acid affect the bath as much or is it compatable with the sulphur base?
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I really don't know how detrimental "other" acids are, but since the black chromate is sulphuric based, at least it's not an "un-natural" ingredient. I suspect that's why black chromate manufacturers recommend only sulphuric acid for bright dips.
Too much of any acid is going to upset the chemical balance, so best to have parts thoroughly rinsed.
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As far as the rinses I don't like the distilled water spray would rater give it a quick rinse with the softened water in my sink and then spray with distilled
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That should work fine. I use running tap water for #1 rinse, then spray down w/distilled.
So how long will the black chromate last? It's not possible to say. It just depends on how many square inches of parts you chromate, and how long you leave them in for (and, how much gets used up by contamination).
When I first started to use my black chromate, parts would turn deep black w/in 15-20 seconds. By the time I had the mishap that ruined the chromate, it was taking 30-40 seconds to get deep black. So it was clear to me the chromate was depleting. By this time I had done about 350-500 sq-inches. YMMV.
After the mishap, this is what parts come out like, even after 5 minutes in the chromate!
Sean