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Indium soldering over chromate conversion coating on copper

November 23, 2016

Q. Hello everyone,

I'm building an assembly of stainless steel with a copper part brazed on it. I need to braze something on the copper part at 120 °C using Indium 1E alloy (52.Sn,48.In). By the time I perform my brazing operation, the copper becomes very tarnished which is very bad for ultra high vacuum application.
We thought about applying chromate conversion (iridite 8p) on copper but I have been told that it would be impossible to do the brazing with the Indium alloy. No flux could break the oxide on the chromate conversion to make the Indium wet the surface.
Is that correct? Is it really impossible to solder on chromate conversion?
Is there a better way to prevent oxidation of the copper and keep the solderability of it?

Thanks for your help!
Simon

Simon Lessard
Product manager - Montreal, Quebec, Canada

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Q. Dear Ted,

Thanks for your reply. I just found that chromate conversion coating would be destroyed at 60 °C so it is another reason no to go this way.
I did some research on benzotriazole ⇦this on eBay or Amazon [affil links] . Mainly on how to apply it. I read that it can be added to detergents solutions containing wetting agents and calcium sequestering polyphosphates in order to prevent tarnishing. I'm far from being a chemist. We are using Liquinox detergent for cleaning the parts, I don't believe it contains these compounds. We could find another detergent.
But first of all, is that right that adding small amount of benzotriazole to the detergent would be enough? Will this protection resist 120 °C? Does it leave a residue? It is a high vacuum application, anything residue is an issue.

Thanks,
Simon

Simon Lessard [returning]
Photon etc - Montreal, Quebec, Canada
November 29, 2016