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Bluing steel generally refers to the process of forming a protective layer of $\ce{Fe_3O_4}$ ("black iron oxide") that prevents normal rust from forming. The chemistry behind the traditional industrial process for bluing steel involving multiple baths in hot alkalis and acids ("hot bluing") is reasonably well documented (and is not the focus of this question).

My question is about a different procedure referred to as "cold bluing" in which steel parts are treated with a solution at room temperature and blacken instantly without heat. The ingredients from the MSDS of a popular cold bluing product are:

  • Phosphoric Acid
  • Copper (II) Sulfate
  • Nickel (II) Sulfate
  • Selenous Acid

(The compounds are listed in descending order by percentage.)

Phosphoric acid seems like a reasonable ingredient. It's well known as a rust remover and iron phosphate protects some steels from oxidation. Unfortunately, from there the ingredients seem to become increasingly perplexing.

Copper sulfate isn't itself an unusual compound, but I'm not sure what it's doing here. Nickel sulfate is even more confusing, but nickel salts are toxic so it must be there for a pretty good reason.

The relevance of selenous acid escapes me. All I really know about it is that it's an oxidizing acid and is basically selenium dioxide dissolved in water.

Treatment of steel with this solution causes a black coating to form almost instantly. I've been starting to wonder if cold-bluing is actually "plating" the similarly colored copper (II) oxide onto the steel rather than forming black iron oxide.

If this is the case, I would expect the reaction to be something vaguely like:

$\ce{Fe + CuSO_4 +\ ?-> FeSO_4 + Cu +\ ?}$ (copper is more noble)

$\ce{Cu + H_2SeO_3\ +\ ?\ -> CuO + Se +\ ?}$ (is selenous acid really strong enough to oxidize copper?)

...but this doesn't involve the nickel sulfate at all and just generally feels wrong.

So, overall my question is:

What is the black compound that forms on the outside of steel when treating it with the solution described above, and how does it form?

edit: Additional thoughts: While there is very little information on this compound online, copper selenide is described by Sigma-Aldrich as being a black powder. I can't find any discussion of its synthesis outside of some articles about nanoparticles, but I wonder if it's a more reasonable outcome than copper oxide.

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    $\begingroup$ Nickel(II) is often used as a catalyst in metal redox reactions. Selenous acid is apparently doesn't do redox quickly, so maybe the nickel speeds that up? $\endgroup$ – SendersReagent Apr 12 '16 at 17:55
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    $\begingroup$ Here's this, but it doesn't mention the nickel. Apparently copper selenide is the final product. $\endgroup$ – SendersReagent Apr 12 '16 at 18:28
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    $\begingroup$ I believe the nickel makes another compound (possibly nickel selenide, but maybe not) that gives the product a desirable color. I base this on this patent for coating metal in copper telluride in which the author speaks of using nickel sulfate to get a desired color. Obviously copper telluride isn't copper selenide, but there are parallels, obviously. $\endgroup$ – SendersReagent Apr 13 '16 at 1:08
  • $\begingroup$ So based on everything here so far, the end products are presumably copper selenide and nickel selenide then. That still leaves me wondering where the iron comes into play. The solution remains pristine until iron is introduced, which would lead me to believe that the selenous acid can only attack elemental copper/nickel and not the salts. It is widely reported in the industry that uses these solutions that they do not work on aluminum, but if iron starts this reaction just by being a reducing agent, I see no reason why raw aluminum (oxide scraped off) wouldn't work just as well. $\endgroup$ – 0x5f3759df Apr 13 '16 at 13:15
  • $\begingroup$ Yeah, it's definitely more complex than I would have expected going into it. I know iron and sulfur have a particular fondness for each other. Maybe it has something to do with the iron-selenium interaction. Or maybe the selenious acid can only reoxidize the aluminum metal back to aluminum oxide, reprotecting it and halting the reaction? $\endgroup$ – SendersReagent Apr 13 '16 at 15:38

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