Keywords:
In article , "DeepDiver"
wrote:
snip
That OP would be me. I was pretty excited (and amazed) when you first stated
that EvapoRust wouldn't hurt finishes (the implication being that it
wouldn't hurt black oxide, since that was the subject of this thread); I
thought perhaps that the chelating reaction was limited only to pure rust.
Oh well...

So now I'm wondering what makes EvapoRust so much better than a bath of
vinegar and salt (besides perhaps the speed of de-rusting)?

Btw, can anyone tell me what is chemically different between rust and black
oxide. I know that black oxide is a more durable oxidation film than rust,
but what makes it adhere to the surface of the steel better? And what makes
it black/blue?

One is Fe3O2, and the other is Fe2O3 I believe. I can never remember
which is which. The red stuff is both porous, and expands as it forms,
which is why it continues rusting & eats holes in things. The black
version is non-porous, and seals the metal underneath, sort of like
anodizing on aluminum does. One thing I hadn't realized until a few
years ago is that one way to make the balck version is to boil the red
version. They sell blueing chemicals that form a thin layer of red rust,
and when you boil it, it turns black. Because the original rust doesn't
cover theings completely, you need to repeat this a few times to get a
really good coat of the black oxide, but it works very nicely on smallish
parts.

Doug White