When you are hot forging, are you using stainless steel hammers and a SS
anvil? If not, you are embedding iron from your tooling in the surface of
the SS, which then leads to rust pits later if it isn't removed, or it can
get transferred to the interior if you fold the metal, and then come back to
the surface by later machining. Same problem if you use a carbon steel wire
brush, or a SS wire brush that has been used on mild steel, or mill and/or
lathe tooling that was used on mild steel first. Citric acid will remove
free iron from the surface but won't take off rust that has already formed.
I think you need a stronger acid pickle after hot forging, then do citric
acid passivation. Oxalic acid might be enough, but a quick hydrochloric
acid (or nitric if you can get it) wash might be needed. Another technique
that hasn't been mentioned is electropolishing. It will clean and passivate
and polish the surface all at one time but is generally limited to the
outside surface, it does not penetrate down holes. It will remove up to a
few 0.001" from a flat surface and can take lots more off of a sharp edge if
you go too long. It works well on small pieces but it is hard to get a
uniform surface finish on large irregular surfaces.

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Regards,
Carl Ijames
wrote in message
...

I think the rust occurs BECAUSE iron particles have been exposed, else there
wouldn't be any rust, right? Isn't the whole idea of a stainless steel that
a chrome oxide of some sort forms over the surface and sorta covers up any
carbon or iron particles?
So, if rust forms, it means that something has removed the chrome oxide
and uncovered the iron, yes?
So I think you have to get the iron GONE and then allow the chrome to
oxidize again to prevent further rust.

I'd vote for a try with the Barkeeper's friend, using the oxalic acid to eat
the iron. As said previously, try in on an out of sight area.

I have tried citric acid on stainless steel that I have hot forged. when
hot forging stainless steel, two problems occur since we are seriously
disturbing the chrome oxide coating:
-iron particles come become exposed (and they rust later)
-carbon particle become exposed (and they stay black forever)
While the citric acid may have some effect on both issues, it is miserably
slow in my limited experience. The oxalic acid that I use for cleaning
erector set girders works slowly too, but a lot faster than does citric
acid.

Pete Stanaitis
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