Alum is an entire class of compounds: from wikipedia…
A is a monovalent cation such as potassium or ammonium
M is a trivalent metal ion such as aluminium or chromium(III).
Sodium bisulphate is not alum, but it is a sulphate. I’m guessing
it’s very mildly acidic in solution, and probably works very
similarly to the alum, in that the acid attacks the iron but is too
weak to attack the non-ferrous metals - either because they are inert
or passivated (form thin stable oxide films)
the “trick” seems to be in getting the Fe2+ or Fe3+ ions away from
the surface, making them soluble, or letting them precipitate (as e.
g. FE(OH)2 ) “somewhere else”.
I am not 100% convinced, but the “story” sounds reasonable. a lot of
technology goes into making this sort of thing not happen - e. g.
which makes me wonder - does it work on stainless steel? or other
metals like Co or Ni or Cr? or just regular steel/iron/high carbon