The older fluxes contained a high amount of metal ions - zinc IIRC.
This plated the metal and the flux that was a chlorine based etched the
metal for the plating process to keep it from oxidizing again.

I think a lot of this has changed through the years with the water based
fluxes and get the lead out solders.

for Stainless steel - HCL 1 part , zinc chloride 1 part.
for cast iron Zinc Chloride 15 parts, sodium chloride 3 parts,
ammonium chloride 2 parts
Al Bronze and Si Bronze - HCL 1 part, zinc chloride 1 part, water 4
parts
Zinc base die casting - not much hope - I suggest electro or flash
plating of copper then solder on that.

Soldering and Brazing by a.r. Turpin - U.K. printing - ISBN 0 85344 098
0 Nice little book.

Naturally the HCL is hydrochloric - you can use swimming pool acid -
Muriatic a lower grade from lab qual.

Martin
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder