I'm with Paul, the electrolysis method works well, and
won't "etch into the base metal" no matter how long ya leave it - if you've got bigger parts to do, just rig a wooden box and line it with sheet poly - there's a good
writeup hereI'm a bit more precise than Paul on the sodium carbonate, about one Tbsp per gallon, even though I happen to have it in 50# sacks, because it's what I use in
my antique bottle washing machine, not a speck of rust on all the bare metal inside

Bill