I use a weak solution of nitric acid to passivate stainless. (I don't
remember, off hand what the mix is. I just keep about 5 gals mixed in a
plastic drum and hang the part in it for a few hours.)

A welder friend has a jelly solution that has nitric acid in it. He brushes
it on a finished stainless weld and lets it set for a couple hours then
rinse it off. Really cleans the weld effected area and never seems to rust
afterwards. Very expensive (about 15 bucks for a half pint jar.)

--
My opinion and experience. FWIW

Steve
s/v Good Intentions