After pickling mild steel

[Sevastian]

I am pickling a mild helmet top to remove scale. I plan on applying gun blue immediately. How should I rinse the metal after it's pickled to prevent flash rust? Water and immediately dry?

[Konstantin the Red]

So you're handling strong solutions of acid -- probably HCl?

Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust- and acid-resistant chromium-nickel steels are pickled in a bath of hydrochloric and nitric acid. Most copper alloys are pickled in dilute sulfuric acid, but brass is pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot.

Sheesh, isn't brass-pickle aqua regia?! -- whether cut with salt and soot or not.

I think you're looking at a neutralizing bath, so water plus a lot of baking soda, immediate drying, and of course rigorous degreasing. You may be wanting a hot bath of this to remove hydrogen from the surface of the metal after the acid bath and to speed the drying. The hydrogen reacts, as does residual chlorine and chlorides, to speed the rusting, so you're wanting to stay ahead of that.

Cold blue?

Wiki: Pickling

[Sevastian]

Yeah. Cold.

[Sevastian]

White wine vinegar for a couple hours in the sun and some love with a scrubby. The hot roll scale just wiped off. Some hot water in the sink then paper towel dryed followed immediately by the cold blue. Interior got primer followed by flat black.

[Konstantin the Red]

Yep, good ole vinegar. Much easier handling than pool acid (HCl in a certain concentration -- not having a pool, I'm a little vague on just how concentrated). I found some refs to using citric acid the same way, and plenty of discussion about the acids evolving hydrogen bubbles to lift off the scale (ferric oxides) flake by flake. Some mild acid pickles add salt to the vinegar as well.