Phosphoric acid is a much better choice than Hydrochloric. Phosphoric (as stated by others) leaves a layer of iron phosphate behind, which provides mild rust resistance by itself and paint adheres well to. HCl will not only cause rust fairly soon once you remove the item (back to air), the fumes (from a closed bottle) will rust everything in reach. Don't keep it near anything made of steel. Sure, you can neutralize it with baking soda, but phos. works much better.
I bought Klean-Strip Phosphoric Prep and Etch at either HD or Lowes a few years ago. A good de-rusting solution is 7% Phosphoric acid, don't use it full strength.
Phos. will also loosen the black scale on steel. After soaking, scrub under running water with a stiff brush and the scale will come right off.
By far my favorite though is electrolysis. Make a fairly strong solution of washing soda in water (not baking soda), connect an old school battery charger to two items in the solution. Negative to the rusty piece, positive to the other electrode. Provide plenty of room around the rusty piece so the current can get to all sides of it (also use a large positive electrode). Don't let the battery clips into the solution or they will go away. You can't do this successfully with a "smart" charger as far as I know. Stainless seems like a good choice for the positive electrode, but puts nasty chemicals in the water (hexavalent chromium). EDM graphite electrodes make great positive electrodes. My setup is in a 25 gallon plastic barrel. I use a 10A battery charger (usually runs 2-4A) with a 4" x 12" piece of EDM graphite from ebay for the positive electrode.
Re: Removing Rust From Free Rusted Steel
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