Citric Acid Pickle
This pickle can be adjusted in strength as needed, and can even be used as a paste for passivating stainless steels! I use one tablespoon of acid powder to one cup of water, however a stronger concentration may work faster. My ratio leaves some undissolved acid in the mix; although' I also use mine unheated. Depending on the amount of scale present, this acid takes 10-20 minutes to descale cold articles, and almost instantly descales hot articles. I rinse in a mason jar before running under the tap to remove traces, so that I don't have too much pollution from the byproducts going into our local water supply. I'm not too worried about a small amount of dilute citric acid-the more concerning thing to me is the metal salts disolved in the solution. I hope to at some point find a way to reclaim those.
Borax paste flux.
This flux works about as well as handy flux. Cleans off of parts easily, and strips oxides really well. I haven't tried it on stainless yet, but it is traditionally used for steel for oxyacetylene welding as well as forge welding.
Recipe is simple-take regular borax, and slowly add in water until the borax is at a paste like consistancy aAs long as the borax stays clean, it should never go bad. Add water or borax as needed to maintain consistancy. Thicker seems to perform better than thinner. Needs to be removed before pickling.