Frankly, I would not bother with sulphuric acid and all the attendant dangers of diluting from concentrated H2 SO4 and think of simpler solutions. If it was just a bit of brass, I would probably rip out a bit of zinc chloride from a dry cell battery. I would probably dunk the bit of brass in a bit of vinegar or even Brown Sauce- both of which contain acetic acid which is far more gentle. Maybe citric acid crystals would suffice. Some of the Brit boiler folks swear by them- but I have no experience personally. Again, if larger quantities were needed I would use something like a pre-clean for car body work which probably contains a mixture of hydrochloric acid and phosphoric acid.
Once you have a clinically clean surface, you can use a variety of fluxes. There are the resin cored solders but you could get away with nothing more than tallow. I used tallow as a boy to mend lead piping bursts and this, given the right solder, it will fill and wipe with fustian cloth.
Personally, I like solder pastes both lead/tin and silver. But that is just me. Presently, I am doing a lot of wiring and a lot of the cables are satellite and ordinary tv cables which have been outside in the snows, rain and assorted atmospheric pollution under trees and so on. Quite often I merely use a pruning knife scrape of the corrosion before tinning again.
Does this help?
