I agree with Noel, coat the insides with something like Waxoyl or old oil, that as Noel says will creep in everywhere and give some protection, I suppose you could also try car underseal.

I've bodged brazed exhausts before and never noticed any problems with corrosion at the interface between the two metals – as Noel says, the HAZ alongside a weld is more likely to give trouble.

Looking in an old book on the corrosion of metals it says that the difference in electro motive force between two mating metals should be kept below 0.25v. In the accompanying table steels have a potential of 0.7-0.78 and phosphor bronze 0.22 (nothing for silicon bronze). So with those figures there is a potential for rusting, and it gets worse when salt is added, as in winter road spray.

I have been doing oxy-acetylene and oxy-propane bronze weld / brazed repairs on mild steel outdoor equipment for over 30 years and have never had the area around the welds corrode, apart from light rust on unpainted areas. A coat of paint or some kind of oil is always a good idea on any exposed steel. Some of the cheaper brand US made mowers had no paint on the deck underside from the factory, so they rusted fast. After bronze welding in mild steel patches, and a dab of spray paint over the welded areas, many have lasted 25 to 30 years without any trouble, usually the cheapy low grade engine wears out before the patched deck rusts out.

My Dad (professional auto mechanic for 50 years) did hundreds of mower decks same way, and many thousands of all sorts of car repairs from rusted oil pans to leaky mild steel exhaust tubes to broken manifolds – with no issues. In his working life today's flux cored MIG sets were either unavailable or very high cost. No doubt the new MIG sets are good, just pointing out they are not the only game in town for successful repairs, and thousands of mechanics got along just fine without them!

I would suggest not overthinking it. If you can paint or oil spray welded repairs, go ahead, if not, don't sweat it, worrying about corrosion. In my opinion corrosion caused by bronze welding in practical repair cases will not be a worry at all desite all the theories.

Moisture and plant juice isn't the same as seawater. If brazing caused significant corrosion outside of marine applications it would never have become so prevalent.