A flux used in braze welding or brazing is made to be corrosive to the metal so that it breaks down oxide films to make the welding easier. If the flux is just brushed off, there is some chemical residue left. Flux residues can usually be removed with hot water.
Some fluxes are oxide-saturated and take more than rinsing to remove.
You usually do not want to put a braze weld in water right after welding. But if the metal and joint design can stand quenching, flux can be removed by quenching the brazement from an elevated temperature. This doen't mean red-hot, but still warm. The metal and flux shrink at different rates making the flux pop off. Nitric acid should not be used on alloys containing copper or silver.