Braze ons are not supplied with a finish already on them before attaching to the frame. FB is correct concerning chrome and welding levels of heat. However silver brazing onto a chromed surface is possible without chrome damage (or brain cell damage either
) Do know that the braze on will only be as well attached as the chrome is, or isn't.
Frame repairs involving torch work on chrome generally don't work out well without some real skills and experience. Even then some cosmetic damage is to be expected. Since the braze on won't be chromed some sort of protective finish will be wanted anyway. Andy