I would be very careful using anything but a Rosin flux. Rosin will work fine if things are clean enough. Other commonly available fluxes may perform better but they are not used in a repair application because they are very corrosive and can not be neutralized. Both Zinc Chloride and Ammonium Chloride are highly corrosive and any flux containing them would be a poor choice for rib repair. You could use an acid flux for tinning ahead of time then scrub the excess flux and neutralize.
https://www.oatey.com/ASSETS/DOCUMENTS/ITEMS/EN/No_95_Tinning_Flux.pdf
