The solvent flux used for base metal alloy is

There are three types of fluxes according to their primary purpose: Type I. Surface protection—covers the metal surface and prevents access to oxygen so that no oxides can form.  Type II. Reducing agent—reduces any oxides present and exposes clean metal.  Type III. Solvent—dissolves any oxides present and carries them away. Fluxes for use with noble metal alloys are generally based on boric acid, boric anhydride, and borax. Chromium oxides that form on base metal alloys are more stable and fluoride containing fluxes are used as solvents to dissolve chromium, nickel, and cobalt oxides. Phillip’s 12th edition page 389