I originally perfected Multi-Etch® for titanium production. It’s a nonacid (pH 6.8) alternative to hydrofluoric acid. I’ve also tested it on other metals with great results.
On niobium, I use Multi-Etch® to remove anodizing mistakes and to clean off contamination—iron from rolling mills, heat oxides, etc. Multi-Etch® will maintain the finish on niobium as long as the metal is not etched too long.