Abstract: Chemical milling of the commercial gamma-gamma prime nickel superalloy IN-100 is carried out by using an aqueous solution comprised of 32.5-85 volume percent HCl, with other ingredients proportioned on the HCl, preferably 40 ml/l nitric acid, 0.0141 M/l metal sulfate ion, 0.0326 M/l metal chloride ion, 0.0168 M/l alkali metal ion and 0.0073 M/l citric acid. The chemical milling is particularly effective in uniformly removing a small amount of surface layer without any deleterious attack. It hereby is useful to improve the effectiveness of surface penetrant inspection, to find small flaws which would otherwise be concealed by a worked surface layer.