Abstract: An aqueous slurry process for producing a diffusion aluminide protective coating in superalloy articles, particularly on internal passages in superalloy articles.Aqueous slurry containing a source of aluminum in particulate form, an inert ceramic particulate, a halide activator compound in particulate form and a viscous aqueous base dispersant is injected into the internal passage or otherwise coated on the internal surface to be protected. The coated article is heated to dry the slurry and remove the aqueous solvent base. The dried, coated article is diffusion heat treated between about 1,350.degree. F. and 2,250.degree. F. for a period of time between approximately 4 hours and 24 hours to transfer the aluminum to the surfaces of the passages and diffuse the aluminum into the substrate material to form the protective coating.