There is provided a method for treating the surface of metals such as nickel based or high alloy steels, austenitic and ferritic stainless steels, copper and aluminum alloys to increase their corrosion resistance by modification of the metal surfaces to inhibit cathodic corrosion processes. In a single step treatment process the metals are immersed into a heated aqueous composition containing a rare earth salt substantially free of any halide compound. Increased corrosion resistance is obtained using nitrates of yttrium, gadolinium, cerium, europium, terbium, samarium, neodymium, praseodymium, lanthanum, holmium, ytterbium, dysprosium, and erbium nitrates. The rare earth salt is present in the range from about 2% by weight to saturation of the solution. The composition includes a pH-modifying substance such as nitric acid to adjust the pH in the range 0.5 to about 6.5 to attack the surface to remove oxides facilitating deposition of the rare earth.