Characterization of air-formed surface oxide film on a Co–Ni–Cr–Mo alloy (MP35N) and its change in Hanks’ solution

Abstract The air-formed surface oxide films used for stents were characterized to determine their composition and chemical state on a Co–Ni–Cr–Mo alloy. The change of the films in Hanks’ solution was used to estimate the reconstruction of the film in the human body. Angle-resolved X-ray photoelectron spectroscopy was used to characterize the composition of the film and substrate, as well as the film's thickness. The surface oxide film on the Co–Ni–Cr–Mo alloy (when mechanically polished) consists of oxide species of cobalt, nickel, chromium, and molybdenum, contains a large amount of OH − , and has a thickness of approximately 2.5 nm. Cations exist in the oxide as Co 2+ , Ni 2+ , Cr 3+ , Mo 4+ , Mo 5+ , and Mo 6+ . Chromium is enriched and cobalt and nickel are depleted in the oxide; however, nickel is enriched and cobalt is depleted in the substrate alloy just under the surface oxide film. Concentration of chromium was low and that of nickel was high at small take-off angles. This indicates that distribution of chromium is greater in the inner layer, but nickel is distributed more in the outer layer of the surface oxide film. During immersion in Hanks’ solution, cobalt and nickel dissolved, and the film composition changed to mostly chromium oxide (Cr 3+ ), along with small amounts of cobalt, nickel, and molybdenum oxides, and calcium phosphate containing magnesium, potassium, and carbonate. After immersion in Hanks’ solution, the thickness of the surface layer containing calcium phosphate increased to more than 4 nm, while the amount of OH − increased. The amount of cobalt and nickel in the surface oxide film and in the substrate alloy just below the oxide decreased during immersion.