Differential Composition Profiles of Passive Films on Inconel 600 and Incoloy 800 Alloys

A differential method for obtaining the exact composition profiles in depth by use of Auger Electron Spectroscopy (AES) and argon ion sputter etching was applied to the surface analyses of Inconel 600 and Incoloy 800 alloys passivated potentiostatically in deaerated 0.5M sulfuric acid solution, pH 6.48 borate solution, and 0.1M sodium hydroxide solution, respectively. The multiple surface layer model was applied to the exact composition profiles in depth, and then the surface excess (Γ_I) of component, chromium, iron, and oxygen, relative to the nickel component was obtained as a function of anodic potential. The values of Γ_Cr for both alloys decreased with increasing pH of solution. The values of Γ_O for Incoloy 800 are always larger than those for Inconel 600 over the whole range of passivity, irrespective of solution pH. Furthermore, the relation between surface tension (7) and surface excess (Γ_O) was derived by assuming that the alloys covered with passive films behave as an oxygen electrode. The decrease in surface tension (Δγ) of the alloys due to the film growth for 1 V change in anodic potentila was of the order of -0.1 ~-0.2 N cm^-1.