THE ANODIC-DISSOLUTION AND PASSIVATION OF NI-CR-FE ALLOYS STUDIED BY ESCA

The surface compositions of two Ni-Cr-Fe alloys [Ni-21Cr-8Fe (at%) single crystal and Ni-17Cr-10Fe] have been characterized by angle-resolved ESCA, after polarization in the passive state or in the active state in 0.05 M H2SO4. The passive layer formed on Ni-21Cr-8Fe(100) has a bilayer structure with an oxide film in the interior of the film and an hydroxide layer on the surface. The film is markedly enriched with chromium oxide and hydroxide. The inner oxide layer consists of 96% Cr2O3 and 4% Fe2O3. There is no detectable Ni2+. The outer hydroxide layer is approximately 100% Cr(OH)3. At 300 mV(SHE), the thickness of the passive film is 11 angstrom, and the thickness of the inner oxide layer is 6.5 angstrom. At higher potentials the film is thicker: the thickness of the inner oxide layer is 9 angstrom at 600 mV(SHE). On the Ni-17Cr-10Fe alloy, similar results were obtained for the passive film composition, but the film was found to be slightly thicker (17 angstrom). An enrichment of chromium is observed in the metallic phase under the passive film. A chromium concentration of 43% was estimated for the first atomic plane of the alloy under the passive film. In the active state, the bilayer structure does not exist. After polarization at +80 mV(SHE), a thin (7 angstrom) layer containing Ni2+ [as Ni(OH)2], Cr3+ [as Cr(OH)3 and Cr2O3], and Fe3+ (as FeOOH) was observed. The layer is enriched with Cr3+. A surface enrichment of chromium in the metallic state was also detected (approximately 75% chromium in the first atom plane of the alloy), which reveals that selective dissolution occurs on this alloy. On the basis of the experimental results, and using a simple model for the surface composition in the stationary state of dissolution, the surface chromium concentration C(Cr)s in the active state can be predicted for Ni-Cr-Fe alloys with different bulk chromium contents C(Cr)b. A relation C(Cr)s approximately 12C(Cr)b/(1 + 11C(Cr)b) is predicted, which leads to a sharp increase in C(Cr)s for C(Cr)b in the range 0-15%.