The anodic dissolution and passivation of NiCrFe alloys studied by ESCA

The surface compositions of two NiCrFe alloys [Ni21Cr8Fe (at%) single crystal and Ni17Cr10Fe] 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 Ni21Cr8Fe(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 ∼100% Cr(OH)3. At 300 mV(SHE), the thickness of the passive film is 11Å, and the thickness of the inner oxide layer is 6.5Å. At higher potentials the film is thicker: the thickness of the inner oxide layer is 9Åat 600 mV(SHE). On the Ni17Cr10Fe alloy, similar results were obtained for the passive film composition, but the film was found to be slightly thicker (17Å). 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 (7Å) 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 (∼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 CsCrin the active state can be predicted for NiCrFe alloys with different bulk chromium contents CbCr. A relation CSCr ∼ 12CbCr/(1 + 11CbCr) is predicted, which leads to a sharp increase in CsCr for CbCr in the range 0–15%.