Features of oxidation of Ni−Cr alloys with additions of Sc and Y

Autor: Braslavskaya, G. S., Maslenkov, S. B.
Zdroj: Metal Science and Heat Treatment; August 1991, Vol. 33 Issue: 8 p578-587, 10p
Abstrakt: 1.The presence of chemical inhomogeneity causes in the initial stage of oxidation local growth on the surface of NiO oxide together with Cr2O3 oxide.2.Under equilibrium conditions in the 700–1200°C range the origin and change in structure of the scale on Ni-Cr alloys and alloys with addition of a rare earth metal are similar. The single-layer Cr2O3 scale gradually changes into scales with a two- and even three-layer structure. The mechanism of scale formation on these alloys is primarily vacancy and grain boundary transfer of cations to the front of the embryo of the oxide layer.3.As the result of the low level of vacancy concentration and mobility of cations at 700°C the formation of a three-layer scale on Ni-Cr alloys is in principle impossible and only a transformation from a single-layer to a two-layer structure is experimentally observed.4.Rare-earth metal additions reduce the vacancy concentration of the alloy and also the grain-boundary mobility of the cations, as the result of which from the first minutes of oxidation the adhesion properties of the primary Cr2O3 layer improve and both the growth rate of Cr2O3 oxide and the rate of transition from a simple Cr2O3 scale to a scale with a more complex structure of Cr2O3 and NiCr2O4 decrease by approximately an order of magnitude.5.On Ni-Cr alloys at 1200°C thin films peel off according to the Wood mechanism at the alloy-scale interface. Such a failure of scale is not observed on alloys with a rare-earth metal addition.6.Thick films with a multilayer structure on Ni-Cr and Ni-Cr-rare-earth metal alloys formed as the result of long oxidation fail by separation within the scale and a dense layer of Cr2O3 oxide always remains on the surface of the alloys and fulfills its protective function.
Databáze: Supplemental Index
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