Oxide scales iron-based alloys

Min] Miner, R.G., Nagarajan, V., The Morphology of Scale Growth on Iron-base Alloys Containing Chromium and Silicon , Oxid. Met., 16(3-4), 295-311 (1981) (Experimental, Morphology, 6)... 

When iron-, nickel-, and cobalt-base alloys containing chromium were oxidized in air at 900 °C for 25 h ° only oxide scales were formed and nitride formation was observed only on a Co-35 wt% Cr alloy. Presumably the inward diffusion of... 

When alloyed with small percentages of certain metals (e.g., aluminum, beryllium, iron, silicon, manganese, tin, titanium, and zinc), copper oxidizes with precipitation of oxide particles within the body of the metal as well as forming an outer oxide scale. Oxidation within the metal is called subscale formation or internal oxidation. Similar behavior is found for many silver alloys, but without formation of an outer scale. Internal oxidation is not observed, in general, with cadmium-, lead-, tin-, or zinc-based alloys. A few exceptions have been noted, such as for alloys of sodium-lead, aluminum-tin, and magnesium-tin [44]. Internal oxidation is usually not pronounced for any of the iron alloys. 

Breakaway corrosion (Fig. 4) commonly occurs if numerous cracks continuously form and extend rapidly through the scale. It can also occur for alloys that have had one component (principally the most stable scale former) either effectively depleted from the alloy through repeated scale formation and spallation or through selective internal compound formation. Breakaway corrosion leaves bare substrate continuously exposed. Mass change measurements are meaningless during breakaway corrosion as scales rapidly form and break away from the substrate. Efforts to predict the onset of breakaway corrosion have been attempted for iron-base oxide dispersion alloys [4],... 

Industrial attention focuses mainly upon iron-base (Fe-Cr—Al) alumina-forming alloys. For gas-turbine applications, AI2O3 scale formation is preferred, but the high-strength turbine-blade alloys do not possess adequate oxidation resistance and hence they must... 

Besides corrosion issues of metal substrates, the use of alloys as mechanical supports of the cells is subject to interdiflfusion of iron, chromium, and nickel between ferritic steel and nickel-containing anodes during cells fabrication and operation. Diffusion of nickel into FSS substrates may cause austenitization of steels, which would result in TEC mismatch with other cell components. Diffusion of iron and chromium into Ni-based anodes may cause formation of oxide scales on nickel particles. This would result in fast degradation of cell performance during operation, as the electrochemically active surface is passivated. In order to overcome these issues, one possibility investigated by MS-SOFC developers is to use protective coatings [1-6, 13]. 

Of course, if the protective scale of chromia or alumina is not penetrated by SO2, sulphide cannot form at the scale-metal interface. This was found for Ni-20 wt% Cr, Co-35 wt% Cr and Fe-35 wt% Cr alloys exposed to pure SO2 at 900 °C and emphasizes the resistance of a chromia scale to permeation. On the other hand, alloys in the Fe-Cr-Al, Ni-Cr-Al and Co-Cr-Al systems were exposed to atmospheres in the H2-H2S-H2O system. These atmospheres had compositions that supported the formation of chromia or alumina together with the sulphides of Fe, Ni and Co at the scale-metal interface. In these cases, a protective layer of chromia or alumina that formed initially was penetrated by sulphur to form iron, nickel, and cobalt sulphides at the scale-metal interface. Furthermore, iron, nickel, and cobalt ions apparently diffused through the oxide layer to form their sulphides on the outside of the protective scale. Thus the original protective scale was sandwiched between base-metal sulphides. 

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