Subscale formation

Figure 5.24. The initial oxide (a) contains all the cations of the alloy surface resulting in 15 wt% NiO-85 wt% Ni(Cr,Al)204 coverage, (b) subscale formation of Cr203 occurs because it is stable at the low oxygen activity defined by the NiO-alloy equilibrium and internal oxidation of A1 occurs ahead of this front since AI2O3 is stable at the even lower oxygen activities here. The high chromium content results in a Cr203 subscale, which may be continuous (c) and defines a lower scale-alloy...

C.J. Wang, T.T. He, Morphological development of subscale formation in Fe-Cr-(Ni) alloys with chloride and sulfates coating, Oxid. Met. 58 (2002) 415-437. 

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. 

Barrett and his colleagues , and Kosakhave summarised existing information on the scales formed on nickel-chromium alloys. Up to about 10% Cr, the thick black scale is composed of a double layer, the outer layer being nickel oxide and the inner porous layer a mixture of nickel oxide with small amounts of the spinel NiO CrjOj. Internal oxidation causes the formation of a subscale consisting of chromium oxide particles embedded in the nickel-rich matrix. At 10-20% Cr the scale is thinner and grey coloured and consists of chromium oxide and spinel with the possible presence of some nickel oxide. At about 25-30% Cr a predominantly chromium oxide scale is... 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

Another limiting case is an alloy, AB, neither component of which initially reacts to form an external scale and in which component A has a much greater affinity for oxygen than B. A classical example is the silver-indium alloy system. Silver doesn t form an oxide at elevated temperatures, but it does dissolve oxygen. Small amounts of indium will therefore form an oxide within the alloy metal, a so-called subscale or internal oxide. The process is diffusion controlled and the diffusion of oxygen in the alloy is rate limiting. Such systems have been studied extensively by R. A. Rapp, especially the transition from only internal oxidation to the formation of an external scale (22),... 

The English version of ASCIv2 was used at the three universities (Bauer, 2008 Xu Lewis, 2011). The instrument is intended to measure students attitude toward chemistry in general in a 7-point semantic differential format, e.g., chemistry is easy vs. hard for item 1 and comfortable vs. uncomfortable for item 4. It includes eight items which can be grouped in two subscales intellectual accessibility (four items) and emotional satisfaction (four items). The entire instrument and instructions can fit on half a page, and it takes at most 5 min to administer. For a copy of the instrument, see the supplementary material (Xu Lewis, 2011) or contact the corresponding author directly. 

Such a duplex microstructure is commonly observed for NiO scales grown at a temperature lower than 1000°C (Peraldi et al., 2002 Haugsrud, 2003). For such duplex scales, inert marker location, 0 experiments and careful analysis of NiO scale microstructure show that the growth of the external columnar subscale is associated with the outward diffusion of Ni cations and occurs at the scale-gas interface, while the inward diffusion of oxygen is involved in the growth of the inner equiaxed subscale. Therefore, the internal interface between the equiaxed and columnar sublayers marks the initial location of the Ni surface before the formation and growth of NiO scales. 

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