High chromia-forming alloys

The electrical conductivity requirement for interconnect applications necessitates the use of chromia-forming (or Cr-rich spinel) oxidation-resistant alloys. One drawback of the chromia-forming alloys for this particular application, however, is the Cr volatility of the chromia or Cr-rich scale. As indicated by many studies [185-189], during high-temperature exposure Cr203 (s) reacts with 02 via the following reaction... 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

High-nickel chromia-forming alloys, such as alloy 600, are particularly susceptible to rapid sulfidation attack at temperatures above 645 °C owing to the formation of a liquid corrosion product. Fig. 5-23 shows the influence of Ni-Fe-Cr alloy composition on the melting temperature of the product sulfide scale. It is apparent that the melting temperature of the sulfide scale generally increases with increasing Fe and... 

As discussed in Section 5.5, chromia forming alloys that are widely considered for use in metal dusting environments suffer from the fact that in many cases a fully protective surface oxide is unable to form, or defected regions in the oxide are unable to reheal themselves for kinetic reasons. While high chromium ferritic alloys exhibit a better tendency for protective chromia film formation, the poor mechanical properties of such alloys render them unsuitable as bulk... 

It was discussed in Section 5.5 that a primary reason why many chromia forming alloys, even with relatively high Cr concentrations, are susceptible to metal dusting is related to the paucity of achieving full surface coverage... 

In addition to the temperature limitation, chromia-forming alloys suffer problems at lower temperatures due to the presence of water vapor in the environment (i.e. steam or combustion gas) (Opila, 2004). This is a significant issue as many high temperature applications involve these environments. The study of water vapor effects has been ongoing for many years (e.g. Caplan and Cohen, 1959 Fujii and Meussner, 1964) and is a very active area of current research (Otsuka et al., 1989 Shen et al 1997 Nickel et al.. 

As expected, no carburisation attack at all was detected on iron-aluminium-chromium alloys after 1000 hours exposure in CH4/H2 environments at 850°C, 1000°C and 1100°C. Since the formation of chromia and iron requires relatively high oxygen partial pressures, alumina is the only stable phase at the low partial pressure of the used gas. If once formed, alumina is impervious to carbon, provided the scale remains intact [20], Excellent resistance to carburisation was also found for other alumina forming alloys like nickel aluminides [21] and Ni-Al-Cr alloys [22], The results of the present work show that 10 wt% aluminium are sufficient to prevent carburisation. It is expected, that the minimum aluminium concentration is even lower than 10 wt%. 

Stanislowski M, Wessel E, Hipert K, Markus T, Singheiser L (2007) Chromium vaporization from high-temperature alloys 1. Chromia-forming steels... 

From 650°C, chromia-forming nickel-based alloys exhibit better corrosion resistance than austenitic steels even with similar chromium content [48]. Indeed, the formation of chromium-rich scale appears to be easier [44,48] and they resist better against carburization [47,48]. The use of alumina-forming nickel-base alloys may be an alternative solution at the highest temperatures, typically >750°C [49]. The consequences of carburization on the mechanical properties at high and low temperature depend on the extent of the damaged zone [47]. 

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. 

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