Alumina-forming alloys

With the problems outlined above for chromia-forming alloys, oxidation-resistant alloy development tends to focus on alumina-forming alloys. A few examples will be given to illustrate some of the potential and a few of the problems with these alloys. 

For Ni-base alloys, the addition of Al to NiCr alloys has been done with a fair level of commercial success. Low levels of Al have been added to alloys 

3 Total mass gain (specimen + spalled oxide) during lOOh cycles at 1200°C in laboratory air for various Ni-Cr-AI alloys. Generally, the mass gain decreases with Al content due to the formation of a protective alumina scale with NiAl + Hf having one of the slowest growth rates of any alumina-former. Two different specimens are shown for alloy 214 to indicate the reproducibility. 

4 Total mass gain (specimen + spalled oxide) after a 500 h isothermal exposure at 1000°C in laboratory air for various precious metal and Hf additions to alloys with 22% Al. NiAl + 0.05% Hf sets a (dashed) baseline in terms of no Ni-rich transient oxide, minimal Hf internal oxidation and slow alumina growth. The alloys with 22% Al primarily show higher mass gains due to the formation of Ni-rich oxide. Pd additions were not as effective as Pt or Ir. Internal oxidation of Hf also could cause increased mass gains. The addition of Cr did not have much effect with -10% Pt, but Y, in addition to Hf, was added to this alloy, which may have resulted in additional internal oxidation. Adapted from Pint et al., 2007d. 

Alloys based on FeCrAl have been discussed for over 40 years (Wukusick and Collins, 1964). Intermetallics (Fe3Al, FeAl) also have been considered (Tortorelli and Natesan, 1998) but exhibit scale spallation problems due to their high CTE (Smialek etal., 1990 Wright etal., 2001a Pint and Wright, 2004) which limit their lifetime and compatibility with other components 

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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%. 

Since the present day TBCs are used in conjunction with alumina-forming alloys, this imposes an additional constraint on the selection of prospective coatings. They must be thermodynamically stable with alumina at high temperature. Although a great effort has been made in searching out low thermal conductivity materials for high temperature applications, there are three difficulties in selection of candidate materials for TBCs. First, an effective model is needed to understand and... 

B.A. Pint, Optimization of reactive-element additions to improve oxidation performance of alumina-forming alloys, Journal of the American Ceramic Society 86 686-695, 2003. 

Lance, Accelerated cyclic oxidation testing protocols for thermal barrier coatings and alumina-forming alloys and coatings. Mater. Corrosion, 57 (2006), 1-13. 

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... 

Ce-modified alumina-forming alloys have been investigated by Amano et al. (1979) yielding evidence for dominant inward O diffusion in the presence of Ce. 

Life Extension of Alumina Forming Alloys in HT corrosion environments, EC project no. BRPR-CT97-0562. 

J.R. Nicholls, Life Extension of alumina forming alloys - background, objectives and achievements of the BRITE/EURAM Programme LEAFA, in Lifetime Modelling of High Temperature Corrosion Processes, EFC Monograph No. 34, The Institute of Materials, London, 3-15 (2001). 

B. A. Pint, P. F. Tortorelli and I. G. Wright, Effect of cycle frequency on high temperature oxidation behavior of alumina-forming alloys. Oxidation of Metals, 2002, 58, 73. 

This section considers the growth of oxide on nickel and chromium, chromia and alumina forming alloys and intermetaUics. 

Table 70.2 Transport mechanisms established from isotopic marker experiments during high temperature oxidation of alumina-forming alloys...

In comparison to its effect in chromia scales, in which the oxidation rate can be decreased by a factor of 100 in the presence of RE, the introduction of RE has a slight influence on the oxidation rate of alumina-forming alloys (Fig. 10.8 and Table 10.4) [33,34,124-127]. The addition of Zr or Y2O3 decreases the oxidation rate of FeCrAl steels by a factor of only 3 [128]. This is confirmed by the results summarised in Figs 10.3 and 10.9 [29-31,34,56,129-133]. 

Table 10.4 Activation energy values of the oxidation reaction of several alumina-forming alloys doped with RE...

Parabolic rate constants determined during the oxidation of some RE-doped alumina-forming alloys. 

Kirchheiner R R, Becker P, Young D J and Durham R (2005), Improved Oxidation and Coking Resistance of a New Alumina Forming Alloy 60 HT for the Petrochemical Industry, NACE Paper 05-A28, Houston, TX, presented at NACE Corrosion 2005, Houston, TX, April 2005. 

Pint B A, Tortorelli P F and Wright I G (2002), Effect of Cycle Frequency on High Temperature Oxidation Behavior of Alumina-Forming Alloys, Oxid Met, 58, 73-101. 

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