Oxidation-resistant alloys

The oxidation and corrosion behavior of metals and alloys has been widely investigated in a range of environments for a myriad of applications. Recently, oxidation resistant alloys have been studied particularly for SOFC interconnect applications. 

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... 

Corrosion of Oxidation-Resistant Alloys nnder SOFC Interconnect... 

This chapter will provide an overview of oxidation and corrosion behavior of candidate oxidation-resistant alloys under SOFC operating conditions and discuss surface modifications for improved stability and performance of metallic interconnects. 

CORROSION OF OXIDATION-RESISTANT ALLOYS UNDER SOFC INTERCONNECT EXPOSURE CONDITIONS... 

The regenerator for a typical 10,000-barrel/day unit has an overall height of 120 ft. and an internal cross-sectional area of about 100 sq. ft. The shells are constructed of carbon steel and are protected by a 6-in. lining of insulation with a refractory surface adjacent to the flowing catalyst. All parts handling flue gas and air inside the regenerator are constructed of oxidation-resistant alloy steel. 

Z. Yang, J.W. Stevenson, and K.D. Meinhardt, Chemical interactions of barium-calcium-aluminosilicate-based sealing glasses with oxidation resistant alloys. Solid State Ionics, 160 (2003) 213-225. 

Irrespective of details of the transport processes, in the development of oxidation-resistant alloys, the aim is to evolve a composition that develops one of the slow-growing oxides as a scale. This is usually done to encourage formation of AI2O3, Si02, or Cr203 and will be discussed in Chapter 5. 

If the alloy is depleted below the concentration that will allow diffusion in the alloy to provide a sufficient fiux of Cr to the alloy-oxide interface to maintain the stability of the chromia layer, relative to Ni-containing oxides, the chromia layer can break down without fracturing. The condition for this is described by Equation (5.2). Because of this depletion effect, oxidation-resistant alloys based on the Ni-Cr system usually contain at least 18-20 wt% Cr. 

This aspect of the theory of defect structures of non-stoichiometric compounds is usually covered in the main text of books on high-temperature oxidation. The subject of doping is interesting for its own sake, and it is vitally important for the study of the physical chemistry and electrochemistry of ionic compounds. In the case of an introduction to high-temperature oxidation our opinion is that, since the control of oxidation rates by controlling the ionic and electronic transport properties of oxides by impurity solution is not generally used as a technique for the development of oxidation-resistant alloys, this subject should be dealt with in an appendix. This allows it to be covered adequately without over-emphasizing its importance. 

OXIDATION-RESISTANT ALLOYS 11.13.1 Reactive Element Effect (REE)... 

The oxidation of alloys can become very complicated as a result of mutual solubility of AO and BO [29], as a result of the formation of double oxides [30], or as a result of all those effects connected with changes in molar volume and with the adherency of the reaction products. Therefore, this matter will not be pursued any further here. In practice, the production of oxidation-resistant alloys must still be based to a great extent upon empirical rules [6], although the theoretical ideas as outlined here are a necessary prerequisite in order to do systematic oxidation - resistant alloy development. 

Superalloys are usually defined as heat- and oxidation-resistant alloys specially developed for servicing at elevated temperatures under both oxidizing atmosphere and severe mechanical stresses. Three main classes of superaUoys are distinguished ... 

Z. Yang, G. -G. Xia, G. D. Maupin, and J. W. Stevenson, Conductive Protection Layers on Oxidation Resistant Alloys for SOFC Interconnect Applications, Surf. Coat.Tech., 201 4476-83... 

It seems that the temperature range for conventional unprotected (no surface coatings) solid-solution titaniiun alloys is upper limited to about600 °C [Ble85] and that futm e development shoiald include combinations of (1) inherently oxidation-resistant alloys, (2) coatings, (3) further development of the a2 phase and y-phase aliuninides and their variants, Euid (4) the use of rapidly solidified alloys. Figure 9.6 outlines the manner in which such developments proceed naturally out of the earlier work. 

Metallic particles such as chromium can be introduced into a metal plating electrolyte (for example, nickel and cobalt), and the deposited composite can be subsequently heat treated to form high-temperature oxidation-resistant alloys. MCrAlY composites have been made by depositing 10 p,m CrAlY powder in a cobalt or nickel matrix. Heat treatment bonds... 

High chemical resist- In the composition of heat-resistant and oxidation-resistant alloys. [657] 1961... 

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