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ALLOY OXIDATION

Still plenty to explore

Nature Materials volume 17, pages 574–576 (2018)Cite this article

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Advanced characterization and modelling techniques provide unique insights into oxidant transport processes in growing scales of high-temperature alloys and alloy design for improving their degradation resistance in harsh environments.

Structural and functional materials used for high-temperature (that is, >600 oC) service must often meet demanding requirements for properties, including mechanical strength, toughness and surface chemical stability. In those applications materials are often exposed to harsh oxidizing environments that can lead to their rapid surface degradation unless chemical stability is bestowed by judicious alloying or a protective coating. For example, Al and/or Cr is added to the bulk or surface of Fe- and Ni-based alloys so that a slow-growing, thermodynamically stable and continuous Al2O3 or Cr2O3 oxide scales form, which act as a surface-diffusion barrier to reduce further attack. The lifetime of a particular alloy or coating material is determined by the tenacity of the resulting thermally grown oxide (TGO) scale and the ability of that scale to heal or reform in the event of damage such as cracking or spalling. To date, the design of oxidation-resistant alloys and coatings remains an empirical process, since one lacks sufficient understanding to address basic questions, such as how can structural materials be resistant to unacceptably rapid surface oxidation under a given set of service conditions? Writing in Nature Materials, Nooshin Mortazavi and colleagues1 report remarkable insights into the interplay between reactive elements (REs), which are conventionally incorporated into the alloys for improving oxidation resistance, and water vapour in the oxidizing atmosphere. The proposed mechanism for oxidant transport in the scale and the inferences made from these observations provide guidance to the design of improved degradation-resistant, high-temperature alloys.

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Fig. 1: Schematics illustrating the oxidant transport process.
Fig. 2: Cross-sections of Ni-8Cr-6Al (at%) after exposure for 1 hour at 1,100 ˚C.

References

  1. Mortazavi, N. et al. Nat. Mater. https://doi.org/10.1038/s41563-018-0105-6 (2018).

  2. Wagner, C. Z. Phys. Chem. 21, 25–41 (1933).

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  3. Young, D. J. High Temperature Oxidation and Corrosion of Metals 2nd edn 549–601 (Elsevier, Oxford, 2016).

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  1. Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA

    Brian Gleeson

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  1. Brian Gleeson
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Correspondence to Brian Gleeson.

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Gleeson, B. Still plenty to explore. Nature Mater 17, 574–576 (2018). https://doi.org/10.1038/s41563-018-0119-0

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