Thermal stability of NiAl and (Ni, Pt)Al coatings on Nimonic 80A alloy at 950°C

M. Reid, M. J. Pomeroy, J. Robinson

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Abstract

Specimens of Nimonic 80A alloy have been aluminised or platinum aluminised to form plain NiAl and single phase (Ni, Pt)Al coatings respectively. Specimens of the coated alloys were exposed in air for 188, 375 and 750 hours at a temperature of 950°C. Surfaces of as coated and oxidised specimens were subjected to X-ray diffraction (XRD). Specimens were then sectioned and polished and examined using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). After 188 hours, the β-NiAl coating showed slight transformation to γ′ (Ni3Al). After 275 hours the transformation levels increased, particularly in the areas of the coating adjacent to the alloy substrate. After 750 hours, significant transformation was noted both at the exterior of the coating and in the regions adjacent to the alloy. Similar effects were noted for the Pt modified coating - alloy system, excepting that there was little transformation of the exterior coating layers when no internal oxidation / pitting effect were present. The development of the coating morphologies with exposure time and the differences in morphologies between the NiAl and Pt modified coatings can be explained in terms of Ni egress from the alloy into the coating and Al ingress from the coating to the alloy as the coating and alloy attempt to chemically equilibrate. For the NiAl coating, Ni egress is the major factor causing chemical and morphological change. For the Pt modified coating, initial changes are brought about by Al ingress but then subsequently controlled by Ni egress from the alloy. The absence of a markedly different exterior morphology and composition for the Pt modified coating is due to Pt accelerating subscale diffusion effects and the fact that a γ′ zone is unable to form.

Original languageEnglish
JournalJournal of Corrosion Science and Engineering
Volume6
Publication statusPublished - 2003

Keywords

  • Aluminide coatings
  • Equilibration
  • Microstructural change
  • Pt-modified aluminide caotings

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