Formation of diffusion barrier coating on superalloy 690 substrate and its stability in borosilicate melt at elevated temperature

Aluminized and thermally oxidized superalloy 690 substrates forming Al2O3 layer on (NiCr)Al + Cr5Al8 types aluminides and bare substrates were exposed in sodium borosilicate melt at 1248 K for 192 h. SEM–EDXS analysis along the cross-section of bare substrate with adhered glass revealed formation of a continuous, thick Cr2O3 layer at the substrate/glass interface due to its low solubility in borosilicate melt. XRD on aluminide coated and thermally oxidized specimen revealed existence of Al2O3 along with NiAl and Cr5Al8 type phases after the exposure in borosilicate melt. SEM–EDXS analysis along the cross-section of aluminide coated and thermally oxidized sample with adhered glass indicated good stability of coating in borosilicate melt without any phase formation at the coating/glass interface. However, some Al enrichment in glass phase adjacent to interface was noticed without any significant Ni or Cr enrichment.