Investigations on role of plasma sprayed NiCrAlY and Ni–20Cr coatings to combat hot corrosion

Abstract

Studies have been conducted to ascertain the role of plasma sprayed Ni–20Cr–10Al–1Y (NiCrAlY) and Ni–20Cr metallic coatings to combat hot corrosion of a Ni based superalloy Superni 75 (Ni–3Fe–19·5Cr–0·3Ti–0·1C). The performance of the uncoated as well as coated superalloy has been evaluated in an aggressive environment of Na₂SO₄–60%V₂O₅ under cyclic conditions at an elevated temperature of 900°C by the thermogravimetric technique for 50 cycles. X-ray diffraction (XRD), SEM/EDAX and electro probe microanalysis (EPMA) techniques were used to analyse the corrosion products. The uncoated superalloy suffered an accelerated oxidation in the form of spalling of its oxide scale, whereas NiCrAlY coated specimen showed marginal spalling towards the end of exposure. The Ni–20Cr coated superalloys did not suffer from spallation of its oxide scale. The Ni–20Cr coated specimen indicated a maximum weight gain among the cases studied, while NiCrAlY coated the least. Both the coatings were found to be useful in protecting the base superalloy against oxidation. Moreover the coatings, in general were successful in maintaining their continuous surface contact with the substrate superalloy during the whole tenure of cyclic exposure. The phases revealed for the oxidised coatings were mainly oxides of chromium/aluminium, and spinel containing nickel–chromium mixed oxides, which are reported to be protective against the hot corrosion.

Keywords:

• HOT CORROSION
• PLASMA SPRAY COATINGS
• SUPERALLOY
• NICRALY
• NI-20CR
• PROTECTIVE COATINGS