Hot Corrosion Behaviour of Refractory and Rare Earth Oxide Reinforced CoCrAlY APS Coatings at 700 °C

Abstract

This paper investigates cyclic hot corrosion of plasma sprayed CoCrAlY + Al<inf>2</inf>O<inf>3</inf> + YSZ (C1) and CoCrAlY + CeO<inf>2</inf> (C2) composite coatings on MDN 321 and Superni 76 substrates in molten salt (Na<inf>2</inf>SO<inf>4</inf>-60%V<inf>2</inf>O<inf>5</inf>) environment exposed to 700 °C. Weight change technique is used to evaluate the corrosion performance. Both C1 and C2 coatings showed better corrosion resistance than uncoated alloy. Both the coatings showed linear weight gain during the initial cycles and parabolic weight gain nature with subsequent hot corrosion cycles. The parabolic rate constant (K<inf>p</inf>) of C1 and C2 coating was observed to be in the range 0.29–0.32 × 10?10 g2 cm?4 s?1 and 1.0–1.13 × 10?10 g2 cm?4 s?1 respectively. In C1 coating, the globular and continuously packed structure on the corroded surface having CoO, Cr<inf>2</inf>O<inf>3</inf>, CoCr<inf>2</inf>O<inf>4</inf> and CoAl<inf>2</inf>O<inf>4</inf> spinel oxides provided superior hot corrosion resistance. While in case of C2 coating, the outward growth of CeVO<inf>4</inf> irregular crystals as a corrosion product of CeO<inf>2</inf> and V<inf>2</inf>O<inf>5</inf> salt deteriorated the oxide scales resulting in higher corrosion rate. © 2018, The Indian Institute of Metals - IIM.