HOT CORROSION BEHAVIOR OF NiCr A1Y AND YSZ COATED T-91 AND T-22 BOILER STEEL BY PLASMA SPRAYING PROCESS IN AIR AND 75%wt Na2SO4+25% wt NaC1 SALT

Abstract

In a wide variety of applications, mechanical components have to operate under severe conditions, such as high load, speed or temperature and hostile chemical environment. In such working condition metals and alloys sometimes experience accelerated oxidation when their surfaces are covered with a thin film of fused salt in an oxidising gas atmosphere at elevated temperatures. This is known as hot corrosion where a porous non-protective oxide scale is formed at the surfaces and sulphides in the substrate. In order to provide better protection against various types of degradation their surface modification is necessary. Ceramic coatings produced by thermal spray techniques are widely used for corrosion protection. In this dissertation work Yttria-Stabilised Zirconia (YSZ) coatings were deposited on a T-91 and T-22 boiler steel. NiCrA1Y was used as bond coat and YSZ as top coat. Hot corrosion studies were conducted on uncoated as well as plasma spray coated specimens in air as well as salt (75%wt Na2SO4 + 25%wt NaCl) at 900°C under cyclic conditions. The thermogravimetric technique was used to establish kinetics of corrosion. X-ray diffraction (XRD) and scanning electron microscopy/energy-dispersive x-ray analysis (SEM/EDAX) techniques were used to analyse the corrosion products. This YSZ overlay coatings enhance resistance to corrosion significantly which can be attributed to formation of zirconium oxides (Zr02) and yttrium oxide (Y203). This coating was more effective in salt environment and there is an extra phase of ZrS. YSZ coated sample also showed better corrosion resistance in Salt spray (Fog) tes