HVOF Sprayed Mullite Coatings for Use In Extreme Environments

Abstract

Products used in industry and marine applications are exposed to extreme environments like high heat, humidity, acidic or alkaline or hyper saline environment, UV and IR radiation. Metals with good corrosion and oxidation resistance may be used but are restricted to Ni and Cr alloys, titanium and super alloys etc., which are costly and have their limitations. Hence ceramic coatings on low cost metals may be an answer to this problem. Ceramics are inherently chemically inert, high temperature resistant, corrosion and oxidation resistant. Flame spraying of ceramics is a good and reliable method for applying ceramic coatings on metallic substrates with good bond strength (> 80 MPa) and 1% porosity. In this work, HVOF technique is applied to obtain 100 microns thick mullite coatings on MS substrates with a NiCr bond coat. Mullite has a high oxidation and corrosion resistance. It is chemically inert. It has high temperature resistance even at 2000 C. These properties are ideal for industrial components exposed to salty environments. Characterization studies like XRD, SEM/EDS, Corrosion tests using polarization technique, coating thickness and surface roughness have been studied and reported. A corrosion rate of 1.55 mm/ year has been achieved in a sea water environment.