DETAILED STUDY OF OXIDATION AND HOT CORROSION MECHANISM OF NI BASED COATINGS

Abstract

The turbines, boilers and incinerators which are used in large units, when subjected to high temperature causes degradation of the material which may lead to the breakdown of the machinery or even shut down of plant. The main cause which is responsible for this material degradation is high temperature oxidation. To counteract the effect of high temperature super alloys are being used in supercritical boilers and also in other energy conversion systems. The super alloys during their use in service withstand very severe environmental conditions. The coatings are generally provided on the super alloy to increase their life and also to protect them from severe degradable environmental conditions which are mostly observed in boilers, incinerators environment. Nickel based coatings are mostly used to increase the life of materials subjected to high temperature conditions. It was also reported that rare earth elements when added in small amounts can increase the efficiency of these coatings when used at high temperatures. in this study, the corrosion resistance was provided by applying Ni-20Cr and Ni- 20Cr+0.5wt.%Dy203 on super alloy Superni 750. The tests of oxidation and hot corrosion in molten salt i.e. 40%Na2So4-60%V205 at 900°C under cyclic conditions have been performed on these coatings. The coatings of 150-250 tm were obtained by D-gun(Thermal Spray process) and were also sound and adherent. The effect of Dy addition on coating is also observed. The cyclic oxidation and hot corrosion tests for 100 cycles were done individually on bare and coated alloys. The graph of weight change/Area vs No. of cycles has been plotted to monitor the oxidation kinetics. The FE-SEM/EDS, XRD and cross-sectional mapping analysis have been done of the scale formed after 100 hours of cyclic tests on bare and coated alloys. The rare earth element Dy increased the oxidation resistance provided by Ni-20Cr when added in small amount