LIFE TIME MODELING OF COATINGS

Abstract

The protection offered by coatings against oxidation and corrosion at elevated temperatures relies on the ability of the coatings to produce and maintain a stable, adherent, slow-growing oxide layer on its surface. In order to select the composition of the coating that are able to withstand high-temperature degradation in aggressive environments, fundamental and comprehensive knowledge is required on the relations between the oxidation mechanisms an&the developing oxide and alloy substrate. The experimental data on corrosion behaviour of super alloy coatings are plenty in the literature. Therefore simple and convenient forecasting methods are essential to assess life time of coating using the available experimental data without resorting to vigorous and time consuming new experimentation. Numerical modeling of the diffusional transport associated with high temperature corrosion processes is reviewed. These corrosion processes include external scale formation due to coating degradation during oxidation. The present work presents a diffusion kinetics model for predicting the degradation of species (Al or Cr) as well as the life time prediction of high temperature coating. A finite difference method (CRNICH) which is based on Crank Nicolson scheme is developed for solving the diffusion equation governed by Fick's second law with parabolic oxidation kinetics as boundary condition, to define the rate of removal of solute at the oxidizing surfaces of the coating. The diffusion equations are then solved over bulk region to obtain the resulting concentration profiles at a series of discrete time steps up to desired exposure time. The fully implicit method developed here, is unconditionally stable and it has reasonable accuracy. The model has been validated with numerical example and existing analytical models. The CRNICH model predictions have also been compared satisfactorily against experimental measurements of Al depletion for Ni5A1 coatings.