Modem energy conversion and power generation systems require materials and coatings with significant capabilities to withstand high temperature degradation by sulphur, oxygen and chlorine containing environments and by hot corrosion. The alloys and coatings capable of functioning in such aggressive environments are invariably multicomponent and multhiphase materials. The protectivity is achieved through the development of adherent, self-healing protective scales by the diffusional transport of the coating and substrate elements. However with prolonged exposure interdiffusion of other elements, originally added, to increase the mechanical properties of the alloys, will act to undermine the integrity of the protective scales. A knowledge of the diffusional transport properties of these elements is essential to predict the protective behaviour of the coatings and the scales developed, and hence to design alloys and coatings with better capabilities to resist HT corrosion.In this work we will present model of heterogeneous reactions controlled by diffusion and reactions at multiple interfaces, e. g., at alloylcoating and coatinglgas boundaries. We describe here the recent progress that has been made in understanding the diffusion processes in single-phase solid solutions and in particular, the diffusion in an reacting (e. g. oxidized) multicomponent alloy. The oxidation of single-phase binary and higher alloys does not always follow the parabolic rate law. It has been found that the selective oxidation of binary alloys after initial transition (incubation) period, frequently leads to a steady-state condition. During the initial reaction period (transient state and variable boundary conditions) an instantaneous parabolic rate constant that varies that with time is often used to describe the rate of the reaction. For alloys with low concentrations of the oxidized metals, the steady-state rate constant is smaller than that of the pure metal and diffusion in the alloy affects the subsequent oxidation process.
The model of chemical interdiffusion enables one to obtain an approximate expression for the evolution of component distributions in open systems, as a result of the thermochemical treatment and/or oxidation. The proposed boundary conditions allow to study the evolution of the alloy composition as a result of its reaction with a surrounding environment, that is to predict the concentration profiles of all elements beneath the growing scale.
The studies of interdiffusion in [3-NiAI(Pt)IMAR M002 diffusional couples allowed calculation of the intrinsic diffusivities of five components. In this paper the information on the evolution of the elements distribution in the [3-NiAI(Pt)IMAR M002 system (Pt modified I3-NiA1 coating on MAR M002 superalloy) is presented. An agreement between the computed and experimental results suggests that the generalised Darken model correctly describe the transport process in open systems.