Slurry coated Ni-plated Fe-base alloys: Investigation of the influence of powder and substrate composition on interdiffusional and structural degradation of aluminides

Abstract

A novel approach to apply slurry aluminides to produce a thermal barrier system based on an aluminum diffusion zone and an alumina foam layer in one step is being studied in the European FP7 project PARTICOAT. The results suggest possible coating and substrate combinations to decrease the degradation of slurry-produced aluminide coatings under high temperature exposure in heat exchangers, boilers or combustion chambers present in waste incineration and power plants.Spherical Al or Al–Si particles were deposited by air brush technique on austenitic (AISI347), ferritic (AISI446) and ferritic–martensitic (P91) steels. Isothermal exposure tests were performed at 600°C and 800°C for up to 1000h to study the microstructural changes and the oxidation behavior in air. After manufacturing, the resulting diffusion coating consisted of different iron–aluminide phases with porous alumina foam on top. The diffusion layer on top of the ferritic steels contained cracks due to the large thermal expansion coefficient mismatch between the substrate and the bondcoat, but was homogenous and continuous on the austenitic alloy. These cracks are generally filled with aluminum oxide in pure oxidizing atmospheres and the recession of aluminum in the diffusion layer is lower for the austenitic than for the ferritic alloys. The advantage of using an Al–Si based compared to a pure Al-slurry is a significantly decreased aluminum interdiffusion into the ferritic steels which contain high amounts of chromium. The latter effect could be related to the formation of a Cr3Si layer, which acts as an aluminum diffusion barrier.