On the effect of steel substrate alloying elements on the in-situ formation of intermediate thermal diffusion barrier layers

The aim of this work was to evaluate the feasibility of a hot-dipping aluminizing process, for the simultaneous production of an iron aluminide coating, a protective alumina topcoat layer and an intermediate thermal diffusion barrier layer, at the aluminide coating/steel substrate interface, the latter being thought in order to limit the detrimental depletion of Al content in the coating layer as a consequence of its inward diffusion towards the substrate. Indeed, the steel substrates were opportunely selected for their high weight percentages of alloying elements (namely Ni, Cr, Mn and Co) potentially able to diffuse from the substrate and react with the inward diffusing Al at the temperatures employed in the coating preparation, thus forming transition metal aluminides able to stop Al depletion, thus potentially prolong the lifetime of iron aluminide coatings in service. The effective in-situ growth of this intermetallic thermal diffusion layer was found to be strictly dependent on several factors, including the enthalpy of formation of the alloying-element aluminides, together with the atomic percentages of the selected alloying element in the steel substrate as well as its diffusion rate. The obtained results, can be considered an important contribution to the selection criteria of iron aluminides-coated steel substrates, to be employed in burdensome scenarios at high temperatures.