Fe-Al Intermetallic Phases Present in Fe Aluminide Coatings

Abstract

Iron aluminide coatings are known to protect steels at elevated temperatures in diverse industrial environments. Depending on the Al content of the Fe-Al phase, a protective Al2O3 scale can be formed and maintained and the coatings are relatively stable up to approximately 900º C. At higher temperatures, Al inwards diffusion causes impoverishment of Al at the surface until it reaches a critical concentration below which the protective oxide will not form. Depending on the coating composition an Al content of 4 to 8 wt. % seems to be sufficient to maintain the protective scale [1-3]. Among the diverse coating deposition techniques, the application of slurries is one of the most economical, practical and versatile. Aluminide coatings have been generated by applying an Al slurry followed by heat treatment which depends on the substrate. For instance, it has been shown that on ferritic steels, deposition of an Al slurry followed by heat treatment at 700º C for 10 h generates a complex aluminide coatings composed of several intermetalllic phases including FeAl3, Fe2Al5 and FeAl (see figure 1). Under the coating, AlN acicular precipitates can form within the substrate, if the substrate contains sufficient N (≥ 0.01 wt. %) [4-6]. The coatings exhibit thickness-through cracks probably due to brittleness of the Fe2Al5 phase and to thermal expansion mismatch between the different phases. Figure 1: SEM cross sections of an "as deposited" Al slurry coating on P92 followed by heat treatment for 10 h at 700º C