formation of NiAl aluminide coating using a single-step high activity aluminizing process, study of microstructure and mechanism of formation

In this study microstructure and formation of NiAl phase using a modified (single-step) high activity aluminizing at 1080°C have been compared with those of common (two-step) high activity aluminizing at 850°C. The mechanism of this process has been investigated by carrying out the aluminizing process on electroplated Nickel on 316 stainless steel substrate at 1080°C in different durations of 0.25, 1, 2, 4 and 8 h (by investigation the Ni content in NiAl). Results showed that even in a short duration of 0.25 h the NiAl phase was formed. In 0.25 h NiAl phase with more than 50%at. of Al was formed while NiAl phase with less than 50%at. of Al was formed at 1, 2, 4 and 8 h. It is concluded that the mechanism of formation of NiAl coatings takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion and In the final stage, the outward diffusion of Ni dominates the coating formation process. The microstructure of the coatings sections were observed by optical microscopy and scanning electron microscopy. The phase analysis and the coating composition were investigated by XRD and EDS respectively.