High temperature oxidation and corrosion behaviour of Ni/Ni–Co–Al composite coatings

In the present study, Ni/Ni–Co–Al composite coatings were developed by a potentially simple, scalable, non-vacuum technique namely electrodeposition. These coatings were characterized for their microhardness, oxidation and hot corrosion behaviour. An increase in Co content in the matrix from 8 wt% to 70 wt% led to an increase in the Al particle incorporation from 12 wt% to 21 wt%. A change in the surface morphology of the coatings with variation in Co content was seen. The oxidation behaviour of the coatings was studied at temperatures in the range of 400 °C to 1000 °C. The influence of vacuum treatment on the high temperature behaviour of the coatings was also investigated. The intermetallic aluminide phase formation was observed in the temperature range of 600–800 °C and a homogenized structure was seen at 1000 °C. The oxidation rate in terms of weight gain was marginally lower for vacuum pretreated Nisingle bondAl coating annealed at 1000 °C. A significant increase in the oxidation rate was exhibited by Ni–70Co–Al coating beyond 800 °C showing its poor oxidation behaviour. The characterization studies revealed the formation of stable alumina in the case of Nisingle bondAl while, metastable alumina was observed in Ni–Co–Al coatings. The hot corrosion studies showed that Co rich Ni–Co–Al exhibited better resistance compared to Ni rich coatings. An optimum cobalt content of 30 wt% was desirable for high temperature oxidation and corrosion resistance.