Relationship between pack chemistry and aluminide coating formation for low-temperature aluminisation of alloy steels

A theoretical analysis is provided to relate the coating layer growth kinetics to the pack chemical composition and processing conditions under a set of defined thermodynamic and kinetic conditions for aluminising alloy steels at temperatures below 700 °C in an effort to increase their high-temperature oxidation resistance whilst maintaining their microstructure and hence mechanical strength and creep resistance. A series of experiments were subsequently carried out on a type of commercial alloy steel P92 (12Cr–1Mo) in AlCl3-activated packs containing Al as the depositing source and Al2O3 as inert filler with the aluminising temperature varying from 500 to 700 °C, pack Al content from 1 to 30 wt.% and aluminising time from 1 to 16 h to determine the effects of these parameters on the coating growth kinetics and microstructure and hence to check the validity of the assumptions made in the theoretical analysis. The applicability of this analysis in the cases of aluminising steels using different halide salts as activators is also assessed, which highlighted the limited choice of activators available for aluminising alloy steels in the low-temperature range concerned.