Pack aluminisation of low alloy steels at temperatures below 700 °C

This study aims to investigate the feasibility of aluminising low alloy steels at temperatures below 700 °C by pack cementation process to increase their high temperature durability in oxidative and corrosive environments without adversely affecting their mechanical strength and creep resistance at elevated temperatures. Packs activated by AlCl3, AlF3, NaCl and NaF were used to carry out coating deposition experiments at 650 °C in an attempt to identify the most suitable activator for the intended pack aluminising process. Once this was achieved, a series of further experiments were undertaken to investigate the effects of pack composition, deposition temperature and time on the kinetics of coating growth process. The pack Al content was varied from 1 to 6 wt.%, deposition temperature from 600 to 750 °C and deposition time from 1 to 16 h. Within the ranges of these parameters, it was observed that these parameters only affected the coating thickness, but not the surface Al concentration and with the packs activated by AlCl3, the coatings formed via an inward reactive Al diffusion mechanism, which led to the formation of a surface Fe14Al86 layer and an inner FeAl3 layer. Thermochemical calculations were also undertaken to analyse the equilibrium vapour pressures of depositing halide species (AlF or AlCl) generated at deposition temperatures in packs activated by different halide salts. The results obtained were discussed in relation to the observed deposition tendencies of these packs and their influence on the kinetics of coating growth.