Cr produced in the above reaction will in turn react with chloride in the pack to generate various Cr-chloride vapour species, which would include CrCl, CrCl2, CrCl3 and Cr2Cl6. Among these species of Cr-chloride vapours, only CrCl functions as the species that deposits Cr on the metal surface; other types of Cr-chloride vapour species merely participate in the vapour transportation processes from the pack powder to metal surface [29] and [30]. Therefore, the partial pressure of CrCl vapour species may be used to compare the activating strength of different chloride salts. A higher CrCl partial pressure would suggest a higher activating strength as it would lead to a higher rate of Cr deposition and consequently result in faster Cr enriched surface layer growth. Thus, the comparison of the calculated partial pressures of CrCl vapour species as shown in Fig. 1b suggests that, in terms of chromising process, the activating strength of AlCl3 is slightly higher than that of NH4Cl, and the activating strengths of these two types of chloride salts are all much higher than that of NaCl.
Thus, for chromising purpose, NH4Cl would be a slightly weaker activator than AlCl3. But, the N2 partial pressure generated in NH4Cl activated packs is much higher than in AlCl3 activated packs, which means that the conditions for chromium nitride coating formation in the former packs are much more favourable than in the latter packs. Therefore, NH4Cl was chosen as the activator for coating experiments in this study.