Effective time of a diffusion source in a pack cementation process on the AZ91D magnesium alloy

The powder thermal diffusion alloying method is an important method to fabricate intermetallic coatings on magnesium alloy substrates. The diffusion source, the diffusion time and the diffusion temperature are key parameters for a diffusion alloying process. While the effective time of an active diffusion source at the diffusion temperature is still unclear. Clarification of the effective time of a diffusion source is important, as the diffusion source could be utilized effectively and economically only if the effective time of the active diffusion source is known. Here, the effective time of a diffusion source at 400 °C was investigated. Results show that, the effective time of the diffusion source which contains 10 g ZnO and 1 g NH4Cl is around 150 h at 400 °C. For a more compact diffusion source, the effective time is relatively shorter. Then, chemical reactions in the diffusion source at elevated temperature were analyzed. The failure of the diffusion source is related to the reduction in the number of active metal ions, while an ineffective diffusion source can be reactivated by addition of new halide powder. These results could deepen the understanding of the role of the diffusion source in a powder thermal diffusion alloying process and indicate a limit under which the diffusion source could be utilized effectively.