Protective diffusion coatings on magnesium alloys: A review of recent developments

This study reviews the recent progress in preparation and properties of diffusion coatings on magnesium alloys. Surface alloying of magnesium alloys by diffusion coatings has been considered as an effective approach to providing the protection of magnesium alloys because of the following distinct potential advantages. Firstly, the diffusion coating has high adhesion strength since there is a strong metallurgical bond between the coating and the substrate. Moreover, the good electrical conductivity of the coated magnesium alloy can be maintained and thus its electromagnetic shielding property will not be lost. In addition, the alloyed layer comprises of intermetallic compounds, which can improved both the corrosion resistance and the mechanical properties. Therefore, various methods have been developed in an effort to fabricate diffusion coatings on magnesium alloys. This review begins with the basic principles and challenges of fabricating diffusion coatings on magnesium alloys. The types and principles of coating methods are next described. The processes of preparing diffusion coatings can be categorized into two groups: (i) one-step processes including pack cementation, vacuum aluminizing, chemical vapor deposition and molten salt bath; (ii) multi-step processes such as surface mechanical attrition treatment plus pack cementation and post heat treatment of the metallic coatings. Based on understanding the principle of each coating process, the varied microstructures depending on the processing methods and parameters, are highlighted. The corrosion and mechanical properties of the coatings are also discussed on the basis of literature data. In particular, the corrosion property especial the passivation behavior of the diffusion coating is strongly influenced by the processing parameters and the subsequent coating microstructure. Through the experimental results obtained by various researchers, the coating process-microstructure-property relationships are discussed. This review closes with an outlook on areas that should be addressed in the future work. (C) 2011 Elsevier B. V. All rights reserved.