Corrosion protection of magnesium and its alloys by metal phosphate conversion coatings

Abstract

This work describes a group of metal phosphate films, including calcium, manganese and strontium phosphate conversion coatings, for corrosion protection of magnesium alloys. These films have been developed by a process that is simple for implementation, chemically stable and non-toxic. The preparation and characterisation of these coatings, along with evaluation of their corrosion performance in 0·1M NaCl, are systematically described. Phase diagram of calcium phosphate was produced to elucidate the coating formation mechanism, which depends upon the chemistry of coating solution, in particular pH and ion concentration. A discussion of the advantages and disadvantages of the coatings is presented, along with perspectives for industrial applications.

Keywords:

• Magnesium alloys
• Conversion coatings
• Surface pretreatment
• Metal phosphates
• Corrosion

Acknowledgements

The following are acknowledged for their financial support: the CAST Cooperative Research Centre and the Australian Research Council (Centre of Excellence for Design in Light Metals). The authors acknowledge the facilities (SEM and X-ray photoelectron spectroscopy) and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the RMIT Microscopy and Microanalysis Facility. The Australian Research Council (Centre of Excellence for Design in Light Metals) and the Victorian State Government for the establishment of the Victorian Facility for Light Metals Surface are gratefully acknowledged. X.-B. Chen also thanks Dr K. Nairn for her contribution to the proof reading.