Self-catalytic degradation of iron-bearing chemical conversion coating on magnesium alloys ---- Influence of Fe content

Abstract

A number of industrial and biomedical fields, such as hydraulic fracturing balls for gas and petroleum exploitation and implant materials, require Mg alloys with rapid dissolution. An iron-bearing phosphate chemical conversion (PCC) coating with self-catalytic degradation function was fabricated on the Mg alloy AZ31. Surface morphologies, chemical compositions and degradation behaviors of the PCC coating were investigated through FE-SEM, XPS, XRD, FTIR, electrochemical and hydrogen evolution tests. Results indicated that the PCC coating was characterized by iron, its phosphates and hydroxides, amorphous Mg(OH)₂ and Mg_3−n(H_nPO₄)₂. The self-catalytic degradation effects were predominately concerned with the Fe concentration, chemical composition and microstructure of the PCC coating, which were ascribed to the galvanic corrosion between Fe in the PCC coating and the Mg substrate. The coating with higher Fe content and porous microstructure exhibited a higher degradation rate than that of the AZ31 substrate, while the coating with a trace of Fe and compact surface disclosed a slightly enhanced corrosion resistance for the AZ31 substrate.

Keywords

magnesium alloy / iron-bearing chemical conversion coating / self-catalytic degradation / galvanic corrosion

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Disclosure of potential conflicts of interest

The authors declare that they have no conflicts of interest to this work.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51571134) and the Scientific Research Foundation of Shandong University of Science and Technology Research Fund (2014TDJH104).

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2020 Higher Education Press