Influence of substrate microstructure on hydroxyapatite coating and corrosion behavior of coated Mg[sbnd]Zn alloys

Abstract Hydroxyapatite (HAp) coatings were formed on as-cast and T4-treated Mg-xZn (x = 1, 5, and 7 wt%) alloys by a chemical solution deposition method. The uniformity of HAp coatings showed an enhancement with finer particle size and lower volume fraction of (α-Mg + MgZn) eutectic cells and Zn-segregated regions. The Mg[sbnd]1Zn sample showed a defect-free HAp coating, while the HAp coating of the Mg[sbnd]7Zn sample showed cracks preferentially over the eutectic cells causing micro-galvanic corrosion with the matrix. Initially, the corrosion rate of the HAp-coated alloys was governed by the uniformity of HAp coating, and eventually it was dominated by the corrosion resistance of substrate alloys after the HAp coatings irreversibly broken. Ultimately, the volume fraction and size of (α-Mg + MgZn) eutectic cells and Zn-segregated regions governed the corrosion behavior of the HAp-coated Mg[sbnd]Zn alloys. The HAp-coated Mg[sbnd]1Zn alloy showed 2–4 times lower corrosion rate than other alloys after 1 day and 4–10 times after 14 days of immersion. The 14-day average corrosion rate of the as-cast Mg-1, 5, and 7Zn alloys decreased by 45%, 5% and 15% with the HAp coatings, respectively. The polarization resistance (R_p) of the as-cast Mg-1, 5, and 7 Zn alloys increased by 10, 7.5 and 5 times with the HAp coatings, respectively. Consequently, the HAp coating effectively improved the corrosion resistance of Mg[sbnd]Zn alloys with low volume fraction of (α-Mg + MgZn) eutectic cells.