Quantifying the influence of calcium ion concentration on the corrosion of high-purity magnesium, AZ91, WE43 in modified Hanks' solutions

The corrosion rate in a modified Hanks' solution (containing no Ca²⁺ ions) was higher than in Hanks' solution. The increase was by a factor of ∼12 for HP Mg and AZ91, and a factor of ∼6 for WE43. This quantitatively highlights the critical role of Ca²⁺ ions for Mg corrosion in synthetic body fluids. The Ca²⁺ ion containing solutions produced a dense corrosion-product layer of hydroxyapatite, Ca₃(PO₄)₂) · Ca(OH)₂, a greater fraction of which stayed on the corroding surface (∼0.2–0.3) compared with the Ca²⁺ ion free solutions which produced magnesium phosphate, Mg₃(PO₄)₂. (paper)