Monolithic conversion of AZ31 Mg alloy plates to struvite in saturated ammonium dihydrogen phosphate solutions

The surface evolution of AZ31 immersed in saturated aqueous solutions of ammonium dihydrogen phosphate (ADP) and various functional adjuvants was investigated by compositional, morphological and gravimetric analyses to understand the underlying mechanisms for the monolithic conversion of the alloy to struvite. The immersion process was monitored by pH and weight measurements at various intervals for a period of 21 days. Saturated aqueous solutions of ADP were initially acidic with a pH around 4 which caused a rapid degradation of the alloy surface. The dissolved cations reacted with the infusing ions within the bulk of the alloy to induce a strong volumetric expansion that increased the thickness of the plates by more than one order of magnitude. Close examination of the cross section by SEM revealed formation of mainly prismatic crystals of struvite perpendicular to the rolling direction of the plates, thus intercalating and simultaneously densifying the microstructure. However such long term struvite growth could not be sustained in any of the studied multicomponent solutions of ADP, which highlights the unique pH evolution of saturated ADP solution that is suitable for continuous struvite formation. Addition of trisodium citrate also provided a balanced pH evolution enabling monolithic conversion but resulted in significant newberyite formation with struvite. The facility of saturated ADP solution to monolithically convert magnesium alloys to struvite is expected to provide improvements in the efficiency of waste-water phosphate recovery processes.