Conversion of the AZ31 surface and its bulk in saturated ammonium dihydrogen phosphate solutions
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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. 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. Apparently the dissolved cations reacted with infusing ions within the bulk of the alloy to induce a strong volumetric expansion that increased the thickness of the plates more than one order of magnitude. Close examination of the cross section by SEM revealed that monolithic crystals of struvite and other magnesium phosphate phases formed perpendicular to the rolling direction of the plates, thus intercalating and simultaneously densifying the microstructure. However such long term 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 bulk conversion.
