Potentiostatic Conversion of Phosphate Mineral Coating on AZ31 Magnesium Alloy in 0.1 M K2HPO4 Solution

• A four layered phosphate coating was electro-deposited on AZ31 alloy in deaerated 0.1 M K2HPO4 solution with pH 9.5.
• The formation process of phosphate conversion coating contains five characterization stages.
• Amorphous Mg(OH)2, MgHPO4 and crystallized KMgPO4
• 6H2O are in sequence deposited to form the conversion coating.
• The innermost layer of the coating consists of Mg(OH)2, and the inner layer consists of Mg(OH)2 and MgHPO4.
• The middle layer consists of Mg(OH)2, MgHPO4 and KMgPO4
• 6H2O, and the topmost layer consists of MgHPO4 and KMgPO4
• 6H2O.

The coating process of a four layered phosphate mineral conversion coating on AZ31 Mg alloy deposited at −0.8 V in 0.1 M K2HPO4 solution with pH 9.5 was studied by in situ potentiostatic polarization, together with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetry (TG), differential scanning calorimetry (DSC), differential thermal gravimetry (DTG) and electron probe micro-analyzer (EPMA). Our results show that the coating process can be divided into five stages. The initial two stages are caused by the rapid decrease of charging current of the electrical double layer capacitance and the slow increase of faraday current density on AZ31 Mg alloy, and Mg(OH)2 was detected in the corrosion product in these two stages. The third process corresponds to the deposition of amorphous MgHPO4 film, followed by the fourth process corresponding to the decrease of the corrosion resistance of MgHPO4. In the last stage, crystallized KMgPO4 · 6H2O (struvite-K) forms. The SEM, XPS, TG, DSC, DTG and EPMA indicate that the deposited coating consists of four layers. The innermost layer is composed of a corosion product, Mg(OH)2, followed by the inner layer composed of Mg(OH)2 and MgHPO4, subsequently the middle layer consisting of Mg(OH)2, MgHPO4 and struvite-K, and finally the topmost layer consisting of MgHPO4 and struvite-K.