Fabrication of hazenite conversion coating on AZ31 Mg alloy

Hazenite coating was developed on AZ31 Mg alloy by immersion in a solution containing 0.1 mol·dm−3 K2HPO4 and 0.1 mol·dm−3 Na2HPO4 with pH 9 held at 45 °C. Its formation mechanism was established through unveiling the evolution of the coating morphology, structure and composition. The results showed that brucite (Mg(OH)2) was first involved in the coating process, followed by the precipitation of newberyite (MgHPO4·3H2O) and farringtonite (Mg3(PO4)2) on top of brucite, subsequently the formation of struvite-K (KMgPO4·6H2O), and finally the transformation of struvite-K into hazenite (KNaMg2(PO4)2·14H2O). The resultant coating was composed of two layers where the inner layer was composed of brucite, newberyite, farringtonite and struvite-K, and the outer layer consisted of brucite, newberyite, farringtonite, struvite-K and hazenite. The produced coating reduced corrosion rate of AZ31 Mg alloy substrate by more than two orders of magnitude. •Brucite, newberyite/farringtonite, struvite-K and hazenite participate in the formation of the coating, successively.•Struvite-K can be transformed from farringtonite while hazenite from struvite-K.•Brucite, newberyite, farringtonite and struvite-K participate in the formation of the inner layer of the coating.•The outer layer consists of brucite, newberyite, farringtonite, struvite-K and hazenite.•The coating reduces corrosion rate of AZ31 Mg alloy by more than two orders of magnitude.