@shebeer thank you, but how many days for the immersion that the result can be viewed under SEM?

Dear Huynh Nguyen

Thank you for interesting question. The surface morphology of AZ31 magnesium alloy may vary with immersion time, since the formation of protective Mg(OH)2-film on rapidly after 2 h immersion in HBSS.

This film layer become more compact with increasing of immersion time and after a certain time (say 24h) the film began to degrade as the Mg(OH)2 layer on the surface is dissolved by Cl- into soluble MgCl2.

This dissolution reaction, in case of stagnant corrosion systems, increases the local hydroxide ions concentration near the surface and so the degradation leads to a local alkalization of the HBSS near alloy surface. After 48h immersion, the degradation promoting a greater tendency for film formation as the local pH increase above 10. At this time and after, a layer of magnesium phosphate Mg3(PO4)2 and hydroxiapatite Ca10(PO4)6(OH)2 may observed on the surface and that covered by MgO and Mg(OH)2 corrosion products .

Now, it is a clear that the microscopy observations of the surface morphology are related by the immersion time and become clearly visible after 2h of immersion and higher up to a week for obtaining full observations.

best regards