This work demonstrates the corrosion behavior of the AZ31 Mg alloy as a function of an immersion time of 48 h in 0.1 M HCl, H
2SO
4, H
3PO
4 and HF solutions, in which pH was adjusted to 6 to exclude the contribution of hydrogen ions (H
+) and hydroxide ions (OH
−). In situ observations, open circuit potential (OCP), weight changes and AC impedance measurements were performed with an immersion time of 48 h and the morphologies and chemical compositions of the surface products after 48 h of immersion were analyzed by SEM, EDS and XPS. In the chloride ion (Cl
−)-containing solution, the corrosion of the AZ31 Mg alloy initiated locally and propagated discontinuously over the surface with immersion time. The OCP value of the AZ31 Mg alloy showed an initial increase from −1.51 V
Ag/AgCl to −1.47 V
Ag/AgCl after about 5 h of immersion and then a decrease to −1.51 V
Ag/AgCl due to corrosion initiation. In the F
−-containing solution, after 48 h of immersion, the OCP showed an extremely large value of −0.6 V
Ag/AgCl, while the relatively lower values of −1.52 V
Ag/AgCl, −1.59 V
Ag/AgCl were seen in the solutions containing SO
42− and PO
43, respectively. In the sulfate ion (SO
42−)-containing neutral aqueous solution, needle-like surface films were formed and there were no changes in the weight of the AZ31 Mg alloy with immersion time. In the phosphate ion (PO
43−)-containing neutral aqueous solution, a vigorous gas evolution occurred, together with the formation of black surface films with cracks, and a high corrosion rate of −13.8018 × 10
−3 g·cm
−2·day
−1 was obtained. In the fluoride ion (F
−)-containing neutral aqueous solution, a surface film with crystalline grains of MgF
2 was formed and the weight of the AZ31 Mg alloy increased continuously with immersion time. In conclusion, the corrosion of the AZ31 Mg alloy occurred uniformly in neutral phosphate solution but locally in chloride solution. No corrosion was observed in either the neutral sulfate or fluoride solutions.
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