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The annealing treatments were conducted at various temperatures and time using hot rolled AZ31 Mg sheets. The effects of annealing temperature and time on the microstructure and corrosion rate were investigated. The rolled AZ31 Mg consisted of coarse deformed grains with high stored energy, deformation twins, and recrystallized grains with free strain, showing the highest corrosion rate of 10.3 mm/year. The annealing effectively slowed down the corrosion rate of AZ31 Mg by increasing the grain size and decreasing the numbers of dislocations, twins and second phase particles. A high annealing temperature of 450 °C and a long holding time of 5 h further reduced the corrosion rate of AZ31 Mg to quite low level of 3.5 mm/year.