The effect of subsequent annealing on the electrochemical properties of an AZ31 magnesium alloy processed by extrusion and equal channel angular pressing (ECAP) was investigated. The electrochemical properties were evaluated using potentiodynamic tests in corrosion solution of 0.1 M sodium chloride. The electrochemical changes after annealing were correlated with microstructure evolution. The microstructure was analyzed by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The evolution of dislocation density was determined by positron annihilation spectroscopy (PAS). The annealing for 1h at temperatures ranging from 150^∘C to 250^∘C resulted in higher polarization resistance in all cases. The polarization resistance of sample annealed at 250^∘C was ~17% higher compared to just ECAPed material. Combination of gradual decrease of dislocation density, grain growth and second phase particles dissolution played the crucial role in the corrosion resistance improvement.