We performed the construction of a equipment, which was adequated to a potentiostat galvanostat for the on-site evaluation of the corrosion under stress of the AZ31-B magnesium alloy modified by the friction-stirring process, with the purpose of determinate the effect of the applied stress and the degradation. Analyses were performed by electrochemical impedance spectroscopy and potentiodynamic polarization curves. It was observed that a welded alloy generates higher corrosion velocities which cause a lower corrosion resistance in saline environments, apparently related with the continuous cracks propagation by anodic dissolution.