The connection between the corrosion characteristics of friction stir spot welded AZ31B and the microstructural changes induced by the joining operation has been examined. Second phase particle dissolution was investigated through temperature measurements during welding, coupled with microstructural characterization using scanning electron microscopy and nuclear magnetic resonance spectroscopy. The electrochemical properties of the stir zone (SZ) region were characterized by microcapillary polarization. The SZ region was noble to the base metal for all welding parameter settings examined. For a constant dwell time, increasing the tool rotational speed increased the corrosion potential and reduced the corrosion current density of the SZ region. The improvement in the corrosion resistance was attributed to increased dissolution of beta-Mg17Al12. Increasing the length of time available for particle dissolution by diffusion had a similar effect for durations up to one second, and longer times had no effect as complete dissolution of beta-Mg17Al12 was already achieved. A model capable of predicting the electrochemical properties of the stir zone was developed based on mixed potential theory, changes in stir zone microstructure, and the electrochemical properties of individual phase constituents. The area fraction that supports the anodic dissolution reaction was decreased with increasing extent of second phase dissolution. (C) 2018 The Electrochemical Society.