Stationary shoulder friction stir processing (SSFSP) is a new surface modification method for magnesium alloys with minimal heat input. However, the corrosion advantage of SSFSP should be evaluated in terms of the electrochemical rate and corrosion morphology. The key issue is to determine if SSFSP prevents localized corrosion in the AZ31B magnesium alloy under chloride environment by creating a homogeneous stirring layer through the entire thickness. Commercial plates of AZ31B of 6.35 mm thick were processed by SSFSP technique with H13 steel tool under stationary shoulder and rotating probe with the speed of 700 rpm and travel of 100 mm/min. The corrosion resistance of base material and the surface of stir layer in the deaerated 3.5 wt% NaCl solution was tested by OCP measurement, potentiodynamic polarization, and SEM/EDS analysis after corrosion. In this paper, experimental results show that the mean grain size in the base material reduced from 25.2 ± 2.2 μm to 6.34 ± 0.12 μm (74.84% reduction with grain-size inversion factor of 3.97). The corrosion current density decreased from 9.815 × 10−3 to 2.032 × 10−3 A/cm2, and the corrosion potential kept stable value around −0.546 V. On the other hand, OCP of processed area formed a steady-state band of −0.40 V ∼ −0.35 V, whereas base metal showed much wider fluctuations between −0.65 and −0.47 V. Morphology is able to answer the primary question regarding SSFSP – it is not simply about slowing down the average electrochemical reaction rate but rather changing the governing corrosion mechanism from a pitting one characterized by enhanced localized action to a corrosion product film mechanism with mud cracking and no pits at all. Hence, the enhancement of corrosion resistance results from the synergy of grain refinement, reduction in microgalvanic continuity, fragmentation of Al-Mn-Fe clusters, and absence of thermal asymmetry through the material thickness.