This work examined how rotation speed influences the microstructure and electrochemical properties of the stir zone (SZ) in 430 ferritic stainless steel joined by friction stir welding (FSW). The findings show that a higher rotation speed improves the formation quality of the initial region of the welded joint, coarsens the SZ grains, reduces the fraction of low-angle grain boundaries (LAGBs), and deteriorates corrosion resistance. At 500 rpm, the SZ achieved an average grain size of 3.8 μm and a self-corrosion current density of 1.367 × 10 −7 A·cm −2, showing the most uniform microstructure and the optimal electrochemical performance. Lower rotation speed helps obtain a fine, uniform grain structure in the SZ, facilitating rapid formation of a dense passive film and improving corrosion resistance.