Galvanic corrosion between 42CrMo steel and 410 stainless steel in low-conductivity Yangtze River freshwater was investigated using electrochemical measurements and secondary current distribution modeling. Experimental polarization data were used to establish the electrode kinetics, and the numerical model was calibrated against the measured galvanic current. The effects of cathode-to-anode area ratio (A_c/A_a) and electrode separation (d) were evaluated across 20 geometric configurations. At d = 50 mm, increasing A_c/A_a from 0.5 to 4 increased the total galvanic current from 1.57 × 10⁻⁴ to 4.97 × 10⁻⁴ A and the maximum local anodic current density from 0.078 to 0.270 A m⁻². At A_c/A_a = 1, increasing electrode separation from 10 to 100 mm reduced the total galvanic current by approximately 41%, while the current-localization factor increased. These results demonstrate that electrode geometry governs both the overall galvanic corrosion intensity and the spatial distribution of anodic current. The findings provide a basis for identifying high-risk configurations and optimizing the geometric design of dissimilar-metal connections in hydropower structures.