The grain boundary engineering (GBE) effect on the erosion–corrosion resistance of 90/10 cupronickel alloys was studied using a self-made erosion–corrosion device. A 90/10 cupronickel alloy with abundant low-Σ coincidence site lattice grain boundaries and random grain boundary disconnectivity was fabricated through GBE. The average corrosion rate of the self-prepared alloy was reduced by 34% to 0.0414   mm/a after the erosion–corrosion test. The corrosion product films of both samples included Cu, Cu2O, Ni(OH)2, NiO, FeO, FeOOH, and trace amounts of Cu2(OH)3Cl. A schematic diagram of the intergranular corrosion mechanism during the erosion–corrosion test was prepared.