Minor Fe additions are necessary to enhance the corrosion resistance of commercial Cu-Ni alloys. The present paper aims at optimizing the Fe content in three alloy series Cu90(Ni,Fe)10, Cu80(Ni,Fe)20, and Cu70(Ni,Fe)30 (at.%) from the viewpoint of their corrosion performance in a 3.5% NaCl solution. An Fe/Ni = 1/12 solid solubility limit line was revealed in the Cu-Ni-Fe phase diagram. Three Fe/Ni = 1/12 alloys, Cu90Ni9.23Fe0.77 (at.%) = Cu-8.6Ni-0.7Fe (wt.%), Cu80Ni18.46Fe1.54 = Cu-17.3Ni-1.4Fe, and Cu70Ni27.7Fe2.3 = Cu-26.2Ni-2.1Fe, show the best corrosion performances in their respective alloy series. The Fe/Ni = 1/12 solubility limit is explained by assuming isolated Fe-centered FeNi12 cuboctahedral clusters embedded in a Cu matrix. The three Fe/Ni = 1/12 alloys can be respectively described by cluster formulas [Fe1Ni12]Cu117, [Fe1Ni12]Cu52, and [Fe1Ni12]Cu30.3. The Fe/Ni = 1/12 rule may serve an important guideline in the industrial Cu-Ni alloy selection because above this limit, easy precipitation would negate the corrosion properties of the Cu-Ni-based alloys.