1,2-Ethane diphosphonates of the general formulaM.sub.2 O.sub.3 P--CH.sub.2 CH.sub.2 --PO.sub.3 M.sub.2wherein M is hydrogen, metal ion, ammonium, alkyl ammonium or mixtures thereof, are disclosed as inhibiting the corrosion of metals by oxygen-bearing waters. The 1,2-ethane diphosphonates can be employed either alone or in combination with certain thiols, 1,2,3-triazoles, zinc salts, chromates, silicates, inorganic phosphates, molybdates, tannins, lignins, lignin sulfonates, certain calcium and magnesium salts and mixtures thereof.