Effect of Oxygen, Chlorides, and Calcium Ion on Corrosion Inhibition of Iron by Polyphosphates

Data are presented showing that sodium polyphosphates are effective corrosion inhibitors for steel, provided the dissolved oxygen concentration is above a critical minimum. Presence of calcium ion appreciably improves inhibition under all conditions. In solutions containing less than the critical amount of oxygen (1 ml/l), 60 ppm sodium polyphosphate accelerates corrosion. This is also true when 60 ppm Ca ++ is added to the polyphosphate solution, although the critical oxygen concentration is now about 0.15 ml/l. Sixty ppm sodium polyphosphate effectively inhibits against corrosion in solutions containing up to ; above this concentration, passivity by dissolved oxygen is lost, and the inhibitor is no longer effective. However, the calcium complex continues to function to some extent even at high chloride ion concentrations. The foregoing corrosion data combined with polarization and potential studies indicate that sodium polyphosphates inhibit by favoring the passivation of steel by adsorbed oxygen. The hydrous film normally present on iron presumably reacts with polyphosphate to form a soluble complex, permitting higher surface concentrations of oxygen for a given concentration in solution. Corrosion control under these circumstances is mixed, and pitting occurs in presence of chlorides. Calcium salt additions lead to formation on cathode areas of a diffusion barrier film of unknown structure containing both calcium and phosphorus. This film appreciably reduces the corrosion rate, with control of corrosion, however, remaining mixed. Reasons for the need of continuous flow and aeration in practical systems are discussed.