Abstract
Over a two-month period, the statistical mean of the corrosion rate of mild steel in tap water was 20 mpy. When the water was adjusted to give a positive Langelier index with increasing HCO3- and Ca2+ contents, the corrosion rate of cleaned steel coupons was reduced. The lowest corrosion rate (viz. 8.5 mpy) was measured in a water containing 17.6 ppm Ca2+ and 38.4 ppm HCO3-. This same water had, however, no effect on the corrosion rate of already rusted steel coupons. Lowering of the corrosion rate of the latter was achieved in presence of disodiumorthophosphate and sodium hexametaphosphate. The lowest corrosion rate, ca. 12 mpy, was obtained in presence of 2 ppm of either additives. Continuous dosing of the orthophosphate to keep a level of 2 ppm over a long term experiment resulted in the further reduction of the corrosion rate to 6-7.5 mpy (ca. 62% inhibition) after 20 days of treatment. The effect extended over 43 more days. On the other hand, similar treatment with the hexa-metaphosphate had no effect.
In view of the large quantities of water that has to be treated daily, the effect of the amounts of Ca2+ and HCO3- on the corrosion rate of rusted steel was re-established for tap water containing 2 ppm of either inhibitor. The lowest corrosion rate, 6.5 mpy, was obtained in presence of 8 ppm Ca2+ and 18 ppm HCO3- in the case of orthophosphate and 12 ppm Ca2+ and 24 ppm HCO3- in the case of hexa-metaphosphate, respectively. Inhibition in the presence of the two additives involved the formation of sparingly soluble phosphates since the solutions had negative Langelier Saturation indexes.