| A series of 7-day batch corrosion tests showed that the addition of commercially formulated products did not decrease the corrosion rate of cast iron specimens suspended in the water system until a sufficient amount of polyphosphate (greater than 20 mg/l as P(,2)O(,5)) had been added. Similar results were obtained in the 28-day studies using continuous-flow corrosion test systems. The rate of corrosion of cast iron in batch tests, at an optimum polyphosphate dosage of 20 mg/l as P(,2)O(,5), can be effectively reduced by increasing total alkalinity, pH, calcium concentration as well as by; decreasing the concentration of dissolved oxygen. Analysis performed on the materials deposited on the cathode and anode, showed that colloidal condensed phosphate compounds (not orthophosphate compounds) which deposited on the cathode and not the anode, play the most important role in inhibiting corrosion. Particle information studies indicated that increasing the dosage of P(,2)O(,5) and the calcium concentration resulted in an increase in colloidal particles in the size range, 5-10 micron, which were believed to form the protective coatings. It was also observed that increases in microbiological growth by 1 to 80 times resulted from the phosphate addition.; The experimental results indicate the possible mechanism of corrosion inhibition by the polyphosphate compounds tested. Both orthophosphate and condensed phosphates appear to react with cations, such as ferrous, zinc and calcium ion, resulting in the formation of charged colloidal particles. The particle formation appears to be favored by increasing pH, calcium ion and polyphosphate concentrations. These colloidal particles are subsequently deposited. It is possible that the condensed phosphate particles deposited on the cathode play the most important role and may serve as a protective coating. The protective coating thus formed on cathodic areas decreases the corrosion rate of cast iron in water by increasing the degree of cathodic polarization in the corrosion cells. |
