Effects of Environmental Variables on the Corrosion Rate of Cast Iron in Water Distribution Systems

摘要

Corrosion rate measurements for cast iron in water were made using a re-circulating water test loop made of austenitic stainless steel. The water in most of the experiments was tap water of Otaniemi (Espoo, Finland) as such or with some changes, mostly in chloride concentration. The cast iron specimens were "aged" galvanostatically. Cyclic polarisation curves were measured from the open circuit potential (Eo) to Eo+30 mV, then to Eo-30 mV and back to the open circuit potential. The corrosion rates were determined from the cyclic polarisation curves with a "manual curve fitting" method. A "hysteresis" observed in the polarisation curves was taken into account by including a correction term in the curve fitting. The corrosion rates calculated with the curve fitting method varied in the range 0.08-0.9 mm/a. A crude model for quantitative prediction of the corrosion rate from the values of the environmental variables was developed by using simple functions for the effects of the individual variables. According to the results, the corrosion rate increases when the conductivity, the oxygen content or temperature increases, and decreases when the hardness and alkalinity increase. The values obtained with the curve fitting method were also compared with ones calculated from the polarisation resistances using fixed anodic and cathodic Tafel slopes (120 mV). The correlation was found to be reasonable.