1998 Volume 47 Issue 5 Pages 326-332
Recently, dissolved oxygen (DO) in water has been removed by deaerators equipped with polymeric hollow fiber membrane. By this membrane deaeration method, it is possible to examine effects of relatively low concentration of DO and of flow velocity on corrosion of carbon steel in tap water, systematically. In this study, concentrations of DO in Matsuyama-city water were adjusted to 0.5, 2.0, 4.0mg/dm3 by mixing deaerated water by the membrane deaeration system with non-deaerated water. Flow velocities were controlled to 0.5, 1.0, 1.5 and 2.0m/s by successively reducing the cross sectional area on a water line, and there was turbulent flow in all cases. The experimental system was designed so that it flowed smoothly, considering hydrodynamic conditions. In the case of DO=0.5mg/dm3, the corrosion rate of carbon steel increased with flow velocity. To the contrary, the corrosion rate in non-deaerated water decreased with velocity, although the corrosion rate was larger with all flow velocities. Within the range of velocity of this experiment, the lowest corrosion rate and the minimum depth of pitting were observed in the case of tap water of DO=0.5mg/dm3. This condition is acceptable to prevent corrosion of water piping. It is also recognized that the behaviour of the corrosion rate and/or pit depth is closely related to the total amount of oxygen supplies, i.e., multiplication of DO value by water velocity.