The authors have published a series of papers on countermeasures against the corrosion of building equipment in water piping systems. In these previous papers (first and second), we showed that tap water treated with anion exchange resin drastically reduced corrosivity. In practice, carbon steel did not corrode for 720 h in the treated water. The time trajectory of corrosion potential versus pH on the Pourbaix diagram was analyzed. At the same time, the water qualities before and after corrosion tests were compared. The corrosion prevention mechanism was established based on these studies. We found that the passivity of carbon steel in treated water plays a key role in corrosion prevention. We also found that the corrosivity of treated water was influenced by the quality of the pumped up water and the conditions of the anion exchange processes. The relation between these variables was studied. The criteria to provide negligible corrosivity by anion exchange treatment could successfully be determined. To express the completeness of anion exchange, the ion exchange ratio was introduced. The corrosivity of the water mixture was evaluated by corrosion testing. When the ratio of treated water was sufficiently high, corrosion did not occur due to the presence of passivation film. When the ratio of untreated water was increased, corrosion occurred abruptly. The water quality required for the passivation of carbon steel was related to corrosion indexes such as the Larson-Skold index and SS ratio. The quality criteria to minimize cororsion were SS ratio ≦ 0.168 and Larson-Skold index ≦ 0.071. The relation between the corrosion potential and corrosion rate for carbon steel was also studied using a water mixture.