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Initia treatment 80 Maintenance treatment Corrosion rate (mg/dm day) 40 20 Without initial treatment With initial treatment where W; - amount of chemicals required for initial treat- ment (kg) C = required concentration of chemicals for initial treatment (mg/l) H = holding water volume of cooling water sys- tem (m) After the completion of initial treatment, the chemical concentration in the circulating water is decreased by the water blowdown. When the chemical concentration reaches the maintenance level, the continuous injection of chemicals is started to keep the maintenance concentration. The injection rate of chemicals for the maintenance treatment is calculated by the equation (3.65) or (3.66): 0 1 2 10 20 25 15 Time (day) Fig. 3.74 Effect of an initial treatment with a zinc- phosphate based inhibitor on carbon steel