Process cooling water systems require effective chemical treatment to protect steel pipelines against corrosion attack. In this work, experiments were conducted to study the inhibition effect of molybdate and nitrite on the corrosion of carbon steel in the presence of open recirculating flow condition. Carbon steel grade 1018 was used as specimens by way of immersion test in a Simulated Cooling Water (SCW) solution, within a rotating cage and mini flow circuit experiments. Corrosion rates were measured by weight loss method after immersion for 72 hours. Results showed that the combined inhibitor treatment of molybdate and nitrite showed a synergistic effect in reducing the corrosion rate to less than 0.01 mm/y with the inhibition efficiency of 99.6. The rotating cage results showed good correlation with mini flow circuit results. Surface morphology showed that inhibited steel was protected by oxide film but still suffered some pitting. © 2019 Taylor's University. All rights reserved.