Effect of pH on the Passivation of Carbon Steel by Sodium Borosilicate Controlled-Release Inhibitor in Simulated Recirculating Cooling Water

A sodium borosilicate controlled-release inhibitor has been prepared via calcination for suppressing carbon steel corrosion in simulated recirculating cooling water (SRCW). The synthesized inhibitor was mainly composed of B2O3 and SiO2, coupling with small quantities of NaB5O6(OH)4·3H2O and NaB5O8·5H2O crystals. The total boron concentration gradually increased to 250 mg L–1 during the 28 days of release (2 g inhibitor in 500 mL SRCW) at 50 °C. A higher controlled-released rate existed in acidic environments (pH 5) than in neutral environments (pH 7) and alkaline environments (pH 9). The potentiodynamic polarization and electrochemical impedance spectroscopy results indicated that the inhibitor showed a better anticorrosion performance in neutral and alkaline environments than in acidic environments. In addition, a passive film with a Fe–O–B structure formed on the carbon steel surface to prevent the attack of the corrosive ions. A passivation-pitting-repassivation process was detected in the passive film...