Effect of phosphate based inhibitor on the threshold chloride to initiate steel corrosion in saturated hydroxide solution

• In alkaline medium, Na3PO4 is an efficient inhibitor against the Cl− ions agressivity.
• Na3PO4 increases the resistance to localized corrosion of reinforcement in concrete.
• Na3PO4 increases the threshold ([Cl−]/[OH−]) of steel corrosion from 0.6 to 15.
• The acoustic emission technique is a powerful tool to monitor the corrosion process.

In this paper, a phosphate-based inhibitor was used to increase the well-known critical ratio [Cl−]/[OH−] (R), inducing the localized corrosion of steel reinforcements in saturated Ca(OH)2 solution. For this purpose, electrochemical measurements and characterization methods (SEM, EDS, XRD, etc.) were first employed to identify the phosphate layer formation, and then to evaluate the inhibition effectiveness for the localized corrosion of steel reinforcements. The process of pitting corrosion is evaluated by cyclic polarization and is followed by acoustic emission.The experimental results show that electrochemical techniques can evaluate the pitting corrosion and reveal that the introduction of phosphate-based inhibitor increases strongly the critical ratio [Cl−]/[OH−] from which the depassivation and pitting corrosion can be initiated. This clearly confirms its positive effect on the localized corrosion resistance of steel reinforcements in saturated Ca(OH)2 solution. A perfect correlation with the evolution of the acoustic activity is also demonstrated. Thus, it was significantly possible to highlight the acoustic signature of depassivation damage and to chloride concentration.