Steel corrosion is one of the main problems associated to the performance of reinforced concrete structures. In particular, it is well-known that chloride-rich environments and chloride-contaminated aggregates accelerate the reinforcements’ corrosion. To minimize risks, inorganic compounds dissolved in water are frequently used as inhibitors, with an excellent cost to benefit ratio. This investigation centers in the performance of two inhibiting agents: sodium phosphate (Na₃PO₄) and sodium nitrite (NaNO₂), evaluated in concrete pore simulating solutions (PSS). Even when they both attain high levels of corrosion inhibition, their mechanism is different. Thus, the impact of these mechanisms on the electrical properties of the passive film will be analyzed.

 The inhibitor concentration is fixed at 0.3 mol L^-1, the inhibitor/chloride ratio is kept at 1 and the pH of the PSS is 13. Anodic polarization curves, weight-loss tests, electrochemical impedance (EIS) and Mott-Schottky plots were used to correlate the performance of these inhibitors to the pitting resistance and to the electronic properties of the passive films grown at open circuit potential (OCP) for 24 h and 7 days. Changes in composition were analyzed by X-ray photoelectron spectroscopy (XPS). Nitrite ions behave as anodic type inhibitors in alkaline solutions contaminated with chloride ions. This type of inhibitor is supposed to promote the formation of a more compact and less defective outer layer, finally leading to an increase in the pitting resistance [1]. In contrast, phosphate ions behave as mixed type inhibitors. Phosphates are proposed to favor the development of an outer phosphate layer, increasing the Fe₃O₄ content in the inner duplex layer. Fe₃O₄is quite insoluble and known to inhibit iron dissolution. In addition, the phosphate layer could delay oxygen diffusion through the duplex interface, hindering the consumption of the electrons produced by the anodic reaction taking place at the metal–film interface [2]. Both inhibiting agents, nitrites and phosphates, show inhibition levels higher than 99%.

As regards the electronic properties, the positive slope in the Mott–Schottky plots is characteristic of n-type semiconductors. The stability of films during MS tests was evaluated by measuring the capacitances in both forward and reverse scan directions. Only a very small hysteresis was observed within the potential window used in these tests. The donor density, calculated from the slope of the Mott–Schottky, together with the passive film resistance, calculated from EIS and the inhibition percentage from weight-loss, are summarized in Table 1.

The presence of chlorides has a marked effect on the Mott–Schottky plots, as compared with the response of passive films grown on steel immersed is non-contaminated PSS for 24 h at OCP. The donor density increases in one order of magnitude when chlorides are incorporated to the PSS. The donor density increases slightly more in the presence of nitrites and even more in the presence of phosphates. In every case, it almost doubles when the immersion time is extended from 1 to 7 days.

 It is generally accepted that the increment in the donor density enhances the susceptibility of the passive film to pitting corrosion [3]. However, our results demonstrate that the donor density also increases when inorganic inhibitors are present, showing that this parameter cannot be taken as a direct proof of film deterioration and risk of failure.

1. M.B. Valcarce, M. Vazquez, Carbon steel passivity examined in alkaline solutions: the effect of chloride and nitrite ions; Electrochimica Acta 53 (2008), p. 5007–5015.
2. L. Yohai, M. Vázquez, M.B. Valcarce, Phosphate ions as corrosion inhibitors for reinforcement steel in chloride-rich environments; Electrochimica Acta 102 (2013), p 88– 96
3. Z. Dong, W. Shi, G. Zhang, X. Guo. The role of inhibitors on the repassivation of pitting corrosion of carbon steel in synthetic carbonated concrete pore solution. Electrochimica Acta 56 (2011) 5890– 5897