INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Correlation between donor density and inhibition efficiency by phosphate and nitrite ions on carbon steel
Autor/es:
M.B. VALCARCE; M. VÁZQUEZ; M.A. FRONTINI
Lugar:
La Haya
Reunión:
Congreso; 67th Annual Meeting of the International Society of Electrochemistry; 2016
Institución organizadora:
International Society of Electrochemistry
Resumen:
Nitrite ion is an anodic type inhibitor of carbon steel in alkaline media contaminated with chloride ions. This type of inhibitor favors the oxidation of Fe2+ to Fe3+ producing a more compact and less defective outer layer rich in insulating Fe2O3 and increasing the pitting resistance to chloride ions[1]. In contrast, phosphate ions can act as a mixed type inhibitor. Phosphates could favor the development of an outer thin phosphate layer, increasing the Fe3O4 content in the inner duplex layer. Fe3O4 is a good conductor; it is 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]. This investigation compares sodium phosphate (Na3PO4) and sodium nitrite (NaNO2) as corrosion inhibitors for construction steel. All the tests are carried out in a solution simulating the composition of the pores in chloride-contaminated concrete. The inhibitor concentration is 0.3 mol L-1 and the pH of the synthetic medium that simulates concrete is 13, while the inhibitor/chloride ratio is kept at 1.Anodic polarization curves, electrochemical impedance (EIS) and Mott-Schottky plots were used to correlate the performance of these inhibitors to the pitting resistance and to the semiconductive properties of the passive films grown at free potential for 24 h. The results indicate that pitting attack is inhibited by both inhibitors. However, the EIS results show differences in the oxide resistance passive layer (Ro). The Ro values decrease in the presence of phosphate ions due to the presence of a more conductive passive layer, probably mainly composed by Fe3O4. Pitting susceptibility has been correlated with the semiconductive properties of passive films on carbon steel in alkaline media [3]. It is generally accepted that the lower the donor density (Nd), less susceptible is the passive film to pitting corrosion. Mott-Schottky plots indicate that in all the conditions tested the passive layer behaves as n-type semiconductor. However, the capacitance values are always higher in the presence of phosphate ions due to the presence of higher concentration of Fe2+ at the passive film. Mott-Schottky plots follow a non-lineal response and this could be related to the presence of shallow and deep donor species, in non-discrete distribution. The Nd values were obtained by regression from the lineal zone from 0 to 0.5 V. The Nd value decreases in the presence of nitrite ions with an increment in the pitting potential following the expected correlation between the semiconductor properties and the pitting resistance. However, this behavior is not observed with phosphate ions. With phosphate ions, an increment of Nd is associated to the lack of pitting attack. This fact could be associated to the incorporation of phosphates to the passive film, since phosphorous with five valence electrons could give electron excess to act as donor and increasing Nd. So, when the inhibitor affects the composition of the passive film Nd can increase even for very effective pitting inhibitors such as phosphates.pH=13ECORR / mVHg/HgOEPIT / mVHg/HgOεNd / cm-30.3 mol/L Cl--229 ± 516 ± 6414 (Fe2O3)20.3 x 1020NO2- + Cl--268 ± 90658 ± 55 14 (Fe2O3)16.0 x 1020PO43- + Cl--279 ± 77----- 20 (Fe3O4)24.0 x 1020 [1] M.B. Valcarce, M. Vazquez. 53 (2008) 5007?5015.[2] L. Yohai, M. Vázquez,1, M.B. Valcarce. Electrochim. Acta 102 (2013) 88? 96[3] J. Williamson, V.J. Azad and O. B. Isgor. J. Electrochem. Soc. 162(12) C619-C629 (2015).