Effect of nitrites as inhibitors for steel corrosion, examined in alkaline solutions with various degrees of HCO3-/CO3= contents

M.B. VALCARCE; C. LOPEZ; M. VAZQUEZ

Cancun, Mexico

Congreso; XVII Internacional Materials Research Congress y VI NACE Mexican section Corrosion Congress.; 2008

Nitrite ions are well-known corrosion inhibitors of wide application in the field of steel corrosion in concrete. This study aims at investigating carbonated concrete pore simulating solutions representative of structures placed in marine environments: slightly alkaline solutions (pH=9) contaminated with chloride ions ([Cl-]=0.1 mol l-1), containing NaNO2 0.2 mol/l as inhibiting agent. It has been suggested that the CO3= and HCO3- contents could influence the properties of the surface layer.Thus, two HCO3-/CO3= ratios are tested in these simulating pore solution. One of them has a so-called low degree of carbonation (LDC) with Na2CO3 0.0015 mol l-1 and NaHCO3 0.03 mol l-1, while the other has a higher degree of carbonation (HDC) with Na2CO3 0.015 mol l-1 and NaHCO3 0.3 mol l-1. Changes in key electrochemical parameters are investigated on steel electrodes, using electrochemical techniques such as polarization curves and electrochemical impedance spectra (EIS), together with weight loss evaluation. In LCD solutions the steel surface undergoes general corrosion. The presence of 0.2 mol l-1 nitrite ions restore the surface passivity, stabilizing an oxide film mainly composed of iron oxides. In the presence of nitrite and chloride ions a passive layer develops on the electrode surface. The composition of this layer is similar to that found in the absence of chloride ions but pitting attack is observed. The nitrite dosage used is effective in regaining surface repassivation after pitting attack In HCD solutions the steel surface remains passive. The presence of 0.1 mol l-1 chloride ions induces pitting. But, when nitrite and chloride ions are simultaneously present the pitting potential moves to much more positive values compared with those of LCD solutions. However, once the pitting process starts, no sign of repassivation can be detected.