Examination of the mechanism of corrosion inhibition by calcium nitrite and calcium nitrate-based admixtures in concrete

Abstract

Chloride-induced corrosion is responsible for the deterioration of many reinforced concrete structures such as roads, bridges, parking garages and marine structures. Calcium nitrite (Ca(NO2)2) inhibitors are increasingly becoming the weapon of choice to address this problem. However, while many studies have shown these inhibitors to be effective in the short term, the lack of understanding of the mechanism of inhibition, and hence of their effectiveness after the many years normally taken for chlorides to penetrate the concrete cover, has made many jurisdictions wary of their use. Previous investigations show mixed results regarding the effectiveness of this inhibitor and most research within a concrete environment has simply determined the time to initiate corrosion and the subsequent corrosion rate without addressing the chemical reaction that may be occurring between the concrete and the inhibitor. A hypothesis for the mechanism by which calcium nitrite delays the onset of chloride-induced corrosion of reinforcing steel in concrete involving these reactions is proposed which is based on previous work by the researcher.

In the present investigation, a series of experiments were conducted to examine the effect of two commerically available calcium nitrate inhibitors (designated CN1 and CN2, respectively) and laboratory grade calcium nitrate. Studies of steel in synthetic concrete pore solution and of cement paste were conducted. Two types of cement were used: white Type 50 and grey Type 10 (Ordinary Portland Cement - OPC). Corrosion measurements in synthetic concrete pore solution found that none of the inhibitors inhibited or delayed the onset of corrosion. These tests also showed that a solid precipitate containing chlorides forms in all solutions, with the highest chloride concentration being in the precipitate from the solution containing CN1. Raman spectroscopy was used to determine the composition of the passive film formed on samples immersed in the control solution (Control) and CN1 solution both with and without exposure to chlorides. CN1 was found to have no beneficial effect to the passive layer formed.