Galvanized coatings are traditionally used for protecting steel rebars in concrete. In some cases, the galvanized steel rebars are also protected using calcium nitrite in the concrete mix. Data from galvanized iron immersed in model solutions of Ca(OH)2, reveals that calcium nitrite does not prevent zinc corrosion in the presence of chloride ions.

From this study of the corrosion rate of galvanized steel in concrete samples in the presence and absence of chlorides, data indicates that calcium nitrite is ineffective in protecting the zinc from dissolving in concrete. Hence, this study aims to show that the use of calcium nitrite to protect galvanized steel is neither economical nor effective.

This studies the inhibitive effect of nitrite ions in protecting galvanized steel. Potential-time data from the galvanized rebar in concrete indicate that calcium nitrite does not aid the formation of a passive layer on zinc in the absence of chloride ions. This is also confirmed by the corrosion rate data. Increasing the concentration of calcium nitrite in the concrete mix has no effect on the corrosion rate of zinc. Similar behavior is also seen in the presence of chloride ions. However, calcium nitrite aids in the formation of a passive film on carbon steel. Hence, once the zinc has dissolved the presence of nitrite ions will protect the underlying steel.

The rebars used for testing were centrally emdedded in concrete as shown in Fig. 1. Four sets of rebars were used in the studies – three based on carbon steel of dia 0.375² (0.9525 cm), 0.5² (1.27 cm), 0.75² (1.905 cm) and one of galvanized steel of dia 0.5² (1.27 cm and 3 mils thick Zn). Concretes with different concentrations of inhibitor in the concrete mix were also prepared. The water to cement ratio was kept constant at 0.5.

The potential values given in Fig. 2a indicate the thermodynamic tendency for iron to corrode. Calcium nitrite is an anodic inhibitor, which acts by promoting the formation of a barrier film through the dissolution of iron in the pore solution. Nitrite ions promote the formation of Fe(OH)+ species, which helps passivate the surface of iron. Fig. 2b shows that increasing concentration of inhibitor is seen to reduce the corrosion rate. These results in conjunction with the potential-time data of Fig. 2a, clearly shows that calcium nitrite aids in the formation of a passive film on iron.

Fig. 3a indicates either the presence of a passive film or active corrosion. The data on Fig.3b clearly shows severe active corrosion of zinc both in the presence and absence of inhibitors. In our previous study, we had seen passivation of Zinc in Ca(OH)2 solutions. But these results are not to be seen for Zinc in concrete.

All the samples are kept in concrete containing 5 gal./yd.3 of calcium nitrite. From the plot, it is clear that the different carbon steel samples exhibit the same potential as a function of time indicating the formation of a stable passive film on the rebar surface. The potential for the galvanized steel becomes more positive with time indicating dissolution of zinc and exposure of underlying iron. Fig. 4b shows that the thicker concrete cover (.375” carbon steel) contributes to the low corrosion rate.

In Fig.5a the corrosion potentials for the samples with inhibitor are more positive as compared to the one with no inhibitor. This indicates that calcium nitrite aids in the formation of the passive Fe2O3 at the rebar/electrolyte interface. From Fig. 5b, it is clear that the increase in corrosion rate for the samples with inhibitor after 36 months corresponds to an increase in the chloride concentration at the concrete/rebar interface beyond the threshold level. It is seen that the inhibitor delays the increase in corrosion rate.

The data here conclusively shows that nitrite does not aid in the formation of a passive film both in the presence and absence of chloride ions in the test solution.

Surface analysis of the sample indicates the presence of Cl, Si, Al, O and Ca on Zn. The presence of Si and Al can be attributed to the concrete cylinder. The presence of chlorides indicates that the corrosion product is zinc hydroxy chloride. The increase in volume due to the formation of zinc corrosion products can produce additional stresses in the concrete structure. Hence, use of galvanized coatings in high chloride environments could be detrimental to the health of the structure.

Reference- http://www.che.sc.edu/