The effect of calcium nitrite on the chloride-induced corrosion of steel in concrete

Laboratory tests were undertaken to assess the effect of calcium nitrite-based corrosion inhibitors in raising the chloride threshold level (CTL) for the corrosion of steel embedded in concrete and hence the time to corrosion initiation. Specimens with a centrally located steel rebar were cast with 0, 1·23, 2·5 and 5·0% nitrite by weight of cement and were cured for 4 weeks. Then they were immersed in 4M sodium chloride solution and the galvanic current between the embedded steel and an external cathode was monitored. The CTL of nitrite-free specimens was typically doubled and trebled by 2·5% nitrite and 5% nitrite, respectively. With and without nitrite, the CTL was reduced with an increase in the air void content at the steel–concrete interface. The time to corrosion depended on the cement content. Use of low cement content (282 kg/m³) increased the CTL as the dosage of nitrite in concrete increased, but did not extend the time to corrosion because it accelerated chloride penetration. For a richer mix (350 kg/m³) the time to corrosion increased with the dosage of calcium nitrite. After corrosion initiation, the corrosion rate for specimens containing calcium nitrite was typically two to three times higher than for nitrite-free specimens.