Effects of corrosion inhibitors on the critical chloride threshold of thermo-mechanically treated (TMT) steel

The critical chloride threshold (Clth) is defined as the concentration of chloride ions required at the surface of embedded reinforcement to initiate corrosion. This is an important parameter in determining the service life of reinforced concrete structures exposed to chloride environments. An increase in Clth indicates an increase in corrosion resistance and service life of structures. Now-a-days, corrosion inhibitors (say, calcium nitrite and bipolar inhibitors) are used to increase the corrosion resistance of reinforced concrete structures exposed to chloride environments. Nevertheless, the manufacturers recommend a wide range of dosage of corrosion inhibitors (e.g., 10 to 30 litres of calcium nitrite inhibitor per m3 of concrete). In addition, the quality control may be poor leading to a significance difference between the prescribed and actual concentrations of corrosion inhibitor used. Also, the effect of changes in the dosage of various types of corrosion inhibitors on Clth is not well understood. In addition, corrosion resistant steels (say, TMT steel) are also widely used with enhanced corrosion resistance in mind. However, very little information is available on the Clth of TMT steels used in India. This paper will present the Clth of TMT steel embedded in mortar with various dosages of two types of corrosion inhibitors. The experimental program used a recently developed accelerated chloride threshold test method in which the corrosion rates are measured by linear polarization resistance technique. In this test, the potential application and corrosion rate measurement are performed in a cyclic manner until the corrosion initiation occurs. After the initiation of corrosion, the test specimen is autopsied and the chloride concentration in the mortar adjacent to the steel surface is determined. This is recorded as Clth values of the embedded steel specimen. Using this method, the Clth of TMT steel embedded in mortar with five different dosages of the calcium nitrite [Ca(NO2)2] and bipolar corrosion inhibitors were determined. The various dosages tested are R-20%, R-10%, R%, R+10%, and R+20%, where R is the average dosage recommended by the manufacturer.