Role of fluoride in accelerating corrosion and pitting of steel in concrete environments

The studies related to corrosion and pitting behaviour of mild steel rebars have been performed in contact with 0.01 N NaOH, saturated lime water and cement slurry having different concentrations of fluoride ions. Weight loss, electrochemical DC cyclic polarisation and polarisation resistance, surface topographic and X-ray diffraction (XRD) techniques have been used to investigate the effects of fluoride ion on the corrosion behaviour of the material. Under all the conditions of the exposure it is observed that a low content of fluoride (less than or equal to 25 ppm) in the corrodent has deleterious action on the performance of the steel, whereas at its higher content (greater than or equal to 100 ppm) the ion has rather an inhibiting effect on corrosion rate. XRD studies of the corrosion product accumulated on the steel surface in the presence of fluoride ion indicate the predominance of magnetite oxide phase. Accelerating effect of the ion is due to depolarisation of cathodic reaction of the corrosion process. Observations show that in accelerating range of fluoride (less than or equal to 25 ppm) it has almost double corrosive effect than that noted for equal concentration of chloride ion.