Use of polarization curves in the study of the corrosion mechanism of the steel in concrete

The corrosion behaviour of steel rods embedded in concrete specimens of different compositions, was examined using various techniques: polarization resistance, resistivity, a.c. impedance and corrosion potential. The first three methods gave results which indicated very low corrosion rates (0.5 - 1.5 μm/y). Moreover, the corrosion potential values varied in a very wide range, from -1 V vs. Ag/AgCl reference electrode, to about zero. This behaviour may be easily interpreted: where oxygen cannot be renewed, steel becomes active, thus showing low potential values, but at the same time it corrodes at a very low rate. Where oxygen diffuses easily enough, the passivation state is clearly shown by the high potential values and, of course, by the low corrosion rate. Starting from the above considerations, it was considered useful to determine the polarization curves relevant to two bars situated in two extreme situations. It was observed that, starting from -1 V and increasing the potential value up to +1 V with a scanning rate of 10 μV/s, two very different trends were obtained. Since it was not clear why such phenomena occurred, a new test was planned in order to quantify the influence of the ohmic drop between the working and reference electrodes. With a suitably designed specimen, the polarization of a bar was imposed, meanwhile measuring the potential at various distances from the bar. Other curves were obtained with a bar immersed in saturated lime in a glove box, with and without oxygen. The results unequivocally showed that the ohmic drop only influences the curve at current densities quite high (>0.5 μA/sqcm) and therefore cannot be considered the cause of the trend observed. Moreover it was confirmed, as was easily envisaged, that a lack of oxygen does not hinder the formation of the passive film, as soon as the potential is raised. Finally, the behaviour of the original specimens was examined again. The curves were obtained potentiostatically, with increasing steps of 5 mV, and with a long time gap between each step