OBTAINING IMPEDANCE INFORMATION ON THE STEEL-CONCRETE INTERFACE

Corrosion of steel in concrete can be assessed using a number of techn iques. In the present work, impedance spectra were obtained by transfo rming time domain coulostatic transient data into the frequency domain . This minimized the risk of perturbation-induced mass transfer in the electrolyte affecting the results as no perturbing current flowed whi le the potential transient was recorded. Furthermore, the low-frequenc y real axis Intercept was given by the area under the potential transi ent divided by the charge in the perturbation. Parameters obtained fro m this analysis were compared with those determined by curve fitting a nalysis of potentiostatic and coulostatic transients using a simple mo del of the interface. Differences in the polarization resistance (R-p) values were generally within the error attributed to the R-p method o f corrosion rate (I-corr) determination. The range of interfacial capa citance (C) was dependent on the method used. However, in all cases, l ower values were estimated when the passive film was intact. Impedance spectra obtained on actively corroding specimens were more depressed than those obtained on passive specimens. This might have resulted fro m a greater position dependence of C and R-p when the partial breakdow n of the passive film occurred.