Obtaining Impedance Information on the Steel-Concrete Interface Available to Purchase

Corrosion of steel in concrete can be assessed using a number of techniques. In the present work, impedance spectra were obtained by transforming 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 while the potential transient was recorded. Furthermore, the low-frequency real axis intercept was given by the area under the potential transient divided by the charge in the perturbation. Parameters obtained from this analysis were compared with those determined by curve fitting analysis of potentiostatic and coulostatic transients using a simple model of the interface. Differences in the polarization resistance (R_p) values were generally within the error attributed to the R_p method of corrosion rate (I_corr) determination. The range of interfacial capacitance (C) was dependent on the method used. However, in all cases, lower 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 from a greater position dependence of C and R_p when the partial breakdown of the passive film occurred.