As application of an electric field to the surface of an electrolytic medium can indirectly polarize a metal, an indirect impedance measurement has been proposed as a means to assess the properties of a conductive element contained within a resistive medium without a direct connection to the conductive element [1-3]. The indirect impedance technique was studied by Zhang et al.  to determine the location and the condition of steel rebar within concrete slabs. They were able to determine qualitatively that the measured surface impedance was a function of the corrosion state of the steel as well as the resistance of the concrete. However, extracting the true impedance of the conductive element from the impedance of the whole system was difficult.
Andrade et al.  used results of finite element models to propose an analogue circuit that accounts for the polarization behavior of the steel and the properties of the mortar in which the steel is embedded. They determined that a suitable circuit must contain a parallel, as well as, a series resistance term associated with the resistivity of the concrete in relation to the impedance of the steel.
Our work has shown that the nonuniform current distributions and ill-defined geometries associated with the grout gives rise to a complex ohmic impedance associated parallel to the steel and another complex ohmic impedance that is in series with the steel. The complex ohmic impedance of the steel limits the indirect impedance measurement to providing only a qualitative assessment of the corrosion of bridge tendons.
- C. Andrade, I. Martnez, “Metal corrosion rate determination of diﬀerent solutions and reinforced concrete specimens by means of a noncontacting corrosion method,” Corrosion 66 (2010) pg. 056001-056001.
- P. J. Monteiro, H. Morrison, Non-destructive method of determining the position and condition of reinforcing steel in concrete, 1999. US Patent 5,855,721.
- M Keddam, X. R. Novoa, V. Vivier, “The concept of floating electrode for contact-less electrochemical measurements: Application to reinforcing steel-bar corrosion in concrete,” Corrosion Science, 51 (2009) pg. 1795-1801.
- J. Zhang, P. J. Monteiro, F.H. Morrison, “Noninvasive surface measurement of corrosion of reinforcing bar in concrete: Part I Experimental results”, ACI Materials Journal, 98 (2001) pg. 116-125.
- C. Andrade, J. Sanchez, I. Martinez, N. Rebolledo, Analogue circuit of the inductive polarization resistance,” Electrochimica Acta 56 (2011) pg. 1874-1880.