High Frequency Impedance Dispersion of Corroding Galvanized Steel in Soils

Electrochemical Impedance Spectroscopy at the open circuit potential of galvanized steel buried in soil showed a marked region of near constant imaginary component element (CIE) behavior [1], illustrated in Figures 1 and 2, representative of results from multiple samples. The near-CIE region spans frequencies ranging from the practical high frequency end (~10 kHz) to ~ 1 Hz. Figure 1 corresponds to a medium size 1.2 cm diameter rod specimen 20 cm long (surface area 60 cm²) inside a soil-containing cylindrical cell 10 cm in diameter. Figure 2 corresponds to a much larger (4060 cm²) rod mesh specimen of the same material, placed inside a large soil box 2.5 m x 1.2 m x 0.9 m. The soil in both cases was typical backfill used in mechanically stabilized earth walls, with 40% of the pore space filled with fresh water and contaminated with ~100 ppm of chloride ions, resulting in a medium of ~ 7000 ohm cm resistivity. In both cases the counter electrode was large enough to result in even excitation current distribution on the working electrode. The reference electrode was a solid metal/metal oxide activated Ti rod placed near the working electrode surface. The observation of near-CIE behavior in both large and small cells of different geometries, and preliminary calculations of possible effects, suggest that macroscopic current distribution in a resistive medium due to counter electrode configuration and placement is not the primary cause of this phenomenon. The low frequency end of the spectrum (1mHz to 0.1Hz) can be well fitted in both cases to a simple constant phase element (CPE) - resistance (R) parallel combination, as shown in both figures. Treating R as a polarization resistance and using a Stearn-Geary constant value B=22 mV [2] yielded in both cases apparent galvanized layer corrosion current density in the order of i_CORR ~0.8 μA/cm² for day 90 of exposure and ~0.4 μA/cm² for day 220, consistent with typical behavior of buried galvanized steel in similar conditions and ages [3]. That analysis implicitly assumes that the high frequency part of the spectrum represents the series combination of two elements. The first is a solution resistance, responsible for the impedance up to about 10 kHz and found to be approximately consistent with the cell dimensions, reference electrode placement, and soil resistivity value. The second is an unknown impedance component Z_UNK responsible for the near-CIE behavior. The overall analog equivalent circuit is then as shown in Figure 3a. Figure 3b shows an alternative combination where an element Z*_UNK is placed in parallel with a polarization resistance R*_p. This alternative is likely to fit the data successfully as well but results in a sometimes appreciably lower value of i_CORR, with consequent uncertainty in the interpretation of the results for nondestructive corrosion rate evaluation. It is therefore of interest to elucidate the cause the near CIE behavior.  Work in progress to explain that behavior is presented including characterization of the Zn oxide film on the surface of the specimens, to assess its electronic properties especially in regard to the effect of localized states on the frequency response of the film, as done in other systems where significant frequency dispersions resulted [4]. In addition, while as indicated above macroscopic current distribution effects do not appear to be a primary cause of the behavior, calculations are in progress to assess the possible effect of microscopic current distribution in the corrosion product/sand particle aggregate immediately next to the galvanized surface.

References

[1] “Impedance of reinforcing steel with disbonded dual polymer–zinc coating”,  K. Lau  and  A. Sagüés, Electrochimica Acta, Vol. 56, pp. 7815-7824 (2011)

[2] "Corrosion of Galvanized Strips in Florida Reinforced Earth Walls", A.A. Sagüés, R. Scott, J. Rossi, J. Peña and R.G. Powers, ASCE's Journal of Materials in Civil Engineering, Vol. 12, p.220, 2000.

[3] Victor Elias. “Durability/Corrosion of Soil Reinforced Structures”. Federal Highway Administration Report N. FHWA-RD-89-1986, 1990

[4] L.V. Taveira et al. “Impedance Behavior of TiO₂ Nanotubes Formed by Anodization in NaF Electrolytes”. Journal of The Electrochemical Society, 155 (6) C293-C302, 2008