Localized Corrosion Analysis Using Complex Potential Distribution Ratio as a New Transfer Function

Furukawa, Noriyuki

An electrochemical impedance spectroscopy (EIS) is powerful technique that has been used for the understanding of the corrosion mechanisms. In order to investigate the surface heterogeneities attributed to the localized corrosion, Lillard et al.¹⁾ have developed a local electrochemical impedance spectroscopy (LEIS). Because the potential differences related to the iR drop in vertical direction to the working electrode (WE) can be measured with dual microprobes in this method, the local impedance on WE can be calculated.

In the present study, our group developed a Complex Potential Distribution Ratio (CPDR) as a new transfer function to investigate the potential distributions in parallel direction to the WE. Assuming that the surface is composed of the dissolution site and oxide film site, namely, an interfacial impedance at dissolution site is lower than that at oxide film site, the current density of each site is different. In this case, the potential differences were observed in parallel direction to the WE near these sites, indicating that the iR drop of each site is different. The CPDR, Ξ, is expressed by the following equation.

Ξ = ΔV_p / ΔV_applied (1)

In this equation, the ΔV_p is the ac-potential differences between the potentials above the dissolution site, V₁, and oxide film site, V₂, in parallel direction to WE and the ΔV_appliedis the ac-potential differences between WE and reference electrode in the bulk solution. In this case, the dual probes are set above the dissolution site and oxide film site to measure the ΔV_p. The impedance (Z) is expressed by the following equation.

Z = ΔV_applied / ΔI_CE (2)

In this equation, the ΔI_CE is the ac-current flowing into CE from WE.

In the present study, the Ξ and Z simulations were performed by using the equivalent circuit based on the concept. The interpretation of calculated spectra was discussed, and we applied the proposed method to the dissolution of metals.

.

Reference:

1) R. S. Lillard, P. J. Moran, H. S. Isaacs, J. Electrochem. Soc., 139 (1992) 1007-1012.