Electrochemical Impedance Spectroscopy (EIS) has been known as a powerful tool for analyzing electrode reaction mechanism. In this review the history of EIS, the definition of electrochemical impedance, the general concept of transfer function, and the measurement method of electrochemical impedance using a transfer function analyzer are first introduced. Then, analytical methods of impedance spectra using electrical equivalent circuits and the kinetic theory of electrochemical reactions are stated. Finally, the future scope of EIS is briefly given.
The instrument and experimental method of electrochemical impedance were described. The simple criteria to select the Nyquist plot or Bode plot was revealed. Some equivalent circuits were presented and its analytical procedure was shown. The physical meaning of inductive behavior was explained by the calculation of Faradaic impedance. Finally, the 3-D plot (real, imaginary and time axes) to compensate the impedance, which does not have time stability, was introduced.
Electrochemical monitoring of corrosion rate is usually carried out by continuous measurement of polarization resistance, the reciprocal of which is theoretically proportional to the corrosion current density in some cases. In Ac impedance method, which is often employed for corrosion monitoring, impedance at low frequency limit is defined as the polarization resistance. Physical meanings of the polarization resistance depend on the corrosion mechanism. In the first half of this paper, the impedance characteristics of three different corrosion systems under charge transfer control, under diffusion control and under mixed charge transfer-diffusion control and the physical meanings of polarization resistance obtained from the low frequency limit are described. In the monitoring of atmospheric corrosion, problems of high ohmic drop and uneven current distribution over working electrode are often encountered because it proceeds under a very thin electrolyte layer. Electrochemical impedance spectroscopy (EIS) is very effective in the atmospheric corrosion monitoring. Because the solution resistance can be determined from high frequency impedance and the ohmic drop can be compensated by subtracting the solution resistance from polarization resistance obtained from low frequency limit. The EIS enable us to clarify how degree the applied voltage (response current) distributes over working electrode. The details of impedance characteristics under a thin electrolyte layer are interpreted in the latter half of this paper.
The corrosion monitoring of reinforcing steel bars in concrete is presented as an attractive application of a-c impedance measurement methods. This paper reviews a-c impedance characteristics of reinforcing steel bars in concrete, its measuring methods and related problems on real concrete structures.
In order to establish a diagnostic technique to check the status of rust formed on weathering steel, the corrosion protection mechanism by rust was studied using EIS technique. The impedance appeared in intermediate frequency range was attributed to ion transfer through the rust, which was concluded as a quantitative measure for its protective properties. Based on these findings, a compact instrument was developed to facilitate in-situ monitoring the surface condition of existing weathering steel structures.
The effect of steel fiber (SF) addition to concrete on the corrosion behavior of steel reinforcement was studied by immersion corrosion tests in a 3mass% NaCl solution for 363 days. The tests were conducted on steel rod/mortar specimens with different SF content (0-2vol%), and corrosion potentials and electrochemical impedance of the specimens were measured during the tests. Corrosion potentials of the specimens were not so changed by the SF content. Electrochemical impedance measured at corrosion potentials was analyzed to get charge transfer resistance (Rct) using an equivalent electrical circuit with the Rct, Warburg impedance and a constant-phase element. The corrosion rate of steel rod in mortar estimated from the reciprocal of the Rct decreased with increasing of SF content in mortar, and correlated well with the rust area of steel rod measured after immersion corrosion tests. This shows that the electrochemical impedance measurement is the useful diagnosis method of corrosion of steel in concrete. The corrosion inhibition of steel rod in mortar by the SF addition was thought to be attributed to the consumption of dissolved oxygen by the corrosion of SF in mortar.
The susceptibility of pure copper to stress corrosion cracking (SCC) depending on pH and Cu2+ concentration in acetic acid solutions has been studied by examining the nominal stress-strain curves obtained by the slow strain-rate technique. The results obtained are summarized as follows: The clear SCC was observed under the solution condition of pH not less than 5 and of Cu2+ concentration not less than a few thousand ppm. This condition is almost identical to that which accelerates the formation of oxide film covering the surface of the copper specimen. This strongly supposes that the SCC mechanism can be interpreted in terms of the tarnish film rupture model.
SUS 347H stainless steel specimens and those coated with an Al2O3 or Al2O3/TiC film by a plasma CVD were subjected to corrosion test in a simulated refuse incinerator environment at 823K for 180ks (50h). Thermal stability of the coatings was evaluated by annealing in Ar at 823K for 1.2 or 180ks. The degree of the defect generated by the annealing was assessed by polarization tests. The Al2O3 coating decreases the corrosion mass loss a little and the Al2O3/TiC dual-layer coating significantly decreases it. However, the implantation of Zr ions to the coated specimens does not affect the protectiveness of the Al2O3 coating and increases the corrosion mass loss to some degree for the Al2O3/TiC dual-layer coating.
In order to discuss how to use internal standard quantitative X-ray diffraction analysis of rust, recently proposed techniques which utilized α/γ, α/γ* ratios for corrosion stability evaluation of weathering steels were assessed. Reported correlation between α/γ (or α/γ*) values and corrosion rates on good exposure test pieces obtained at limited number of sites in industrial and rural atmospheres, did not always stand when this technique was applied to the cases of much wider environmental ranges. Thus, a further improved method for approximate assessment of inactivity of rust, in which both (α+am)/γ* value and thickness of rust are taken into account, is proposed based on the discussion in terms of rust formation processes, corrosion protection mechanisms, and actual performances of weathering steels.
With the aim of studying the kinetics and mechanism of environmental assisted cracking such as stress corrosion cracking and delayed fracture in small diameter rod specimens, we developed a new signal processing method of the guided waves monitored by resonant-type AE sensors mounted on the outer surface of the rod. Source location of AE events can be identified by utilizing the ratio A/S of the maximum amplitude A of zero-th order anti-symmetric mode to the peak amplitude S of the first zero-th order symmetric mode waves. Here, the guided waves must be measured by four AE sensors mounted on the surface in 90° step at the same distance from the source. Fracture kinetics, limited to only the Mode-I fracture, were estimated by the So-packet waveform simulation by utilizing the overall transfer function determined as a response to a small compression-mode PZT element excited by a pulse. This simulation method, though needs some special techniques and depends on the frequency of the element, enables us to estimate the fracture kinetics with source rise time less than 1μs. The proposed method was adopted to tetrathionic SCC of sensitized SUS304 stainless steel rod, and revealed that fast cleavage-type fractures with rise time of 0.4μs to 2.5μs occurred during the SCC progression.