Florian Mansfeld

Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings

A discussion of the requirements for hardware and software necessary for collection and analysis of electrochemical impedance spectroscopy data for polymer coated metals is presented. Most authors agree that a simple model can describe the frequency dependence of impedance spectra for polymer coated metals exposed to corrosive environments. The water uptake of the coating can be estimated from the time dependence of the coating capacitance Cc, The pore resistance Rpo depends both on the resistivity ρ of the coating and the disbonded area Ad. The polarization resistance RP of the corroding area under the coating and the corresponding capacitance Cdl both depend on Ad. The breakpoint frequency method is discussed in detail and the dependence of the breakpoint frequency fb on ρ and Ad is derived. In addition to fb other parameters can be obtained which depend on the ratio Ad/ρ or only on Ad or ρ. Since these parameters can be obtained at frequencies exceeding 1 Hz without the need for an analysis of the impedance spectra in the entire frequency region, this approach is considered especially useful for corrosion monitoring. The concepts proposed for the analysis and interpretation of EIS data for polymer coated metals are illustrated using data for Al alloys, Mg and steel exposed to NaCl. For an alkyd coating on cold rolled steel the time dependence of Ad and ρ during exposure to 0.5 m NaCl has been determined qualitatively using the modified breakpoint frequency method.

Determination of corrosion rates by electrochemical DC and AC methods

Abstract A brief review is given of DC and AC techniques which can be used to determine corrosion rates. The advantages and disadvantages of the extrapolation method of Tafel lines, polarization resistance measurements and impedance measurements are discussed. In particular it is shown that the intercept of the capacitive impedance loop with the real axis of the complex impedance diagram does not correspond to the charge transfer resistance of complicated corrosion systems exhibiting several time constants in the capacitive and inductive loop(s). Therefore, the correlation between this intercept value and the corrosion rate is not generally valid. Experimental data are presented for two types of iron (Marz and Johnson-Matthey) and 4340 steel in de-aerated and aerated 0.5 M H 2 SO 4 and 1 M HCl in the absence and presence of the following inhibitors: triphenylbenzylphosphonium-chloride (TPBP + ), propargylic alcohol (PA), 2-butyne-1,4 diol (BD) and hexynol (H). Corrosion rates have been determined by applying DC and AC measurements and solution analysis by atomic absorption. The results, in the absence of inhibitors and in the presence of TPBP + , show that the corrosion rate is unequivocally correlated to electrochemical DC data and to the extrpolated value of the inductive loop of AC measurements to zero frequency. However, in systems containing the other inhibitors the corrosion rate cannot be correlated to the polarization resistance because of an irreversible desorption of the inhibitor in the close vicinity of the corrosion potential, which gives an unpolarizable behaviour of the system in the anodic range. The relatively low inhibition efficiencies in the presence of PA and BD can be explained by a superimposed fast electrochemical reduction of the additives. Moreover, it is shown that the inhibition efficiency depends on the hydrodynamic conditions. The results of these investigations show that electrochemical DC and AC techniques can successfully be applied for determination of metal corrosion rates in systems that have simple corrosion kinetics. In more complicated systems knowledge of the details of the mechanism is necessary for interpretation of the experimental data.

The corrosion protection afforded by rare earth conversion coatings applied to magnesium

The corrosion protection afforded by cerium, lanthanum and praseodymium conversion coatings formed on pure magnesium and a magnesium alloy, WE43, has been studied using dc polarisation and ac impedance techniques. The coatings, which were formed by immersion in rare-earth salt containing solutions, reduced significantly the dissolution of magnesium in a pH 8.5 buffer solution. With continued immersion of the treated electrodes in the aggressive pH 8.5 solution, the coatings first appeared to become more protective, but after periods exceeding 60 min began to deteriorate. This is attributed to the formation of magnesium hydroxy corrosion products and mixed rare earth/magnesium oxide/hydroxide coatings, which on continued immersion became consumed by the formation of magnesium corrosion products.

Exploring the use of electrochemical impedance spectroscopy (EIS) in microbial fuel cell studies

Electrochemical impedance spectroscopy (EIS) is a powerful nondestructive technique that can act as a beneficial addition to the current techniques for studying microbial fuel cells (MFCs). Its application in MFC research should be further explored in the analysis of the internal resistance of MFCs, electrode materials, catalyst coatings on electrodes, biofilm development and electrochemical reactions on the anodes and the cathodes of MFCs.

Evaluation of corrosion behavior of coated metals with AC impedance measurements

Abstract AC impedance measurements have been performed in 0.5 N NaCl for coated steel and aluminum alloys which had been subjected to different surface treatment procedures. These procedures includ...

Electrochemical impedance spectroscopy (EIS) as a new tool for investigating methods of corrosion protection

Abstract The application of EIS as a new tool for the investigation of methods of corrosion protection is illustrated for corrosion inhibitors, conversion coatings, polymer coatings, polymer coating and oxide layers as well as for cathodic protection of stainless steels in seawater. It is pointed out that it is essential for all these cases to develop the appropriate models for the impedance which can be used to fit the experimental data and extract the parameters which characterize the corrosion process.

Determination of the long term corrosion behavior of coated steel with A.C. impedance measurements

A previous study of the a.c. impedance of thin polymer films on aluminum alloys, steel, and phosphated steel, determined over a wide frequency range, has resulted in a general model for the electrochemical impedance across the coated surfaces. According to this model, penetration of the coating by the electrolyte results in an impedance behavior which is typical of a dielectric film short-circuited by conducting electrolytic paths perpendicular and parallel to the polymer/metal interface. Comparison of the time dependence of the impedance of coated steel substrates with the impedance across a free film has demonstrated that the corrosion of the substrate enhances the development of parallel paths of ionic conduction in the coating. A.c. impedance measurements carried out on polybutadiene-coated steels pretreated by different procedures determine quantitatively the resistance Rp of the conducting paths. The time dependence of Rp for the coated substrates has been determined for exposure periods of up to three weeks. Visual observation of the corroded and delaminated areas according to ASTM D610 has been carried out simultaneously, and correlations have been established with Rp. The results are discussed in terms of the possibility to predict the corrosion behavior of a coated metal, based on the measurement of Rp.

Effect of electrolyte pH on the rate of the anodic and cathodic reactions in an air-cathode microbial fuel cell

The measurement of electricity generation from an air-cathode microbial fuel cell (MFC) with a mixed bacteria culture at different pH showed that this MFC could tolerate an initial (feed solution) pH as high as 10. The optimal initial pH was between 8 and 10 with higher current generation compared to lower or higher pH. The bacterial metabolism exhibited a buffer effect and changed the electrolyte pH. The impedance spectra of the anode and cathode of the MFC at the open-circuit potential (OCP) revealed that the anodic microbial process preferred a neutral pH and microbial activities decreased at higher or lower pH; while the cathodic reaction was improved with increasing pH.

The use of electrochemical impedance spectroscopy (EIS) in the evaluation of the electrochemical properties of a microbial fuel cell.

Electrochemical impedance spectroscopy (EIS) has been used to determine several electrochemical properties of the anode and cathode of a mediator-less microbial fuel cell (MFC) under different operational conditions. These operational conditions included a system with and without the bacterial catalyst and EIS measurements at the open-circuit potential of the anode and the cathode or at an applied cell voltage. In all cases the impedance spectra followed a simple one-time-constant model (OTCM) in which the solution resistance is in series with a parallel combination of the polarization resistance and the electrode capacitance. Analysis of the impedance spectra showed that addition of Shewanella oneidensis MR-1 to a solution of buffer and lactate greatly increased the rate of the lactate oxidation at the anode under open-circuit conditions. The large decrease of open-circuit potential of the anode increased the cell voltage of the MFC and its power output. Measurements of impedance spectra for the MFC at different cell voltages resulted in determining the internal resistance (R(int)) of the MFC and it was found that R(int) is a function of cell voltage. Additionally, R(int) was equal to R(ext) at the cell voltage corresponding to maximum power, where R(ext) is the external resistance that must be applied across the circuit to obtain the maximum power output.

The Polarization Resistance Technique for Measuring Corrosion Currents

In 1938 Wagner and Traud1 published a paper: “Concerning the Evaluation of Corrosion Reactions by Superposition of Electrochemical Partial Reactions and Concerning the Potential Formation on Mixed Electrodes.” This paper is today considered the classical paper on mixed potential theory, which postulates that, even without the assumption of the existence of local anodes and cathodes, one can explain corrosion reactions by assuming that cathodic and anodic partial reactions occur at the phase boundary metal/electrolyte in constant change with statistical distribution of location and time of the individual reaction. Wagner and Traud showed further that under certain conditions (yielding certain forms of the partial potential-current curves) it is possible to calculate the rate of a given reaction using electrochemical measurements. These principles were then applied to the dissolution rate of zinc amalgam in acid solutions. Good agreement was found between corrosion rates calculated from polarization curves using the technique, which today is called “Tafel slope extrapolation” and from the amount of hydrogen evolved. These experimental results were considered proof for the general postulations of the mixed potential theory.