Since a corrosion process is a nonlinear electrochemical phenomenon, a potential perturbation signal by one or more sine waves will generate current responses at more frequencies than the frequencies of the applied signal. Current responses can then be measured, for example, at zero, harmonic, and intermodulation frequencies. This simple principle offers various possibilities for corrosion rate measurements, like the intermodulation or electrochemical frequency modulation (EFM) technique in which the potential perturbation signal consists of two sine waves of different frequencies. With this novel EFM technique, the corrosion rate can be determined from the corrosion system responses at the intermodulation frequencies. With the EFM technique a corrosion rate can be obtained instantaneously, without prior knowledge of the so-called Tafel parameters. The EFM approach requires only a small polarizing signal, and measurements can be completed in a short period. A special advantage of the EFM technique is its capability of inherent data validation control using "causality factors" (parameters introduced for the first time in this paper). It is shown that the EFM technique can be used successfully for corrosion rate measurements under various corrosion conditions, such as mild steel in an acidic environment with and without inhibitors and mild steel in a neutral environment.
The ac impedance measurement has proved to be a useful electrochemical technique for mainly qualitative studies of electrochemical and corrosion systems. Even for complicated systems such as coated metals in corrosive environments this technique has been used with success. The system chosen for the present study is an ARMCO iron plate, coated with a
SrCrO4‐normalpigmented
styrene acrylic polymer, and immersed for several weeks in an aqueous
normalNaCl
solution. Impedance measurements analyze a system under test into its constituting phenomena. The dependence of system parameters on coating layer thickness,
normalNaCl
concentration, and pigmentation of the coating during the immersion time provides insight into the corrosion and protection mechanisms at the coating/metal interface, besides the behavior of the coating itself.
In order to study the anodic and cathodic corrosion process of painted iron separately, two similar specimens are mounted as a differential aeration couple. Application of the electrochemical impedance measurement technique, scanning electron microscopy, and x-ray analysis demonstrate a different paint-degradation rate according to the anodic or cathodic polarization. The blistering phenomenon is shown to be a consequence of the anodic process. The electrochemical impedance points out diffusion control in the cathodic cell and charge transfer control in the anodic cell.The study of the quality of organic corrosion inhibiting coatings assumes an investigation of the interaction between the corrosive environment, the coating itself, and the metal substrate. More particularly, the course of the subcoating corrosion process and the mechanism of the coating deterioration are important topics. Interaction means that one should investigate the metal/coating/environment system preferably as an integral one. Furthermore, the system is time dependent, so one should be able to monitor it in situ. Since the underlying corrosion phenomenon has an electrochemical nature, first of all electrochemical techniques provide a proper method.The electrochemical impedance technique is especially useful in examining interface processes, and coating performance (1-10). Two important advantages of this technique are that: (i) it often displays the constituting characteristics of the system separately (surface layers, charge transfer, mass transfer, etc.) and (ii) there is no pronounced perturbation of the system, since one usually works with small amplitude signals.Still, the interpretation and modeling of electrochemical impedance data is not an easy matter.In order to evaluate what role oxygen plays in corrosion of the integral metal/coating/environment system, an attempt is made to subdivide the system artificially into the oxidation and reduction process. Impedance data, together with scanning electron microscopy and x-ray analysis, are used to investigate the paint degradation and to determine the corrosion mechanism under the coating.
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