Coulostatically-Induced Transient Technique And Data Analysis Method

Corrosion science and engineering have benefited tremendously from the explosion in the use of electrochemical methods that can probe the thermodynamic and kinetic aspects of corrosion, including the rate of corrosion. Enhanced prevention and understanding of corrosion have been achieved over the past several decades by developing new electrochemical methods in both the laboratory and field. The coulostatic method, based on Faraday's law, enables determination of instantaneous parameters for interface reaction. In the case of a coulostatic measurement, a potential transient is recorded instantly after the inputting of a small known amount of charge. The absence of any flow of current between the working and counter electrodes while the potential is recorded means that electrolyte resistance usually does not influence the result of coulostatic measurements. Now this technique has since been applied to more and more complex systems in the electroanalysis and corrosion science. However, most of the publications focused on the applications of rapid measurements and no flow of current during the sampling data process, only few works aimed at developing new coulostatic measurement & automation method, and the studies on automatic corrosion monitoring technique based on the coulostatic principle is even rare. Therefore, the above problems are main barriers of coulostatic technique applied for the corrosion science and engineering. In this thesis, the coulostatic method was employed for determinations of the electrochemical kinetic parameters of various corrosion systems, and the new analysis methods for the coulostatically-induced transients (CITs) were presented. In addition, the practical designs both for portable coulostat and the automatic optimizing test conditions were conducted. The accuracy of the Coul-1 system was demonstrated by comparing the experimental results with that obtained from other methods, including potentiodynamic polarization curve, electrochemical impedance spectroscope (EIS) and solution analysis data from atomic absorption spectroscopy, and using the statistical analysis technique to perform the validity check. The main conclusions are as follows: The CITs analysis has been used as a tool to evaluate the corrosion parameters of metallic materials, and a novel algorithm has been developed to calculate the Tafel slopes (ba and bc). This algorithm is based on the coulostatic strong polarization integration (CSPI) method. Experimental results for carbon steels exposed to acidic solution show excellent agreement between the proposed method and the Tafel line extrapolation method with IR drops correction. Furthermore, the improved CSPI algorithm shows superior performance over the differential (CSPD) and conventional (Kanno) algorithm due to its simplicity and higher signal/noise ratio. Additionally, the validity of the proposed method was also verified by a series of diffusion-limited system. For example, for carbon steel corroded in seawater, the electrochemical corrosion parameters obtained by the CSPI method coincide with those obtained from the polarization curve with the consideration of concentration polarization. Therefore, coulostatic method is so rapid that the measurement of potential transient is much less influenced by diffusion limitation. Thus, the experimental curves are mainly related to the interfacial impedance of the working electrode and not affected by the bulk electrolyte. Obviously, this technique differs from the steady-state methods such as polarization curve measurements. The coulostatically-induced impedance spectra (CIS) can be obtained from Laplace transformation of CIT. The equations for the coulostatic response were also derived from the network analysis in the Laplace domain. Compared with the curving fitting method for CIT, CIS analysis is more convenient to interpret the response of coulostatic impulsive signal, and thus greatly facilitates the evaluation for the impedance of electrode and its corrosion process. With respect to the complex corrosion system involving multi-time constants, the CIS is not only helpful to judge and determine the number of time constants, but also gives aid to setup initial input values and add some constraints to the fitting process. It is possible to eliminate the problem of diverse results for multi-parameter of NLLS fitting. Thus the electrode impedance can be evaluated accurately. Based on the above knowledge, a simple procedure for the evaluation of corrosion inhibitors was developed based on the CITs analysis. Since coulostatic perturbation is so rapid that the slow processes such as inhibitor adsorption on the metallic electrode surface cannot respond perceptibly. Consequently, Rt is always close to Rp in coulostatic measurements, which allows the use of a simple circuit model to analyse CITs. Furthermore, the coulostatic method is useful for the rapid measurement of Tafel slopes and Rt, which enables accurate assessment of the exact corrosion current density at any time. Therefore, this method facilitates the rapid procedure for the evaluation of corrosion inhibitors, and simultaneously obtains more accurate instantaneous corrosion rate. Two methods of de-noising based on wavelet thresholding or Empirical modedecomposition (EMD) for CIT were presented. This filtering was illustrated with CIT and its associated impedance spectra, and comparisons have been made with two other filtrations: data smoothing and digital filtering. The results demonstrate that wavelet de-noising or EMD is applicable to CIT in the time domain, where traditional filtering can also be successful. Moreover, in cases where traditional methods fail, EMD or wavelet de-noising can extract noisy data from impedance spectra of CIT. Therefore, the proposed method shows consistently superior de-noising performance over traditional methods in domains of both time and frequency. Practically, the wavelet de-noising method is more effective when the decomposition levels are from 5 to 7. Based on the above studies and design, Model Coul-1 coulostatatic corrosion measurement system was developed. Experimental results for carbon steels exposed to acidic solution or seawater show high precision and reliability. To simplify the operation of Coul-1 system, a solution designed for automatic optimizing test conditions was implemented, and the whole system was built in a rugged portable box, therefore paved the way for the convenient corrosion monitoring based on the coulostatic method.