A. Aballe

Using wavelets transform in the analysis of electrochemical noise data

Various concepts developed in the framework of wavelet transforms have been adapted to the analysis of electrochemical noise measurements (ENM). Wavelet transforms are proposed as an alternative tool to overcome the limitations of FFT in the analysis of ENM data. In this context, the theoretical basis of wavelet analysis is briefly reviewed. Then, both methods are applied to the study of various different corrosion systems covering a wide range of ENM signals. The results reported here demonstrate, firstly, that wavelet analysis is applicable to those systems in which FFT techniques also work. Moreover, in cases where FFT fails, wavelet analysis provides valuable knowledge about the behaviour of the system. Using wavelets, the different ENM components contributing to the original signal can be characterised. Each component is defined by a set of wavelet coefficients, which contain information about the time scale characteristic of the associated corrosion event.

CeCl3 and LaCl3 binary solutions as environment-friendly corrosion inhibitors of AA5083 Al–Mg alloy in NaCl solutions

Abstract Lanthanum and cerium chloride and their binary mixtures have been investigated as corrosion inhibitors of AA5083 Al–Mg alloy in aerated 3.5% NaCl aqueous solution. Their inhibitor power was evaluated by using electrochemical techniques together with full immersion tests. The highest protection degree was found for the binary solution doped with 250 ppm CeCl3 and 250 ppm LaCl3. Scanning electron microscopy and energy dispersive spectroscopy enabled us to confirm the cathodic nature of the inhibition process and to establish some features of the inhibition process.

Localized alkaline corrosion of alloy AA5083 in neutral 3.5% NaCl solution

The corrosion process of the alloy AA5083 in an aerated solution of NaCl at 3.5% has been studied. The results obtained indicate that the main process that this alloy undergoes, under the conditions of exposure studied, is related to localized corrosion that takes place as a consequence of the process of alkalinization around the cathodic precipitates existing in the alloy. The pitting formed presents a hemispherical morphology that is clearly different from crystallographic pitting. The formation of crystallographic pitting has not been observed, even in samples submitted to tests of very long duration. In order to obtain the formation of crystallographic pitting, it is necessary to polarize the alloy at the nucleation potential of pitting and, in addition, the density of the current must be above a critical value. Only when the layer of oxide is eliminated does the formation of crystallographic pitting take place by simple exposure in an aerated solution of NaCl at 3.5%.

WAVELET TRANSFORM-BASED ANALYSIS FOR ELECTROCHEMICAL NOISE

Abstract Wavelet transforms are presented as a useful tool to analyse electrochemical noise data. Various concepts developed in the framework of wavelet transforms have been adapted to study electrochemical noise measurements. The most relevant feature of this method of analysis is its capability of decomposing electrochemical noise records into different sets of wavelet coefficients, which contain information about corrosion events occurring at a determined time-scale. Thus, this mathematical approach could become an alternative tool which solves the limitations of other more established procedures for the analysis of electrochemical noise data, such as statistical or Fourier transform-based methods.

Use of wavelets to study electrochemical noise transients

Two kinds of time records are found when measuring electrochemical noise: those presenting characteristic transients or spikes, and others in which neither transients nor spikes can be distinguished. Although the former are likely to be more informative than the latter, most of the methods developed for analyzing electrochemical noise are focused on studying the latter. This paper suggests a methodology for automatically studying the appearance of the transient, which could represent a significant advance in the field of corrosion monitoring and laboratory investigation. The proposed methodology is based on a mathematical tool called the wavelet transform, whose principal characteristic is that it enables a simultaneous analysis of signals in both time and scale. Thus a subsequent interscalar analysis of the signals can reveal the existence of transients and even provide a comparative characterization of their size and scale.

Influence of the cathodic intermetallics distribution on the reproducibility of the electrochemical measurements on AA5083 alloy in NaCl solutions

The corrosion behavior of alloy AA5083 in solutions of NaCl at 3.5% is controlled by the density of the cathodic precipitates of Al(Mn,Fe,Cr) present in the alloy. These precipitates are not distributed homogeneously over the surface of the alloy. Further, their presence conditions the electrochemical response of the alloy. For these reasons, in order to guarantee the reproducibility of electrochemical tests on this alloy, it is necessary to determine the minimum surface area exposed to the medium which represents the average behavior of the system. In the present study, a systematic analysis has been conducted on the degree of reproducibility of electrochemical tests as a function of the area of surface exposed, for the alloy AA5083 in solutions of NaCl at 3.5%. A high dependence between the dispersion of the electrochemical data and the exposed area has been established. This behavior contrast with those of other alloys, like AA1050, which is not conditioned by the values of exposed area. This has been related to the much lower cathodic precipitates density in this alloy.

Electrochemical noise applied to the study of the inhibition effect of CeCl3 on the corrosion behaviour of Al–Mg alloy AA5083 in seawater

Electrochemical noise measurements have been used to study the corrosion behaviour of aluminium alloy AA5083 in 3.5% NaCl solution with and without doping with CeCl3 at 500 ppm. Information on the evolution of corrosion activity over a period of 4 days and on the corrosion mechanism itself has been obtained for the two systems. Experimental data obtained have been analysed using robust statistical parameters and wavelet transform; transient shapes were also studied. The various mathematical techniques applied to analyse the electrochemical noise data have been proposed recently. This paper thus not only characterises the corrosion systems studied but also illustrates the usefulness of these new methods.

Inhibition of the corrosion process of alloy AA5083 (Al‐Mg) in seawater by cerium cations. An EIS study

The principal corrosion process of the alloy AA5083 in aerated NaCI solutions mainly affects the zones occupied by Al(Mn,Fe,Cr) cathodic precipitates, over which the oxygen reduction reaction takes place. Thus, the design of an effective protection system could be based on the use of cathodic inhibitors. In previous papers, the inhibition efficiency of various lanthanide chlorides, particularly CeCl 3 , has been proved. These compounds act as cathodic inhibitors, blocking these cathodic sites by means of the precipitation of a lanthanide oxide or hydroxide film. In this paper, EIS has been used to detect the blocking effect of the cathodic sites. This fact is reflected in the electrical response of the system through the minimisation of the values of those elements of the electrical loop that are related to the response of the cathodic intermetallics. In addition, the dielectric properties of the anodic film have been evaluated by measuring the value of the associated capacitance. From these values, it can be concluded that the thickness of the film produced over the matrix when the inhibitor is added is much lower than that produced in the bare solution. This finding is in good agreement with the results obtained with other techniques and may be related to the minimisation of the anodic reaction as a consequence of the decrease in the intensity of the cathodic process by the action of CeCl 3 .

On the mixed nature of cerium conversion coatings

Alternative pretreatments are currently under development in order to avoid the environmental impact produced by using surface finishing processes based on chromates. Some of the environmentally friendly alternatives proposed involve the use of lanthanide based compounds. In this study, conversion coatings on AA5083 (Al-Mg) samples developed using full immersion pretreatments in 500 ppm CeCl 3 aqueous solutions have been investigated. Their microscopic and compositional features have been analyzed using SEM images and EDS spectra. From this analysis it has been determined that this layer over the surface of the samples is of a heterogeneous composition. This coating is formed by an alumina coating covering the aluminum matrix and dispersed cerium-rich islands deposited over the cathodic sites of the alloy. A characterization methodology is proposed based on the utilization of different electrochemical techniques, such as Open Circuit Potential monitoring (OCP), Linear Polarization (LP) and Electrochemical Impedance Spectroscopy (EIS).

Using EIS to study the electrochemical response of alloy AA5083 in solutions of NaCl

The corrosion behavior of alloy AA5083 in aerated solutions of NaCl has been studied. Special attention has been paid to the analysis of the electrochemical response of this alloy using Electrochemical Impedance Spectroscopy (EIS). Analysis of the obtained data has enabled an evaluation of the contributions made by each of the corrosion processes active in the system: localized alkaline corrosion in the areas surrounding the intermetallic precipitates and growth of the layer of oxide on the matrix. It has been determined that the process of localized corrosion has the greatest verifiable intensity, but this process reduces with length of exposure time. Einsatz der Elektrochemischen Impedanzspektroskopie (ElS) zur Untersuchung des elektrochemischen Verhaltens der Legierung AA5083 in NaCl-Losungen Das Korrosionsverhalten der Legierung AA5083 wurde in belufteten NaCl-Losungen untersucht. Besondere Aufmerksamkeit wurde der Analyse der elektrochemischen Reaktion dieser Legierung mittels Elektrochemischer Impedanzspektroskopie (EIS) gewidmet. Die Analyse der erhaltenen Daten ermoglichte eine Beurteilung der Anteile der einzelnen aktiven Korrosionsprozesse in dem System: lokale alkalische Korrosion in Bereichen, die die intermetallischen Ausscheidungen umgeben, und Wachstum der Oxidschicht auf der Matrix. Es wurde festgestellt, dass der Prozess der lokalen Korrosion die groste nachprufbare Intensitat aufweist, wobei dieser Prozess aber mit der Auslagerungsdauer abnimmt.