Electrochemical Current Noise Research Articles

DETECTING PITTING CORROSION AND ITS SEVERITY USING WAVELET ENTROPY IN ELECTROCHEMICAL NOISE MEASUREMENT

Entropy as a measure of uncertainty was used to represent the results of the wavelet technique in electrochemical noise analysis. The experimental signals were obtained by recording the electrochemical potential and current noise of 7075 aluminum alloy in 3.5%wt NaCl solution. The electrochemical potential and current noise were decomposed into 16 levels using Daubechies wavelets. Wavelet output entropy was calculated using a kernel density estimator and the Shanon equation. The entropy contribution to high frequency scales decreased with the appearance of pits and as the pits increased and enlarged on the electrode surfaces (detected by scanning electron microscopy). The amount of reduction in entropy magnitude was attributed to the severity of pitting corrosion. It appears that a waveletentropy plot is a promising method to differentiate between different types and the severity of corrosion.

Detection of microbiologically influenced corrosion by electrochemical noise transients

This work investigates the electrochemical processes involved in pitting corrosion induced by microbiologically influenced corrosion by using time-resolved instantaneous frequency information of electrochemical current noise (ECN) transients obtained from Hilbert spectra. In addition to the time-frequency analyses, also the open corrosion potential is investigated and microscopic examinations of the specimens are performed after the tests. Hilbert spectra of the ECN signals indicated the development of transients in one of the two electrochemical cells containing sulphate-reducing bacteria with a different instantaneous frequency decomposition as compared to the background ECN signal, which resulted from the anaerobic general corrosion process. After day 13, the transients in the ECN signals developed towards consistent instantaneous frequency decompositions in the Hilbert spectra that are typical for relatively fast pitting corrosion processes. Post-exposure microscopic observations confirmed the existence of pits underneath the attached biofilms at the working electrodes.

Cover Picture: Materials and Corrosion. 5/2014

Cover: The determination of critical pitting temperatures (CPTs) in specific test media is a common testing method for the pit‐ting corrosion resistance of high alloyed materials. Com‐monly used test solutions for high alloyed steels like the ferric chloride test (ASTM G48) or the “Green Death” test have limitations using them for the highest alloyed Ni‐Cr‐Mo alloys. Furthermore the iterative test procedures limit their accuracy and effectiveness. To overcome these problems CPTs were determined by the examination of electrochemical current noise signals in a continuously heated 4.5 M calcium chloride solution under potentiostatic control, taking into account the limitation of pitting in the anodic range on alloys with a very high content of molybdenum. The new testing procedure also enables a significant better accuracy allowing an effective screening of cladding parameter and surface pre‐treatments on the resulting corrosion protection performance.More detailed information can be found in: S. Schmigalla, A. Heyn; Determination of critical pitting temperatures for Ni‐Cr‐Mo alloys using electrochemical noise measurements; Materials and Corrosion 2013, 64, 700.

Study of corrosion protection of mild steel by eco-friendly silane sol–gel coating

The effects of dipping duration in the silane solution and the sol pH on the protective performance of an eco-friendly silane sol–gel coating consisting a mixture of tetraethoxysilane, methyltriethoxysilane and glycidyloxypropyltrimethoxysilane applied on mild steel substrate were studied in this paper using electrochemical techniques and surface analysis. In consistency with the data obtained from electrochemical impedance spectroscopy and polarization curves, electrochemical current noise fluctuation analysis, appearance of the power spectral density plots and noise resistance indicated the most efficient sol pH and immersion time in the silane solution. In this regard, field emission type scanning electron microscopy images and water contact angle values revealed impact of the sol pH on the film structure. The enhancement in protective performance of the hybrid coating resulting from optimum immersion in the silane solution with appropriate pH was connected to the film homogeneity and higher cross linking and thickness as well. Moreover, a good trend correlation was observed between the noise resistance and low-frequency impedance modulus data.

Measuring electrochemical noise of a single working electrode for assessing corrosion resistance of polymer coated metals

Abstract Electrochemical noise measurement (ENM) of the spontaneous perturbation of current and potential of coated samples immersed in electrolyte determines the resistance of the coating system. ENM offers several advantages: the measurement is relatively simple to make, it is completely non-interfering with the natural process occurring on the surface and the data are simple to interpret. The original standard arrangement for ENM using a pair of samples has limitations for practical applications because two separate and nominally identical working electrodes are needed and this requirement is very hard (if not impossible) to fulfil in on-site application. This paper describes an alternative approach for electrochemical noise measurement to measure the noise resistance ( R n ) of protective coatings based on use of just one working electrode. In this so-called “Single Cell” (SC) arrangement the electrochemical noise current and electrochemical noise potential between the working electrode and a non-noisy reference electrode is measured separately and consecutively. This new approach has been tested for a range of coating resistances. Also, the coating's resistance has been measured using DC resistance and EIS (at low frequency) and the results were compared with the R n obtained from the single cell (SC) set up.

Application of transient analysis using Hilbert spectra of electrochemical noise to the identification of corrosion inhibition

This study validates the ability of Hilbert spectra to investigate transients in an electrochemical noise signal for an aqueous corrosion inhibition process. The proposed analysis procedure involves the identification and analysis of transients in the electrochemical current noise signal. Their decomposition into instantaneous frequencies in a Hilbert spectrum allows detection of changes in corrosion characteristics, i.e. the evolution of corrosion inhibition with time. The effectiveness of the proposed analysis procedure is investigated for AA2024-T3 exposed to aqueous 10-1 M NaCl solutions with or without the addition of Ce ions at various concentrations. Examination of specific features in the electrochemical noise signals shows the presence of three characteristic regions, which represent surface activity regimes ranging from active (localized) corrosion to the inhibited state. Hilbert spectra of the electrochemical current noise signals allow identification of transients occurring in these successive regions.The analysis procedure introduced in this work yields improved applicability of electrochemical noise measurements for the identification of an inhibition effect in corrosion processes.

Electrochemical Noise Analysis of Pitting Corrosion of Type 304L Stainless Steel

Electrochemical current and potential noise were simultaneously acquired from Type 304L stainless steel (UNS S30403) in 0.05 M ferric chloride (FeCl3) using a three-electrode configuration. Power spectral, statistical, and wavelet analyses have been used to know the uniqueness of the parameters proposed for the identification of various types of corrosion processes. Roll-off slopes derived from power spectral analysis and statistical parameters such as standard deviation, localization index, and kurtosis corroborated with pitting as the corrosion mechanism. Energy distribution plots (EDP) obtained from wavelet analysis of current noise was found to be useful to derive mechanistic information on the progress of corrosion. Discrete wavelet transform was used to decompose the signals into a D1, D2, D3…D8, S8 set of coefficients. The EDP showed that the contribution from the medium time scale crystal, D5, prevailed over the smaller time scale crystals and larger time scale crystals during the initial stages of immersion. With an increase in the time of immersion, the energy deposition on the larger time scale crystals increased and the maximum energy was concentrated on the D8 crystals, indicating that the dominant process occurring on the specimen surface was stable pitting. The results of the investigation are detailed in the paper.

The Features of 304 Nitrogen Controlled Stainless Steel during Slow Strain Rate Test by Electrochemical Noise

An investigation was carried out on the features of electrochemical noises of 304 nitrogen controlled stainless steel (304NGSS) in solution 4mol/LNaCl +0.01mol/LNa2S2O3under slow strain rate testing (SSRT) conditions. Electrochemical voltage and current noise were measured simultaneously in the Zero Resistance Ammeter (ZRA) mode. After experiment, there are several cracks on the edge of pit on the specimen. In order to characterize the stress corrosion cracking (SCC) processes of 304NGSS, time analysis of data alongwith the spectral estimation using FFT method, the results show SCC processes consisted of breakdown of the oxide film, the formation of pitting, crack initiation and crack propagation. According to the varieties of spectrums in time and frequency, the different state can be distinguished. The work in this paper will contribute to the research of indicator for real-time detection of SCC in plant.

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.

Electrochemical impedance spectroscopy and electrochemical noise measurements as tools to evaluate corrosion inhibition of azole compounds on stainless steel in acidic media

Abstract The corrosion inhibition function of two azole derivatives namely benzotriazole (BTR) and benzothiazole (BNS) on stainless steel in 1 M HCl was investigated using electrochemical techniques and SEM surface analysis as well. In consistency with the data obtained from EIS and polarization curves, electrochemical current noise transient analysis, noise resistance and characteristic charge from shot noise theory indicated effectiveness of the inhibitors. The corrosion damage mitigation was also confirmed through SEM in the presence of BNS. To remove the DC trend from noise data, the appropriate p value was proposed based on the correlation with polarization and EIS data.

Degradation process of coated tinplate by phase space reconstruction theory

The degradation process of organosol coated tinplate in beverage was investigated by electrochemical noise (EN) technique combined with morphology characterization. EN data were analyzed using phase space reconstruction theory. With the correlation dimensions obtained from the phase space reconstruction, the chaotic behavior of EN was quantitatively evaluated. The results show that both electrochemical potential noise (EPN) and electrochemical current noise (ECN) have chaotic properties. The correlation dimensions of EPN increase with corrosion extent, while those of ECN seem nearly unchanged. The increased correlation dimensions of EPN during the degradation process are associated with the increased susceptibility to local corrosion.

Analysis of Electrochemical Current Noise From Metastable Pitting of SS304L in NaCl Solutions

Electrochemical noise emanating from a corrosion situation gives indication about the nature and form of corrosion. An attempt has been made to analyse electrochemical current noise signal generated under potentiostatic condition (for metastable pitting) for SS304L–NaCl system. To begin with polarisation plots of SS304L were obtained in various test solutions to precisely know pitting potential of SS304L. It is found that Epit and ipass increase on increasing chloride content. The electrochemical current noise was measured at potentials 20–30 mV below the pitting potentials. The current time record shows two types of current transients; (i) slow rise and rapid decay and (ii) rapid rise and slow decay. Power spectral density analysis of current noise shows that the power (A2/Hz) of the signal measured at metastable pitting range increases with increasing chloride. Sampling frequency has to be properly selected otherwise some of the spikes are not recorded and as a result, size and shape of few current transients is altered.

Study of pitting corrosion on mild steel during wet–dry cycles by electrochemical noise analysis based on chaos theory

Electrochemical noise (EN) was used to investigate the corrosion behaviour of mild steel (Q235) in 0.1M Na2SO4 and 0.1M NaCl aqueous solutions during wet–dry cycles. The positive fraction and value of the Largest Lyapunov Exponent (LLE) of electrochemical current noise (ECN) were found out to represent the number and isolation degree of the pits formed in two electrolyte conditions. The calculated results indicate that metastable pits are more plentiful and uniformly distributed in wet cycles and in Na2SO4 solution than those in dry cycles and in NaCl solution respectively.

Comparison of Symmetrical and Asymmetrical Cells by Statistical and Wavelet Analysis of Electrochemical Noise Data

The most common cell configuration for electrochemical current noise (ECN) measurements consists of two identical working electrodes (WE) known as symmetrical cell, connected by a zero-resistance ammeter (ZRA). The analysis of ECN data is complicated by its reflection of the behavior of two identical WE, but it would be facilitated greatly if the behavior of one of two WE were measured separately. According to an earlier theoretical model, an asymmetrical cell with a large difference in size between two WE, which are otherwise identical, allows the measurement of ECN on the small electrode while allowing free corrosion to proceed. In this paper, the ECN signals obtained on the symmetrical cells are compared with those carried out on the asymmetrical cells by statistical and wavelet analysis methods. Three advantages of the asymmetrical cells for measurement of ECN data are clearly seen: the time scale preserving of current transients, the increase of amplitude of current transients, and the detection of a higher number of events.

Time–frequency methods for trend removal in electrochemical noise data

Electrochemical current and potential noise signals in many cases exhibit a DC drift that should be removed prior to further data analysis. The theoretical ability of wavelet analysis and empirical mode decomposition to effectively remove only the DC drift component is evaluated based on their mutual performance for the first time. The correlation coefficient between individual signals after both trend removal techniques proved to be superior compared to the correlation coefficients between these and the signals after moving average, polynomial and linear trend removal. The residual power of these signals was compared and again the two time-frequency methods acknowledged their theoretical ability of removing only a well-defined part of the data. © 2012 Elsevier Ltd. All rights reserved.

Electrochemical noise analysis on multiple dissimilar electrodes: Theoretical analysis

The electrochemical potential and current noise generated by multiple electrodes are analyzed by considering the current flowing across each electrode and the electrochemical potential of the electrode array. By introducing the concept of a “virtual electrode”, the analysis of the electrochemical noise generated by an array of electrodes is reduced to the case of two dissimilar electrodes. For each electrode, an apparent impedance, Zρ*, can be determined as the square root of the power spectral density of potential divided by the power spectral density of the individual electrode current. When two dissimilar pairs of nominally identical electrodes are used and it is possible to assume that the pair of electrodes corroding more produces higher noise levels and displays lower impedance, the actual electrode impedance can be obtained with acceptable precision from the value of the nominal impedance. Further, the asymmetry between the two dissimilar pairs can also be quantitatively evaluated.

Comparison between ED and SDPS plots as the results of wavelet transform for analyzing electrochemical noise data

Two valuable results of wavelet transform are energy distribution (ED) plot and standard deviation of partial signal (SDPS) plot. In this work, after brief review of theoretical basis of wavelet transform, the ED and SDPS plots are compared using both calculated and experimental signals. The experimental signals are obtained by recording electrochemical current noise of two Al alloys, AA2024 and AA5052. Both ED and SDPS plots can detect the predominant transients of each signal by the position of the maximum peak between d1 and d6 crystals and thereby may categorize several corroding systems according to the pit size. The slope value of the SDPS plot in the region after the maximum peak can be a measure of the frequentness of the predominant transients. Based on these slope values it might be possible to classify several corroding systems according to the pit density. The SDPS plot seems as an alternative tool to overcome the limitations of the ED plot in the corrosion monitoring applications. Employment of the ED plot along with the SDPS plot is recommended to provide a further insight into the electrochemical behavior of a corroding system.

Detection and characterisation of intergranular stress‐corrosion cracking on austenitic stainless steel

AbstractIntergranular stress‐corrosion cracking (IGSCC) on a sensitised type AISI 304 stainless steel specimen was monitored simultaneously by acoustic emission, electrochemical noise, elongation measurements and a digital imaging system. The specimen was exposed to an aqueous sodium thiosulphate solution in combination with a constant load. It was established that before the final fracture two large cracks and numerous smaller cracks had developed. Detection and characterisation of the stress‐corrosion processes which generated these cracks are discussed. The results confirm and generalise previously established correlations between various parameters obtained by the implemented characterisation methods and IGSCC processes. Additionally, a clear differentiation between crack related and crack non‐related AE signals was made based on an analysis of the AE signals. The relationship between the crack lengths calculated by means of digital image correlation analysis and the electrochemical current noise was also established.

Micro- and Nano-scale Tribo-Corrosion of Cast CoCrMo

Previous studies have established that some of the wear damage seen on cast CoCrMo joint surface is caused by entrained third-body hard particles. In this study, wet-cell micro-indentation and nano-scratch tests have been carried out with the direct aim of simulating wear damage induced by single abrasive particles entrained between the surfaces of cast CoCrMo hip implants. In situ electrochemical current noise measurements were uniquely performed to detect and study the wear-induced corrosion as well as the repassivation kinetics under the micro-/nano-scale tribological process. A mathematical model has been explored for the CoCrMo repassivation kinetics after surface oxide film rupture. Greater insights into the nature of the CoCrMo micro-/nano-scale wear-corrosion mechanisms and deformation processes are determined, including the identification of slip band formation, matrix/carbide deformation, nanocrystalline structure formation and strain-induced phase transformation. The electrochemical current noise provides evidence of instantaneous transient corrosion activity at the wearing surface resulting from partial oxide rupturing and stripping, concurrent with the indent/scratch.

Ruido electroquímico de la erosión-corrosión en cobre: su relación con los parámetros hidrodinámicos

This work presents the electrochemical noise results obtained of the surface degradation on copper, due to erosion corrosion phenomena, which were a function of the hydrodynamic parameters of the system (fluid movement). A modified rotating cylinder (RC) comprising three ring electrodes under two rotating speeds (880 and 1750 rpm with a Reynolds numbers 1486 Re and 2972 Re, respectively) were used. Characteristic electrochemical noise spectra as a function of the hydrodynamic parameters were found, as well as surface attack intensities the noise signal. An increase and a more uniform attack due to particle impact was related to larger particle size and lesser erosion corrosion intensity, in the form of more localized attack over the surface, was obtained for smaller ones. Erosion corrosion attack presents characteristic electrochemical current and potential noise signals, according to the laminar or transitional turbulent regime and particle size added.