Comparative Electrochemical Noise Study of the Corrosion of Different Alloys Exposed to Chloride Media

This paper describes the corrosion behavior of aluminum, copper, and mild steel when exposed to chloride media using both electrochemical noise analysis (ENA) and electrochemical impedance spectroscopy (EIS). Analysis of electrochemical noise (EN) data demonstrated the need for removal of drifts in both potential and current fluctuations. Statistical analysis such as noise resistance, localization index, skewness and kurtosis has been evaluated. Noise resistance showed a good agreement with polarization resistance. Fast Fourier transformation (FFT) has been applied to convert EN data from the time domain to the frequency domain. Spectral noise plots showed a good agreement with impedance spectra for the different alloys determined at the same exposure time. Spectral and statistical analysis can extract useful information from EN data.

STUDY THE INFLUENCE OF H_(3)BO_(3)ON THE ZINC ELECTROPLATING USING ELECTROCHEMICAL NOISE TECHNIQUE

The influence of H3BO3 on the zinc electroplating was studied using electrochemical noise technique, cyclic voltammetry and steady-state polarization method. The results showed that, under the experimental conditions, the deposition of zinc followed the mechanism of two-dimensional nucleation and subsequent grain growth. The addition of H3BO3 into the electroplating solution prominently changes the nucleation and growth kinetics of zinc deposits, which is directly related to the features of electrocrystallization noise and the corresponding structure of the electrodeposits. The results also shown that the electrochemical noise (EN) technique can give more information about the electrodeposits structure and electroplating mechanism than other normal electrochemical measurements can give, such as steady-state polarization method and cyclic voltammetry technique.

ELECTROCHEMICAL NOISE ANALYSIS OF THE INFLUENCE OF THE Zn^(2+)IONS CONCENTRATION ON ZINC ELECTROPLATING

The influence of the concentration of Zn2+ ions on zinc electroplating process was investigated by means of electrochemical noise (EN) and cyclic voltammetry methods in conjunction with the scanning electron microscopy (SEM) technique. It was found that the EN generated during the electroplating of dentritic or large polymeric zinc deposit has large potential oscillation amplitude and positive potential drift while the compact zinc deposit possesses small noise amplitude and little potential drift. With the change of rate determining step from diffusion-control through mixed-control to activation-control, the maximum relative energy obtained from wavelet analysis defined from the region with larger scales to those with smaller scales, and the EDP (relative energy distribution plot) can be us, as 'fingerprints' of EN to characterize the electroplating process and the deposit structure. The results also showed that electrochemical noise technique can give more information about the electrodeposit structure than other normal electrochemical measurements, such as linear potential sweep method and cyclic voltammetry technique.

Wavelets analysis of electrochemical noise generated during corrosion processes of pure Al in NaCl solution

The potential noise during corrosion of pure aluminum in sodium chloride solution was recorded and analyzed with wavelet transform technique. The typical results show that the electrochemical noise (EN) signal is composed of distinct type of events, which can be classified according to their scales, i.e. time constants. And the process underlying the rapid fluctuations of EN, which is characterized by a small scaling value, i.e. high frequency components and which is usually used for local analysis, is not consistant with time; whilst those associated with slower processes or characterized by a large scaling value, which are usually used for global analysis, are continuous.

CORROSION MONITORING OF LY12 IN SODIUM CHLORIDE SOLUTION WITH ELECTROCHEMICAL NOISE TECHNIQUE

Spontaneous electrochemical noise (EN) can be a rich source of information concerning the processes simultaneously occurring on a corroding interface. But the noise signal is often difficult to be analyzed due to the complicated nature of the specific systems being investigated. In this paper, the potential noise fluctuations during the free corrosion of commercial aluminum alloy LY12 in sodium chloride solution was recorded and analyzed with different techniques. The typical results showed that the fractal dimension (D,n) obtained from spectral power density (SPD) is mainly directly proportional to the intensity of pitting corrosion and to the value of pitting parameter (SE) derived from dimensional analysis, while the fractal dimension (DE) obtained from EIS is mainly related to the uniform corrosion.

ELECTROCHEMICAL NOISE ANALYSIS OF PURE ALUMINUM IN SODIUM CHLORIDE SOLUTION WITH WAVELET TRANSFORM TECHNIQUE

Wavelet transforms (WT) are proposed as an alternative tool to overcome the limitations of Fourier transforms (FFT) in the analysis of electrochemical noise (EN) data. 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 the time scale characteristic of the associated corrosion event. In this context, the potential noise fluctuations during the free corrosion of pure aluminum in sodium chloride solution was recorded and analyzed with wavelet transform technique. The typical results showed that the EN signal is composed of distinct type of events, which can be classified according to their scales, i.e. their time constants. Meanwhile, the energy distribution plot (EDP) can be used as 'fingerprints' of EN signals and can be very useful for analyzing EN data in the future.

ELECTROCHEMICAL NOISE FEATURES OF PURE ALUMINUM DURING PITTING CORROSION IN NEUTRAL NaCl SOLUTION

Spontaneous electrochemical noise (EN) can be a rich source of information concerning the processes simultaneously occurring at a corroding interface. Potential noise fluctuations during the free corrosion of pure aluminum in different concentration of neutral sodium chloride solution are investigated, and the breakdown and restoration of passive metal's film are studied using potentiodynamic scanning (PDS) measurements and electrochemical impedance spectroscopy (EIS) technique. Two capacitance loops are observed in the Nyquist plots in two kinds of concentration, and the corrosion process is under activation control at first, then become diffusion control within the oxide film and corrosion products of (Al(OH)p-mCl-m) accumulated on the surface of the corroding electrode. It is suggested that the pitting corrosion is much easier to occur for pure aluminum in 7.0wt% than in 2.0wt% NaCl solution, and the high concentration of chloride ion in solution inhibits the repassivation of a metastable pit. The corrosion rate deterministic step does not involve Cl-.

Characterization of stress corrosion crack growth of 304 stainless steel by electrochemical noise and scanning Kelvin probe

The fatigue pre-cracking 304 stainless steel （SS） specimens with lengths of 1.002 mm （L-crack） and 0.575 mm （S-crack） were prepared. Their corrosion behavior was studied by electrochemical noise （EN） in 4 mol/L NaC1 ＋ 0.01 mol/L Na2S203 solution under slow-strain-rate-testing （SSRT） conditions. Moreover, the characteristics of L-crack＇s surface morphology and potential distribution with scanning Kelvin probe （SKP） before and after SSRT were also discussed. Compared with S-crack, L-crack is propagated and the features of crack propagation can be obtained. After propagation, the noise amplitudes increase with increasing stress and accelerating corrosion, the white noises at low and high frequencies （WE and WH） of the later stage are one order of magnitude larger than that at early stage in the current power spectral densities （PSDs）. The potential PSDs also increase, but WH disappears. In addition, the crack propagation can be demonstrated according to variation of probability distribution, surface morphology and potential distribution.

Online evaluation of electroless deposition rate by electrochemical noise method

Continuous noise resistance calculation(CNRC)technique was used for online determination of the electroless nickel deposition rate on zirconium pretreated magnesium alloy.For this purpose,the noise resistance(R_n) variation with time was calculated for the pretreated alloy surface in the electroless plating solution.The CNRC results were described by energy dispersive X-ray spectroscopy(EDS)and scanning electron microscopy(SEM)techniques.Also,potentiodynamic polarization and gravimetric measurements were used for determination of the electroless deposition rate at the same time period and the results were compared with the CNRC results.The Rn variation with plating time shows that the electroless plating consists of different stages with various deposition rates.The results of the CNRC and polarization methods were not in acceptable agreement due to the limitations of the polarization method for online monitoring of the deposition rate.However,the results of the gravimetric measurements were in complete agreement with the CNRC technique and so,the CNRC can be considered as suitable tool for online evaluation of the electroless deposition rate.

Galvanic corrosion behavior of plain carbon steel-B_4C composite in 3.5% NaCl solution with electrochemical noise

The galvanic corrosion behavior of metal-matrix composite plain carbon steel/boron carbide （B4C） in 3.5% NaCl solution was studied. The composite was locally produced as a weld band on carbon steel by means of the gas tungsten arc welding process and using nickel as the wetting agent. Samples from the weld band, heat-affected zone and parent metal region were extracted precisely and DC/AC electrochemical tests in combination with techniques such as scanning electron microcopy and energy dispersive spectrometry were conducted. The results of the electrochemical tests show that the corrosion resistance of the parent metal sample is higher than that of the welded composite and the HAZ samples. However, as the corrosion potential （Eco^r） of the parent metal is more positive than other two samples, this becomes the cathode in galvanic couples with two other samples. On the other hand, the weld composite sample is also cathodic due to its more positive Ecorr compared to HAZ sample. This means that the HAZ can be particularly at risk of preferential dissolution. The approach can be used in specific areas on plain carbon steel to locally increase hardness and resistance to abrasion and reduce manufacturing costs.

Electrochemical noise characteristics in corrosion process of AZ91D magnesium alloy in neutral chloride solution

The corrosion process of AZ91D magnesium alloy in neutral 1%(mass fraction) sodium chloride aqueous solution was investigated by electrochemical noise(EN),SEM and EDX. Fractal theory was primarily used to depict the corrosion process of the alloy. The fast wavelet transform(FWT),as well as the fast Fourier transform(FFT),was employed to analyze the EN data. The results show that the overall corrosion process can be described by three stages. The first stage corresponds to the pit nucleation and growth; the second stage involves the growth of a passive oxide layer; and the third stage involves reactivation. With increasing immersion time,fractal dimension increases fast initially,fluctuates in the medium and increases again at last. Pitting corrosion and fractal dimension increase due to the initiation and formation of pits in the initial and the end of immersion,while depresses due to the passivation in the medium period. The results of SEM and EDX support the above conclusions.

Portable 4-Channel Electrochemical Noise Test System

A portable 4-channel electrochemical noise(EN) test system with high precision was developed.The modular instrument cRIO was used as its core and the signal conditioning module included zero resistance ammeter(ZRA),fly line,screening box and shielded wire.The EN data were acquired from two Q235 carbon steel specimens placed in 0.5 mol/L H2SO4 solution and 0.1 mol/L NaCl solution.The experimental result shows that this system can achieve an accuracy of 10 pA and 10 μV,and it can be applied to on-site multi-channel EN test.

In situ characterization of natural pyrite bioleaching using electrochemical noise technique

An in situ characterization technique called electrochemical noise（ECN） was used to investigate the bioleaching of natural pyrite.ECN experiments were conducted in four active systems（sulfuric acid,ferric-ion,9k culture medium,and bioleaching solutions）.The ECN data were analyzed in both the time and frequency domains.Spectral noise impedance spectra obtained from power spectral density（PSD）plots for different systems were compared.A reaction mechanism was also proposed on the basis of the experimental data analysis.The bioleaching system exhibits the lowest noise resistance of 0.101 MΩ The bioleaching of natural pyrite is considered to be a bio-battery reaction,which distinguishes it from chemical oxidation reactions in ferric-ion and culture-medium（9k） solutions.The corrosion of pyrite becomes more severe over time after the long-term testing of bioleaching.

In-situ monitoring of nickel electrodeposit structure using electrochemical noise technique

The nickel electroplating process was investigated by means of electrochemical noise(EN), cyclic voltammetry in conjunction with the scanning electron microscopy(SEM) technique. The results show that, in the experimental conditions and with the increase of current density, the growth mechanism of nickel crystallites changes from 2-D to 3-D with the potential turning point of about -1.15 V, and the potential for the onset of diffusion control of the ensemble nickel electroplating process was about -1.4 V. In the case of activation-control, the two-dimensional (2-D) nucleation / growth process of nickel often results in the electrocrystallization EN features of only slowly small positive potential drift and the corresponding compact layer-by-layer deposit structure, and the maximum relative energy of the RP-EDP (re-plotted relative energy distribution plot), which is obtained from wavelet analysis, defined in the region with smaller scales. While under the diffusion-control, the three-dimensional (3-D) nucleation / growth process of nickel, often results in the electrocrystallization EN features of both the fast positive potential drift and subsequent remarkable negative potential drift and the corresponding dentritic/large conglomerate structure of nickel deposit, and the maximum relative energy of the RP-EDP defined in the region with larger scales. The electroplating time affects the nickel deposit structure mainly through its influence on the growth rate of crystallites and the Ni2+ ions diffusion process around each crystallite.

Monitoring localized corrosion of Inconel 82 weld overlay on 304L SS weld by electrochemical noise

The manuscript presents the results from the electrochemical noise (EN) monitoring of Inconel 82 weld overlay on Type 304L stainless steel (SS) weld in 0.01M FeCl3. The microstructure of the weld overlay obtained from optical and scanning electron microscopy (SEM) showed an austenite structure, containing equiaxed dendrites and secondary phases at the interdendritic region. Energy dispersive spectroscopy (EDS) attached to SEM revealed the secondary phases to be Nb rich Laves phase. The electrochemical potential noise was monitored using a three identical electrode configuration. The acquired signals were detrended, and wavelet analysis was employed to encode useful information from the noise transients. Visual examination of the potential noise-time record contained distinct high amplitude transients typical of localized corrosion attack. The energy distribution plots (EDP) of the potential noise derived from wavelet analysis depicted maximum relative energy on D6-D8 crystals, which represent large time scale events such as those occurring from localized attacks. Also, repassivation events too could be divulged from the potential EDP. The micrographs of the post electrochemical noise experimented specimens revealed the occurrence of localized attacks along the interdendritic region and none inside the dendritic cores. The presence of secondary phases along the interdendritic regions was found to be detrimental in chloride medium, imparting inferior localized corrosion resistance to the weld overlay.

Quantifi cation of the Atmospheric Corrosion of 304 and 2205 Stainless Steels Using Electrochemical Probes Based on Thevenin Electrochemical Equivalent Circuit Model

Stainless steel(SS)is one of the most widely used engineering materials in marine engineering.However,its corrosion in the marine atmospheric environment due to the high concentration of Cl-is a problem.The SS corrosion is a threat to the development and security of marine industry;therefore,evaluating the corrosion resistance of SSs is necessary.In this work,atmospheric corrosion detection probes based on a symmetrical electrode system were used to study the corrosion behaviors of 304 SS and 2205 duplex stainless steel(DSS)in a simulated marine atmosphere.A theoretical model for electrochemical noise(EN)data analysis based on the Thevenin electrochemical equivalent circuit(EEC)model was established.The relationship between the EN characteristic parameters and the corrosion rate was obtained.The Thevenin EEC model analysis showed that the relationship between the noise resistance(Rn),the noise impedance[Rsn(f)],and the impedance modulus(|Z(f)|)was Rn≈Rsn=■.Thus,Rn and Rsn can be used as indicators for quantitative corrosion evaluation.The results of EN detection for the 304 SS and 2205 DSS showed that in a simulated marine atmospheric environment,the passive fi lms on the two SSs were relatively intact at the initial exposure stage,and their dissolution rates were slow.The corrosion resistance of the 2205 DSS was higher than that of the 304 SS.With the deposition of Cl-on the SS surface,pitting was initiated and the dissolution rate increased.The pitting initiation process on the SS surface was random,and part of the active pores could be repassivated.

Atmospheric Corrosion Monitoring of Field-exposed Q235B and T91 Steels in Zhoushan Offshore Environment Using Electrochemical Probes

This work aimed to in-situ monitor the atmospheric corrosion of steels exposed to Zhoushan offshore environment by using electrochemical noise（EN） technique. A portable EN monitoring system was established and two electrochemical probes（named as Q235 B and T91） were designed. Experimental results indicated that the noise resistance of T91 steel was higher than that of Q235 B steel, revealing that the corrosion resistance of T91 was higher than that of Q235 B. A 60-day monitoring result indicated that the noise resistance was well correlated with the weight loss data. Wavelet analyses results of EN data indicated that Q235 B underwent uniform corrosion and T91 suffered from localized corrosion, which was further confirmed by the surface observation. It is concluded that EN can be used as a new method to identify the corrosion form and corrosion resistance in atmospheric conditions.

LOW-FREQUENCY LOW-NOISE CIRCUITS DESIGN USING AN E_n-I_n MODEL

In view of the limitations of a Rn-Gn model in the low frequency range and the defects of an En-In model in common use now, this paper builds a complete En-In model according to the theory of random harmonic. The parameters for the low-noise design such as the equivalent input noisy voltage Ens, the optimum source impedance Zsopt and the minimum noise figure Fmin can be calculated accurately by using this En-In model because it considers the coherence between the noise sources fully. Moreover, this paper points out that it will cause the maximum 30% miscalculation when neglecting the effects of the correlation coefficient 7. Using the series-series circuits as an example, this paper discusses the methods for the En-In noise analysis of electronic circuits preliminarily and demonstrates its correctness through the comparison between the simulated and measured results of the minimum noise figure Fmin of a single current series negative feedback circuit.

Effect of pH and applied stress on hydrogen sulfide stress corrosion behavior of HSLA steel based on electrochemical noise analysis

The influence mechanism of pH and the externally applied stress on sulfide stress corrosion cracking behavior based on the joint analysis of the in situ electrochemical noise and microstructure was studied.The results showed that{\mathrm{H}}^{+}in solution changes the composition and structure of corrosion product film by affecting the concentration of{\mathrm{S}}^{2-}and{\mathrm{Fe}}^{2+}near the anode surface.When the pH increased from 2.6 to 3.6 and 4.6,the corrosion product film changed from porous Mackinawite to dense and stable FeS.The change in corrosion product type delayed the crack initiation time by 10.5 and 45.5 h,while the uniform corrosion time was prolonged by 6.1 and 46 h,respectively,delaying SSC behavior.After increasing the applied stress,the local plastic deformation on the material surface increases the porosity and crack rate of the corrosion product film and becomes a fast propagation channel for SSC cracks.When the applied stress is 110%of the actual yield strength of the material,the initiation time of stress corrosion cracking is 6 and 18.1 h earlier than that of 90%and 100%,respectively.The local corrosion time was extended by 23.5 and 8.2 h,respectively,accelerating SSC behavior.

Shot noise analysis on corrosion behavior of zinc alloy (ZnAl4Cul) under dry-wet cycles

The corrosion behaviors of zinc alloy （ZnAl4Cul） in 3.5% （mass fraction） NaCl, 7.3% （mass fraction） Na2SO4 and simulated acid rain solutions were investigated using electrochemical measurements. The potential noise during dry-wet cycle was monitored and analyzed by fast Fourier transform （FFT）, fast wavelet transform （FWT）, shot noise theory and stochastic theory. Cumulative probability curves of event frequency fn indicate that the corrosion events in the dry cycles are greater than those in the wet cycles. Uniform corrosion was observed in the NaCl solution compared with more localized corrosion in the Na2SO4 solution, which is evidenced by FWT and SEM. Conditional events generation rate r（t） for diffusion controlled reactions decreases with increasing the time. r（t） values for uniform corrosion and diffusion controlled process are the largest in the wet cycle in 3.5% NaCl solution. The values of r（t） for pitting corrosion in Na2SO4 solution are observed to become large during spraying periods, and r（t） for pitting corrosion has the largest value in the Na2SO4 solution. The intergranular corrosion of zinc is serious in simulated acid rain solution.