Evaluation of the corrosion resistance of anodized aluminum 6061 using electrochemical impedance spectroscopy (EIS)

The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated using electrochemical impedance spectroscopy (EIS). The scanning electron microscope (SEM) was employed to determine the surface structure and the thickness of the anodized layers. The EIS data revealed that there was very little change of the properties of the anodized layers for samples that were hard anodized in a mixed acid solution and sealed in hot water over a 365 day exposure period in a 3.5 wt% NaCl solution. The specific admittance A_s and the breakpoint frequency f_b remained constant with exposure time confirming that the hard anodizing process used in this study was very effective in providing excellent corrosion resistance of anodized Al 6061 over extended exposure periods. Some minor degradation of the protective properties of the anodized layers was observed for samples that were hard anodized in H₂SO₄ and exposed to the NaCl solution for 14 days.     

Another approach for ranking and evaluating organic paint coatings via electrochemical impedance spectroscopy

This paper proposes an approach for ranking organic protective coatings via electrochemical impedance spectroscopy (EIS). The areas under the Bode plots have been determined and used as useful parameters to evaluate and to rank six commercial paints with high, intermediate, and poor qualities. Decreasing percentages (DP) of the areas under Bode plots were determined during the immersion periods. It is found that DP of the areas under Bode plots are useful parameters for evaluating coating degradation during the immersion times. Results from this work were in good agreements with the results from electrical equivalent circuit (EEC) models.     

In situ characterization by localized electrochemical impedance spectroscopy of the electrochemical activity of microscopic inclusions in an X100 steel

A localized electrochemical impedance spectroscopy (LEIS) technique was used to characterize in situ the micro-electrochemical activity of inclusions contained in an American Petroleum Institute (API) X100 steel in a near-neutral pH solution. It is found that there exists an electrochemical heterogeneity between inclusions and the adjacent steel matrix. Consequently, a galvanic couple is formed to result in the locally preferential dissolution. The local electrochemical activity of the inclusion depends on its composition. A Si-enriched inclusion is associated with a high electrochemical activity, and the preferential dissolution of the inclusion generates a local microvoid, whose further dissolution initiates a corrosion pit. An aluminum oxide-enriched inclusion is more stable than the adjacent steel matrix. The preferential dissolution would occur on the steel, causing the “drop-off” of the inclusion and generating a corrosion pit.     

Corrosion study of Ni/Zn compositionally modulated multilayer coatings using electrochemical impedance spectroscopy

Ni/Zn compositionally modulated multilayer (CMM) coatings were deposited using dual bath technique. Coatings corrosion performance was evaluated using electrochemical impedance spectroscopy (EIS) during extended immersion times up to 48 h. The results of electrochemical impedance spectroscopy showed that Ni/Zn CMM coatings had better corrosion resistance compared to that of the zinc single layer coating. The modified corrosion product which is formed on the Ni/Zn CMM coatings during extended exposure times and also a good barrier effect of the nickel layer against aggressive species in these coatings can be two important reasons for high corrosion performance and so protection performance of the Ni/Zn CMM coatings.     

The effects of sulfate reducing bacteria on corrosion of carbon steel Q235 under simulated disbonded coating by using electrochemical impedance spectroscopy

The effect of sulfate reducing bacteria (SRB) on the corrosion of the carbon steel Q235 has been investigated in the crevice under the simulated disbonded coating in the soil-extract solutions (SES). The results of electrochemical impedance spectroscopy (EIS) show that the corrosion rate is inhibited in the SES with SRB during the stationary phase of SRB, but enhanced during the death phase. The comparison of the polarization (R_p) and the charge transfer resistances (R_t) has indicated that the biofilm seriously influences the reactive procedure of metal/solution interface. SRB is found in the pits on the surface of the steel.     

Contribution to a better understanding of different behaviour patterns observed with organic coatings evaluated by electrochemical impedance spectroscopy

In the present paper, three different coatings (epoxy, alkyd, polyurethane paints) were characterized by electrochemical impedance measurements, permeability tests, free-standing film impedance and local impedance measurements. The increase in resistance with immersion time of alkyd paint was linked to the nature of the polymeric network and not to phenomena occurring at the metal/paint interface. For polyurethane paint, local impedance was not able to detect defects in the paint, which was attributed to the fact that the defects are smaller than those observed in alkyd paint and also that they are less active and homogeneously distributed through the coating. Although electrochemical impedance methods (global and local) are excellent tools to monitor the behaviour of organic coatings, these techniques alone are not sufficient to screen different paints.     

EIS study of a self-repairing microarc oxidation coating

The self-repairing microarc oxidation (MAO) coating consisting of a bottom nanocrystalline layer covered by a top conversion ceramic coating was fabricated on 2024 Al alloy by a duplex process with surface mechanical attrition (SMAT) prior to microarc MAO treatment. A 20 μm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated by SMAT, and on which covered by a top MAO coating of 5 μm. The self-repairing property caused by the formation of a dense passive film at the damaged regions contacting the bottom nanocrystalline layer enhances the corrosion reisitance of the SMAT-MAO coating.     

Evaluation of microstructural evolution of thermal barrier coatings exposed to Na2SO4 using impedance spectroscopy

Impedance spectroscopy as a non-destructive evaluation technique was employed to study the microstructural evolution of thermal barrier coatings exposed to Na₂SO₄ at 950 °C. The results showed that the resistance and capacitance of yttria-stabilized zirconia top coat increased with corrosion time. The resistance of thermally growth oxide (TGO) from 20 to 60 h decreased slowly, which indicates there is little change in the composition of TGO. The fast decrease in the resistance of TGO from 60 to 100 h may correspond to the compositional change of TGO from a-Al₂O₃ to more conductive NiO and Ni(Cr, Al)₂O₄ with corrosion time.     

Corrosion stability of epoxy coatings on aluminum pretreated by vinyltriethoxysilane

The electrochemical and transport properties and adhesion of epoxy coatings electrodeposited on aluminum pretreated by vinyltriethoxysilane (VTES) were investigated during exposure to 3% NaCl. The VTES films were deposited on aluminum surface from 2% and 5% vinyltriethoxysilane solution. The electrochemical results showed that the pretreatment based on VTES film deposited from 5% solution provides enhanced barrier properties and excellent corrosion protection. The values of diffusion coefficient of water through epoxy coating on this substrate and water content inside the epoxy coating were the smallest, indicating the low porosity of the coating. In addition, the good adhesion was maintained throughout the whole investigated time period.     

Effect of sintering temperature on corrosion properties of sol–gel based Al2O3 coatings on pre-treated AZ91D magnesium alloy

A novel anti-corrosion sol–gel based Al₂O₃ coating was developed on the AZ91D magnesium alloy. The morphology, microstructure and composition of the coatings were investigated by scanning electron microscope coupled with energy dispersive spectroscopy, Fourier transform infrared spectrum analysis, X-ray diffraction, thermo-gravimetric and differential thermal analysis. The corrosion resistance of the coatings in 3.5 NaCl wt.% solution was studied using electrochemical measurements. The results demonstrated that a homogeneous Al₂O₃ coating could be obtained and the sol–gel coated samples sintered at 380 °C had the best corrosion resistance properties as compared to the specimens sintered at 120 and 280 °C.     

Direct electrosynthesis of polyaniline–montmorrilonite nanocomposite coatings on aluminum alloy 3004 and their corrosion protection performance

Homogeneous and adherent polyaniline–montmorrilonite (MMT) nanocomposite coatings were electrosynthesized on aluminum (Al) alloy 3004 (AA 3004) by using the galvanostatic polarization method. The synthesized coatings were characterized by UV–Vis absorption spectrometry, Fourier transform infrared spectroscopy, X-ray diffraction patterns and scanning electron microscopy. The corrosion protection effect of the coatings was demonstrated by performing a series of electrochemical experiments of potentiodynamic and impedance measurements on Al in 3.5 wt% aqueous NaCl electrolytes. The corrosion current (i_corr) values decreased from 6.55 μA cm⁻² for uncoated Al to 0.102 μA cm⁻² for nanocomposite-coated Al under optimal conditions.     

Validation of electrochemical impedance measurements for water sorption into epoxy coatings using gravimetry and infra-red spectroscopy

In this paper, the sorption/diffusion of water from an aqueous electrolyte into a commercial epoxy paint coating was compared using gravimetry, electrochemical impedance spectroscopy (EIS) and attenuated total reflection infra-red spectroscopy (ATR-IR) techniques as a function of electrolyte composition. All three techniques show an apparent two-stage diffusion process. Diffusion coefficients for water sorption obtained by ATR are consistent with those obtained using (conventional) gravimetric methods and impedance spectroscopy (EIS). We discuss the methods used to derive diffusion coefficients, and concentration of the sorbed species, from capacitance measurements. As such, we postulate that the cause of anomalous diffusion behaviour is due to the fact that the penetrant enters the polymer because of classical diffusion down the concentration gradients, which are modified a result of the time dependence of the surface concentration. Hence, an apparent two-stage sorption profile is not necessarily significant in a heterogeneous coating and Brasher’s equation remains the most appropriate for data analysis.     

In-situ study of the formation process of stannate conversion coatings on AZ91D magnesium alloy using electrochemical noise

The formation process of stannate conversion coating (CC) on AZ91D alloy was in-situ investigated by electrochemical noise (EN). The wavelet transform, as well as noise resistance (R_n) and spectral noise resistance (R_sn), had been employed to analyze the EN data. It was revealed that there exist two distinguishing stages of stannate CC formation process on AZ91D alloy, including an incubation stage companying with the nucleation and nuclei dissolution process, a periodical growth stage involving hemispherical particles growth and coating dissolution process. Furthermore, the results demonstrated that EN was a powerful tool to investigate rapid electrochemical process, such as CC formation process.     

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

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.     

Comparative study on the corrosion behavior of milled and unmilled magnesium by electrochemical impedance spectroscopy

The corrosion behavior of milled Mg prepared by high-energy ball milling for 10 h has been investigated in alkaline solutions by electrochemical impedance spectroscopy and compared with that of unmilled Mg. X-ray powder diffraction indicates a crystallite size of 34 nm for the milled Mg compared to >100 nm for the unmilled powder. Chemical analyses show no significant iron contamination in milled Mg powder, indicating the absence of tools erosion during the milling procedure. In contrast, significant MgO enrichment in the milled powder is observed (6.5 wt.% after 10 h milling compared to 1.0 wt.% before milling). The oxygen contamination is mainly attributed to the powder oxidation occurring during milling. From XPS analyses, no MgO enrichment is detected on milled Mg electrode surface, confirming that MgO is dispersed homogeneously in the bulk of the material rather than to segregate on its surface. Electrochemical impedance spectroscopy demonstrates clearly the better corrosion resistance of milled Mg compared to unmilled Mg in passive conditions (KOH solution, pH=14) and in more active corrosion conditions (borate solution, pH=8.4). This is illustrated by a nobler corrosion potential and by a significant increase of the interfacial resistance related to the film and charge-transfer reaction. Moreover, the variation of the different electrochemical parameters (corrosion potential, interfacial resistance and capacitance) with immersion time is less accentuated and tends more rapidly to a steady state with milled Mg, suggesting an enhancement of the Mg(OH)₂ formation kinetic. The origin of the distinctive passivation behavior of ball-milled Mg is discussed.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

The effects of immersion time on morphology and electrochemical properties of the Cr(III)-based conversion coatings on zinc coated steel surface

The main purpose of this paper is to develop a dynamic and non-destructive method to quantify and correlate the microstructure changes of the Cr(III) layer by electrochemical techniques. The open circuit potential (OCP) analysis reveals the nucleation growth mechanisms of the Cr(III) layer and the dissolution phenomena of Zn. In addition, the effects of immersion time to the corrosion behavior of Cr(III)-based conversion coatings (TCCCs) on electrogalvanized steel were studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5% NaCl solution. Furthermore, surface morphology of the Cr(III) coatings under different immersion times was examined using both a scanning electron microscope and an atomic force microscope.From the potentiodynamic polarization experiment, the corrosion current density (I_corr) of the specimen with immersion time of 60 s was found appreciably small, representing the inheritance of the best anticorrosion performance. Additionally, the corrosion resistance of the Cr(III)-coating for the specimens obtained between 30 s and 60 s is two order higher than those of the untreated specimen from the EIS experiments. Results show that the quality of Cr(III)-based conversion coatings was strongly influenced by the immersion time of Cr(III) solution. And the optimal immersion time is recommended in the range of 30–60 s.     

Influence of temperature and applied potential on the electrochemical behaviour of nickel in LiBr solutions by means of electrochemical impedance spectroscopy

The effects of temperature, applied potential and hydrogen generation on the passive behaviour of nickel were investigated in lithium bromide aqueous solution using different electrochemical techniques: open circuit potential (OCP), potentiodynamic and potentiostatic measurements, and electrochemical impedance spectroscopy (EIS). From the polarization curves, it is observed that localised corrosion resistance decreases with temperature, the repassivation of nickel is more difficult at 75 °C and the hydrogen evolution reaction is favoured with an increase in temperature. Impedance results showed that the most suitable corrosion mechanism of nickel in LiBr solutions includes the double layer and the passive film formed on the nickel surface. The passive film of nickel partially disappears when a low cathodic potential is applied.     

Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions

The corrosion deterioration process of plasma electrolytic oxidation (PEO) coatings on AM50 magnesium alloy prepared from two different based electrolytes, i.e., an alkaline phosphate electrolyte and an acidic fluozirconate electrolyte, were investigated using electrochemical impedance spectroscopy (EIS) in a 0.1 M NaCl solution with pH of 3, 7 and 11, respectively. It was found that the PEO coating formed in alkaline phosphate electrolyte, which was composed mainly of MgO, suffered from rapid chemical dissolution and lost its protection capability very quickly in acidic NaCl solution (pH 3). The chemical dissolution of this PEO coating was retarded in neutral NaCl solution (pH 7) and the corrosion damage was localized in this environment. On the other hand, in the alkaline NaCl solution (pH 11), the MgO coating underwent only slight degradation. The PEO coating produced in acidic fluozirconate electrolyte, the failure was marked by the flaking-off of the large areas of coating in acidic NaCl solution (pH 3). However, in the neutral and alkaline NaCl solutions, the coating underwent only a slight degradation without any observable corrosion damage in the 50 h test. The results showed that the deterioration process of PEO coated magnesium alloy was governed mostly by the pH of NaCl solution and it was also strongly related to the microstructure and composition of the PEO coatings.