The effect of chromated and organic layers on corrosion resistance of galvanized steel sheets

Chromated zinc steel sheets have been used widely for corrosion resistance performance in various waters. The coating system includes a chromated layer and both a primer and topcoat, a commonly applied system for better corrosion performance. Electrochemical impedance spectroscopy results show that the corrosion resistance of chromated zinc is due to its passivity after water uptake during the exposure. In galvanized steel sheets with a primer, the electrolyte solution can only reach the metallic substrate through the coating pores; thus, a double layer could build up locally at the bottom of the pores, resulting in a good corrosion resistance for the sheet. On the other hand, the high-pore-resistance system of primer and topcoat provides no evidence of corrosion even after 8 weeks’ exposure.     

电化学阻抗谱在缓蚀剂研究中的应用进展

概括了电化学交流阻抗谱的基本原理,介绍了电化学阻抗谱在缓蚀剂研究中的应用进展,并对电化学阻抗谱在腐蚀科学领域其他方面的应用进行了展望。     

Stainless steel electrode characterizations by electrochemical impedance spectroscopy for dye-sensitized solar cells

Electrochemical impedance spectroscopy (EIS) was used to understand the electrochemical mechanisms which appear in dye-sensitized solar cells (DSSCs). This qualitative and quantitative technique permits identification of the phenomena proceeding within the different elements composing the cell and at their interfaces.In this study, the classical conducting glass substrate was replaced by a protected stainless steel (304 type) substrate as the counter-electrode (cathode) in dye-sensitized solar cells. Platinum was deposited at the substrate surface to optimize the charge transfer resistance of the electrode.After a few days of immersion in the electrolytic solution, stainless steel substrates coated with low thickness of Pt show pitting corrosion due to iodine. Defects in the Pt layer such as discontinuity of the film and micro-cracks may explain the corrosion of the stainless steel substrate. However the Pt layer degradation is retarded for thicker films. On the other hand, polished substrates show a better behaviour probably due to the elimination of the defects on the stainless steel surface.Electrolytic solution was optimized. For this, components such as 1-butyl-3-methylimidazolium iodide (BMII), guanidine thiocyanate (GT) and 4-tert-butylpyridine (TBP) were added. No corrosion phenomena on stainless steel 304 appeared within 3 days when TBP was added. This means that TBP acts as a corrosion inhibitor.A schematic equivalent circuit is also proposed.     

Using DC electrochemical techniques to assess the relative corrosiveness of water-based coatings and their ingredients

It has been common to assess corrosion inhibitors by ranking their relative corrosion currents at some arbitrary potential above the open circuit potential via DC potentiodynamic anodic scans. Groseclose et al.1 demonstrated that when bare steel panels are subjected to such scans, the difference in mV between the open circuit potential and breakdown potential is a sensitive measure of the panels’ tendency to corrode once coated and exposed to a corrosive environment. The present authors show that a combination of the two DC electrochemical techniques can be used to assess the relative corrosion (or inhibition) contribution of various ingredients in water-based coatings, and that those results translate into the same order of behavior when coatings utilizing those ingredients were applied to steel and tested in a corrosive environment. This technique is also shown to be capable of ranking finished liquid paints for their subsequent corrosion resistance as dried coatings on steel, and verifying the batch consistency of liquid paints in production.

An electrochemical impedance spectroscopy study of fuel cell electrodes

Electrochemical impedance spectroscopy (EIS) was used to study the capacitance and ion transport properties of fuel cell catalyst layers. It was found that limiting capacitance correlates with active area. The capacitance per gram of catalyst was calculated and is proposed as a measure of catalyst utilization. Results obtained with catalyst layers immobilized on glassy carbon electrodes agree very well with results obtained with gas diffusion electrodes. EIS was also used to study ion conductivity and active area in fuel cell electrodes that contain the electroactive probe Os(bpy)₃²⁺. Together, these results validate the hypothesis that the non-ideal behavior of fuel cell electrodes is due to variations of conductivity across the layer, rather than variations in capacitance.     

Corrosion behavior of different alloys exposed to continuous flowing seawater by electrochemical impedance spectroscopy (EIS)

The corrosion of four types of alloys, under a dynamic condition, has been studied in continuous fresh seawater system using electrochemical impedance spectroscopy (EIS) technique. The materials used in this study were stainless steel 304, Cu-Ni 70-30, Hastelloy G-30, and titanium. The total exposure time of the test was 180 days, in continuous fresh seawater of the Gulf in Kuwait. The EIS tests were carried out by using EG&G software and hardware instrument. Electrochemical parameters such as the polarization resistance (R_P), solution resistance (R_Sol), and the double layer capacitance (C_dL) of these alloys were determined. Then the obtained EIS parameters were used to study the effect of the seasonal change of the Gulf seawater on the corrosion behavior of the tested materials. All the obtained EIS parameters showed that the seasonal changes of the Gulf seawater have a significant effect on controlling the rate of the formation of the marine bio-film on the surface of tested materials. Consequently, the corrosion behavior of the materials tends to vary as a function of the rate formation of the marine bio-film on the surface of tested materials.     

Identification of inductive behavior for polyaniline via electrochemical impedance spectroscopy

Polyaniline (PANI) film electrodeposited in HCl medium using cyclic voltammetry (CV) with an upper potential limit of 0.90 V, exhibited an inductive behavior. PANI films deposited with different conditions were subjected to various applied potentials and the impedance characteristics were recorded through electrochemical impedance spectroscopy (EIS). The impedance results clearly reveal the existence of inductive behavior to PANI. Inductive behavior was observed for PANI films deposited with conditions which favor benzoquinone/hydroquinone (BQ/HQ) formation and further evidenced by X-ray photoelectron spectroscopy (XPS). A comparative analysis of the EIS and XPS results of PANI films prepared under similar conditions with the upper potential limits of 0.75 and 0.90 V, respectively, clearly documented that the presence of BQ/HQ, the degradation product of PANI, formed during the electrochemical polymerization at the upper potential limits causes inductive behavior to PANI.     

The corrosion resistance of coated steel dowels determined by impedance spectroscopy

The corrosion resistance of coated smooth steel dowels in simulated pore solution with and without 3.5% sodium chloride solution is reported. The dowels are coated with a double-layer structure composed of a 180 μm of Nickel-Chromium-Boron (referred to as NiCrB) that is overlaid by a 20 μm inorganic layer made of silicon powder, silica fume, blast furnace slag, and combinations thereof. The composition of the synthetic pore solution is based on the composition of pore fluids expressed from Type I portland cement that is characterized by pH of about 13.4 and an ionic strength of 0.4.Alternating current impedance spectroscopy measurements conducted in the range of 10 mHz to 10 kHz were found to be instrumental in detecting any change in the resistance of the coatings. It was found that the NiCrB sub-layer provides a good corrosion resistance that outperforms that of the inorganic coating. Of the five inorganic compositions tested, the combination of slag and silica fume has the best performance. The protection against corrosion is attributed mostly to particle packing rather than densification of the matrix due to pozzolanic reaction.     

The electrochemical synthesis of poly(aniline-co-o-anisidine) on copper and their corrosion performances

The copolymer films PANI-co-POA and poly(aniline-co-o-anisidine) were carried out on copper (Cu) electrode, by applying two different scan rates (20 and 50 mV s⁻¹) and using two different thicknesses at high scan rate. Synthesizes were achieved under cyclic voltammetric conditions from 0.075 M aniline and 0.075 M o-anisidine containing sodium oxalate solutions. The synthesized copolymer films were strongly adherent and homogeneous in both cases. AC impedance spectroscopy (EIS), anodic polarization plots and open circuit potential–time curves were used to evaluate the corrosion performance of copolymer coated and uncoated electrodes in 3.5% NaCl. It was shown that the copolymer film coated at low scan rate exhibited a better property initially when compared with the copolymer film produced at high scan rate. However, it could not resist the attack of corrosion products, in longer time and meanwhile its barrier property significantly diminished. It was found that the thin copolymer film produced at high scan rate by its catalysing effect led to the formation of highly protective copper oxides on the surface whereby providing a better protection for long exposure times. It also emerged that the corrosion resistances of thin copolymer film produced at high scan rate and copolymer film synthesized at low scan rate were almost same and relatively higher for much longer periods when compared with the one observed for bare copper electrode.     

Characterization by electrochemical impedance spectroscopy of magnetite nanoparticles supported on carbon paste electrode

Magnetite nanoparticles were supported on carbon paste electrode and characterized by low scan rate voltammetry and electrochemical impedance spectroscopy (EIS) to obtain mechanistic information related to its oxidation and reduction in acid media.The voltammograms showed only one reduction and one oxidation peak for the supported magnetite, which were attributed to formation of ferrous ion and ferric oxide, respectively. Both peaks are fairly wide, indicating complex mechanisms.Using EIS, a mechanism showing up to three time constants, capacitive all of them, was evidenced, both in anodic and cathodic domain. These were attributed to charge transfer at the highest frequencies, adsorption of generated species at intermediate frequencies, and proton adsorption at low frequencies. Discussion about the nature of the adsorbed species and the concerned mechanism for each domain is developed.     

Defect dimension evaluation in organic coated galvanized steel by electrochemical impedance spectroscopy

The protective effectiveness of organic coatings, in controlling corrosion processes by the barrier effect, is dominated by the absence of defects passing through the coating and reaching the substrate. It is, however, difficult in general to identify and quantify the presence of defects. This work is an effort to reach a more precise quantification of the size of defect in organic coatings by means of electrochemical impedance spectroscopy (EIS) measurements. Artificial defects with controlled dimensions between 60 and 200 m were produced on organic coated galvanized steel (coil coating). After optimization of the experimental procedure for EIS data acquisition, the parameters obtained, according to a classical electrical model, were correlated with the defect dimensions. The results show that the double layer capacitance (C _dl) values depend in linearly on the defects area, while this is not true for the pore resistance (R _p) values, as the electrolyte resistivity inside the defects is a function of the defect size. Further work is necessary to extend the results to smaller defects and different systems, that is, different organic coatings and substrates.     

Corrosion of 7A04 aluminum alloy under defected epoxy coating studied by localized electrochemical impedance spectroscopy

In this work, corrosion of the coated 7A04 aluminum alloy with and without ultraviolet (UV) aging was studied by salt spray test, surface observation and localized electrochemical impedance spectroscopy measurements. Results demonstrated that, in the absence of UV aging, the impedance at coating defect increases with time, indicating the formation and accumulation of corrosion product, while the impedance in the adjacent coating decreases, which is attributed to the permeation of corrosive solution from defect and the resultant disbondment of coating. Upon UV aging, the impedance values at both defected area and the coating decrease. The coating contains a significant number of wrinkles. The wrinkles provide micro-pathways where the corrosive species, such as chloride ions, could permeate and reach the aluminum alloy surface for corrosion reaction. Different from the coating without UV aging where corrosion is limited to the defected area, a layer of corrosion product would not be formed to cover the defected area when coating experiences UV aging. Therefore, corrosion occurs continuously with time.     

Electrochemical behaviour of PANi/polyurethane antifouling coating in salt water studied by electrochemical impedance spectroscopy

Electrochemical behaviour of polyaniline–polyurethane (PANi–PU) antifouling coating in 3.5 wt% NaCl is studied by electrochemical impedance spectroscopy (EIS). A thick coating (∼1 mm) of 10, 15 and 20% PANi in marine grade PU, is cast over corrosion resistant aluminium alloy 2024 and its impedance characteristics are measured by EIS and compared with neat PU. On addition of 10% PANi, the impedance of the coating drastically comes down from 10⁹ to 10⁷ Ω. 20% is the maximum processable amount of PANi for the selected PU system. The coatings are exposed to 3.5 wt% NaCl and its impedance characteristics are monitored as a function of time. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all cases, though their evolution with time showed marked differences with PANi content. Water sorption and break down frequency are derived from the experimental results and analysed.     

Study on the corrosion behavior of reinforcing steel in simulated concrete pore solutions using in situ Raman spectroscopy assisted by electrochemical techniques

In situ Raman spectroscopy, electrochemical impedance spectroscopy (EIS) and polarization curves were used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions (saturated Ca(OH)₂ solutions). Results indicated that the reinforcing steel remained passive in chloride-free SCP solutions. However, the anodic polarization curve of the steel did not exhibit a stable passive region in the SCP solution with 0.5 M NaCl, the corrosion current density exceeded 0.1 μA cm⁻², the steel surface was unstable with chloride attack and localized corrosion appeared on it with FeCO₃ and Fe₂O₃ as the main corrosion products.     

电化学阻抗谱在电沉积研究中的应用(一)

介绍了电化学阻抗谱在各种金属及合金的电沉积研究中的应用。文章分3期连载。第一部分介绍了电化学阻抗谱的基础知识,包括复数、复阻抗的概念,以及在各种常见条件下电解池的等效电路图。     

Simulation of transient electrochemical impedance spectroscopy due to water uptake or oxide growth

Coatings are often employed to prevent, or at least delay, corrosion of a metallic substrate. The topcoat is often also intended to be a barrier to environmental moisture and any entrained, corrosive ions reaching the substrate. The protective ability of a coating system is very well monitored by electrochemical impedance spectroscopy since it can measure changes as the protective coatings deteriorate, as they absorb water and as they permit the substrate to corrode. Infiltration of water and ions into the coating can also be measured by EIS to deduce approximate water uptake. EIS analysis in coating systems is often predicated on the system being stable over the timeframe of the spectral measurement, which if taken from 100 kHz to 0.01 Hz, or even 0.001 Hz, may take as long as an hour, or more. EIS measurement is most often accomplished by the use of an aqueous electrolyte solution as the electrical contact at the top of the coating, with the metallic substrate used as the other electrical contact. The ingress of aqueous solution is likely to result in time-dependent dielectric or chemical changes to the coating/substrate system. This work shows how these changes, during the spectral measurement period, produce a Bode plot that has a slope of magnitude slightly less than 1. This capacitive response to non-steady state water uptake is commonly characterized by a constant phase element (CPE) in an equivalent circuit model for the material, and equations developed here can be used to deduce the diffusion coefficient based on the CPE.     

Comparative EIS study of pretreatment performance in coated metals

Various coated metal samples with different pretreatments were investigated by electrochemical impedance spectroscopy (EIS). Variables were the substrate (cold-rolled steel and hot-dipped galvanized steel), phosphate system (iron and zinc phosphate), post rinse (chromate and silane/zirconium rinse) and paint systems. The corrosion performance was determined on the basis of coating degradation, water uptake and interface delamination of the tested samples. The zinc phosphate performed better than iron phosphate on CRS. The silane/Zr rinse did not perform well in the CRS/iron-phosphate system. However, it showed a better performance than the chromate when used as a post rinse of zinc phosphate. Salt spray test (SST) and adhesion test results of the same samples are also reported in this paper and compared to the EIS data. The correlation among three test methods was poor.     

Interaction of BSA protein with copper evaluated by electrochemical impedance spectroscopy and quartz crystal microbalance

The interaction of bovine serum albumin (BSA) protein with copper in phosphate buffer solution has been studied by a combination of electrochemical impedance spectroscopy (EIS) close to the open circuit potential, with simultaneous monitoring by the electrochemical quartz crystal microbalance (EQCM), in order to throw light on BSA adsorption. Copper films were electroplated onto gold quartz crystals and mounted in the EQCM. Experiments were conducted in the presence and absence of dissolved oxygen and of BSA and the results show the influence of O₂ on the protein/metal interaction and also show specific interactions between BSA and copper. The good reproducibility obtained in these experiments suggests future application to other systems and which should lead to a better understanding of the use of such types of protein as corrosion inhibitors.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

Corrosion of steel under the defected coating studied by localized electrochemical impedance spectroscopy

Corrosion of steel under the defected coating in near-neutral pH solution was investigated by localized electrochemical impedance spectroscopy (LEIS) measurements. The LEIS response is dependent on the size of the defect. For small defects, e.g., less than 200 μm in diameter, localized corrosion process and mechanism of steel, as indicated by the measured LEIS plots, change with time. The diffusion process dominates the interfacial corrosion reaction, which is due to the block effect of the deposited corrosion product combined with the geometrical factor of a large coating thickness/defect width ratio. In the presence of a big defect, e.g., up to 1000 μm, the LEIS responses measured at the defect are always featured by a coating impedance in the high-frequency range and an interfacial corrosion reaction in the low-frequency range. The block effect of corrosion product does not apply due to the relatively open geometry. Conventional EIS measurements on a macroscopic-coated electrode reflect the “averaged” impedance results from both coating and defect. The information of the localized electrochemical corrosion processes and mechanisms at the small defect is lost, and the coating impedance information is “averaged” out when a big defect is contained. LEIS measurement provides an essential technique to characterize microscopically the local electrochemical corrosion reaction of steel under the defected coating.