Uses of scanning electrochemical microscopy for the characterization of thin inhibitor films on reactive metals: The protection of copper surfaces by benzotriazole

Scanning electrochemical microscopy (SECM) was used to study the film formation of benzotriazole towards corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times to freely attain its open circuit potential in the test environment. Following exposure to aggressive electrolytes the anticorrosion abilities of the layers were characterized by image analysis and by an electrochemical method derived from the experimental approach curves. Changes in the shape of the approach curves were clearly observed during the inhibitor film formation process. They showed the transition from an active conducting behaviour towards ferrocinium reoxidation typical of unprotected copper, to a surface exhibiting insulating characteristics when the metal was covered by a surface film containing the inhibitor. This supports that SECM is a practical technique in the investigation of corrosion inhibitor performance. However, a consistent tendency for the characterization of inhibitor film formation using SECM measurements in the positive feedback mode for the copper-benzotriazole system was only found if the experiments were conducted when the inhibitor molecule was not present in the test solution. That is, inhibitor molecules were found to interact not only with the copper surface during the monitoring process, but with the SECM tip as well, this effect being significantly enhanced when chloride ions were present in the electrolyte. Finally, a procedure to image the chemical activity of copper surfaces partially covered with the inhibitor film with SECM is proposed.     

Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy

In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion.     

Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode

Herein, the redox competition mode of scanning electrochemical microscopy (RC-SECM) has been applied to in situ monitor the local reactivity arising from a circular holiday operated in a painted metal. The metal-coating system consisted of a carbon steel substrate coated with an epoxy-polyamine polymeric film containing glass flakes as pigment. The present work demonstrates the possibility to use RC-SECM for investigation of the corrosion reactions occurring when protective coatings are applied on a metal surface without the addition of a redox mediator to the experimental system. Oxygen reduction was employed to monitor the reactive metal-polymer system, though the onset of a redox competition for this redox species between the SECM tip and the bare metal inside the holiday could be found in certain conditions. Whether attack predominated inside the holiday or the system became non-reactive depended on the composition of the test electrolyte, with borate ions acting as corrosion inhibitor.     

Scanning microelectrochemical characterization of the anti-corrosion performance of inhibitor films formed by 2-mercaptobenzimidazole on copper

The aim of this work is to explore the applicability of the scanning electrochemical microscope (SECM) to characterize the inhibiting effect of 2-mercaptobenzimidazole against the corrosion of copper. SECM was operated in the feedback mode by using ferrocene-methanol as redox mediator, and the sample was left unbiased at all times. The kinetic changes in the corrosion processes were monitored over time from the Z-approach curves. Furthermore, inhibitor-modified copper samples presenting various surface finishes were imaged by SECM and the scanning vibrating electrode technique (SVET), allowing changes both in the surface activity of metal-inhibitor films and in the extent of corrosion attack to be spatially resolved. Differences in the local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample were successfully monitored.     

The use of scanning electrochemical microscopy for the characterisation of patinas on copper alloys

The scanning electrochemical microscopy (SECM) was employed in the feedback mode for the visualisation of the local changes of morphology and reactivity, occurring on the surface of a quaternary copper-based alloy, due to exposure to environment. First, samples artificially aged by exposure to leaching acid rain were tested. The layer of corrosion products (“patina”) was investigated by performing SECM scans and approach curves, and the information provided by this electrochemical technique was compared with SEM–EDX–Raman characterisation. Successively, to highlight the early stages of localised corrosion processes, in situ examination of the surface exposed to an acidic environment was performed. The results show that SECM is a powerful characterisation tool of the deterioration process and is able to map the precursor sites of corrosion on the bronze surface.     

Visualisation of the local electrochemical activity of thermal sprayed anti-corrosion coatings using scanning electrochemical microscopy

Scanning electrochemical microscopy was used to study the electrochemical activity of anti-corrosion coatings formed from Inconel 625, a Ni-Cr-Mo alloy commonly used in engineering applications. The coatings were formed using a high velocity oxygen fuel thermal spraying technique. Upon spraying the alloy onto mild steel substrates, clear splat boundaries were formed at the interface between adjacent droplets as they cooled on the substrate surface. Scanning electrochemical microscopy in the feedback mode, employing ferrocenemethanol as redox mediator, was used to study the local electrochemical activity of samples of the wrought alloy, the sintered alloy and the thermal sprayed coating. The wrought and sintered materials showed responses typical of that expected for a purely insulating material. However, SECM approach curve data showed that the electrochemical activity of the thermal sprayed material was higher than that of the bulk alloy. Local variations in the coating's electrochemical activity were then visualised using SECM imaging, which appear to be related to the splat boundaries formed during the thermal spray process.     

Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM)

The redox competition mode of scanning electrochemical microscopy (RC-SECM) has been utilized to visualize the local electrocatalytic activity of metalloporphyrin spots towards oxygen reduction in 0.1 M phosphate buffer as electrolyte solution. The metalloporphyrin spots were obtained by electrochemically induced deposition using a droplet cell. Tetratolyl porphyrins (TTPs) of Mn, Fe and Co have been investigated, with that containing Mn as central metal atom showing highest catalytic activity. The multiple stable oxidation states of Mn were seen as a key factor in the influence of the metal ion on the catalytic activity. From the RC-SECM results, it is shown that oxygen reduction at a manganese TTP (MnTTP) modified electrode surface yielded the least amount of H₂O₂ when compared to iron TTP (FeTTP) and cobalt TTP (CoTTP). As further confirmed by means of rotating disc electrode (RDE) measurements this was attributed to the high activity of MnTTP for H₂O₂ reduction.     

Extending surface Raman spectroscopy to transition metals for practical applications IV. A study on corrosion inhibition of benzotriazole on bare Fe electrodes

The emphasis in the present study was placed on developing Raman spectroscopy into a versatile technique, which offers an opportunity for investigating the inhibition effect on the corrosion process of bare Fe surfaces. Several surface pretreatments have been developed to bare Fe electrodes in order to obtain a surface of optimal surface-enhanced Raman scattering (SERS). It has been shown that the surface enhancement factor (SEF) of a bare Fe electrode can reach about two to three orders, depending on the roughening procedure. Therefore, SERS can be extended successfully to study some Fe electrode systems of practical importance. Here we present a study on the film formation process and inhibition effect of benzotriazole (BTA) on Fe surfaces. The results show that BTA interacts with Fe surface through its two N atoms of the triazole ring and surface complex polymer of [Fe_n(BTA)_p]_m is formed, which may suppress the dissolution and oxidation of Fe effectively. In addition, the solution pH, the synergetic effect of I⁻ with BTA was revealed to have a significant influence on the inhibition efficiency.     

Scanning electrochemical microscopy for the investigation of localized degradation processes in coated metals

Scanning electrochemical microscopy has been employed to obtain spatially-resolved information regarding surface topology, water uptake and blister formation at intact coatings, as well as the onset and progress of corrosion reactions within coating defects. Topographical lines and maps as well as chronoamperometric plots were measured during operation in the feedback mode. Next, the release of metal ions at the anodic sites and the consumption of oxygen at the cathodic sites developed in holidays could be monitored during operation in the generator–collector mode. Furthermore, the surface topography of defective coatings was imaged by using the redox competition mode.     

Photoelectrochemical kinetics of Eosin Y-sensitized zinc oxide films investigated by scanning electrochemical microscopy under illumination with different LED

The overall efficiency of the light-induced charge separation in dye-sensitized solar cells depends on the kinetic competition between back electron transfer and dye regeneration processes by a redox electrolyte. In a previous study, the reduction of the intermittently formed photo-oxidized dye molecules by iodide ions in the electrolyte phase was investigated using the feedback mode of a scanning electrochemical microscope (SECM) and a quantitative model had been derived. Here we provide a more thorough experimental verification of this model by variation of the excitation wavelength, light intensities and mediator concentrations. Nanoporous ZnO/Eosin Y films prepared by self-assembly were used as model electrodes and were used with an iodide/triiodide electrolyte. The experimentally found effective rate constants could be related to the rate constant for the reaction of the dissolved donor with photo-oxidized Eosin Y bound to ZnO and the absorption spectrum of the dye and confirmed the assumption made in the derivation of the model. For the regeneration process of Eosin Y, a rate constant of k_ox with different light emitting diodes and light intensities is determined.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Visualization of electrocatalytic activity of microstructured metal hexacyanoferrates by means of redox competition mode of scanning electrochemical microscopy (RC-SECM)

Transition metal hexacyanoferrates are versatile inorganic compounds widely employed for the assembling of sensors and biosensors in a variety of different electroanaytical applications. A modified version of the recently introduced redox competition mode of scanning electrochemical microscopy (RC-SECM) was exploited to visualize the local electrocatalytic activity of microstructured Prussian blue (PB) films towards the reduction of H₂O₂ with improved lateral resolution. The PB films were electrochemically deposited in a spot on glassy carbon surfaces using a droplet cell. The influence of the potential applied to the PB modified surface on the current at the SECM tip was evaluated when both the sample and the SECM tip were competing for H₂O₂ in solution. Thus, high local electrocatalytic activity is indicated by low currents at the SECM tip. The same strategy was successfully employed for the characterization of the performances of a biosensor employing the enzyme glucose oxidase (GOx) immobilized within a polymer hydrogel matrix on the top of PB-modified glassy carbon electrodes.     

高分子材料微观形态结构的扫描电镜研究

利用扫描电镜对聚乙烯(PE)、丙烯腈-丁二烯-苯乙烯共聚物(ABS)等高分子材料填料的分散性及表面与断裂界面进行了研究。结果表明,扫描电镜能够详细观察研究高分子材料的表面结构、微观相分离等,便于表征高分子材料微观结构形态,是分析高分子材料微观结构形态的有效手段。     

Generation of platinum microstructures on non-conducting surfaces by means of the scanning electrochemical microscope (SECM)

The scanning electrochemical microscope (SECM) was used to form platinum microstructures. For this purpose, a thin layer of platinum dichloride was deposited as precursor on different substrates by evaporating it in high vacuum. For the reduction of this precursor the SECM provided methyl viologen radical cations locally as reducing agent. The mechanism of the reduction is discussed.     

Investigation of the corrosion protection of SiOx-like oxide films deposited by plasma-enhanced chemical vapor deposition onto carbon steel

The paper reports on the corrosion behavior of carbon steel coated with thin SiO_x-like oxide films. The SiO_x-like coatings were deposited by plasma-enhanced chemical vapor deposition (PECVD) and their thickness was varied between 20 and 200 nm. The coated carbon steel interfaces were investigated for their corrosion protection efficiency when immersed in an aqueous saline solution of 3% NaCl. FTIR measurements and electrochemical impedance spectroscopy (EIS) experiments revealed that thin SiO_x-like coating layers (20 nm thick) do not prevent the carbon steel from corrosion, while thicker silica layers (d ≥ 100 nm) protect efficiently carbon steel interfaces in highly saline media with a protection efficiency of about 96% for a 200 nm thick coating.     

Cyclic voltammetry and scanning electrochemical microscopy studies of methylene blue immobilized on the self-assembled monolayer of n-dodecanethiol

Electron transfer (ET) kinetics through n-dodecanethiol (C₁₂SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN)₆^4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC^0/+) through the C₁₂SH monolayer yielding standard tunneling rate constant () of (4 ± 1) × 10⁻¹¹ and (3 ± 1) × 10⁻¹⁰ cm s⁻¹ for Fe(CN)₆^4−/3− and FMC^0/+ respectively. Decay tunneling constants (β) of 0.97 and 0.96 Å⁻¹ for saturated alkane thiol chains were obtained using Fe(CN)₆⁴⁻ and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C₁₂SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k_BI = (5 ± 1) × 10⁶ and (4 ± 2) × 10⁷ cm³ mol⁻¹ s⁻¹ for the bimolecular ET and and (7 ± 3) × 10⁻² s⁻¹ for the standard tunneling rate constant of ET using Fe(CN)₆^4−/3− and FMC^0/+ probes respectively.     

双向脉冲电镀纳米级镍镀层耐腐蚀性能研究

用直流电沉积法制备了普通光亮镍镀层,同时用双向脉冲电镀制备了纳米级镍镀层。用X射线衍射(XRD)、扫描电镜(SEM)等方法研究了镀层的晶粒尺寸、组织结构和表面形貌,通过孔隙率测定、盐雾试验、静态浸泡腐蚀失重试验和电化学方法等测试了镀层的耐蚀性能。结果表明,采用双向脉冲电流制备的纳米级镍镀层的耐蚀性明显优于普通直流镍镀层。     

Influence of pH on the synthesis and properties of polypyrrole on copper from phytic acid solution for corrosion protection

Polypyrrole (PPy) films were deposited on copper from “green” inhibitor of phytic acid solution for corrosion protection of copper. The corrosion protection property of the PPy layer was studied by an immersion test in a NaCl aqueous solution. The polymerization process of PPy on copper changed with the pH values of phytic acid solution and current density applied. When one oxidized bare copper in phytic acid solution at various pHs containing pyrrole monomer, a thin layer consisting of complex compound of Cu-phytate was firstly formed, followed by the formation of the PPy layer doped with phytate anion on the complex compound layer. The complex compound layer passivated the copper surface and its thickness increased with the lower pH value of the solution and the lower current density applied. It was found that the PPy coating prepared in the phytic acid solution at pH 4 exhibit the most protective property against copper corrosion.     

An electrochemical impedance spectroscopy and scanning electron microscopy study of the influence of positive plate compression on the electrochemical behaviour of lead-acid batteries

This study has developed an electrochemical impedance spectroscopy (EIS) method for the in situ investigation of the influence of positive plate compression on the electrochemical behaviour of lead-acid batteries during charge/discharge cycling. The EIS data for a fully charged and fully discharged battery are internally consistent with the expected kinetics of a battery in the opposite states of charge, and demonstrate that EIS measurements may be recorded with a high level of reproducibility. Furthermore, this study has necessitated the development of a special cell incorporating horizontally orientated battery plates that can be subjected to elevated pressure through the stacking of lead bricks on top of the cell, as well as a physically robust reference electrode system that can withstand the application of pressure. For this purpose, a platinum-wire pseudo-reference electrode has been developed, and has been shown to exhibit sufficient electrode stability over the period of an EIS recording, enabling the measurement of reproducible and meaningful EIS data. Additionally, the influence of positive plate compression on the behaviour of the lead-acid battery has been investigated by using scanning electron microscopy (SEM). Clearly, the experimental data show that plate compression enhances significantly the kinetics and concomitant performance of the lead-acid battery, and this is related to the enhanced reactivity of the active material, as rationalized by using the agglomeration-of-spheres (AOS) model.