Electrochemical technique for investigating temporarily protective oil coatings

A wire-beam electrode using mild steel wires has been developed and used to study temporarily preventive oil coatings in terms of the heterogeneous electrochemical states after immersion in a salt solution. It has been found that the electrochemical states on the surface of the wire-beam electrode are homogeneous compared with an oil-painted electrode. The distribution of corrosion potential on oil-painted wire-beam electrodes is heterogeneous and follows a discontinuous binomial probability distribution. Polarization resistance of each oil-coated wire sensor is randomly distributed over a given range of resistance and follows a log-normal probability distribution. The distribution of corrosion potential over a high potential range can be increased, distribution of polarization resistance over a low resistance range can be decreased and eliminated by adding oil-soluble inhibitor. With increasing concentration of oil-soluble inhibitor, the distribution of anodic polarization resistance of oil coatings may change from discontinuous binomial distribution to exponential distribution and to log-normal probability distribution, the distribution of cathodic polarization resistance of oil coatings will transform from discontinuous binomial distribution to log-normal probability distribution.     

A study on the electrochemical inhomogeneity of organic coatings

The inhomogeneity of three organic coatings, phenolic resin, alkyd resin and poly(urethane) varnish, was studied using a wire beam electrode and a high resistance measurement technique by measuring the distribution of d.c. resistances on different surface areas of the coatings in a 3% NaCl solution. Two types of areas which showed a significant difference in their d.c. resistance were found to be a basic pattern of this inhomogeneity. Experimental findings in this work nicely proved the reality of ‘D' and ‘1' areas in organic coatings. The results are in good agreement with those of Mayne et al.. Results about the influence of the coating's thickness and layers on its inhomogeneity are also reported in this paper.     

环氧树脂涂层钢筋混凝土的电化学研究

用4×4阵列丝束电极模拟研究混凝土中钢筋的腐蚀,将钢筋混凝土做铁丝表面涂抹树脂和混凝土表面涂抹树脂的涂层处理。通过测量铁丝的自腐蚀电位,研究不同涂层方式对混凝土防腐方面的影响。结果表明,只在混凝土表面涂抹树脂是最好的保护方式,它可以使钢筋的腐蚀电位一直处于钝化范围内,从而保证了钝化膜的完整性。环氧树脂涂层能够非常有效地防止钢筋锈蚀,可有效减少使用期间的维修管理费用,且延长了结构的使用寿命,其经济效益和社会效益是十分巨大的。     

A novel,two-phase medium for electrochemically induced conversion coatings

A method to facilitate electrochemical coatings from a non-aqueous, poorly conductive (and therefore normally difficult to work with) electrolyte is presented. The process involves loading the electrolyte with pretreated ion-exchange beads to provide multiple low-resistivity paths for ionic but not electronic conduction between a counter-electrode and workpiece. Using the formation of iron phosphate conversion coatings from an oil-based lubricant as a test case, the effects of bead moisture content and oil additive concentration on the coating process are described. With this new approach electrochemical coating is possible, even in poorly conductive fluids, over a far larger range of electrode separations than could otherwise be accomplished.     

Real time mapping of corrosion activity under coatings

Current accelerated testing of aircraft coating systems for corrosion protection relies heavily on salt spray methods. Electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM) provide insight into the global properties of a coating system, and both techniques are being used on a limited basis. However, there is a need to investigate corrosion events with greater spatial resolution under coatings at the metal/coating interface. Such corrosion activity may be related to coating defects and variations in the surface chemistry of the underlying metal.

The scanning vibrating electrode technique (SVET) has been developed to allow high spatial resolution investigation of localized corrosion activity that may be associated with coating defects or galvanic coupled regions of the metal surface. The SVET offers high resolution in current measurements of the order of 0.5 μA/cm² and is able to detect in-situ initiation and progress of corrosion activity under a protective coating. Using the SVET, minute variations in d.c. current associated with localized corrosion activity are detected and used to map both anodic and cathodic corrosion activities in a localized area. The difference in initial corrosion activity under various coatings can be correlated to the performance life of the coatings. The application of SVET to aircraft coatings and corrosion is reported to demonstrate the utility of this important new electrochemical tool.

In the current study, the SVET was used to discriminate the corrosion protection performance of selected sol–gel based coating systems. Sol–gel based surface treatments are being developed as part of an environmentally compliant coating system alternative to the currently used chromate-based systems. The SVET results are compared with data obtained from chromium inhibition coating systems. The SVET analyses are compared with electrochemical impedance measurements. The comparison of such data will provide the basis to adopt SVET measurements as an early performance discriminator for newly developed coating systems.     

Study on the wire electrochemical groove turning process

In this study a copper wire is proposed as a tool electrode in electrochemical groove turning process (WECGT). The working parameters, namely, radial feed rate, wire diameter, and rotational speed are investigated to study the performance criteria via MRR, groove width, and roundness error. An experimental study is presented through performing series of designed experiments. Key features of a WECGT setup that was designed and developed incorporating several unique features are also highlighted. The experimental results are statistically analyzed and mathematically modeled through response surface methodology (RSM). The mathematical model adequacies are checked using analysis of variance (ANOVA). Furthermore, optimal combination of working parameters has been evaluated to maximize MRR and minimize roundness error. The results reveal that using wire as an electrode in electrochemical turning instead of using a profiled tool proved its powerfulness to produce circular grooves. The results also demonstrate that the groove width has greatly increased by increasing the wire diameter, while it is decreased by increasing both the radial feed rate and rotational speed. Lower roundness errors are obtained by increasing both radial feed rate and wire diameter. The optimum combination of parameters setting is: radial feed rate of 0.08 mm min⁻1, wire diameter of 2.3 mm, and rotational speed of 578 rpm.     

An overview of techniques for characterizing inhomogeneities in organic surface films and underfilm localized corrosion

This paper presents an overview of techniques for characterizing inhomogeneities in various forms of organic surface films including organic coatings, corrosion inhibitor and anti-corrosion oil films, and for understanding the effects of surface film inhomogeneities on underfilm localized corrosion. Particular focus is on technological innovations that have been made over the past two decades, including scanning probe techniques and the wire beam electrode method.     

The combined use of scanning vibrating electrode technique and micro-potentiometry to assess the self-repair processes in defects on “smart” coatings applied to galvanized steel

Model weldable primer coatings for galvanized steel were modified with submicron containers loaded with corrosion inhibitors. This procedure aims at introducing a new functionality in the thin coatings self-repair ability. The assessment of this property demands new protocols and new approaches, combining conventional electrochemical methods with electrochemical and analytical techniques of micrometer spatial resolution. Thus, in this work model defects were created in the coatings by using a focused ion beam (FIB). The coated samples, containing the model defects, were immersed in a NaCl 0.05 M solution and the corrosion inhibition ability was studied using the scanning vibrating electrode technique (SVET) and the scanning ion-selective electrode technique (SIET). SVET–SIET measurements were performed quasi-simultaneously. Qualitative chemical analysis was performed by SEM combined with EDS. Complementary studies were carried out by electrochemical impedance spectroscopy (EIS) to assess the effect of the containers filled with corrosion inhibitors on the barrier properties of the coatings. The electrochemical results highlight the importance of the combined use of integral and localized electrochemical techniques to extract information for a better understanding of the corrosion processes and corresponding repair of active microscopic defects formed on thin coatings containing inhibitor filled containers.     

Matériaux anodiques pour électrolyseurs

This paper is a review of the main anodic materials (including coatings) used in industrial electrochemical cells. The mechanism of the electrochemical reactions is given in detail; many examples of applications as well as the probable new developments in electrode materials are discussed. An important part of the paper is devoted to carbon and precious metal oxides coatings on a valve metal substrate such as titanium, leading to dimensionally stable anodes (DSA). The mechanism of the deactivation process of DSA is described: the passivation is due to the formation of a passivating TiO₂ layer between the titanium substrate and the electrocatalytic coating.     

微区电化学方法在金属腐蚀研究中的应用进展

本文综述了扫描Kelvin探针技术和丝束电极技术的测试原理、特点及应用现状,重点探讨了这两种微区电化学测试技术在腐蚀研究中的应用现状和发展趋势。扫描Kelvin探针(SKP)具有高技术精度和解析度,为微区扫描测量技术在腐蚀领域的研究提供了新的途径,得到了越来越广泛的应用。丝束电极技术可以准确表征电极表面的电化学不均匀性。     

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

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.     

Electrochemical characterisation of protective organic coatings for food packaging

A very common material for food packaging is steel, in the form of metallic containers (cans), in particular for beverage packaging. The corrosion degradation of the packaging must be carefully controlled, not only because the packaging integrity must be preserved, but also in order to avoid any significant contamination of the food or drink, compromising the flavour. In order to increase the coating performance and the food compatibility, new organic coatings are under development with very high protective properties, with the final aim to increase the shelf life of the product. An electrochemical characterisation is often used to study the protective performance of organic coatings on metal substrate for various applications. Some different coatings for food packaging were considered in the present study, including materials with different chemical composition and different pigments content. The protective properties were quantified using electrochemical impedance spectroscopy (EIS) measurements, comparing the electrochemical substrate activity with electrochemical noise (EN) and scanning Kelvin probe (SKP) measurements. The influence of mechanical deformations on the protective properties was also investigated. The results obtained on the studied coatings confirmed the validity of the electrochemical approach and showed that, in general, the coatings containing pigments (TiO₂) have better performance than clearcoats, while comparing the different polymers, epoxy–phenolic coatings have a better corrosion protection than epoxy–melamine coatings.     

Effect of the experimental setup in the behaviour of sol–gel coatings

Electrochemical behaviour of the sol–gel-coated AA2024-T3 samples was evaluated using electrochemical impedance spectroscopy (EIS) in a three-electrode arrangement cell using Na₂SO₄ 0.1 M solution as electrolyte. The effect of the reference electrode leakage was examined using as reference a low-leakage Ag/AgCl electrode and a platinum wire. The results show that very low-chloride concentrations in solution are able to induce heavy dissolution of intermetallic precipitates. That corrosion process masks information on the barrier properties of sol–gel coatings otherwise available through EIS measurements.     

New methods for the study of organic coatings by EIS: New insights into attached and free films

Organic coatings are widely used for the protection of structures against marine corrosion. Based on long-term service experience, their thickness reaches often as high as several hundreds of micrometers. However, for primer coatings, where the barrier effect is believed to play a minor protective purpose, the surface preparation and the nature of the substrate seem to play an important role.Experiments have been performed on free-standing and supported coatings and the comparison showed considerable differences. It is postulated that the adhesion mechanisms provide an important part of the protection afforded by the coating. The insertion of a conducting probe inside the coating allowed separating the impedance of the inner part of the coating in contact with the substrate from that of the outer part in contact with the solution. Information on the influence of the adhesion on the protection was then evidenced. A gold electrode, deposited on the surface of the inner half-layer of the coating, was used as a potential probe to perform electrochemical impedance measurements. The first results presented in this paper show a clear difference between the behaviour of these two parts of the coating.     

Mechanisms of non-toxic anticorrosive pigments in organic waterborne coatings

Investigations have been carried out concerning the mechanism of the behaviour of non-toxic anticorrosive pigments belonging to the group of phosphates, ferrites and ion exchange pigments in waterborne systems. The mechanism controlling the protective effectiveness of organic coatings is complex and results from simultaneous activity of various agents, from among which the kind of the corrosion inhibitor and the structure of the coating are of fundamental importance. The effect of pigments on the protective properties of coatings was tested by means of electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET) as well as the salt spray and Prohesion tests. For the investigation of the structure of coatings the porosymetric method and modulated-force thermomechanical analysis (mf TMA) were applied. The results of these investigations have shown that calcium zinc phosphate and zinc ferrite are the most effective. These pigments take part in the passivation of steel, which has been proved by the results of electrochemical investigations and by the presence of the passive layers as has been found out by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDXA) and scanning electron microscopy (SEM). Calcium zinc phosphate and zinc ferrite affect the structure of the coatings, increasing the glass transition temperature (Tg) of the coatings. Zinc phosphate and calcium-exchanged silica do not act in compliance with electrochemical mechanism neither do they improve the barrier properties of the binder.     

聚脲涂层对储油罐耐冲击性影响的研究

为避免储油罐发生意外碰撞造成巨大损失，考察了新型聚脲防护涂层对油罐车储油罐破坏变形的防护作用。通过储油罐耐冲击实验，分析了不同涂覆方式对聚脲复合涂层耐冲击性的影响，最后通过 LS-DYNA有限元模拟进行验证。结果表明：相比无涂层防护，聚脲涂层对储油罐具有良好的耐冲击防护效果，涂覆聚脲试样比无涂层试样最大变形位移减少了 18. 3 mm。在相同涂层厚度情况下， 2 mm Q190m和 2 mm Q413m聚脲复合涂层耐冲击效果最优，适合作为储油罐的防护涂层。     

Styrene butadiene co-polymer based conducting polymer composite as an effective corrosion protective coating

Electroactive conducting polymer composite coatings of polyaniline (PANI) are electrosynthesized on styrene–butadiene rubber (SBR) coated stainless steel electrode by potentiostatic method using aqueous H₂SO₄ as supporting electrolyte. The protective behaviour of these coatings in different corrosion media (3.5% NaCl and 0.5 M HCl) is investigated using Tafel polarization curves, open circuit potential measurements and electrochemical impedance spectroscopy. The results reveal that SBR/PANI composite coating is much better in corrosion protection than simple PANI coating. The corrosion potential of composite films shifts to more noble values indicating that SBR/PANI composite coating act as an effective corrosion protective layer.     

Electrochemical impedance spectroscopy as a tool to measure cathodic disbondment on coated steel surfaces: Capabilities and limitations

The disbondment of protective organic coatings under excessive cathodic protection potentials is a widely reported coating failure mechanism. Traditional methods of evaluating cathodic disbondment are based on ex situ visual inspection of coated metal surfaces after being exposed to standard cathodic disbondment testing conditions for a long period of time. Although electrochemical impedance spectroscopy (EIS) has been employed as an effective means of evaluating various anti-corrosion properties of organic coatings; its application for assessing the cathodic disbondment resistance of coatings has not been sufficiently exploited. This paper reports an experimental study aimed at developing EIS into a tool for in situ measurement and monitoring of cathodic disbondment of coatings. A clear correlation between EIS parameters and the disbonded coating areas has been confirmed upon short term exposure of epoxy-coated steel electrodes to cathodic disbondment conditions; however the degree of this correlation was found to decrease with the extension of exposure duration. This observation suggests that EIS loses its sensitivity with the propagation of coating disbondment, and that in order to achieve quantitative determination of the coating cathodic disbondment localized EIS measurements are required to measure the parameters related to local disbonded areas.     

A new approach for monitoring corrosion and flow characteristics in oil/brine mixtures

Monitoring of corrosion processes and hydrodynamic flow pattern in oil/brine mixtures of various compositions between 0 and 80% in volume of oil has been investigated by means of electrochemical impedance and noise measurements. In such two-phase electrolytes the electrode behaviour is mostly characterized by the fact that the state of the corroding metallic surface evolves in a highly dynamic way as a result of local heterogeneities yielded by the non-uniform phase distribution near the electrode. This is why conventional electrochemical current and/or potential noise, as well as electrochemical impedance measurements, showed very poor reproducibility and weak sensitivity to electrolyte composition and flow changes. In contrast to conventional techniques, the electrolyte resistance fluctuations have shown to be a highly sensitive and reliable tool conveying real-time information about the two-phase flow pattern and electrolyte composition. Such measurements can be an indirect way to evaluate the environment aggressiveness in industrial plant and field applications.     

环保型金属三防涂料的制备及其性能研究

采用聚合物硅溶胶为成膜物质,制备了具有室温固化特性的环保型硅系三防涂料,并开展了该硅系三防涂料与传统过氯乙烯涂料在耐水性、耐油性、耐溶剂性等防护性能方面的对比研究。结果表明:该硅系三防涂料在铝合金表面具有优异的附着力,其涂层的硬度、耐冲击性、耐水性和耐油性远高于过氯乙烯涂层。