应用EFM技术检测碳钢在海水中的腐蚀行为

电化学频率调制（electrochemical frequency modulation,EFM）是一种腐蚀速度检测的新方法,能够同时测得腐蚀电流密度和Tafel常数。本文以恒电位仪、笔记本电脑、数据采集卡和LabVIEW开发的应用程序建立了EFM测试系统,将其用于低碳钢/海水体系,研究了扰动频率等测试参数对EFM测试结果的影响。在实验室应用EFM方法检测了Q235钢在NaCl溶液和舟山海水中的早期腐蚀速度,并与其他电化学方法进行了比较。结果表明,在低碳钢/海水体系中EFM方法测得的腐蚀速度因界面电容行为的影响而偏大,但EFM技术和测试系统仍然可以作为海洋腐蚀现场检测的一种理想选择。     

Evaluation of electrochemical frequency modulation as a new technique for monitoring corrosion and corrosion inhibition of carbon steel in perchloric acid using hydrazine carbodithioic acid derivatives

Electrochemical frequency modulation, EFM is a new technique for corrosion rate measurements. With the EFM technique, the corrosion rate and corrosion kinetic parameters can be obtained instantaneously without prior knowledge of Tafel slopes, which makes this method an ideal technique for application as a corrosion monitoring tool. Results obtained with the EFM technique were shown to be in agreement with chemical (weight loss) and electrochemical methods (Tafel extrapolation and electrochemical impedance spectroscopy, EIS) for corrosion rate measurements. New synthesized hydrazine carbodithioic acid derivatives namely, N′-furan-2-yl-methylene-hydrazine carbodithioic acid (A), N′-(4-dimethylamino-benzylidene)-hydrazine carbodithioic acid (B) and N′-(3-nitro-benzylidene)-hydrazine carbodithioic acid (C) were examined as corrosion inhibitors for carbon steel in 1 M perchloric acid solution. The results obtained from both chemical and electrochemical measurements show that these compounds suppressed both anodic and cathodic processes of carbon steel corrosion in 1 M HClO₄ by adsorption on the electrode surface. The adsorption mode follows the Langmuir adsorption isotherm. The efficiency of the inhibitors increases in the order C > B > A.     

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.     

Microbial consortium influence upon steel corrosion rate, using polarisation resistance and electrochemical noise techniques

The microbiologically influenced corrosion (MIC) is a process, which affects the oil industry, particularly the hydrocarbons extraction, transport and storage. MIC evaluation has been normally based upon microbiological tests, and just a few references mention alternating methods, such as the electrochemical techniques, which can be used as criteria for their evaluation.In this work, two different electrochemical laboratory techniques, polarisation resistance and electrochemical noise were used, in order to determine the corrosion behaviour of a microbial consortium, obtained from a gas transporting pipeline, located in the southeast of Mexico.The bacteria population growth was found to be different for sessile and plancktonic microorganisms. Moreover, long incubation times were required to reach the maximum concentration of sessile bacteria.The electrochemical techniques used in this study exhibited a similar tendency on the corrosion rate behaviour with time, and values above 0.3 mm year⁻¹ were observed at the end of the experiments.The experiments were complemented with surface analysis. Scanning electron microscope observation of APIXL52 steel coupons, exposed to the consortium action, revealed bacteria presence, as well as a damaged steel surface. A type of localized corrosion was observed on the metal surface, and it was associated to the bacteria effect.     

The adsorption and corrosion inhibition of 8-hydroxy-7-quinolinecarboxaldehyde derivatives on C-steel surface in hydrochloric acid

The protection influence of 8-hydroxy-7-quinolinecarboxaldehyde derivatives against C-steel corrosion was studied in 2 mol· L-1 HCl solutions at 30 ℃.Measurements were conducted under various experimental conditions using weight loss,potentiodynamic polarization,electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) techniques.These studies have shown that 8-hydroxy-7-quinolinecarboxaldehyde derivatives are very good "green",mixed-type inhibitors.Corrosion rates obtained from both EFM and EIS methods are comparable with those recorded using Tafel extrapolation method,confirming validation of corrosion rates measured by the latter.The inhibitive action of these 8-hydroxy-7-quinolinecarboxaldehyde derivatives was discussed in terms of blocking the electrode surface by adsorption of the molecules through the active centers contained in their structures following Langmuir adsorption isotherm.Quantum chemical method was also employed to explore the relationship between the inhibitor molecular properties and its protection efficiency.     

Corrosion behaviour of multiwall carbon nanotube/magnesium composites in 3.5% NaCl

Corrosion behaviour of multiwall carbon nanotube (MWNT) modified pure magnesium (Mg) composites (MWNT/Mg) prepared by melt stirring technique was investigated by weight loss, H₂ gas collection and pH measurements, as well as electrochemical methods such as open circuit potential (OCP) monitoring, polarisation curves and electrochemical impedance spectroscopy (EIS). Results show that the corrosion rate of MWNT/Mg composites is significantly higher than the corrosion rate of pure Mg. The corrosion rate of the composites also depends on the degree of dispersion of MWNTs during melt stirring process. The role of the MWNTs in increasing the corrosion rate clearly can be attributed to their high cathodic activity. Characterization of the corrosion product layers indicates that dispersion of MWNTs in the matrix leads to a more homogeneous coverage of the surface by corrosion products, and the lowest thickness of the corrosion product layer. The corrosion product layers in all cases provide poor protection of the surface. The corrosion rate of pure Mg as well as Mg/MWNT composites significantly increases during 1-week immersion; this effect is the strongest for the composite with dispersed nanotubes and can be attributed to the increased cathodic kinetics with time.     

Stainless steel as new substrate for coil coating

Increasing needs of very high resistance to cosmetic corrosion, of more extended service life and reduced maintenance costs for infrastructures, civil and industrial buildings open new fields of application for coil coated stainless steel. This paper describes the adhesion and corrosion properties of new coil coated stainless steel materials produced in industrial coil coating lines. The use of an electrochemical test (electrochemical impedance spectroscopy) can give detailed information on the reactivity of the system and allow the performance of different substrates (AISI 409, 430, 316 and 304) coated with different polymers (polyvinylidene and polyester) to be compared. The results obtained show the interesting properties of this new class of coil coated products. The materials were tested for a long time (about 200 days) in an aggressive environment (3.5% sodium chloride solution) also in the presence of macrodefects. In particular, VIVINOX 430, 304 and 316 revealed no reactivity, corrosion or disbonding, thus supporting the expectancy of very long trouble free exposure also in very aggressive natural environments. (VIVINOX is the brand name of the AST (Acciali Speciali Terni) line of coil coated stainless steel.)     

Electrochemical noise applied to the study of the inhibition effect of CeCl3 on the corrosion behaviour of Al-Mg alloy AA5083 in seawater

Electrochemical noise measurements have been used to study the corrosion behaviour of aluminium alloy AA5083 in 3.5% NaCl solution with and without doping with CeCl₃ at 500 ppm. Information on the evolution of corrosion activity over a period of 4 days and on the corrosion mechanism itself has been obtained for the two systems. Experimental data obtained have been analysed using robust statistical parameters and wavelet transform; transient shapes were also studied. The various mathematical techniques applied to analyse the electrochemical noise data have been proposed recently. This paper thus not only characterises the corrosion systems studied but also illustrates the usefulness of these new methods.     

Dimensional analysis applied to pitting corrosion measurements

In this work, two new indices, S_E and S_G are proposed to evaluate pitting corrosion by dimensional analysis of three parameters of PSD, the slope of high frequency linear region (k), the critical frequency (f_c) and the low-frequency plateau level (W). And then they are employed to analyze the potential electrochemical noise obtained during the corrosion processes of stainless steel 1Cr18Ni9Ti and pure Al, two materials most susceptible to pitting corrosion in NaCl solutions. Meanwhile another two mathematical methods, statistics and wavelet transform techniques, are also carried on the same potential oscillation data to estimate the function (meaning) of these two indices (S_E and S_G) in monitoring corrosion processes. The results suggest that, the index S_E is directly proportional to the rapider potential oscillating and higher fluctuation amplitude and can be used to depict the distribution behavior of electrochemical noise, while the index S_G can be used to depict the slower corrosion processes.     

Electrochemical noise (EN) technique: review of recent practical applications to corrosion electrochemistry research

The ability to analyze different electrochemical corrosion phenomena, in–situ, without requiring any form of electrode perturbation has strongly attracted the attention of corrosion researchers towards the application of electrochemical noise (EN) analysis method. With the ability to analyze stochastic and chaotic electrode potential and current fluctuations at E_OCP, the EN analysis method is capable of providing information about the kinetics and mechanism of metallic corrosion with accuracy that can match conventional electrochemical techniques. Herein, we review the recent applications of EN analysis method to electrochemical corrosion research. We discuss briefly the theory behind the measurement of EN, and then highlight some of its application in the evaluation of corrosion inhibitors, pitting corrosion, coatings on metals, microbiologically–induced corrosion, as well as CO₂–corrosion. The drawbacks of the EN analysis method have also been highlighted and future considerations on the use of this important electrochemical technique have also been proposed.     

Synthesis,characterization and electrochemical evaluation of anticorrosion property of a tetrapolymer for carbon steel in strong acid media

A novel tetrapolymer(TP) consisting of carboxylate, sulphonate, phosphonate and sulfur dioxide based comonomers was synthesized using Butler cyclopoymerization technique. The synthesized tetrapolymer was characterized using FTIR,1 H-NMR,~(13)CNMR and elemental analysis. The performance of the tetrapolymer as a corrosion inhibitor for St37 carbon steel in 15% HCl and 15% H_2SO_4 acid media was assessed using electrochemical impedance spectroscopy(EIS), linear polarization resistance(LPR), potentiodynamic polarization(PDP) and electrochemical frequency modulation(EFM) techniques. The influence of addition of a small amount of KI on the corrosion inhibition efficiency of TP was also assessed. Results obtained showed that the tetrapolymer moderately inhibited the corrosion of St37 steel in the acid media with protection efficiency of 79.5% and 61.1% at the optimum concentration of 1000 mg·L~(-1) studied in HCl and H_2SO_4 media respectively. On addition of 5 mmol·L~(-1) KI to the optimum tetrapolymer concentration, the protection efficiency was upgraded to 90.6% and 93.5% in HCl and H_2SO_4 environment, respectively. The enhanced performance of the polymer in the presence of KI is due to synergistic action deduced from synergism parameter(S1) which was found to be greater than unity.The tetrapolymer afforded the corrosion inhibition of St37 steel in the acid media by virtue of adsorption of the polymer molecules on the steel surface which was confirmed by ATR-FTIR analysis of the adsorbed film extracted from the steel surface. TP + KI formed complex with St37 steel surface in H_2SO_4 solution but not in HCl solution.     

Electrochemical study of effect of the concentration of azole derivatives on corrosion behavior of stainless steel in H2SO4

In order to improve corrosion resistance of the stainless steel structures exposed to acidic media, a variety of corrosion inhibitors particularly organic ones have been examined. In this work, the corrosion inhibition performance of two azole derivatives namely benzotriazole and benzothiazole on stainless steel in 1 M sulfuric acid was studied through taking advantage of electrochemical techniques as well as SEM surface analysis. Revealing effectiveness of the two inhibitors, the AC impedance spectra indicated no change in corrosion mechanism. The noise resistance and average current density as parameters extracted from electrochemical noise measurements revealed the direct proportion of inhibition function to the inhibitor concentration. In accordance with the polarization curves, benzotriazole and benzothiazole appeared to act as mixed type inhibitors. The adsorption of the two corrosion inhibitors was shown to obey Langmuir isotherm. Moreover, it was deduced from the isotherm that the type of adsorption can be physical and chemical in nature. The corrosion damage mitigation was also confirmed through SEM in the presence of benzothiazole.     

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.     

Electrochemical noise and impedance study of aluminium in weakly acid chloride solution

The electrochemical noise (EN) characteristics of pure aluminium in unbuffered potassium chloride solution and with acetic acid/sodium acetate buffer at pH 5.4 and 4.3 have been analysed to throw light on the influence of pH and of the presence of buffer at the aluminium surface on chloride ion-induced corrosion. Comparison has been made with results obtained by electrochemical impedance spectroscopy (EIS) and quantitative deductions made concerning the values of the noise resistance and the magnitude of the electrochemical impedance. Deviations between results obtained by the two experimental techniques are discussed.     

Monitoring and measuring electrochemical behavior of engineering alloys by optical interferometry

In a previous study, an optical corrosion meter was built based on a mathematical model relating to surface and bulk behaviors of metals in aqueous solution. The optical corrosion meter was established based on principles of holographic interferometry for measuring microsurface dissolution, i.e., mass loss, and on those of electrochemistry for measuring the bulk electronic current, i.e., corrosion current. The corrosion meter consists of an electrochemical cell in which the sample is immersed in aqueous solution. Furthermore, the corrosion meter has a holographic camera with a thermoplastic film for in situ processing holograms in order to obtain real-time holographic interferograms of the sample in the electrochemical cell. Results of the present work indicate that optical holography is a very useful technique for measuring the corrosion current density of different alloys in aqueous solutions. As a result, the corrosion current density of aluminium, stainless steel, low carbon steel, copper, and brass in 1M KCl, 1 M NaCl, 1 M NaOH, seawater and seawater solutions, respectively, were obtained. The obtained corrosion data from the optical corrosion meter (interferometric technique) were compared with corrosion data obtained on the same alloys in the specified solutions from the linear polarization method as well as from the weight-loss method. The comparison among the three techniques indicates that there is contrast in the results among the investigated alloys. In general, the new method of optical interferometry can be considered a more conservative method of corrosion measurement with respect to the linear polarization and weight-loss methods.     

Synthesis and evaluation of the inhibitor effect of a new class of triazole compounds

In this work, a new class of triazole molecules, containing heteroatoms such as nitrogen and sulphur, is prepared. The effect of these molecules on the corrosion behaviour of iron was investigated. Electrochemical studies were performed in aerated chloride acid solution (1 M) by potentiodynamic polarisation curves. Surfaces were characterised by scanning electron microscopy.

The recorded electrochemical data showed that the corrosion resistance was greatly enhanced in the presence of inhibitor. The maximal protection efficiency exceeded 94%. The corrosion protection could be explained by the adsorption of inhibitor through the S- or N-atom and formation of a protective layer attached to the metal surface.     

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.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H2S

In this work, the corrosion behavior of SAE 1018 carbon steel in buffered acetic acid (HAc) solutions containing chlorides, with and without H₂S, was studied. Polarization curves obtained by different electrochemical techniques, indicate negligible modification of anodic slopes when adding H₂S; however, the cathodic branch is more sensitive showing an accelerated reduction reaction in the presence of H₂S. Interface characterization was performed by electrochemical impedance technique (EIS) in the absence and presence of H₂S and near to the corrosion potential (E_corr). Analysis of results shows no film of corrosion products, since the impedance spectra characteristics indicate a great activity of steel in the solutions studied, with differences only at low frequencies. The adsorbed complexes formed in the solution containing HAc, acetate and chlorides control the corrosion process and prevent passive film formation, even in the presence of H₂S.     

A new technique for online visualization of the electrode surface under electrochemical corrosion processes

A new method for the online visualization of corrosion processes has been developed. It relates two techniques, electrochemical studies (potentiodynamic, potentiostatic, chronoamperometric etc.) and image processing. The main advantage of this method is that it is possible to relate the morphological changes of the electrode surfaces to the electrochemical signal measured without disturbing the electrochemical system. The online visualization technique is based on a horizontal electrochemical cell. The cell allows observation of the surface of the electrodes in a horizontal position by means of a triocular microscope–stereoscope assembled to an image acquisition system. The methodology was applied to the study and understanding of the surface changes of a copper and an AISI 316L electrode, respectively, when their potentiodynamic curves were taken in a lithium bromide solution. The corroded area of an AISI 316L electrode was determined by image processing in order to calculate the local current density in a potentiostatic test. The visualization method proposed can be used to gain a better understanding of electrochemical corrosion processes.