Pitting behavior on super 13Cr stainless steel in 3.5% NaCl solution in the presence of acetic acid

The pitting behavior was investigated on super 13Cr stainless steels in 3.5% NaCl solution in the presence of HAc. The electrochemical measurements including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and chronoamperometric experiments were carried out, as well as the SEM surface analysis. The parameters such as E _corr and I _corr were obtained by fitting technique. Moreover, the equivalent circuit model was applied in the analysis of Nyquist plots, and ZsimWin software was used to analyze the EIS data. The results indicate that the pitting corrosion is accelerated with increasing the amounts of HAc. In addition, pitting nucleation and growth mechanisms were described in this work.     

Influence of caffeine and temperature on corrosion-resistance of CoCrMo alloy

The inhibitory activity of caffeine (1,3,7-trimethyl xanthine) on artificial saliva was studied on a CoCrMo alloy using different electrochemical methods: open circuit potential (OCP), potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). The results show that caffeine produces an inhibitory effect on the anodic currents due to its adsorption on the surface of the alloy. Temperature is another parameter with an influence on corrosion processes, so thermodynamic data were obtained from Arrhenius plots and Langmuir adsorption isotherms. The protective action of caffeine is enhanced at high temperatures at OCP, while for potentiodynamic experiments high temperatures block the inhibitory activity of caffeine and the corrosion rate increases. The process may also be studied by a simulation, determining the functional dependence between OCP, corrosion current density (i _corr), corrosion potential (E _corr), breakdown potential (E _bd) and temperature and amount of caffeine in artificial saliva, for Heraenium® CE. The neural network-based methodology applied in this work provides accurate results, thus proving to be an efficient modelling technique.     

Electrochemical performance of Mg–9Al–1Zn alloy in aqueous medium

A systematic study on the corrosion and passivation behavior of AZ91D alloy in relation to the influence of concentration, temperature, pH, and immersion time was made in aqueous sulfate solution using electrochemical techniques including open-circuit potential, potentiodynamic polarization and impedance spectroscopy. It was found that the corrosion and pitting potentials (E _corr and E _pit) of the alloy drift to more active values with increasing either concentration (0.01–1.0 M) or temperature (278–338 K) of the test solution, suggesting that sulfate solution enhances the alloy dissolution, particularly at higher temperatures. On the other hand, values of the total film resistance (R _T) indicate that neutral solution (pH 7.0) supports the formation of a better protective layer on AZ91D surface than alkaline (pH 12.5) or acidic (pH 1.0) medium. The growth of a protective film on the alloy surface at short immersion times (up to ∼50 h) is evinced by a rapid positive evolution of E _corr and fast decrease in the corrosion rate (i _corr). However, for a long-term exposure (up to 500 h) E _corr drifts negatively and i _corr increases due to breakdown of the protective film, which causes a decrease in the alloy stability. Fitting the impedance data to equivalent circuit models suitable to each behavior assisted to explore the mechanism for the attack of the sample surface at various testing times. The results obtained from the three studied electrochemical techniques are in good agreement.     

饱和NaCl溶液浸泡下混凝土试块中有机阻锈剂对钢筋腐蚀行为的影响

利用电化学阻抗谱(EIS)、半电池腐蚀电位(Ecorr)和宏观电池腐蚀电流密度(Icorr)测量技术,在饱和NaCl溶液浸泡的硬化混凝土试块中,研究了4种醇胺基阻锈剂对钢筋电极腐蚀电化学行为的影响和长期阻锈性能.在浸泡初始的100d内,与空白样相比,添加阻锈剂后钢筋电极腐蚀电位升高,阻抗膜值增大,腐蚀电流密度值降低,表明电极表面处于钝态,阻锈剂表现出良好的阻锈性能.随浸泡时间延长,电极腐蚀电位和阻抗膜值下降,腐蚀电流密度增大.浸泡后期,除添加醇胺基CI-4样外,电极电位和腐蚀电流密度与空白样相比无明显差别,表明电极由钝态转变为活性腐蚀状态.但添加CI-4样品,钢筋电极始终保持在钝化状态,阻锈性能最好.基于阻锈剂与Cl-间的竞争吸附,分析探讨了可能的阻锈机理.     

Corrosion mechanism of pure aluminium in aqueous alkaline solution

The corrosion of pure aluminium in alkaline solution has been explored using an open circuit potential transient, potentiodynamic polarization experiment and a.c. impedance spectroscopy. The steady-state value of the open circuit potential (E _ocp ^ss ) of pure aluminium in alkaline solution was observed to decrease with increasing rotation rate of the specimen, which is ascribed to the enhanced anodic reaction. The extent of anodic polarization for the aluminium dissolution reaction on pure aluminium at E _ocp ^ss was found to be greater than that of cathodic polarization for the water reduction reaction. This indicates that the rate of corrosion of pure aluminium is mainly determined by the anodic reaction in alkaline solution. Based upon the experimental results, a corrosion mechanism for pure aluminium has been proposed in the presence of the native surface oxide film in alkaline solution, involving consecutive oxide film formation and dissolution, and simultaneous water reduction.     

Determination of corrosion rate of molybdenum,rhenium and their alloys in sodium chloride solution by the method of Tafel extrapolation

Based on the measured voltammograms, the corrosion potentials and corrosion current densities of Mo, Re, and several their alloys in the NaCl solutions were determined by the method of Tafel extrapolation. The dependences of E _corr and i _corr on the alloy composition, including the corresponding values for the metals (alloy components), were plotted. The obtained values of i _corr and the corrosion rates in the linear units were compared with the literature data.     

iR correction: Part II. Effect on corrosion monitoring

The effect of iR drop on the corrosion rates obtained by curve fitting of the polarisation data around the corrosion potential is an increase in the corrosion current calculated, which is in direct contrast to linear polarisation measurements. The b_a values and, for an activation-controlled cathodic process the b_c values, are increased by added ohmic resistance, R_u. This more than offsets the increase in R_p and so the i_corr increases. For a diffusion-controlled cathodic process the b_c becomes negative with added R_u. All the results are not unduly affected by up to ±2% random error. A current-interrupt method of iR correction has been used for steel corroding in a deaerated H₂SO₄ solution to demonstrate the effect of iR correction.     

Dissolved carbon dioxide effect on the behavior of carbon steel in a simulated solution at different temperatures and immersion times

Dissolved carbon dioxide effect on the behavior of carbon steel (0.4%carbon), in a simulated solution at different temperatures and immersion times, has been investigated using different techniques as potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy. The observation of the steel surface was done by scanning electron microscopy. All measurements reveal that the corrosion resistance is strongly dependent on both the temperature and immersion times. The corrosion resistance of carbon steel decreases in the solution considered as the temperature increases from 20?°C to 50?°C. Potentiodynamic polarization curves showed that cathodic and anodic current densities increase with the increased temperature. Besides, the study concluded that the addition of CO₂ gas to the simulated solution affects negatively the corrosion resistance in one hand and that, on the other hand, the higher the time of immersion the higher is the resistance to corrosion.     

Electrochemical and dissolution behavior of the Ti-alloy VT-9 in H2SO4 solution in the presence of the organic inhibitor (2-phenyl-4-[(E)-1-(4-solphanylanilino)methylyden]-1,3-oxazole-5(4H)-one

This investigation potentiodynamically evaluates the corrosion behavior of a high strength titanium alloy, VT-9, in 4 M sulfuric acid solution containing different concentrations (10, 20, 30 ppm) of the organic inhibitor, 2-phenyl-4-[(E)-1-(4-sulfanylanilino)methylidene]-1,3-oxazole-5(4H)-one (L-SH), at different temperatures (293, 303 and 313 ± 1 K). The open circuit potential values noted before and after each experiment varied appreciably with time. These values, in the presence of L-SH, were negative before polarization, but after completion of the experiment turned positive and remained stable over a long period of time. The cathodic current density values increased with increasing cathodic potential (more negative). The corrosion potential (E_corr) increased remarkably with the addition of L-SH. The corrosion current densities (I_corr), critical current density (I_cr), and passive current density (I_p) all decreased when L-SH was used. However, only the decrease in the I_corr with increasing amounts of inhibitor was significant compared to that of I_cr and I_p. L-SH expanded the range of the passive potential. SEM micrographs and open circuit potential measurements revealed the formation of a uniform and protective film on the alloy surface in the presence of L-SH, which acted as an efficient inhibitor.     

Synergistic effect of activated carbon from disposed plastics and polybenzoxazine composite coating on corrosion inhibition

The fabrication of activated carbon dispersed polybenzoxazine (BXP) composite through a single step melt condensation technique is reported. Employing green strategy, activated carbon (AC) was obtained from various types of worn-out plastics such as computer keyboard, sponge, electrical switches, and automobile tyres. Structural features of all the polymeric composites were explored using Fourier transform infrared (FTIR) spectroscopy, Field Emission Scanning Electron Microscope (FESEM) with Energy Dispersive X-ray Analysis (EDAX). Thermal stability and hardness of the composites was analyzed by recording thermogravimetric analysis (TGA) and Rockwell hardness measurements. The results of thermal studies revealed that AC–BXP composites show higher thermal stability (301.98°C) than BXP (220.71°C). Rockwell hardness study (RHN) for ACWCKB@BXP is 71 whereas pristine BXP showed only 20. The corrosion protection ability of coated steel was examined through potentiodynamic polarization and electrochemical impedance spectroscopy analysis. From the Tafel plot, more shift in E_corr value (from −0.6286 to −0.5065 V) towards positive side confirms its corrosion protection ability. Furthermore, the I_corr, the corrosion rate and the corrosion protection efficiency of the ACWTYR@BXP composite are 0.83 × 10⁻⁶ A/cm², 0.0098 mm/year, and 99.54%, respectively, which inhibit the rate of corrosion to a greater extent in 3.5% NaCl solution. The current study applies principles of green chemistry like minimum number of synthesis steps, non-usage of solvents and simply adoptable coating procedure.     

Mechanical polishing,surface roughness,near‐surface deformation,and electrochemical corrosion of Alloy 690TT

The effects of mechanical grinding/polishing, surface roughness, and near‐surface deformation on the electrochemical corrosion behavior of thermally treated (TT) Alloy 690 were studied in a sodium chloride solution. The X‐ray photoelectron spectroscopy and transmission electron microscopy analyses revealed that mechanical grinding/polishing can change the ratio of the elements at the surface of the as‐received Alloy 690TT specimen by removing its Cr‐rich outer layer and causing deformation at the near‐surface microstructure, something which has a direct impact on the rate of the oxygen reduction reaction (ORR), the pitting potential (E_pit), and the corrosion potential (E_corr) of Alloy 690TT. It was observed that the ratio of Cr in the surface is a significant factor that controls the rate of the ORR and the corrosion parameters such as E_corr. Higher amounts of Cr at the surface accelerate the ORR. The near‐surface deformation shifts the E_pit values towards less positive potentials. It was also found that due to the different near‐surface chemical composition of the as‐received Alloy 690TT specimen compared with the ground and the polished specimens, the surface roughness parameters do not have a regular correlation with the rate of the ORR and the values of the E_corr and the E_pit. Only the passive current density increases when the surface roughness is increased. Copyright © 2015 John Wiley & Sons, Ltd.     

Differential Pulse Polarography for a First‐Order EC Process and Its Diagnostic Parameters

Theoretical expressions for differential pulse polarography (DPP) for a reversible electron transfer coupled with an irreversible follow‐up first‐order chemical reaction (E_rC_i) is derived approximately. The peaks as given by the current expressions are analyzed in terms of several parameters such as a ratio of anodic‐to‐cathodic peak‐currents (i_p^a/i_p^c), a separation of peak‐potentials (E_p^c?E_p^a), and a ratio of anodic‐to‐cathodic half‐peak‐widths (W_1/2^a/W_1/2^c) in order to characterize the E_rC_i process and distinguish it from other types of electrode processes. The anodic peak is found to be more susceptible to the post kinetics than the cathodic peak. The new parameter of W_1/2^a/W_1/2^c ratio is much more sensitive to the post kinetics than the peak separation (E_p^c?E_p^a). The peak current ratio (i_p^a/i_p^c) and the peak‐width ratio (W_1/2^a/W_1/2^c) have comparable sensitivities to the kinetics. Hence, W_1/2^a/W_1/2^c ratio is a better diagnostic parameters than (E_p^c?E_p^a) which has a poor sensitivity. This phenomenon is different from cyclic voltammetry (CV) in which E_p^c?E_p^a is as sensitive as i_p^a/i_p^c. The new criteria for EC with DPV is tested and successfully applied to several Co(III) complex systems, including coenzyme B₁₂. The homogeneous rate constant (k) for the follow‐up step is estimated from the measurements of the experimental values of the parameters. The present treatment is valid quantitatively at lower values of k, yielding relatively larger errors for higher k values (k>10 s^?1).     

Electrochemical properties and corrosion inhibition of AA6061 in tropical seawater

The corrosion inhibition of aluminum and its alloys is the subject of tremendous technological importance due to the increased industrial applications of these materials. This study reports the results of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) on the corrosion inhibition of AA6061 aluminum alloys in seawater using sodium benzoate as an inhibitor. The electrochemical measurements for aluminum alloys in seawater after varied immersion period showed that the presence of sodium benzoate significantly decreases the corrosion currents densities (i_corr), corrosion rates and double layer capacitance (C_dl), as simultaneously increase the values of polarization resistance (R_p). Charge transfer process and development of thin film on the specimen have been proven by morphology study using SEM.     

Experimental and theoretical studies of acridine orange as corrosion inhibitor for copper protection in acidic media

The corrosion process commonly limits the use of copper in practical applications. The use of corrosion inhibitors is one of the effective methods to reduce the corrosion rate of copper. In this research, the inhibition effect of acridine orange (3,6-bis(dimethylamine)acridine) (AcO) for the protection of copper in 0.5 ?M ?H₂SO₄ solution was studied. For this aim, the change of open circuit potential with exposure time (E_ocp-t), electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), anodic and cathodic potentiodynamic polarization measurements (PP) and chronoamperometry (CA) techniques were used. Some quantum chemical parameters (E_HOMO, E_LUMO and dipole moment) were calculated and discussed. The AcO film formed over the copper surface was examined by SEM, EDX, AFM and contact angle measurements. The electrochemical data showed that AcO is an effective corrosion inhibitor even at low concentrations (ranging between 99.1% and %99.4 ?at concentrations from 0.01 ?mM to 1 ?mM). The corrosion rate of copper decreases in the presence of the inhibitor by reducing both anodic and cathodic rates, which is depended on its concentration. This compound behaves as mixed-type corrosion inhibitors with predominantly cathodic type. Its adsorption on the copper surface obeys Langmuir adsorption isotherm. The value of adsorption equilibrium constant (K_ads) and the standard free energy of adsorption were ΔG_ads 1.298 x 10³ ?M^?1 and -27.71 ?kJ/mol in the case of 0.5 ?M ?H₂SO₄ solution containing 1.0 ?mM AcO, which shows the adsorption is high and spontaneous. The adsorbed inhibitor film over the metal increase contact angle of the surface, which suggests the more hydrophobic properties of the surface are increasing coming from the orientation of hydrophobic sites to the electrolyte. The zero charge potential (E_pzc) studies showed that the surface charge of the metal is positive in the corrosive media containing the inhibitor. Quantum chemical calculations showed that the binding of inhibitor molecules to the metal surface takes place through N atoms of the inhibitor.     

甲基纤维素(MC)疏水作用的电化学研究

应用电化学循环伏安法, 以电活性小分子亚甲基蓝(MB)为探针, 研究了不同温度下修饰在玻碳电极表面的甲基纤维素(MC)凝胶的疏水性. 研究发现, 在45～70 ℃温度范围内, MB在MC凝胶修饰电极上的式电位E⁰⁽较相应的裸电极均正移, 氧化峰电流i_pa和还原峰电流i_pc分别较相应裸电极增大, 且随温度升高而增大. 这些结果表明MC分子之间发生了疏水相互作用, 且随温度的升高, 疏水作用增强. 此外, 在上述温度下, MC凝胶修饰电极上峰电流的比值i_pc/i_pa均小于1, 为0.70, 且没有观察到MB单独的吸附峰, 因此MB分子在凝胶修饰电极上发生了弱吸附. 本文研究显示电化学方法是研究该类多糖凝胶机理的一个补充手段.     

Fundamental aspects of the polarization resistance technique—the early days

The work of Oldham and Mansfeld dealing with the concept of the polarization resistance technique that can be used to determine corrosion rates from potential E—current I curves measured in the vicinity of the corrosion potential E _corr has been summarized. In addressing the interpretation of this technique as the “linear polarization (resistance) technique,” Oldham and Mansfeld pointed out that curvature of polarization curves in the vicinity of E _corr has to occur in order for the Butler–Volmer equation, as modified for corrosion reactions, to be valid. They proposed a modification of the Stern–Geary linear polarization equation and presented a new graphical method for the calculation of corrosion rates from polarization curves. Some of the earliest computer programs for the determination of corrosion current densities, and Tafel slopes from polarization data collected in the vicinity of E _corr and examples of their application have been discussed. Dedicated to the 80th birthday of Keith B. Oldham.     

Electrochemical response of natural and induced passivation of high strength duplex stainless steels in alkaline media

The passivation of two high strength duplex stainless steels (HSSS) was investigated in alkaline solutions simulating the pore solution of concrete by the growth of natural and induced passive films. Induced passive films were generated both by cyclic voltammetry and by chronoamperometry. Natural passive films were spontaneously grown by the immersion of the steel in the alkaline electrolyte. These passive layers were characterised by electrochemical impedance spectroscopy, corrosion current density (i _corr) and corrosion potential (E _corr) monitoring. The effect of significant parameters, such as the pH in the HSSS/alkaline solution interface, the composition of the duplex stainless steels and the ageing of the passive layer, on the electrochemical performance of both induced and spontaneously grown passive films has been analysed. The increase of alkalinity highly influences the electrochemical performance of the passive film by promoting the formation of a passive layer with a less resistant electrochemical response. The electrochemical behaviour of the passive layer is also affected by the alloying elements like Mo or Ni. Both natural and induced passive films show similar electrochemical trend with respect to significant parameters such as the pH and the composition of the steel. The ageing of the spontaneously grown passive layer promotes a higher resistive electrochemical response which might be related to the enrichment of the passive layer in non-conducting (or semi-conducting) oxides.     

Barium Sulfate Effects on the Electrochemical Behaviors of Nanostructured Lead Dioxide and Commercial Positive Plates of Lead-Acid Batteries

Positive electrode with uniform lead dioxide nanostructures directly synthesized by cyclic voltammetry (CV) method on the lead substrate in 1 M sulfuric acid solution including different concentration of barium sulfate. The effect of potential scan rate, sulfuric acid and barium sulfate concentration were studied on the morphology and particle size of lead dioxide using scanning electron microscopy (SEM) and X-ray diffraction techniques (XRD). The effect of barium sulfate was studied on the CV parameters including anodic peak current (I _p^a), cathodic peak current (I _p^c), anodic peak potential (E _p^a) and cathodic peak potential (E _p^c) during synthesis process. Finally, the effect of barium sulfate on the discharge capacity and cycle life of nanostructured positive electrodes and commercial positive plates was investigated. Both CV and battery test results showed that barium sulfate with concentration of 1 × 10⁻⁵ M can be used as suitable additive for positive paste of lead-acid batteries.     

Electrochemical corrosion behaviors and corrosion protection properties of Ni–Co alloy coating prepared on sintered NdFeB permanent magnet

In this study, a protective Ni–Co alloy coating was prepared on sintered NdFeB magnet applying electrodeposition technique. A pure nickel coating was also studied for a comparison. The microstructure, surface morphologies, and chemical composition of coatings were investigated using X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy, respectively. The corrosion protection properties of coatings for NdFeB magnet in neutral 3.5 wt.% NaCl solutions were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The microstructure and surface morphologies analysis showed that the addition of cobalt element into matrix metal Ni altered the preferential orientation of pure nickel coating from (2 0 0) crystal face for pure nickel coating to (1 1 1) crystal face for Ni–Co alloy coating, and made the surface morphologies more compact and uniform due to the grain-refining. The results of potentiodynamic polarization test showed that compared with pure nickel coating, Ni–Co alloy coating exhibited much nobler corrosion potential (E _corr) and lower corrosion current density (j _corr), indicating better anticorrosive properties. The long-term immersion test by dint of EIS indicated that the Ni–Co alloy coating still presented high impedance value of 1.9 × 10⁵ Ω cm² with the immersion time of 576 h indicating the excellent anticorrosive properties, and corrosion protection properties of nickel coating for NdFeB magnet practically disappeared with the immersion time of 144 h, which also indicated that the Ni–Co alloy coating provided better corrosion protection properties for the NdFeB magnet compared with nickel coating.