用旋转电极研究低碳钢阳极溶解机理

用旋转电极研究低碳钢阳极溶解机理吴庆余（武汉工学院武汉４３００７０）１前言钢铁的溶解过程非常复杂,不同的研究者得出的溶解机理不同［１－６］。其原因主要是由于所用材料及所处介质不同等多种因素造成。本工作参照工业锅炉盐酸酸洗的实际工况,在含有较大ＣＩ－浓度（１．５ｍｏｌ／Ｌ）的酸性介质中,测定低碳钢阳极溶解过程的电化学参数,以探求反应历程和动力学方程式。２实验方法研究电极用２０钢制成圆盘状,四周?...     

Prediction of pathways for the dissolution of iron

Previous mechanisms for the anodic dissolution of iron have been reviewed, and experimental evidence to distinguish between them is given. Of the various values found in the literature for the anodic Tafel slope, the values of 30 and 40 mV at (30°C) seem to be the most accurate. These two values have been taken as criteria for the possible mechanisms. New reaction pathways have been suggested which also involve the formation of Fe(OH)₂, FeOH⁺, and Fe₂OH⁺ as intermediates, and both the Tafel slope and the reaction order with respect to OH^? for these pathways have been deduced from steady state kinetics. The data thus obtained show that many of the proposed rate-determining steps give rise to the same results derived for previous mechanisms, thus indicating that such previous mechanisms are not the only possible ones. Difficulties associated with the work on iron, and with the present state of steady state kinetics are also outlined.     

Impedance investigation of the anodic iron dissolution in perchloric acid solution

The anodic iron dissolution in 0.5 mol dm⁻³ perchloric acid (HClO₄) was investigated by the electrochemical impedance spectroscopy, the potentiodynamic sweep and the scanning electron microscopy measurements. The anodic polarization behavior of iron in HClO₄ solution showed that the strong current oscillations occurred in a narrow potential region, particularly the pitting corrosion was observed in the active dissolution region. These characteristics were quite different from those of iron in the sulfuric acid (H₂SO₄). At the potentials 82 and 132 mV more positive than the corrosion potential (−482 mV vs. SCE), the impedance spectra for the iron in HClO₄ displayed two inductive arcs; however, by gradually increasing potential the lower frequency inductive arc disappeared at −300 mV at first, and then the higher frequency inductive arc changed into a capacitive arc at −250 mV. Based on the impedance display of iron at various potentials, a reaction model involving two adsorbed intermediate species was proposed, in terms of which the impedance behavior at different potentials were described. Occurrence of the pitting corrosion in active dissolution region was explained.     

Die Inhibitorwirkung von Phenyl-trithioniummethosulfat auf die Korrosion des Eisens in Salzsäure

The inhibition effect of phenyl-tri-thionium-metho-sulphate on the corrosion of iron The following measurements were made in order to test the inhibitor effect of phenyltrithioniummethosulphate as an example of a film-producing inbibitor: determination of galvanostatic polarization curves, oscillographic cut-off measurements, impedance determinations at very small measuring voltage (8 mV), determination of the dissolved iron quantity per time unit by chemical analysis, polarographic test of the inhibitor. The measurements have shown:

• 1 The a/m substance is definitely a filmproducing inhibitor.
• 2 The inhibition in an air-saturated solution is relatively better than in deaerated solutions; however in deaerated acides the absolute dissolution velocities are consistently smaller than in aerated acides.
• 3 The covering of the iron surface with an inhibitor film causes a strong increase in the slope of the cathodic Tafel line; the better is approximately proportional to the logarithm of the inhibitor concentration. A hypothesis is developed in order to explain this phenomenon.
• 4 The Ohmic resistance of the inhibitor film is very small.
• 5 The dissolution velocities determined by means of the polarization curves show in a deaerated acid a good correspondence with the data found by chemical analysis. No such correspondence exists in air-saturated acid.

     

The polarization behaviour of FeCr alloys in acidic sulphate solutions in the active region

The polarization behaviour of six FeCr alloys was studied in sulphate solution of variou pH, with emphasis on the hydrogen evolution, active dissolution, and evaluation of active corrosion current, in comparison with results on the individual components. The exchange current densities of hydrogen evolution on FeCr alloys were found to increase with increasing Cr content up to 30Cr%. The rate of the anodic dissolution of low Cr alloys (< 10%Cr) strongly depended on pH and exhibited a Tafel behaviour with the slope of 40 mV/decade. That of high Cr alloys (> 10%Cr) was independent of pH and the Tafel slope was 60 mV/decade, similar to that of pure Cr, thus there is a threshold Cr content above which the dissolution of the alloy is controlled by the Cr component. The peak current at the active-passive transition decreased with increasing pH or Cr content. It is highly probable that the pre-passive state commences with the formation of chromic hydroxide, which promotes the formation of a ferrous oxide to shift the Flade potential in the negative direction.     

Anodic dissolution of nickel in acidic chloride solutions

The anodic dissolution of nickel was studied galvanostatically in hydrochloric acid solutions of various concentrations. The reaction orders of chloride ion and hydrogen ion concentrations were found to be 0.5 and 1.0, respectively. An anodic Tafel slope equal to 120 ± 10 mV · decade⁻¹ was obtained. The dissolution rate of nickel at constant acid concentration was increased with stirring of the solution and increasing temperature. The activation energy, ΔH, for the anodic dissolution process was found to be 12 kcal · mol⁻¹. The presence of oxygen in solutions assisted the passivation process. The effect of addition of aniline and some of its derivatives (o-, m-, and p-anisidine) as inhibitors on the dissolution kinetics of Ni in 1 M HCl was also investigated. These compounds inhibited the anodic dissolution of nickel without affecting the Tafel slope, indicating that the adsorption of such inhibitors could not interfere with the mechanism of metal dissolution.     

In-situ hydrothermal synthesis of Ni–MoO2 heterostructure on porous bulk NiMo alloy for efficient hydrogen evolution reaction

The Ni–MoO₂ heterostructure was synthesized in suit on porous bulk NiMo alloy by a facile powder metallurgy and hydrothermal method. The results of field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) reveal that the as-prepared electrode possesses the heterostructure and a layer of Ni(OH)₂ nanosheets is formed on the surface of Ni–MoO₂ electrode simultaneously after hydrothermal treatment, which provides abundant interface and much active sites, as well as much active specific surface area. The results of hydrogen evolution reaction indicate that the Ni–MoO₂ heterostructure electrode exhibits excellent catalytic performance, requiring only 41 mV overpotential to reach the current density of 10 mA/cm². It also possesses a small Tafel slope of 52.7 mV/dec and long-term stability of electrolysis in alkaline medium.     

Reproducibility of corrosion parameters for the acidic dissolution of pure iron: potentiostatic polarisation

Polarisation curves were determined potentiostatically for pure polycrystalline iron corroding in oxygen-free 0.5 M H₂SO₄. Four different working electrode pre-treatments (abrasion/polishing, pre-polarisation and time to establish E_corr) were employed and the reproducibility of E_corr and calculated corrosion parameters (i_corr, Tafel slopes and i₀) for each treatment was determined. Electrode pre-treatment effects changes in working electrode catalytic activity with subsequent variation in the reproducibility of polarisation curves and measured and calculated corrosion parameters. A method incorporating abrasion/polishing followed by anodic/cathodic pre-polarisation resulted in general in improved parameter reproducibility and cathodic and anodic Tafel slopes close to those predicted by the reaction mechanisms.     

A Study on the Electrodeposition Mechanism of Palladium from Ammoniacal Solution Using a Rotating Pt Ring-Cu Disk Electrode

The electrodeposition of palladium in ammonium phosphate solution (pH=10) containing 0.025 M tetramminepalladium(II) ion has been studied at room temperature by means of cyclic voltammetry, potentiostatic current transients and ac impedance measurements on a rotating Pt ring-Cu disk electrode, Cyclic voltammograms and potentiostatic current transients exhibited the characteristic features of the mechanism of progressive nucleation and growth under interfacial electron transfer and diffusion control. The interfacial reaction-controlled growth condition was established on the reverse sweep of the cyclic voltammogram. From the reverse sweep curve of cyclic voltammetry done at various rotation speeds, a Tafel plot of -210 mV per decade and diffusivity of tetramminepalladium(II) ion of 1.4 × 10^?5 cm² s^?1 were obtained. During the electrodeposition of palladium on the Cu-disk electrode, the ring current increased in the anodic direction, indicating the oxidation of reaction intermediates produced on the disk electrode. Complex-plane impedance spectra obtained in the potential range of-0.76 to -0.8 V(SCE) showed a semicircle in the high frequency region and an inductive loop in the low frequency region. Based upon the experimental results, it is suggested that the electrodeposition of palladium from tetrammine palladium(II) ion proceeds partly via two consecutive reactions with the formation of a monovalent reaction intermediate and partly via a direct two electron discharge reaction in which the intermediate acts as a catalyst associated with propagating kink sites.     

N-Phenylcinnamimide and some of its derivatives as inhibitors for corrosion of lead in HCl solutions

The anodic and cathodic polarization behavior of lead electrode was studied galvanostatically in HCl solutions of various concentrations. Increasing the acid concentration enhances the rate of the anodic dissolution of the metal and the rate of hydrogen evolution reaction, with anodic and cathodic Tafel slopes equal to 30 and 115 ± 5 mV decade⁻¹, respectively. The effect of addition of N-phenylcinnamimide and some of its derivatives, N-(p-nitrophenyl)cinnamimide, N-(p-methylphenyl)cinnamimide, and N-(p-methoxyphenyl)cinnamimide, as inhibitors on the kinetic of the anodic and cathodic reactions of lead in 0.1 M HCl solutions was also studied. These compounds inhibit both the rate of anodic dissolution and the rate of hydrogen evolution reactions without affecting the Tafel slopes. This result indicates that the used compounds were of mixed-type. The inhibition efficiency of these additives increases in the order: N-(p-nitrophenyl)cinnamimide < N-phenylcinnamimide < N-(p-methylphenyl)cinnamimide < N-(p-methoxyphenyl)cinnamimide. The equilibrium constant and the free energy of adsorption process have been calculated and discussed.     

Corrosion and corrosion inhibition of Cu-20%Fe alloy in sodium chloride solution

The electrochemical behavior of copper (Cu), iron (Fe) and Cu-20%Fe alloy was investigated in 1.0 M sodium chloride solution of pH 2. The effect of thiourea (TU) addition on the corrosion rate of the Cu-20%Fe electrode was also studied. Open-circuit potential measurements (OCP), polarization and electrochemical impedance spectroscopy (EIS) were used. The results showed that the corrosion rates of the three electrodes follow the sequence: Cu < Cu-20%Fe < Fe. Potentiostatic polarization of the Cu-20%Fe electrode in the range −0.70 V to −0.45 V (SCE), showed that iron dissolves selectively from the Cu-20%Fe electrode surface and the rate of the selective dissolution reaction depends on the applied potential. At anodic potential of −0.45 V, thiourea molecules adsorb at the alloy surface according to the Langmuir adsorption isotherm. Increasing thiourea concentration (up to 5 mM), decreases the selective dissolution reaction and the inhibition efficiency η reach 91%. At [TU] > 5 mM, the dissolution rate of the Cu-20%Fe electrode increases due to formation of soluble thiourea complexes. At cathodic (−0.6 V), the inhibition efficiency of thiourea decreases markedly owing to a decrease of the rate of the selective dissolution reaction and/or desorption of thiourea molecules. The results indicated that thiourea acts mainly as inhibitor of the selective dissolution reaction of the Cu-20%Fe electrode in chloride solution.     

Hydrogen evolution reaction on spheroidal graphite cast iron with different pearlite areas in sulphuric acid solutions

The rate equation of hydrogen evolution reaction of spheroidal graphite cast iron with different pearlite area has been studied in sulphuric acid solutions at 298 K. The cathodic Tafel slope of ?0·130 V/decade and the reaction order with respect to the activity of hydrogen ion of 1 are obtained by linear potential sweep technique. The rate equation of hydrogen evolution reaction does not depend on the area of pearlite. There is no difference in hydrogen evolution reaction mechanisms between pure iron and spheroidal graphite cast iron.     

The corrosion behaviour and hydrogen evolution reaction of Fe in non-aqueous formic acid

The corrosion of Fe in de-oxygenated non-aqueous formic acid was studied by the impedance method. It was confirmed that the cathodic reaction is a hydrogen evolution reaction (h.e.r.) and that formaldehyde is not formed. The rate-determining step (r.d.s.) of the h.e.r. is the formation of adsorbed hydrogen. The presence of adsorbed hydrogen was confirmed by the appearance of the low frequency capacitive arc obtained in the cathodic Tafel region of the impedance diagram. The inductive arc regarded as being due to the anodic dissolution of Fe was seen in the potential range between E_corr and cathodic Tafel region.     

Oxygen reduction on teflon-bonded carbon electrode

1　INTRODUCTIONOxygenreduction (OR)inconcentratedKOHsolutionisanimportantprocessinvolvedinalkalinefuelcellsandmetal airbatteries .SomefundamentalstudiesofthekineticsandmechanismsofORincon centratedalkalineelectrolytesthoughrelativelylimit ed .Inalkalineelectrolytes ,OR proceedsbytwopathwaysasfollows :1) 4electrons (4e) pathwayleadingdirectlytoOH- ;2 ) 2electronspathwaywithhydrogenperoxideasintermediateorfinalproducts ,dependingonelectrodesurfacestatesuchastheoxy genadsorption ,oxideco…     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more negative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

The corrosion and passivation of iron in the presence of halide ions in aqueous solution

The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential (E_c) was noted in each solution, the value of E_c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E_c, indicating that the kinetics of pit initiation are independent of mass-transfer effects.During anodic dissolution at potentials more regative than E_c, an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH⁺ (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH^? to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.     

pH值和氯离子浓度对铁在酸溶液中的腐蚀电化学行为的影响

发展了一种根据弱极化区的单支极化曲线确定腐蚀电流密度Ⅰ_(corr)和阳极反应与阴极反应的Tafel斜率b_a与b_c的方法。利用这种方法研究了溶液pH值与Cl~-浓度对铁在酸溶液中的腐蚀电化学行为的影响,并用强极化区的实验数据进行了校验。结果表明,OH~-和Cl~-都参与了铁的阳极溶解过程,反应级数分别为1和0.5;b_a和b_c分别为40mV和120mV。提出了一个铁在含Cl~-的酸中阳极溶解反应的动力学模型。实验结果还证明,在pH值与Cl~-浓度两者对E_(corr)和Ⅰ_(corr)的影响之间并无交互效应。     

Role de la diffusion dans les phenomenes de passivation et de corrosion localisee du fer en milieu acide

By using a rotating disc electrode and a new device for the control of interfacial polarization, an experimental study has been performed concerning the anodic dissolution and the passivation of iron in sulphuric medium. A model is given which interprets the Z-shaped current-potential curve and the impedance diagrams measured. As a matter of fact, it is proved and understood that a passive state and an active state can take place for a same potential. In the same way, active dissolution in localized corrosions at the surface of passive iron is due to the slowing down of the diffusion of passivating species in the hole.     

Potentiometric response of polyaniline electrode to dissolved nitrate

Nitrate-sensitive electrodes were prepared by aniline electropolymerization on platinum substrate from a nitric acid solution. The electrodes show a linear response of potential to nitrate concentration in the range of 10⁻¹–10⁻³ M, and a slope up to − 44 mV/decade.