Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions

The reaction of H₂O₂ on SIMFUEL electrodes has been studied electrochemically and under open circuit conditions in 0.1 mol l⁻¹ NaCl (pH 9.8). The composition of the oxidized UO₂ surface was determined by X-ray photoelectron spectroscopy (XPS). Peroxide reduction was found to be catalyzed by the formation of a mixed U^IV/U^V (UO_2+x) surface layer, but to be blocked by the formation of U^VI (UO₂²⁺) species on the electrode surface. The formation of this U^VI layer blocks both H₂O₂ reduction and oxidation, thereby inhibiting the potentially rapid H₂O₂ decomposition process to H₂O and O₂. Decomposition is found to proceed at a rate controlled by desorption or reduction of the adsorbed O₂ species. Reduction of O₂ is coupled to the slow oxidative dissolution of UO₂ and formation of a corrosion product deposit of UO₃·yH₂O.     

Corrosion inhibition of steel in chloride-containing alkaline solutions

Some organic inhibitors of steel corrosion in saturated calcium hydroxide solutions containing 0.1 M chlorides were investigated to elucidate their interactions with the steel surface. The compounds studied were dicyclohexylammonium nitrite (DCHAMN), dicyclohexylamine (DCHA), sodium -glycerophosphate (GPH) and 5-hexyl-benzotriazole (C6BTA). Sodium nitrite (SN) was also studied as a reference. The techniques applied were electrochemical impedance spectroscopy (EIS), polarization curves, cyclic voltammetry (CV) and Fourier transform infrared (FTIR) spectroscopy. FTIR spectra showed that DCHAMN, DCHA, GPH and C6BTA interact with the steel surface by chemisorption and some information about the mechanism also emerged. From DCHAMN solution, DCHA chemisorption is induced by slow salt hydrolysis. CV tests show that, among chemisorbed substances, only GPH avoids chloride penetration on cycling, most likely due to a quick chemisorption, while DCHAMN can only enlarge the passive potential range. Addition of SN also prevents chloride attack on cycling. Twenty days of immersion in inhibited solutions revealed that, besides SN, GPH and DCHAMN also form an impervious surface film on steel, which blocks any localized corrosion attack, whereas in the case of DCHA and C6BTA solutions, pitting corrosion is slowed down, but not avoided. DCHAMN exhibits the highest inhibiting efficiency at long immersion times, as a result of a synergetic inhibitive action which develops between nitrite and chemisorbed DCHA.     

Electrochemical and corrosion studies of poly(nickel-tetraaminophthalocyanine) on carbon steel

Nickel-tetraaminophthalocyanine [TAPcNi] was electropolymerized from the complex monomeric solution, onto carbon steel substrates, yielding thin adherent films of poly[TAPcNi]. The investigation of such polymer-modified electrodes was carried out by means of cyclic voltammetry, UV-vis spectroscopy, FT-IR spectroscopy and spectroelectrochemistry. The preparation of TAPcNi modified electrode was also carried out by electropolymerization of a preformed molecular film of TAPcNi, after applying a drop of TAPcNi dimethylsulphoxide solution onto carbon steel, and allowing it to dry. The comparison of the corrosion behavior of the two types of polymer-coated electrodes was carried out by electrochemical impedance spectroscopy (EIS) in acid medium. It was found that the structure and morphology of each polymer greatly influence their redox behavior and corrosion inhibition performance for steel in hydrochloric acid. The film prepared by the drop-dry method offered a better corrosion protective efficiency while the electropolymerized film presented a more conductive behavior.     

Electrochemically grown passive films on carbon steel (SAE 1018) in alkaline sour medium

Corrosion films were prepared by applying cyclic potential pulses to the 1018 carbon steel-sour medium interface (1 M (NH₄)₂S, 500 ppm CN⁻) for 1 min. Electrochemical behavior and surface morphology of these films were determined using electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and scanning photoelectrochemical microscopy (SPECM). EIS diagrams and SPECM images show the passive properties and homogeneity of the films. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize their chemical nature and structure. XPS results show that different oxide and sulfur structures were developed during the electrochemical oxidation of carbon steel in concentrated sour media. The analysis of O 1s data indicated that, during film growth, H₂O and/or hydroxyl groups are incorporated into the film structure. The XPS spectra of Fe 2p show iron bonds with S as iron sulfide (FeS₂ and FeS) and the corresponding peak of O 1s shows those bonds with oxygen as Fe₂O₃ and/or FeO. XPS depth profile analyses for the film showed that the ratio of FeS and FeO increases from film surface to film-carbon steel interface. This corroborates the diffusion of iron ions through the film during its electrochemical growth. The chemical shift through the film for the peak associated with Fe 2p signal proves that transport mechanism of iron ions through the film is carried out by chemical diffusion.     

Electrochemical study of benzene on Pt of various surface structures in alkaline and acidic solutions

The electrochemical behaviour of benzene on platinum electrodes (polycrystalline and single-crystal electrodes) has been studied in acidic and alkaline solutions. In acid solutions the reduction of benzene to cyclohexane takes place in all the platinum surface structure employed, however it does not occur in alkaline media (0.1 M NaOH). In this case, the hydrogen adsorption-desorption processes displace the adsorbed benzene from the electrode surface. The oxidation of benzene is also affected by the pH of the electrolyte and also by the surface structure of the platinum electrode used. In acid solutions, this oxidation at higher potentials (1.4 V vs. RHE) yields CO₂, benzoquinone and α,β-unsaturated esters or lactones, however in alkaline media carbonate anions coming from CO₂ and salts of carboxylic acids have been detected by in situ FTIR spectroscopy using a platinum polycrystalline electrode. Bulk electrolysis of benzene solutions using a platinum electrode in acid and alkaline media was performed in order to confirm the results obtained by spectroscopic measurements.     

Passivity and passivity breakdown of 304 stainless steel in alkaline sodium sulphate solutions

The passivity and passivity breakdown of 304 stainless steel were investigated in 0.25 M Na₂SO₄solutions of pH 10. The effect of applied potential and the presence of Cl^– ions in the electrolyte were also studied. Different electrochemical methods such as open circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS) were used. The results showed that the steel electrode passivates under open circuit conditions and also under potentiostatic control. The rate of passive film thickening under open circuit conditions follows a simple logarithmic law. Addition of Cl^– ion shifts the polarization curves in the active direction and above a critical chloride concentration, [Cl^– ] 0.15 M, pitting corrosion occurs and the pitting potential, E _pit, decreases linearly with the logarithm of [Cl^–]. The addition of sulphate ions to the chloride-containing solutions was found to inhibit the pitting process, and at [SO^2- ₄] 0.25 M, a complete immunity to pitting corrosion was recorded. The impedance measurements provided support for film thickening and film breakdown reactions. An equivalent circuit model which consists of a pure resistor, R , in series with a parallel combination of a pure resistor, R _p, and a constant phase element, Q, was proposed to describe the electrode/electrolyte interface. The passive film thickness was found to increase with applied potential up to a critical value of 0.3 V. At higher voltages, breakdown of the passive film occured.     

Inhibition of nickel coated mild steel corrosion by electrosynthesized polyaniline coatings

Polyaniline (PANI) coatings were electrochemically synthesized on nickel (Ni) coated mild steel (MS) and their corrosion protection properties were investigated. In this work, the Ni layer (∼1 μm thick) was electrodeposited on MS under galvanostatic condition. Thereafter, the PANI coating was deposited over the Ni layer from aqueous salicylate medium by using cyclic voltammetry. These bi-layered composite coatings were characterized by cyclic voltammetry, UV–vis absorption spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The corrosion protection properties of Ni coated MS (Ni/MS) and PANI coated Ni/MS (PANI/Ni/MS) were investigated in aqueous 3% NaCl by using open circuit potential (OCP) measurements, potentiodynamic polarization technique and electrochemical impedance spectroscopy (EIS). It was shown that the top layer of PANI exhibits a lower porosity behavior with respect to Ni coating and reduces the corrosion rate of Ni/MS almost by a factor of 3500 and increases the lifetime of Ni coating.     

Effect of resistivity on the corrosion mechanism of mild steel in sodium sulfate solutions

Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of mild steel samples immersed in 1, 0.1, 0.01 and 0.001% Na₂SO₄ aqueous solutions at room temperature in order to analyze the corrosion mechanism and obtain representative values of corrosion rates in environments with different resistivity. The EIS technique was used to measure corrosion current densities in 1 to 0.01% Na₂SO₄ solutions, and the measurement corresponding to 0.001% concentration gave a scattered Nyquist plot, whereas in the Bode representation a response associated with solution resistance was obtained. Other electrochemical techniques such as linear polarization resistance, LPR, Tafel extrapolation method and electrochemical noise (EN) were used in order to compare the results obtained by EIS. The charge-transfer resistance (R _ct) parameters obtained in the EIS technique agree well with the corresponding values of polarization resistance (R _p) values obtained by the LPR technique, whereas the electrochemical noise resistance (R _n) parameters obtained by the EN technique gave the highest values for all concentrations. These parameters generally tended to increase as the concentration of the solution decreased.     

Electrosynthesis of polyaniline–molybdate coating on steel and its corrosion protection performance

Electrosynthesis of polyaniline–molybdate (PANI–MoO₄²⁻) on mild steel was achieved in oxalic acid medium using cyclic voltammetry technique. Adherent and homogeneous PANI–MoO₄²⁻ coating was obtained. The corrosion behavior of steel with PANI–MoO₄²⁻ coatings in 1% NaCl solutions was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The coating was characterized by SEM, XPS, EDAX and FTIR. The self-healing ability of PANI–MoO₄²⁻ coating was confirmed by SVET technique. It has been found that the PANI–MoO₄²⁻ coating is able to offer higher corrosion protection in comparison to that of pure PANI coating due to inhibitive nature of molybdate ions.     

Influence of adsorbed carbon dioxide on hydrogen electrosorption in palladium-platinum-rhodium alloys

Carbon dioxide electroreduction was applied to examine the processes of hydrogen electrosorption (adsorption, absorption and desorption) by thin electrodeposits of Pd-Pt-Rh alloys under conditions of cyclic voltammetric (CV) experiments. Due to different adsorption characteristics towards the adsorption product of the electroreduction of CO₂ (reduced CO₂) exhibited by the alloy components hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. Reduced CO₂ causes partial blocking of hydrogen adsorbed on surface Pt and Rh atoms, without any significant effect on hydrogen absorption into alloy. It reflects the fact that adsorbed hydrogen bonded to Pd atoms does not participate in CO₂ reduction, while hydrogen adsorbed on Pt and Rh surface sites is inactive in the absorption reaction. In contrast, CO is adsorbed on all alloy components and causes a marked inhibition of hydrogen sorption (both adsorption and absorption)/desorption reactions.     

The influence of different surface conditions in the corrosion process of carbon steel in alkaline sour media

Different electrochemical methodologies were established to induce general corrosion and blistering on homogeneous and heterogeneous carbon steel surfaces similar to the corrosion damage in a catalytic oil refinery plant. In one case, the film porosity and the iron sulphide stoichiometry were modified and in other case, the surface conditions were changed with sulphur films and microblisters. Additionally, we studied the influence of 1018 carbon steel surface conditions on the corrosion process in a medium simulating the average composition of sour waters in catalytic plants of PEMEX Mexico (0.1 M (NH₄)₂S, 10 ppm CN^– as NaCN, pH 8.8). Using the impedance spectra, from 10 kHz to 0.01 Hz, it was possible to qualitatively identify the carbon steel surface condition in an alkaline sour environment and to suggest the same corrosion process steps for this system, despite different surface conditions: charge transfer resistance of steel oxidation in the metal/corrosion product film interface and Fe²⁺ ion and H° diffusion through the corrosion product film. Finally, scanning electron microscopy of a freshly polished surface showed the formation of a homogeneous film immediately after introducing the carbon steel into the sour media. The other surface changes depended on the induced corrosion process and corroborated the electrochemical impedance predictions.     

Iodide surface coverage and its effect on the rate constants during hydrogen charging and permeation in iron

This paper demonstrates the use of the new Iyer/Pickering/Zamanzadeh/Al-Faqeer (IPZA) analysis to determine the effect of the iodide surface coverage (θ_I⁻) on the hydrogen surface coverage (θ_H) and the rate constants of the hydrogen evolution reaction (HER) and hydrogen absorption reaction (HAR). Iodide ions have an inhibiting effect on the HER but they enhance the HAR. The IPZA analysis was used to determine θ_I⁻ in addition to the usual parameters: hydrogen discharge (k₁) and recombination (k₂) rate constants, hydrogen kinetic-diffusion constant (k) and θ_H. Inclusion of the iodide coverage is important as shown by a comparison with literature values that were obtained using the original IPZ analysis, which does not include the θ_I⁻ parameter. The θ_I⁻ values from the IPZA analysis were within reasonable agreement with θ_I⁻ values measured using the electrochemical quartz crystal microbalance (EQCM) and θ_I⁻ values calculated from the measured corrosion current densities before and after adding iodide to the electrolyte.     

Iodide surface coverage and its effect on the rate constants during hydrogen charging and permeation in iron

This paper demonstrates the use of the new Iyer/Pickering/Zamanzadeh/Al-Faqeer (IPZA) analysis to determine the effect of the iodide surface coverage (θ_I⁻) on the hydrogen surface coverage (θ_H) and the rate constants of the hydrogen evolution reaction (HER) and hydrogen absorption reaction (HAR). Iodide ions have an inhibiting effect on the HER but they enhance the HAR. The IPZA analysis was used to determine θ_I⁻ in addition to the usual parameters: hydrogen discharge (k₁) and recombination (k₂) rate constants, hydrogen kinetic-diffusion constant (k) and θ_H. Inclusion of the iodide coverage is important as shown by a comparison with literature values that were obtained using the original IPZ analysis, which does not include the θ_I⁻ parameter. The θ_I⁻ values from the IPZA analysis were within reasonable agreement with θ_I⁻ values measured using the electrochemical quartz crystal microbalance (EQCM) and θ_I⁻ values calculated from the measured corrosion current densities before and after adding iodide to the electrolyte.     

Effects of hydrogen on the corrosion of pure magnesium

Electrochemical measurements and capacitance measurements were performed for better understanding of the effects of hydrogen on the corrosion of pure magnesium. Anodic polarization curves, activation energy (E_a), pitting initiation time and electrochemical noise (EN) were carried out, which showed that hydrogen had a strong influence on the corrosion of magnesium. There existed a highest corrosion resistance of magnesium, when a series of cathodic charging current density were applied to the specimens due to the optimum hydrogen concentration in the hydride coating (MgH₂) on the surface. Mott-Schottky results confirmed that there was a hydride coating on the charged magnesium. Hydrogen ionized as H⁻ and depleted donor/electron, which induced the inversion of semi-conductivity from N-type to P-type.     

Anodic growth of passive layers on steel rebars in an alkaline medium simulating the concrete pores

Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques have been used to study passive layers anodically grown on steel rebars in an aqueous alkaline solution simulating the electrolyte of the concrete pores. Nyquist diagrams recorded by EIS at the different stabilization potentials show a diffusional tail at low frequencies. The analysis of the impedance measurements has been made by means of an equivalent circuit with a Warburg component and within the framework of the point defect model (PDM) theory. It is observed that the calculated concentration of vacancies is a function of the potential in accordance with the theoretical prediction of the PDM.     

Electrochemical stability of Cu-10Al-5Ni alloy in chloride-sulfate electrolytes

The electrochemical behavior of the Cu-10Al-5Ni alloy in simulated electrolytes like those used in the industrial processes under normal working conditions was investigated. Conventional electrochemical techniques including open circuit potential measurements, polarization techniques, and electrochemical impedance spectroscopy were used. The alloy was found to be more stable against corrosion when the chloride-sulfate solution had a pH of 7-9. A sulfate concentration of ≅0.1 M in the chloride-sulfate electrolyte leads to the passivation of the alloy surface. The activation energy of the corrosion process was found to be 8 kJ mol⁻¹ assigning a diffusion controlled reaction. The impedance measurements and impedance data fitting to equivalent circuit models have shown that the passive film posses a duplex nature.     

氢对不锈钢堆焊层的力学性能和断裂形貌的影响

研究了在模拟加氢反应器工况条件下,国产加氢反应器壁材料2(1/4)Cr-1Mo内壁上的309L和347L不锈钢堆焊层的力学性能和断裂形貌.结果表明：热渗氢后,明显降低了其断裂应力(σ_f)和塑性.对于309L和347L堆焊层,未经热渗氢的光滑试样与缺口试样(除缺口位置在309L区域试样外)均断裂在347L区域内.而经渗氢后,无论是起裂于2(1/4)Cr-1Mo和309L的熔合线,还是309L和347L的熔合线处,最后都断在309L区域内(除缺口位置在347L区域试样外).在对短裂纹的低周疲劳特性研究中,计算了裂纹扩展速率(da/dN)和门槛值(ΔJ_th).经热渗氢后,ΔJ_th有明显下降,da/dN-ΔJ曲线也向左移,提高了da/dN.SEM断口分析结果与上述结论一致,裂纹扩展以穿晶为主,热渗氢后有二次裂纹存在,说明热渗氢降低了其力学性能,增加了加氢反应器壁材料的断裂敏感性.     

Compatibility between cataphoretic electro-coating and silane surface layer for the corrosion protection of galvanized steel

The compatibility between a cataphoretic electro-coating and a silane layer applied on galvanized steel was evaluated by performing electrochemical impedance measurements on coated and uncoated samples. During electro-deposition, the water electrolysis induces hydrogen production. This process can induce degradation or destruction of the silane layer. This process was simulated by reproducing the application conditions of electro-coating in an aqueous solution of same pH (6) and conductivity (1600 μS) than the electro-coating bath, but without any pigments and binder. A current of 2 mA/cm² was applied between the sample and the counter-electrode during 10 and 20 s. These conditions are representative of the mean real application conditions just before the coating formation. The loss of the barrier effect offered by the silane layer was evaluated by EIS before and after simulation. This simulation shows whether it is possible to conveniently design the properties of the silane layer to maintain its protection and adhesion promotion properties after polarization. The barrier properties and the water uptake of the electro-coated samples were evaluated by EIS as a function of immersion time in a sodium chloride solution (0.1 M). The coated silane pre-treated samples show a good behaviour compared to the samples coated without pre-treatment. By properly managing the deposition conditions of sol–gel films it is possible to obtain cataphoretic coating with improved corrosion resistance. Silane sol–gel films of different thicknesses and curing temperature were produced. It was demonstrated that a 120 nm thick silane sol gel film cured at 180 °C ensures a very good compatibility with the electro-coat. In fact, this system shows a very high corrosion resistance even after 50 days of immersion in a sodium chloride solution. Also the resistance in the salt spray chamber of the electro-coated thin silane layer cured at 180 °C is remarkable. The results confirm that, if conveniently designed, silane sol–gel film properties, the silane layer is a good adhesion promoter of the cataphoretic coating on galvanized steel and this property is maintained for long exposure times.     

The mathematical model of the stripping voltammetry hydrogen evolution/dissolution process on Pd layer

The advanced two-plate mathematical model of electrochemical hydrogen evolution/dissolution process has been presented and discussed. The model, with Langmuir adsorption equation, has been experimentally verified by the use of the glassy carbon/Pd layer electrode system at different scan rates. The two cathodic-anodic stages of hydrogen evolution/dissolution process in 0.1 M and 0.001 M HCl solutions have been interpreted and discussed. The thickness of the layer and the way of deposition were also investigated. The fundamental kinetic problem of a change of electrode properties during electrode process as an effect of the elementary hydrogen presence in the solid electrode is presented and interpreted. The isopotential point phenomenon, an electrochemical analog of isosbestic point in absorption spectroscopy, was unexpectedly discovered as experimental effect of hydrogen adsorption and α variability.     

钢铁零件碱性高温氧化膜的封闭处理

采用HT-200封闭工艺处理钢铁零件表面的高温氧化膜,处理后的零件存于库房2个月之内无锈蚀.对比了HT-200封闭工艺与传统肥皂液填充和浸油工艺的成本和能耗.结果表明,按0.8元/(kW.h)计算,HT-200工艺一年可节省12.6万元,实现了节能降耗、保护环境的要求.