Effect of carbonation process on the passivating products of zinc in Ca(OH)2 saturated solution

The effect of carbonation process on the passivating layer of zinc in Ca(OH)₂ saturated solution was studied. The investigation was performed by means of corrosion potential, corrosion current density, and impedance measurements. To analyze the changes in the passivating layer, X‐ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM) were utilized. The results obtained indicate that the layer of calcium hydroxyzincate (Ca[Zn(OH)₃]₂ · 2H₂O) (CHZ), which determines the passivity state of zinc both in Ca(OH)₂ saturated solution and in concrete, is destroyed by the carbonation process, in agreement with previous results obtained for galvanized steel embedded in concrete. X‐ray diffractometry and Raman spectroscopy showed that CHZ reaction with CO₂ leads to the formation of Zn₅(CO₃)₂(OH)₆ (hydrozincite) and CaCO₃ (calcite). SEM observation confirms the deep transformation in the passivating layer caused by carbonation. Corrosion potential and corrosion current density measurements show that zinc maintains its passive state also after carbonation. However, impedance measurements indicate that hydrozincite has lower passivating properties than calcium hydroxyzincate in the carbonated solution.     

The effect of fluoride on corrosion of reinforcing steel in alkaline solutions

The influence of fluoride content of alkaline solutions on the corrosion of reinforcements has been studied, by means of electrochemical techniques, in saturated solutions of Ca(OH)₂ and same pH NaOH solutions with additions of NaF, thus simulating the liquid phase of concrete. Fluoride can be present as a minor component in low-energy cements. The joint presence of chlorides and fluorides in the corrosion process of steel has also been analysed. It has been established that fluoride anions are able to produce pitting of reinforced steel in alkaline media of high pH. In Ca(OH)₂ saturated solution, the precipitation of CaF₂ lowers F⁻ concentration below the minimum that promotes a pitting corrosion process; if there is not enough Ca(OH)₂ to precipitate all F⁻ ions, the presence of fluoride in these media causes an increase in passive film dissolution and corrosion rates.     

First-principle investigation on mechanism of Ca ion activating flotation of spodumene

Different hypotheses of the activation mechanisms of metallic ions for silicate minerals flotation were stated in this work. Spodumene and Ca ion were taken to illustrate this issue. Flotation test results and diagram of Ca ion species distribution vs pH show that the dominant hydrolyzed species are Ca（OH）？and Ca（OH）2, which is in correspondence with the maximum flotation recovery of spodumene. Density functional theory（DFT） calculation results reveal further that the precipitation of calcium hydroxide on spodumene surface is the effective process to activate the flotation of spodumene.     

Influence of N,N′‐dimethylaminoethanol as an inhibitor on the chloride threshold level for corrosion of steel reinforcement

The aim of this study is to examine the influence of N,N′‐dimethylaminoethanol (DMEA) as an inhibitor on the chloride threshold level for corrosion of steel in a concrete contaminated by chlorides. The experiment has been carried out in a saturated Ca(OH)₂ solution and chloride contaminated concrete containing different chloride and DMEA level. The critical point of corrosion onset is concluded by combining the open‐circuit potential (E_corr) with corrosion current (I_corr), which is decided by electrochemical impedance spectra (EIS) in the solution. Besides, the EIS has also been applied to determinate the chloride threshold level in the chloride contaminated concrete. It has been found that the presence of DMEA represented as an amino‐alcohol inhibitor, exerts little influence on the chloride threshold level for corrosion of steel in the solution. Similarly, the effect of the DMEA on the chloride threshold level in the chloride contaminated concrete, is also negligible.     

On the corrosion behaviour of phosphoric irons in simulated concrete pore solution

The corrosion behaviour of three phosphoric irons P₁ (Fe-0.11P-0.028C), P₂ (Fe-0.32P-0.026C) and P₃ (Fe-0.49P-0.022C) has been studied in simulated concrete pore solution (saturated Ca(OH)₂ solution) containing different chloride concentration. This has been compared with that of two commercial concrete reinforcement steels, a low carbon steel TN (Fe-0.148C-0.542Mn-0.128Si) and a microalloyed corrosion resistant steel CS (Fe-0.151C-0.088P-0.197Si-0.149Cr-0.417Cu). The beneficial aspect of phosphoric irons was revealed from potentiodynamic polarization experiments. The pitting potentials and pitting nucleation resistances for phosphoric irons and CS were higher than that for TN. Electrochemical impedance spectroscopy (EIS) studies revealed thickening and growth of passive film as a function of time in case of phosphoric irons and CS in saturated Ca(OH)₂ pore solutions without chloride and in the same solution with 0.05% Cl⁻ and 0.1% Cl⁻. In case of TN, breakdown of passive film resulted in active corrosion in simulated pore solution containing 0.1% Cl⁻. Linear polarization resistance measurements complemented EIS results. Visual observations indicated that phosphoric iron P₃ was immune to corrosion even after 125 days of immersion in saturated Ca(OH)₂ solution containing 5% NaCl. The good corrosion resistance of phosphoric irons in simulated concrete pore solution containing chloride ions has been related to the formation of phosphate, based on ultraviolet spectrophotometric analysis and Pourbaix diagram of phosphorus-water system.     

Environmental factors affecting the corrosion behavior of reinforcing steel II. Role of some anions in the initiation and inhibition of pitting corrosion of steel in Ca(OH)2 solutions

Potential-time curves are constructed for the steel electrode in naturally aerated Ca(OH)₂ solutions simulating the corrosion behavior in concrete. Cl⁻ and SO₄²⁻ ions cause the destruction of passivity and initiation of pitting corrosion. The rate of oxide film growth by Ca(OH)₂ and oxide film destruction by Cl⁻ and SO₄²⁻ ions follows a direct logarithmic law as evident from the linear relationships between the open-circuit potential and the logarithm of immersion time. Chromate, phosphate, nitrite, tungstate and molybdate ions inhibit the pitting corrosion of steel. The rate of oxide film healing and thickening increases with their concentrations. In presence of constant inhibitor concentration, the efficiency of pitting inhibition increases in the order: (weak) CrO₄²⁻ < HPO₄²⁻ < NO₂⁻ < WO₄²⁻ < MoO₄²⁻ (strong).     

Measurement of the resistivity of the electrode NiOOH/Ni(OH)<Subscript>2</Subscript> solid phase active substance during the discharge process

The quantitative relation between the resistivity of the solid phase active substance NiOOH/Ni(OH)₂ and its bulk resistivity, which can be measured experimentally according to the potential jump when the current is switched on, is established with the simplified mathematical model of a thin porous electrode with a solid phase reagent. The dynamics of the resistivity of the NiOOH/Ni(OH)₂ particles in the course of the discharge is ascertained with the help of the proposed method.     

Bildungsgeschwindigkeit kathodisch erzeugter Hydroxid-Deckschichten

Rate of formation of cathodically produced hydroxide surface layers The cathodic hydrogen evolution shifts the pH values in the vicinity of the electrode to ward higher values. If a metal ion is present in the solution and if the pH shift results in a condition where the solubility product of the metal oxide or hydroxide respectively is attained at the electrode surface a further current density increase may give rise to the precipitation of the metal hydroxide which then forms a suface layer on the electrode. The rate of formation of such layers is assessed whereby it is presumed that the rate controlling step is exclusively the transport of the ions taking part in the reactions. Tracing current density overtension curves and weighing the surface layers formed enables the rate of formation to be measured as a function of current density, pH value, metal ion concentration and diffusion layer thickness. The results obtained for layers of Ni(OH)₂, Cd(OH)₂, Co(OH)₂, Zn(OH)₂, Mg(OH)₂, Ce(OH)₃ and La(OH)₃ are in good agreement with theoretical considerations. In the case of the formation of Al(OH)₃ layers it is necessary to take account of the contribution to charge transport by aluminium ions dissolved in the form of hydroxo-complexes.     

Corrosion behaviour in alkaline medium of zinc phosphate coated steel obtained by cathodic electrochemical treatment

The present work evaluated the ability of zinc phosphate coating, obtained by cathodic electrochemical treatment, to protect mild steel rebar against the localized attack generated by chloride ions in alkaline medium. The corrosion behaviour of coated steel was assessed by open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. The chemical composition and the morphology of the coated surfaces were evaluated by X-ray diffraction and scanning electron microscopy. Cathodically phosphated mild steel rebar have been studied in alkaline solution with and without chloride simulating the concrete pore solution. For these conditions, the results showed that the slow dissolution of the coating generates the formation of calcium hydroxyzincate (Ca(Zn(OH)₃)₂·2H₂O). After a long immersion time in alkaline solution with and without Cl⁻, the coating is dense and provides an effective corrosion resistance compared to mild steel rebar.     

In situ synchrotron X-ray diffraction study of scale formation during CO2 corrosion of carbon steel in sodium and magnesium chloride solutions

In situ synchrotron X-ray diffraction was used to follow the formation of corrosion products on carbon steel in CO₂ saturated NaCl solution and mixed NaCl/magnesium chloride (MgCl₂) at 80 °C. Siderite (FeCO₃) was the only phase formed in NaCl solution, while Fe(OH)₂CO₃ was also detected when MgCl₂ was present. The proposed model is that siderite precipitation, occurring once the critical supersaturation was exceeded within a defined boundary layer, caused local acidification which accelerated the anodic dissolution of iron. The current fell once a complete surface scale was formed. It is suggested that MgCl₂ addition decreased the required critical supersaturation for precipitation.     

Environmental factors affecting the corrosion behavior of reinforcing steel III. Measurement of pitting corrosion currents of steel in Ca(OH)2 solutions under natural corrosion conditions

Using a simple electrolytic cell, the pitting corrosion current of reinforcing steel is measured in Ca(OH)₂ solutions in presence of chloride and sulfate as aggressive ions. Pitting corrosion current starts to flow after an induction period which depends on the concentration of both the aggressive and the passivating anions. The pitting corrosion current densities reach steady-state values which depend also on the type and concentration of the corrosive and passivating anions. The corrosive action of the aggressive species decreased in the order: SO₄²⁻ > Cl⁻. Corrosion of the steel is found to be governed by a single electron transfer reaction. Raising the temperature decreases the induction period associated with pit initiation and increases the corrosion current associated with pit propagation. From Arrhenius plots, the activation energies for both pit initiation and pit propagation in presence of chloride and sulfate ions are calculated.     

The beneficial secondary effects of conventional and pulse cathodic protection for reinforced concrete,evidenced by X‐ray and microscopic analysis of the steel surface and the steel/cement paste interface

The present study reports on the investigation of conventional and pulse cathodic protection (CP), in terms of steel surface analysis and investigation of the product layers at the steel/cement paste interface, after a long term (460 days) of conditioning and monitoring. The techniques used were: X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy‐dispersive X‐ray (EDX). Wet chemical analysis was used as supportive evidence for ion concentrations in the vicinity of the steel re‐bars. Generally, CP promotes beneficial secondary effects i.e. enhanced OH^? concentration and reduced Cl^? concentration near the steel surface. Cathodic polarization also results in additional deposition of portlandite, which stabilizes the protective properties of the product layer on the steel surface. Consequently, the fundamental mechanisms, underlying the efficiency of CP techniques in reinforced concrete, are strongly related to beneficial secondary effects of CP, affecting the morphology and transformations of these product layers. Since the experimental evidences to support the aforementioned beneficial effects are rather limited, this study investigates the morphology and composition of the “naturally” formed steel surface layers, along with the properties of the steel/cement paste interface, on corroding and protected steel reinforcement (in comparison to reference, non‐corroding, non‐protected conditions) after 460 days of conditioning. For the corroding specimens, the formation and substantial deposition of voluminous corrosion products, with low adherence to the steel surface is relevant (low protective ability), whereas in the protected specimens, a compact and adherent product layer of more stable high valent iron oxides, or calcium substituted such, was observed. To this end, the present work provides the experimental evidence for the fundamental mechanisms, related to the otherwise recognized positive secondary effects of CP in reinforced concrete.     

Cyclic metallurgical process for extracting V and Cr from vanadium slag: Part I. Separation and recovery of V from chromium-containing vanadate solution

The separation and recovery of V from chromium-containing vanadate solution were investigated by a cyclic metallurgical process including selective precipitation of vanadium, vanadium leaching and preparation of vanadium pentoxide. By adding Ca(OH)₂ and ball milling, not only the V in the solution can be selectively precipitated, but also the leaching kinetics of the precipitate is significantly improved. The precipitation efficiency of V is 99.59% by adding Ca(OH)₂ according to Ca/V molar ratio of 1.75:1 into chromium-containing vanadate solution and ball milling for 60 min at room temperature, while the content of Cr in the precipitate is 0.04%. The leaching rate of V reaches 99.35% by adding NaHCO₃ into water according to NaHCO₃/V molar ratio of 2.74:1 to leach V from the precipitate with L/S ratio of 4:1 mL/g and stirring for 60 min at room temperature. The crystals of NH₄VO₃ are obtained by adjusting the leaching solution pH to be 8.0 with CO₂ and then adding NH₄HCO₃ according to NH₄HCO₃/NaVO₃ molar ratio of 1:1 and stirring for 8 h at room temperature. After filtration, the crystallized solution containing ammonia is reused to leach the precipitate of calcium vanadates, and the leaching efficiency of V is >99% after stirring for 1 h at room temperature. Finally, the product of V₂O₅ with purity of 99.6% is obtained by calcining the crystals at 560 °C for 2 h.     

α-Ni(OH)2 electrodeposition from NiCl2 solution

α-Ni(OH)₂ was synthesized from a NiCl₂ solution by electrodeposition method. In order to conduct a systematic study on the effects of experimental parameters, a series of electrolyte initial pH values, current densities, electrodeposition temperatures, and electrodeposition time were used. Cyclic voltammetry results demonstrated a side reaction of Ni²⁺+2e→Ni. The X-ray diffraction analysis, Fourier-transform infrared spectrum, and the color of the product showed that pure α-Ni(OH)₂ could be obtained in the initial pH value range of 2–5.86, current density range of 10–25 mA/cm² electrodeposition temperature range of 25–35 °C, and electrodeposition time range of 1.0–3.0 h. When electrodeposition temperature increased to 45 °C, a mixture of α-Ni(OH)₂ and metallic Ni was obtained. A current density higher than 30 mA/cm² resulted in the sample with features of β-Ni(OH)₂. A small amount of metallic Ni existed in the as-prepared sample when current density decreased to 5 mA/cm². A slight increase of electrolyte pH was observed with increasing initial solution pH and current density. Electrodeposition mass revealed a slight decrease with initial pH decreasing and showed an almost linear increase with current density increasing. The slope of the curve for electrodeposition mass versus electrodeposition time remained stable in the first 2.0 h and then decreased.     

Facile fabrication of spherical flower-like Mg(OH)2 and its fast and efficient removal for heavy metal ions

Spherical flower-like Mg(OH)₂ was fabricated from MgSO₄ effluent and its adsorption performance for heavy metal ions was evaluated. The appropriate fabrication conditions are as follows: Mg²⁺/NH₄OH molar ratio of 1:0.5, temperature of 120 °C and time of 1 h at Mg²⁺ concentration of 2 mol/L. Spherical flower-like Mg(OH)₂ composed of ultra-thin sheets exhibits an excellent adsorption ability for Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Fe³⁺ and Co²⁺, and the adsorption reaches the equilibrium in 6 min. The maximum adsorption capacities of the studied heavy metal ions onto Mg(OH)₂ at 20 °C are 58.55, 85.84, 44.94, 485.44, 625.00 and 27.86 mg/g, respectively. The adsorption is well fitted by the Langmuir model, indicating that the adsorption is monolayer. The adsorption kinetics follows the pseudo-second- order model. Chemisorption is the operative mechanism. Spherical flower-like Mg(OH)₂ is a qualified candidate for heavy metal ions removal.     

Environmental factors affecting the corrosion behaviour of reinforcing steel: I. The early stage of passive film formation in Ca(OH)2 solutions

Oxide film thickening on reinforcement steel at early stage of formation is followed in naturally aerated Ca(OH)₂ solutions, recalling the natural behaviour in concrete, by measuring the open-circuit potential, E, with time up to 4 h. The final potentials, E_fin, are reached from negative values indicating oxide film growth. E varies with the Ca(OH)₂ concentration according to a straight line relationship. Oxide film thickening, at early stage of immersion, follows a direct logarithmic growth law as evident from the linear relationship between E and log t. The rate of oxide film thickening deceases by increasing the concentration and pH of the solution and by raising the temperature. The free activation energy of oxide film thickening is determined and found to be 29.28 kJ/mole, indicating that the process of oxide film growth is under diffusion control.     

Recovery of Mo from Ni-Mo ore leach solution with carrier co-precipitation method

In this paper, “nascent” Fe(OH)₃ as carrier precipitation and NaHCO₃ as buffer agent were used to extract molybdenum from the complex Ni-Mo ore leach solution. The effects of different variables on the molybdenum extraction, such as the dosage of FeCl₃ and NaHCO₃ and the reaction temperature and time, were studied. The results showed that over 99% of molybdenum were extracted in 2 h at 25 ± 1 °C under the conditions of mole ratio of Fe³⁺/Mo 2.2-2.5 and mass ratio of NaHCO₃/Mo 0.7-1.5. About 92% of Mo in the Fe(OH)₃ precipitation can be leached by NH₃⋅H₂O to prepare the ammonium molybdate solution with about 100 g/L Mo. After treating with NH₃⋅H₂O, the Fe(OH)₃ precipitation was dissolved with HCl to obtain the FeCl₃ solution (FeCl₃ 500 g/L) which can be reused for the next round of experiments.     

Evaluation of the uniform current density assumption in cathodic protection systems with close anode‐to‐cathode arrangement

Cathodic protection modelling often involves making assumptions about geometric features and material characteristics that directly impact accuracy of solutions. In the present paper, predictive power of the model using approximate uniform current boundary condition on the cathode is validated against the model using nonlinear cathode polarization curves representative of low‐carbon steel structure of common geometry, buried in soil or immersed in seawater. In order to explore the worst case scenario, the present example deals with a large diameter pipeline (Ø 1.2 m) and a wire anode (Ø 0.05 m), separated by a distance d, both embedded in an infinite space of conductivity κ. The calculation is performed for the two sets of parameters – κ and limiting current density of oxygen reduction, i_l. For simulation of CP systems in seawater κ = 4.79 S/m and i_l = ?86 µA/cm² and for CP system in soil, κ = 10^?3 S/m and i_l = ?1.1 µA/cm². The other physical parameters were identical for both systems (Tafel slopes b_a = 60 mV/dec, b_c = 120 mV/dec and equilibrium potentials , ). The results were visualized to best exemplify the general trends in potential and current distributions that appear upon switch between uniform and nonlinear cathodic boundary conditions.     

An electrochemical and ellipsometric investigation of surface films grown on iron in saturated calcium hydroxide solutions with or without chloride ions

Ellipsometric studies combined with electrochemical measurements of films grown on Armco iron in de-aerated saturated Ca(OH)₂ solution were performed. It was found that the surface film formed on iron in contact with aqueous saturated solution of calcium hydroxide at anodic potentials exhibits a complex index of refraction of 1.7 (1-0.07i), which is not affected by small additions of chloride ions to the electrolyte. However, at CaCl₂ concentrations larger than 0.05 M, the refractive index is reduced to n = 1.6. Presumably, under these conditions a layer of iron oxyhydroxide with incorporated Ca(OH)₂ forms on the surface of iron. The outer gelatinous layer of corrosion products formed on the top of the oxyhydroxide layer exhibits a still lower index of refraction n = 1.34. At open circuit potential, in Cl^?-free solution, the film exhibits n = 1.43, suggesting that it is composed of iron hydroxide. This layer is absent in solution containing Cl^?. In this case, the refractive index of the film is about 1.34.     

Preparation and electrochemical performance of nano-scale Ni(OH)2 doped with zinc

Nano-scale Ni(OH)₂ doped with Zn was prepared by precipitation transformation method and characterized by XRD and TEM. The electrochemical performance was investigated by cyclic voltammetry (CV) and constant current technology. The measurement results indicate that the lattice parameters of nano-scale Ni(OH)₂ are changed and the agglomeration of particles becomes obvious with the increased Zn-doped content. Compared with un-doped one, the discharge specific capacities of nano-scale Ni(OH)₂ doped with 10% Zn are enhanced by 8% and 6%, respectively, at the discharge rate of 0.2C and 3C. After 110 cycles, the discharge specific capacity of the sample doped with 10% zinc is still above 85% of its initial capacity discharged at 0.2C. Therefore, a suitable Zn-doped content is beneficial to improving the discharge performance of nano-scale Ni(OH)₂.