Mass-balance and Eh-pH diagrams of Fe green rust in aqueous sulphated solution

The properties of a Fe^II-Fe^III mass-balance diagram are presented. Each compound is represented by a point P(x, R), where x=n_Fe^III/[n_Fe^II+n_Fe^III] is the ferric molar fraction and R=n_OH^-/[n_Fe^II+n_Fe^III] the number of mole of hydroxyl species necessary to precipitate one mole of iron in a given compound. If a chemical reaction leads to a given point P(x, R) in the diagram, the nature and relative abundance of the various iron species can be predicted by using simple rules that resemble those commonly used when studying binary and ternary phase diagrams. Reactions of oxidation of Fe^II species or coprecipitation reactions of Fe^II and Fe^III species are represented by different routes in the diagram. The evolution of the measured pH and redox potential E_h values during a controlled Fe^II oxidation experiment using sodium persulphate is explained by comparing the mass-balance and the E_h-pH Pourbaix diagrams. These values are also measured at the intersecting points of various routes and identical equilibrium conditions are often obtained. By displaying the involved chemical reactions, the Fe^II-Fe^III mass-balance diagram allows to understand the reaction path consequently followed in the E_h-pH Pourbaix diagram.     

Revised pourbaix diagrams for iron at 25–300 °C

The Pourbaix diagrams, potential pH diagrams, for iron at 25–300 °C have been revised. The selection of thermochemical data has taken into account the experimental solubility of magnetite at T = 100–300 °C. Temperature extrapolations of thermochemical data for aqueous species have been performed with the revised Helgeson-Kirkham-Flower model, which also allows uncharged aqueous complexes to be handled. The Pourbaix diagrams show a stability region for ferrous hydroxide, Fe(OH)₂(cr), at 10⁻⁶m and T ≤ 85 °C, implying that the Schikorr reaction is not thermodynamically possible above this temperature. A corrosion region between iron and magnetite exists at all temperatures in high purity water (10^∞ m), due to the hydrolysis products of iron(II).     

Pourbaix diagrams for chromium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Cr-Br⁻-H₂O system at 25 °C were developed in 400, 700, 850, and 992 g/L (4.61, 8.06, 9.79, and 11.42 M) LiBr solutions, common concentrations in different parts of absorption devices. The diagrams were compared with the simple Cr-H₂O system at 25 °C. Equilibria for the Cr-Br⁻-H₂O system at 25 °C were determined for bromide ion activities of 15.61, 194.77, 650.06, and 2042.65, which corresponded to the 400, 700, 850, and 992 g/L LiBr solutions, respectively. Activities of all the dissolved species containing chromium were plotted for 10⁻⁶, 10⁻⁴, 10⁻², and 10⁰. Comparison of the simple Cr-H₂O system at 25 °C with the diagrams for Cr-Br⁻-H₂O system at 25 °C showed that the dominant aqueous Cr(III) species in acid solutions was Cr⁺³ for Br⁻ activities of 15.61, 194.77, and 650.06, whereas it was CrBr⁺² for Br^- activity of 2042.65. Aqueous CrBr⁺² formed at a Br⁻ activity higher than 943.05. The chromium solubility range in the acid area of the diagrams extended slightly to higher pH values with increasing Br⁻ activity and decreasing water activity, as a result of destabilization of Cr₂O₃.     

Electrochemical characterization of sintered magnetite electrode in LiOH solution

In relation to the mitigation of flow assisted corrosion (FAC) occurring at the carbon steel materials at power plants, magnetite powder was sintered into pellets and used to construct electrodes for electrochemical property measurements in LiOH solutions at ambient conditions. Measurements indicated adsorption of hydroxide ions on the magnetite surface. Slow cathodic scanning removed the adsorbed OH⁻ and returned the electrode surface to its original state. It is proposed that the cause of cyclic voltammetric (CV) peaks is the reduction of H₂O involving the conduction band edge (E_C) and valence band edge (E_V) of the magnetite. Negative polarization by the CV produced hydrogen atoms and injected them into the magnetite body; these were oxidized when the sweep was reversed and then dissipated at the magnetite surface by contact with OH⁻ ions. Results support the interaction of magnetite energy levels with adsorbed species, rather than the dissolution of magnetite.     

Pourbaix diagrams for titanium in concentrated aqueous lithium bromide solutions at 25 °C

Pourbaix diagrams (electrode potential-pH diagrams) for Ti–Br⁻–H₂O system at 25 °C in the absence and presence of titanium hydrides were developed in 400, 700, 850, and 992 g/L LiBr solutions. The diagrams were compared with the simple Ti–H₂O system at 25 °C. Comparison of the simple Ti–H₂O system with the diagrams of the Ti–Br⁻–H₂O system at 25 °C showed that the titanium solubility range in the acid, neutral, and weak alkaline areas of the diagrams extended slightly to both higher pH values and potentials with increasing bromide ion activity and decreasing water activity.     

The characterisation of the protective film formed by benzotriazole on the 90/10 copper-nickel alloy surface in H2SO4 media

The nature of the protective film formed by benzotriazole (BTAH) on the surface of the 90/10 CuNi alloy in deaerated 0.5 mol L⁻¹ H₂SO₄ solution containing Fe(III) ions as oxidant was investigated by weight-loss, calorimetric measurements, and by surface-enhanced Raman spectroscopy (SERS). The SERS measurements show that the protective film is composed by the [Cu(I)BTA]_n polymeric complex and that the BTAH molecules are also adsorbed on the electrode surface. A modification of the BET isotherm for adsorption of gases in solids is proposed to describe the experimental results obtained from weight-loss experiments that suggest an adsorption in multilayers. Electrochemical studies of copper and nickel in 0.5 mol L⁻¹ H₂SO₄ in presence and absence of BTAH have also been made as an aid to interpret the results. The calculated adsorption free energy of the cuprous benzotriazolate on the surface of the alloy is in accordance with the value for pure copper.     

A SERS investigation of the electrodeposition of Au in a phosphate solution

The spectroelectrochemical behaviour of cyanide and phosphate ions adsorbed on a Au electrode during electrodeposition and corrosion in a KAu(CN)₂ phosphate bath was studied by in situ SERS (Surface Enhanced Raman Spectroscopy) and cyclic voltammetry. SERS spectra exhibiting features corresponding to CN⁻ and PO₄⁻ vibrations were recorded under potentiostatic control in an interval from − 1600 to + 1500 mV vs. Ag/AgCl. Surface CN⁻ species are the dominating ones under both cathodic and anodic polarisations. As far as the phosphate vibrations are concerned, bands related to PO₄⁻ are visible in both cathodic and anodic conditions. A fluorescence background was evident in the potential range − 1300 ÷ + 1300 mV. At cathodic potentials phosphate ion adsorption on electrode surface prevails over cyanide for potentials more cathodic than − 1000 mV. At − 700 mV a discontinuity is noticed in the Stark tuning of CN⁻, probably due to a change of Au(CN)₂⁻ adsorption mechanism. At anodic potentials bands due to cyanide oxidation could be observed.     

Prevention of passive film breakdown on iron by coverage with one-dimensional polymer films of a carboxylate ion self-assembled monolayer modified with alkyltriethoxysilanes

A film composed of a one-dimensional polymer was fabricated by modification of a 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) adsorbed on a passivated iron electrode with octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. The pitting potential, E_pit of the passivated electrode coated with this film was measured by anodic polarization in a borate buffer solution containing 0.1 M of Cl⁻. The E_pit value of the electrode coated with the film was markedly shifted from the value of the bare electrode in the positive direction, indicating prevention of passive film breakdown. No breakdown occurred over the potential range of passive region in some cases. Structure of the modified SAM was discussed by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement of the electrode surface covered with the film. Suppression of Cl⁻ accumulation at a defect of the passive film was revealed by electron-probe microanalyses of the surfaces uncoated and coated with the SAM modified with octyltriethoxysilane C₈H₁₇Si(OC₂H₅)₃ after anodic polarization in the borate buffer containing Cl⁻.     

Impedance spectroscopic study of corrosion inhibition of copper by surfactants in the acidic solutions

The inhibitive action of the four surfactants, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate, sodium oleate and polyoxyethylene sorbitan monooleate (TWEEN-80), on the corrosion behavior of copper was investigated in aerated 0.5 mol dm⁻³ H₂SO₄ solutions, by means of electrochemical impedance spectroscopy. These surfactants acted as the mixed-type inhibitors and lowered the corrosion reactions by blocking the copper surface through electrostatic adsorption or chemisorption. The inhibitor effectiveness increased with the exposure time to aggressive solutions, reached a maximum and then decreased, which implies the orientation change of adsorbed surfactant molecules on the surface. CTAB inhibited most effectively the copper corrosion among the four surfactants. The copper surface was determined to be positively charged in sulfuric acid solutions at the corrosion potential, which is unfavourable for electrostatic adsorption of cationic surfactant, CTAB. The reason why CTAB gave the highest inhibition efficiency was attributed to the synergistic effect between bromide anions and positive quaternary ammonium ions. C₁₆H₃₃N(CH₃)₄⁺ ions may electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. On the basis of the variation of impedance behaviors of copper in the surfactant-containing solutions with the immersion time, the adsorption model of the surfactants on the copper surface was proposed.     

Computer simulation of the corrosion inhibition of copper in acidic solution by alkyl esters of 5-carboxybenzotriazole

A series of alkyl esters (methyl, butyl, hexyl, and octyl) synthesised from a mixture of 4- and 5-carboxybenzotriazole (4-CBTAH and 5-CBTAH) inhibited copper corrosion in aerated solution (pH∼0). Inhibition efficiency (IE%) of the protonated esters (CBTAH₂⁺-R) increased with hydrocarbon chain length and this is attributed to chemisorption (through azole ring N) and increased physical adsorption as more methyl groups are introduced. A modelling package employing molecular mechanics and molecular dynamics has been used to simulate the docking of a single protonated species (5-CBTAH₂⁺-R) onto a clean copper (1 1 0) surface. A decrease in potential energy was associated with the flattening of the ester ring system onto the surface and further decreases in energy were associated with the extension of the aliphatic chain onto the surface. The crude binding energy (E_bind) of each ester with the surface was estimated and this energy also increased regularly with carbon chain length. The study suggests that molecular modelling and calculation of E_bind of a single molecule on a specified metallic surface can be used to predict the inhibition performance of compounds whose structures change in a regular way.     

Localised dissolution of iron in buffered and non-buffered chloride containing solutions

Pitting corrosion of pure iron was studied by using conventional samples, and artificial pit electrodes. Experiments were conducted in solutions of 0.01, 0.1 and 1 mol dm⁻³ NaCl at pH values of 7, 10, 11 and 12; and in borate buffer solutions with the same chloride concentrations and pH 8.7 and 9.2. Four times higher concentration of borate salt was required to reach inhibiting capacity of the hydroxyl anions, as determined via pitting potentials. From measurements of solution resistance, the increase in local conductivity due to dissolved corrosion products exuded from the pit was calculated for each bulk solutions. For the artificial pit electrodes, contribution of the borate species to the internal pit solution conductivity in low chloride solution was associated with the difference between transition potential, E_T, values for buffered and non-buffered solutions, and this contribution was also used to explain the non-linear dependence of E_T on [Cl⁻]. Further analysis was conducted using the concept of the critical i.x parameter for stable pit propagation.     

Adsorption of octanohydroxamic acid at fluorite surface in presence of calcite species

The surface properties of fluorite are often affected by dissolved gangue species (e.g., calcite) during the flotation process. Microflotation testing with and without the addition of calcite supernatant was conducted using octanohydroxamic acid (OHA) as the collector. The results revealed that dissolved calcite species significantly affected the flotation behavior of fluorite. Fourier transform infrared spectra confirmed that the decrease in flotation recovery was linked to lower OHA adsorption. Solution chemistry analysis indicated that CaCO₃ and Ca²⁺ from the calcite supernatant were the most favorably adsorbed species, and X-ray photoelectron spectroscopy analysis confirmed the surface adsorption of calcite species. Density functional theory simulations provided a detailed analysis of the multidentate adsorption configuration of OHA, which was the most favorable for adsorption on the fluorite surface. The adsorption energy calculation showed that the calcite dissolved species were more stably adsorbed on the fluorite surface than OHA. The pre-adsorption of calcite dissolved species hindered the adsorption of OHA due to electrostatic repulsion.     

Adsorption of quinonediimines on aluminum(1 0 0): A computational and FT-IR study

Density functional theory (DFT) calculations were performed on a series of symmetrically substituted quinonediimines, both free and adsorbed on a 42 atom aluminum(1 0 0) lattice. Enthalpies of adsorption for quinonediimines in the emeraldine base oxidation state were calculated to vary from −4.07 to −8.47 kcal mol⁻¹, with the emeraldine salt value at −78.00 kcal mol⁻¹. The enthalpy of absorption for the pernigraniline is calculated to be 30 kcal mol⁻¹. Adsorption onto the aluminum surface caused a change in hybridization of the central imine nitrogen from sp² to sp³, with concomitant formation of a coordinate covalent bond to two electron deficient aluminum atoms of the surface. Experimental IR studies of three quinonediimines show shifts in absorption bands consistent with strong interaction with the aluminum surface, and consistent with frequencies calculations using DFT.     

Some aspects of copper corrosion in presence of benzotriazole and chloride ions

This paper deals with the investigation of electrochemical behaviour of copper in a sodium tetraborate (borax) solution in the presence and absence of chloride ions. The influence of benzotriazole (BTA) on the anodic behaviour of copper was also investigated. It was found that Cu₂O and CuO oxides were formed on copper and that peak current moved to the positive side with increased immersion time in a sodium tetraborate solution. After 1 h immersion of the copper electrode in a sodium tetraborate solution containing various chloride ion concentrations (0.001, 0.010, 0.050, and 0.100 mol/dm³ Cl⁻) the activation effect of those ions was found, whereas after 6 h immersion of copper in the above mentioned solutions, chloride ions were found to have not only activation but also passivation effect. The investigation of the effect of benzotriazole led to a conclusion that BTA has an inhibiting effect with increasing immersion times (15 min, 1 h, 4 h, and 48 h) as well as with increasing BTA concentrations (8.4 × 10⁻⁶, 8.4 × 10⁻⁵, 8.4 × 10⁻⁴, and 8.4 × 10⁻³ mol/dm³). In addition, at the lowest investigated BTA concentration (8.4 × 10⁻⁷ mol/dm³) an activation effect of this compound on copper dissolution was observed. Based on the Langmuir adsorption isotherm, the value of the standard free energy adsorption was found to be ΔG⁰ = −35.4 kJ/mol.     

Investigation of the specific adsorption of HSO4(SO4) and Cl ions on Co and Fe by radiotracer technique in the course of corrosion of the metals in perchlorate media

The adsorption of sulphate and chloride ions on Co and Fe was studied in 0.5 mol dm⁻³ NaClO₄ supporting electrolyte at various pH values by radiotracer technique in the course of the corrosion (dissolution) of the metals. The effect of the reduction of ClO₄⁻ ions (leading to the formation of Cl⁻ ions) on the radiotracer measurements was investigated. It was found that the adsorption of chloride and sulphate ions occurs in different pH ranges. This phenomenon was explained by the assumption that the adsorption strength of the two species is very different on oxide covered and “pure” metal surfaces.     

Morphology influence of the oxidation kinetics of carbon nanofibers

This paper reports the stability and oxidation rate of five types of carbon nanofiber (CNF) with distinctly different orientation of their graphite sheets based on conversion to CO₂ when heated in the presence of oxygen. A non-isothermal technique was used to determine the oxidation kinetic parameters including the activation energy (E_a). Graphite shows a similar activation energy (E_a = 158 kJ/mol⁻¹) to CNF with longitudinal alignment (E_a = 156 kJ/mol⁻¹). CNF type herringbone (E_a = 126 kJ/mol⁻¹) and platelet (E_a = 145 kJ/mol⁻¹) show the lowest oxidation resistance which improved dramatically after a heat treatment at 3023 K of the herringbone (E_a = 216 kJ/mol⁻¹) and platelet (E_a = 174 kJ/mol⁻¹) structures.     

The atmospheric corrosion of copper at two sites in Portugal: a comparative study

This paper presents a comparative study on the atmospheric corrosion of copper, at two sites, in Portugal, with exposures started in two different seasons (summer and winter). Particular attention is devoted to the initial stages of the corrosion process.The levels of pollutants, namely of SO₂ and chlorides, in both atmospheres, have been measured, over the periods from August 1999 to July 2000 and from November 2000 until July 2001. Climatic data for both sites and both periods has been collected and analysed.Kinetics of the corrosion process (weight losses) have shown to be described by: Δm=kt^0.6 and Δm=kt^0.4, with k equal to 3.4 (g m⁻² month^−0.6) and 17 (g m⁻² month^−0.4), for the one year exposures, started in summer, at the urban and maritime atmospheres, respectively. Exposures started in winter, at the urban atmosphere, have lead to kinetics described by Δm=kt^0.7 with k=5.0 (g m⁻² month^−0.7).The chemical composition and the morphology of the copper patinas, corresponding to exposures of 2 and 12 months, at the two sites, are compared as well as the morphology of the corroded surfaces.     

In vitro corrosion degradation behaviour of Mg-Ca alloy in the presence of albumin

The in vitro degradation behaviour of Mg-Ca alloy was studied in the presence of albumin by in-situ observation, hydrogen evolution method and various electrochemical techniques. The corrosion and hydrogen evolution rates decreased and the formation of filiform corrosion induced by Cl⁻ was also significantly inhibited due to the adsorption of albumin molecule. Moreover, the higher the concentration of albumin, the higher is the inhibitive effect. The EIS results further show that addition of albumin enhanced the charge transfer resistance and film resistance at the open-circuit potential. The synergistic effect of the negatively charged adsorbed albumin molecule and OH⁻ has been discussed.     

Influence of silicon species on the transformation of green rust I(Cl) in aqueous solution by oxidation

X-ray diffraction (XRD) and solution analysis were used for characterizing the influence of different silicon species on oxidation of green rust (GRI(Cl⁻)) suspension. While addition of silicon to metallic iron enhanced the formation of β-FeOOH, GRI(Cl⁻) in aqueous solution oxidized into lepidocrocite and oxidation was delayed in presence of silica and silicate species as noticed from potential, pH, and dissolved oxygen (DO) measurements. Transmission electron micrographs showed that the particle size of lepidocrocite was reduced due to silicate addition. The influence of silicate was attributed to its adsorption on GRI(Cl⁻) and lepidocrocite particles as confirmed from ICP-AES analysis of supernatant solution.     

Electrodeposition of Copper-Containing Polymer Coatings from Sulfate Electrolytes Containing N-Methylpyrrolidone

In sulfate electrolytes for copper plating, N-methylpyrrolidone (MP) was found to form stable 2 : 1 complexes with the metal. The nature of electroactive species at the copper cathode depends on the MP concentration. In a concentration range from 10⁻⁴ to 10⁻² mol/l, the electrode reaction involves complexes of discharging metal ions with adsorbed MP molecules, the complexes being formed in the surface layer. At higher MP concentrations, a slow chemical step associated with dissociation of complex species in the bulk of the solution precedes ion discharge. The inhibitive effect of MP on copper electrodeposition is due to adsorbed MP molecules at the electrode; the adsorption value determines the efficiency of the additive and the wear resistance of the coating. __________ Translated from Zashchita Metallov, Vol. 41, No. 5, 2005, pp. 463–466. Original Russian Text Copyright ? 2005 by Kuznetsov, Skibina, Kuznetsova, Loskutnikova, Sokolenko.