Studies on the anodic polarization of composite amalgams. The behaviour of manganese—Zinc amalgams in alkaline solutions

The anodic oxidation of manganese-zinc amalgams of different concentrations was studied in 0·1, 1·0, 2·0 and 4·0N NaOH solutions at 25°C. The polarization curves obtained under constant currents exhibited several steps corresponding to the formation of Zn(OH)₂, Mn(OH)₂, MnO₃ and oxidation of mercury before O₂ evolution took place. The effect of presence of zinc on the oxidation step of manganese is explained.     

Studies on the anodic polarization of amalgams: The behaviour of nickel amalgams in alkaline solutions

Nickel amalgams of varying composition were oxidized in 0.1, 2, 4 and 6N NaOH solutions. Two different oxidation patterns were distinguished. The first describes the behaviour of concentrated nickel amalgams, and the second the behaviour of dilute nickel amalgams in NaOH solutions. The anodic curve for the concentrated amalgams showed regions for the charging of the anodic double layer, then oxidation of nickel and mercury, then oxygen evolution. NiO is first formed which is oxidized to Ni₃O₄, then to Ni₂O₃. For dilute amalgams the oxidation curves showed only one arrest corresponding to the system NiO/Ni₃O₄ before oxygen evolution. The relation between the polarizing current and the time of passivation was found to fit the equation

$\text{log}\text{τ = A ? n}\text{log}\text{i}$

, where A and n are constants. With cathodically pretreated electrodes the oxidation curves showed regions for the dissolution of the sodium amalgam formed during precathodization, the charging of the anode double layer and a prolonged indefinite arrest at the potential corresponding to the reaction

$\text{NiO + H}{}_2\text{O = Ni}{}_3\text{O}{}_4\text{+ 2H}{}^{\mathrm{+}}\text{+ 2e?}$

.     

Influence of brushing and abrasion on the in vitro corrosion resistance of dental amalgams

In order to evaluate the influence of toothbrushing on the in vivo corrosion resistance of dental amalgams, immersion tests were carried out in vitro in artificial saliva at 37 °C under static and under dynamic conditions.Three types of dental amalgam were employed: a conventional type, a dispersion-type amalgam and a ‘single-composition alloy’ amalgam.Brushing of the surfaces avoids the accumulation of corrosion products and causes an accelerated corrosion of the amalgam. This phenomenon is particularly evident in the conventional amalgam. Observation of the surface by scanning electron microscopy (SEM) after 100 000 brushing strokes shows deep corrosion of the γ₂-phase.     

The anodic behaviour of copper in static and flowing hydrochloric acid solutions

The dissolution of copper in 0·36 to 3·6 HCl was studied both in static and in flowing solutions using flow rates between 0·098 and 1·14 ms⁻¹, all in the region of laminar flow.Steady state anode potential current curves, potentiodynamic sweep and potentiostatic pulse techniques and impedance measurements, in the range of 0·02–7 kHz, were used. In both static and flowing solutions dissolution of copper occurs to a monovalent state, as chloro-complexes CuCl⁻₂ and CuCl²⁻ with exchange currents for the two reactions of 1·9 × 10⁻⁵ A mm⁻² and 8 × 10⁻⁷ Amm⁻² respectively in 1·44 MHCl. The cuprous chloride layer first forms a monolayer and subsequently grows to considerable thicknesses. The double layer capacity is constant at 0·17 ± 0·03 μFmm⁻² at potentials below multilayer formation and this is interpreted as implying that there is no specific adsorption of chloride ions prior to formation of the cuprous chloride layer. As the flow rate increases the film becomes thinner so delaying the formation of the film of critical thickness required for passivation.     

Role of thermal stability and vapor pressure of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium(II) and its triamine adduct in producing magnesium oxide thin film using a plasma-assisted LICVD process

Thin films of magnesia were deposited on various substrates using plasma-assisted liquid injection chemical vapor deposition with volatile Mg(tmhd)₂·2H₂O (1) (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione). The precursor complexes, Mg₂(tmhd)₄·(2), and Mg(tmhd)₂·pmdien (3) (pmdien; N,N,N′,N″,N″-pentamethyldiethylenetriamine) were prepared from Mg(tmhd)₂·2H₂O (1). The temperature dependence equilibrium vapor pressure (p_e)_T data yielded a straight line when log p_e was plotted against reciprocal temperature in the range of 360–475 K, leading to standard enthalpy of vaporization (Δ_vapH°) values of 59 ± 1 and 67 ± 2 kJ mol^− 1 for (2) and (3) respectively. Thin films of magnesium oxide were grown at 773 K using complex (1) on various substrate materials. These films were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray for their composition and morphology.     

A thermometric study of the reaction between Fe and HNO3

The reaction between Fe and HNO₃ is studied under a wide variety of conditions by the thermometric technique. Up to 4N HNO₃ ΔT varies linearly with the normality of HNO₃, while in solutions from 6 to 10N HNO₃ it is independent of the concentration. Passivity sets in solutions ≥ 10·8N HNO₃. Calculations of the reaction number (R.N.) reveal that the maximum rate of metal dissolution occurs in 7·3N HNO₃. Fe dissolution in dilute HNO₃ is promoted by additions of NO₃⁻ and NO₂⁻. That the rate-determining step of the autocatalytic process involves HNO₂ is supported by the results of addition of urea to the solution. This additive lowers the maximum measured temperature, without affecting the corresponding time necessary to reach it.Additions of HCI, NaCl, H₂SO₄ and Na₂SO₄ to dilute HNO₃ reduce the dissolution rate of Fe. The effect produced by the salts exceeds that of the acids.In contrast to its action in dilute solutions, the Cl⁻ion induces pitting corrosion in concentrated HNO₃. The attack starts after an induction period which decreases in length as the concentration of HCI is increased. Concentrated HNO₃ can tolerate a certain amount of the aggressive agent before attack starts. The concentration “N_HCl” which can be tolerated depends on that of the passivator according to logN_HCl=a+blog(N−N°)_HNO3 where a and b are constants, and N^o is the least concentration of HNO₃ necessary to cause passivity. Pitting corrosion in concentrated HNO₃ can be initiated also through NaCl. In one and the same acid solution more of NaCl is needed to cause the attack than of HCI.     

The magnetic ordering in the Ho5Si3 and Ho5Ge3 compounds

A neutron diffraction investigation has been performed on the Ho₅Si₃ and Ho₅Ge₃ compounds (hexagonal Mn₅Si₃-type, hP16, P6₃/mcm) to study their magnetic structure. The results prove that these intermetallic phases show a complex sine-modulated type magnetic ordering, both presenting on cooling two subsequent antiferromagnetic orderings (T_N1 and T_N2).Between T_N1 = 24(2) and T_N2 = 16(4) K Ho₅Si₃ shows a first antiferromagnetic ordering of the sine-modulated type with a propagation vector K₁ = [0, 0, ±0.284(1)]; then, below T_N2 the sine-modulated type ordering is conserved, but by the two propagation vectors K₁ = [0, 0, ±0.2773(7)] and K₂ = [±1/5, ±1/5, 0].For Ho₅Ge₃, between T_N1 = 27(2) and T_N2 = 18(4) K, the sine-modulated ordering is realized through the propagation vectors K₁ = [0, 0, ±3/10] and K₂ = [0, 1/2, 0], whilst below T_N2 the ordering takes place with propagation vectors K₁ = [0, 0, ±3/10], K₂ = [0, 1/2, 0], K₃ = [0, 0, ±2/5] and K₄ = [±1/5, ±1/5, 0]. For both the Ho₅Si₃ and Ho₅Ge₃ compounds, the dimensions of the magnetic unit cell are 5a × 5a × 10c times the crystal unit cell of Mn₅Si₃-type.     

Effect of Potassium Dichromate on the Passivation of Stainless Steel (AISI 321) in Sulphuric Acid

Abstract

The effect of potassium dichromate on the passivation of stainless steel in sulphuric acid has been investigated. Tests with K₂Cr₂O₇ up to 4N showed that passivation could be achieved in 3·4 N H₂SO₄ containing 0·1 N K₂Cr₂O₇, and that the quality of the passivating film increased with the concentration of K₂Cr₂O₇ and with the duration of the passivation treatment. Although passivity could be induced in these conditions at 30°C, at higher passivation temperatures the film quality diminished as shown by long-term decay studies after transfer of the metal to sulphuric acid solutions containing no dichromate. The metal could be passivated in all concentrations of H₂SO₄. Passivation was also possible in 3·4 N H₂SO₄ containing 0·2 N K₂Cr₂O₇ and up to 5% NaCl. However, the breakdown of the film produced in the presence of NaCl occurred earlier the higher the concentration of NaCl present during passivation.     

The Anodic Passivation of Iron in Solutions Of Inhibitive Anions*

Abstract

The anodic passivation of iron in air-saturated and de-aerated solutions of a number of inhibitive anions has been studied by the determination of potentiostatic polarisation curves. The critical passivation potential, critical passivation current density and leakage currents through the passive films have been found to depend on pH and on the nature of the anion present in the solution. The critical passivation potential (N.H.E. scale) in solutions of anions, where passivation occurs relatively easily, varies with pH in the range 6–12 according to E_p = + 0.09 – 0.06 pH volt For the anions studied, the critical passivation current densities in de-aerated 0·1 M-solution increase (and the difficulty of passivation increases) in the order: nitrite < hydroxide < chromate < borate < phosphate < carbonate < benzoate < bicarbonate < nitrate.     

Corrosion Behaviour of Manganese-Containing Stainless Steels. III. Their susceptibility towards pitting corrosion

The pitting susceptibility of stainless steels with (%) 17.39 Cr, 5.3 Ni, 0.3–13.9 Mn in chloride solutions have been studied by a potentiodynamic cyclic polarization technique in solutions 0.1 N H₂SO₄ + 0.9 N Na₂So₄ + various amounts of NaCl. At low NaCl concentrations no pronounced influence of the chloride can be found and even at medium concentrations the passivation behaviour is generally maintained; in this region, however, there is an increase of dissolution and passivation current densities. High chloride concentrations, however, result in pitting which may be completely suppressed by a cyclic (anodic/cathodic) polarization technique. As to pitting resistance an alloy containing (%) 17.3 Cr, 5.3 Ni, 5.6 Mn is comparable to the steel CrNi 18 9.     

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni shape memory alloy

The corrosion behaviour of Fe-15Mn-7Si-9Cr-5Ni (mass%) shape memory alloy at 25 °C in 0.5 M H₂SO₄ and 3.5% NaCl solutions has been studied using potentiodynamic polarization and electrochemical impedance techniques. Three different microstructures viz., single-phase γ, γ-δ and γ-Fe₅Ni₃Si₂, were produced by heat-treating the alloy in different equilibrium phase fields. The corrosion behaviour in 0.5 M H₂SO₄ solution is almost same for all three microstructures, barring a slight difference in the passivation range. Although, the passivation current in 0.5 M H₂SO₄, is in the same range as that of SS 304, the critical current required for onset of passivation is almost three orders higher and the passivation range is much shorter. In 3.5% NaCl solution the corrosion behaviour of all three microstructures of the Fe-15Mn-7Si-9Cr-5Ni shape memory alloy was that of general dissolution without passivity or localized attack (pitting). The best corrosion resistance in both H₂SO₄ and NaCl solutions is shown by the single-phase γ microstructure.     

Fabrication and performance of La0.8Sr0.2MnO3/YSZ graded composite cathodes for SOFC

The performance of multi-layer (1 − x) La_0.8Sr_0.2MnO₃/x YSZ graded composite cathodes was studied as electrode materials for intermediate solid oxide fuel cells (SOFC). The thermal expansion coefficient, electrical conductivity, and electrochemical performance of multi-layer composite cathodes were investigated. The thermal expansion coefficient and electrical conductivity decreased with the increase in YSZ content. The (1 -x)La_0.8Sr_0.2MnO₃/x YSZ composite cathode greatly increased the length of the active triple phase boundary line (TPBL) among electrode, electrolyte, and gas phase, leading to a decrease in polarization resistance and an increase in polarization current density. The polarization current density of the triple-layer graded composite cathode (0.77 A/cm²) was the highest and that of the monolayer cathode (0.13 A/cm²) was the lowest. The polarization resistance (R_p) of the triple-layer graded composite cathode was only 0.182 ω·cm² and that of the monolayer composite cathode was 0.323 ω·cm². The power density of the triple-layer graded composite cathode was the highest and that of the monolayer composite cathode was the lowest. The triple-layer graded composite cathode had superior performance.     

Properties of Mn-doped FINEMET

Structural, thermodynamical and magnetic properties of Fe_73.5−xSi_13.5B₉Cu₁Nb₃Mn_x amorphous alloys, with Mn content x=1,3–15, were studied by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC), Mössbauer spectroscopy (MS) and energy-dispersive X-rays (EDX), as-quenched and after annealing. The alloys with x≤7 suffer primary (above 550 °C) and secondary (below 680 °C) crystallisation, whereas the alloys with x≥9 only at 580 °C. Mn doping results in increase (for x≤7) and then (for x≥9) in decrease of grain size. The hyperfine field of the amorphous precursor and remainder substantially decrease with increase of Mn content x, whereas the fields in crystallites remain nearly independent of x. A paramagnetic component appears at x9 and grows with x.     

Studies on accumulation of chloride ions in pits growing during anodic polarization

The concentration of Cl⁻ ions within pits grown on 18Cr---12Ni---2Mo---Ti austenitic stainless steel specimens immersed in vertical position in 0·5N NaCl + 0·1N H₂SO₄ and polarized to 860 mV_NHE was studied at 20°C. The Cl⁻ concentration within pits increases with time to a maximum and then decreases (for example, after 6 h 2N Cl⁻ is observed). The higher accumulation of Cl⁻ within pits, the slower their development. For slowly growing pits a maximum value of about 12N Cl⁻ was observed. The low pH values of the solution within pits are the consequence of high Cl⁻ contents occurring there.     

The application of 1-hydroxyimidazole-3-N-oxides as aluminium corrosion inhibitors in alkaline solutions

The influence of the 1-hydroxyimidazole-3-N-oxide (HOI), 1-hydroxy-2-benzoyl-5-phenylimidazole-3-N-oxide (HOBFI) and 1-hydroxyimidazole-2,4,5-trimethyl-3-N-oxide (HOTMI) on aluminium corrosion in 0.01 M NaOH, (0.01 M NaOH + 3% NaCl) and 0.1 M Na₂CO₃ (pH 12) aqueous solutions was investigated on the basis of the polarization log 1 = f(E) curves, a.c. impedance measurements, and electrochemical noise experiments. The 3-N-oxides shift the corrosion potential of aluminium to more positive potentials (i.e. HOTMI shifts E_corr about 900 mV) and decrease the corrosion current (i_corr) by approximately two orders of magnitude. These results may be interpreted as the adsorption of inhibitors coupled with the redox reactions of the 3-N-oxides and the creation of a polymer layer on the aluminium surface.     

In situ gravimetry of nickel thin film during potentiodynamic polarization in acidic and alkaline sulfate solutions

The passivation process of nickel thin films during potentiodynamic polarization in acidic and alkaline sulfate solutions was analyzed by an electrochemical quartz crystal microbalance (EQCM) to separate the partial current density of nickel dissolution through the passive film, i_Ni²⁺, and the partial current density of film growth, i_O^2-. The values of i_Ni²⁺ and i_O^2- during potentiodynamic polarization (20 mV s⁻¹) in the passive potential region could be separated by comparing the mass change rate and net polarization current as a function of electrode potential. It is found that i_Ni²⁺ is larger than i_O^2- in pH 3.0, 0.5 M sulfate solution, while the situation reverses in pH 12, 0.5 M sulfate solution. The sulfate concentration dependences of i_Ni²⁺ and i_O^2- in pH 3.0, sulfate solution were significant as compared to those in pH 12, sulfate solution.     

Effect of Sulfide Ions on the Anodic Dissolution of Copper in Acetate Media

It is voltammetrically found that a covering film forms on copper in S^2–-ion-containing acetate media. It follows from thermodynamic equilibria, crystallographic parameters, and x-ray spectra of the metal and its compounds that the overall anodic process is composed of two separate processes and yields Cu₂S and Cu₂O. The reaction order in S^2– ions (log i/log C _i)_{E = const}is 0.37. Possible stages of the electrode process are suggested.     

Zum Einfluß der Oberflächenbearbeitung und der chemischen Zusammensetzung auf die Korrosion von Dental-Amalgamen

Influence of mechanical treatment of the surface and of chemical composition on the corrosion of dental amalgams The influence of mechanical treatment of the surface and of chemical composition of amalgams on the corrosion behaviour is examined in artificial saliva by determination of useful characteristical dates using potentiodynamical polarization measurements on little (0.03 V min^?1) and high (0.6 V min^?1) polarization rate. In the case of the amalgams the corrosion resistance increase with smaller roughness of the surface and smaller concentration of γ₂-phase. The concentration of γ₂-phase is influenced by the chemical composition and to a certain extent by the mechanical treatment of surface. Therefore dental amalgam with little or without γ₂-phase and polishing of amalgam are required for therapy of filling.     

The preparation and thermodynamic properties of Fe(II)---Fe(III) hydroxide-carbonate (green rust 1); Pourbaix diagram of iron in carbonate-containing aqueous media

Carbonate-containing green rust 1, GR1(CO₃²⁻), is prepared by oxidation of Fe(OH)₂ in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO₄·7H₂O solutions and carbonate ions are added as a Na₂CO₃ solution. For sufficiently large concentrations of sodium carbonate, SO₄²⁻ ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)₂ oxidizes into GR1(CO₃²⁻). In the second stage of reaction, GR1(CO₃²⁻) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO₃²⁻), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 Å and C = 22.45 ± 0.05 Å. From the Mössbauer analysis of the stoichiometric GR1(CO₃²⁻), which leads to a Fe²⁺:Fe³⁺ ratio of 2:1, the chemical formula is established to be: [Fe₄^(II)Fe₂^(III)(OH)₁₂][CO₃·2H₂O]. The 78 K Mössbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe²⁺ doublets d₁ and D₂ corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s⁻¹ and quadrupole splittings (QS) of 2.93 and 2.67 mm s⁻¹, respectively, and one Fe³⁺ doublet D₃ with an IS of 0.47 mm s⁻¹ and QS of 0.43 mm s⁻¹. These three doublets were already used to fit the Mössbauer spectrum of chloride-containing GR1(Cl⁻) [see J.M.R. Génin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Génin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO₃²⁻) is estimated to be ΔG °_f = − 966.250 cal mol⁻¹. Knowing the chemical formula and ΔG °_f of GR1(CO₃²⁻) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.     

Thermodynamic studies on LaFeO3(s)

The enthalpy increments and the standard molar Gibbs energies in the formation of LaFeO₃(s) have been measured using a high-temperature Calvet micro-calorimeter and a solid oxide galvanic cell, respectively. The corresponding expression for enthalpy increments is given as:

H°(T)−H°(298.15 K)(J mol⁻¹)(±1.2%)=−36887.27+103.53 T(K)+25.997×10⁻³T²(K)+11.055×10⁵/T(K).

The heat capacity, the first differential of H°(T)−H°(298.15 K) with respect to temperature, is given as:

C_p,m°(T)(JK⁻¹mol⁻¹)=103.53+51.994×10⁻³T(K)−11.055×10⁵/T²(K).

From the measured e.m.f. of the cell, (−)Pt/(LaFeO₃(s)+La₂O₃(s)+Fe(s))//CSZ//(Ni(s)+NiO(s))/Pt(+), and the relevant Δ_fG_m°(T) values from the literature, the Δ_fG_m°(LaFeO₃, s, T) was calculated, and is given as:

Δ_fG_m°(LaFeO₃, s, T)(kJmol⁻¹)(±0.72)=−1319.2+0.2317T(K).

The calculated Δ_fH_m°(LaFeO₃, s, 298.15 K) and S°(298.15 K) values obtained using the second law method are −1334.7 kJ mol⁻¹ and 128.9 J K⁻¹ mol⁻¹, respectively.