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

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

.     

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.     

The behaviour of copper-zinc alloys in alkaline solutions upon alternate anodic and cathodic polarization

Galvanostatic cyclic anodic and cathodic polarization curves for four CuZn alloys are traced in alkaline solutions of different concentrations. The anodic behaviour of the first three alloys, with 15.9, 46.8 and 50.5 wt% Zn, resembles that of pure Cu. Oxidation arrests, corresponding to the formation of Cu₂O, Cu(OH)₂, HCuO₂? and Cu₂O₃, are recorded before the evolution of O₂ on the passive electrode. The Zn of the electrodes does not develop its oxidation arrest. It affects, however, the behaviour of the alloys in a number of ways. The results are explained on the basis of kinetic interactions, and in relation to the phase diagram of the Cu-Zn system.The fourth alloy, with 85.9 wt% Zn (ε + η phases), yields upon oxidation in concentrated alkali solutions a series of five or six arrests. The first two of these represent the oxidation of the Zn of the η- and ε-phases, respectively. Calculations have shown that the activity of the Zn in the ε-phase is ～ 4.6 × 10^?10 times that of the free η-phase. The other oxidation steps correspond to the oxidation of the Cu of the alloy. In 0.1N NaOH the same alloy behaves as if it was pure Zn.Critical current densities for the passivation of Cu, Zn and the four alloys are determined in 0.1M Na₂SO₄. The ability of the tested materials to withstand electrochemical dissolution decreases in the succession: Alloy I > Alloy III > Cu > Alloy II > Alloy IV > Zn.     

The polarization behaviour of chromium in acidic sulphate solutions

The polarization behaviour of chromium was studied in solutions of various pH, with particular regard to the hydrogen evolution reaction and chromium dissolution. Hydrogen evolution occurred on active and passive chromium and the exchange current density was ca. 10^?6 A/cm² at pH 1.0 on both electrodes. The rate of active dissolution of chromium did not depend on pH, an observation that indicated the absence of a hydroxyl-containing intermediate in the rate determining step. The transpassive dissolution of chromium was strongly dependent on pH, and it was suggested that the rate was controlled by a base-accelerated anodic dissolution of an oxide film.     

Kinetics of anodic behaviour of Pb in HCl solutions

The galvanostatic polarization technique was used to study the electrochemical behaviour of lead electrode in HCl solutions. The general shape of the resulting anodic potential-time curves is found to depend on magnitude of the imposed current density, the acid concentration and the temperature of the solution. The data of the curves were characterized by the presence of various regions due to different electrochemical processes. (i) A rapid and almost linear change of potential due to the decay of hydrogen overpotential and the subsequent charging of the electrical double layer at the metal/solution interface, (ii) a potential arrest corresponding to anodic dissolution of the lead to Pb²⁺ ions, (iii) a linear rise in the electrode potential indicating the onset of passivity due to the formation of PbCl₂, (iv) a second potential arrest due to the chemical dissolution of the passive PbCl₂ layer, and (v) finally, a continuous rise in the Pb electrode potential with time. The electrode potentials corresponding to the different arrests were found to depend on the magnitude of the imposed current density, the acid concentration and the temperature of the solution. The duration time, τ, of the dissolution process was found to increase with the decease of the solution acidity and the current density and also with the increase of solution temperature.     

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.     

Factors affecting the anodic behaviour of zinc electrode in borate solutions

The anodic behaviour of Zn electrode in 1 × 10⁻² M Na₂B₄O₇ solutions in the absence and presence of various concentrations of Na₂SO₄, Na₂S₂O₃ or Na₂S as aggressive agent was studied by galvanostatic polarization technique. In the absence of sulphur-containing anions in solution, the polarization curves are characterized by one distinct arrest corresponding to Zn(OH)₂ and/or ZnO, after which the potential increases linearly with time due to the formation of barrier oxide film before reaching the oxygen evolution reaction. The duration time of the arrest decreases with increasing current density while the rate of oxide film formation increases. On the other hand, the duration time of the arrest increases with the number of anodic cyclization while the rate of oxide film formation decreases. Additions of low concentration of the aggressive anions have no effect on the passive film formed on the metal surface. The potential starts to oscillate within the oxygen evolution region with increases in the concentration of these aggressive anions. Further increases in the concentration of these aggressive anions are associated with impaired Zn passivity that might indicate pitting attack. The aggressiveness of the sulphur species decreases in the order: . The effect of raising pH of the solution on the anodic behaviour of Zn electrode in the presence of anions was also investigated. It was found that the raising the pH of the solution affecting on the rate of oxide film formation and the breakdown potential value.     

The effect of stress on the corrosion behaviour of mild steel in alkaline solutions

The influence of stress, type of aggressive anion and anodic current density on the corrosion behaviour of mild steel in lime water has been studied. Under open circuit condition the passivity of steel in lime water containing low concentrations of C1?, SO₄^2? or S^2? ions was impaired by the application of stress. In the presence of high concentrations of these ions corrosion was enhanced by the application of stress. Sulphide ions were found to be more dangerous than C1- or SO₄^2? ions at the free corrosion potential. With anodic polarization, S^2? ions are inhibitive both in the absence and in presence of stress. In early stages at low current densities, the anodic process was stimulated by the application of stress.     

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.     

Electrochemical behaviour of iron in alkaline sulphide solutions

Abstract

Cyclic voltammograms and galvanostatic polarisation curves were traced for the iron electrode in alkaline solutions containing sulphide species. In sulphide free NaOH solution, the galvanostatic anodic polarisation curves are characterised by two well defined steps before tending finally to values characteristic for the oxygen evolution reaction. However, using cyclic voltammetry technique, three anodic peaks are distinct. On the other hand, the cathodic half cycles using both techniques are characterised by two cathodic reduction steps before hydrogen evolution commences. In the presence of increasing sulphide content the polarisation curves are characterised by the presence of several anodic and cathodic regions. The anodic regions correspond to the simultaneous formation and oxidation of different Fe sulphide and oxide species. A mechanism is thus proposed for the formation and reduction of iron sulphide species.     

Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization

High strength steels used in prestressing concrete structures are not exempt from the effects induced by corrosion on the normal concrete reinforcement. Carbonation of surrounding concrete or mortar is not unlikely for prestressing tendons and strands. Moreover, these steels undergo to brittle fracture as a consequence of stress corrosion cracking phenomena. To evaluate if concrete carbonation can promote this kind of failure, constant load tests in bicarbonate aqueous solutions under anodic polarization were carried out on high strength steel wires. Microscopic examination pointed out that the wires exhibited a brittle fracture mode, while its natural feature is ductile, as indicated by air testing. Failure mechanism was evaluated by a fracture mechanic approach. Cracks initiation was attributed to an anodic dissolution mechanism, while its propagation, interpreted by means of the surface mobility theory, was related to interaction between hydrogen atoms and magnetite at a crack tip.     

Effects of a magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in weakly alkaline solutions with or without halides

The effects of a 0.4 T horizontal magnetic field on the anodic dissolution, passivation and transpassivation behaviour of iron in bicarbonate solutions of various concentrations and in dilute bicarbonate solutions with or without halides are investigated by electrochemical polarisation measurements. The applied magnetic field does not affect the activation-controlled anodic current, the steady passive current and the transpassive current, but significantly affects the activation-passivation transition processes for iron in bicarbonate solutions without halides. The effects of magnetic field are strongly dependent on passivation mechanisms that result in different types of surface films and corresponding rate determining steps of film dissolution. There is a synergistic effect between the applied magnetic field and halides, chlorides or bromides, on attacking the passivation of iron in dilute bicarbonate solutions. The effects of the magnetic field are analysed based on the previously proposed electrochemical kinetics equations. The magnetic field affects the anodic polarisation behaviour through its enhancing effects on mass transport processes at the precipitation-dissolution type surface film/solution interfaces. The magnetic field shows little or no effects on continuous and steady passivation films where the oxidation rate is controlled by mass transport processes within surface films. Magnetoelectrochemistry measurements are suggested as a prospective method for researches on corrosion or passivation mechanisms.     

Magnetic field effects on anodic polarisation behaviour of iron in neutral aqueous solutions

Abstract

The presence of a magnetic induction field with a flux density of 0·4 T caused the anodic active-passive transition potential of iron in neutral aqueous solutions of sodium sulphate to move in the noble direction and resulted in a reduction of the passive potential range. Anodic current densities at the active-passive transition potential and in the passive potential range increased in the presence of the magnetic field. There was a synergistic effect between the magnetic field and the presence of chloride in solution on the breakdown of iron passivity.     

The anodic behaviour of chromium steel X20Cr13 in organic solutions of sulphuric acid

Anodic polarization measurements have been carried out on X20Cr13 steel in anhydrous solutions of H₂SO₄DMF and H₂SO₄CH₃OH. Anodic films are formed on the steel surface, which inhibit the process of steel dissolution. The films differ through their electrochemical behaviour from passive films formed in water solutions of sulphuric acid. These films are of salt-like and not oxide-like character. The solvent as well as its water content affects the solubility of “salt” films and hence the anodic behaviour of steel in investigated environments.     

Some particularities of the corrosion behaviour of Mg-Zn-Mn-Si-Ca alloys in alkaline chloride solutions

The influence of silicon and calcium on corrosion behaviour of Mg-6Zn-Mn alloys was investigated in alkaline solutions. The corrosion behaviour of the alloys with varied silicon and calcium content mainly depends on volume fractions and morphology of the intermetallics. In Cl-ion-free solutions silicon increased the corrosion resistance of alloys causing the growth of the surface passive film mostly composed of hydroxide products. Calcium had caused trouble to the formation of the stable protective film through the formation of Ca-Mg-Si intermetallics. In Cl-ion containing solutions intermetallic Ca-Mg-Si phase was responsible for extreme initial corrosion of Mg-6Zn-Mn-Si-Ca alloys.     

The anodic dissolution of beryllium in non-aqueous solutions

The anodic dissolution of Be was studied in Cl^? and Cl0₄^? solutions of methanol, ethanol and N,N-dimethylformamide with water contents < 0.1 g/l at 25°C. The apparent valences of Be ions going into solution ranged from 0.37 to 1.8. The lower values were obtained in alcohol solutions with Cl^?. Considerable gas evolution was noted from the Be surface. The low and anomalous valences are attributed mainly to a chemical dissolution of the metal by reaction with the solution. This reaction is reported to be catalysed by Cl^?. Theeffect of the low watercontent is to hinder the reforming of an oxide or hydroxide film that protects the Be from chemical attack. The numerical value of the valence is mainly dependent on the rate that the Be will chemically react with the media once a current flows that disrupts the surface film.     

Influence of the anion specificity on the anodic polarization of titanium

The anodic polarization of titanium in the presence of selected electrolytes at 0.1 mol/L concentration has been investigated. The results were interpreted according to the categorization of anions based on their cosmotrope/chaotrope character. The aggressiveness of the anions is found to be related for a part to their proneness to loose their hydration shell during the penetration of the passive film, provided they can access sufficiently to the passive surface.     

Effect of high gradient magnetic fields on the anodic behaviour and localized corrosion of iron in sulphuric acid solutions

The influence of high gradient magnetic fields on the anodic dissolution of iron in sulphuric acid solutions and the localization of the corrosion attack is investigated by means of potentiodynamic and potentiostatic polarization experiments and subsequent surface profile analysis. A localization of the material loss is observed in every potential region of the anodic Fe dissolution except from the passive region. The impact of the magnetic field on the anodic current density and the localization of the corrosion attack are explained by the action of the Lorentz force and the magnetic field gradient force.