Corrosion and passive behaviour of aluminium in weakly alkaline solution

The title subject has been studied by stationary and transient polarization measurements on high-purity aluminium electrodes in acetate solutions of pH 7–10 at 25°C and by Auger electron spectroscopic investigations of oxide films formed. In such deoxygenated solutions, aluminium corrodes under hydrogen evolution and is in some state of active/passive transition, or close to it. Oxygen (when added) acts as a passivator and mostly lowers the corrosion rate. The stationary passive current is potential independent, but varies with pH. It goes through a minimum at a pH of about 8. Oxide films formed have a thin inner barrier layer and an outer non-barrier layer whose thickness increases with increasing pH. Transients yield information on the barrier layer and on charge transfer reactions occurring.     

Corrosion and passive behaviour of aluminium in weakly acid solution

The title subject has been studied by stationary and transient polarization measurements on high-purity aluminium electrodes in acetate solutions of pH 3.7–6.7 at 25°C and by some Auger electron spectroscopic investigations of passive films formed. The potential range covered is ?1.6 to 2.0 V (sce). Aluminium is found to exhibit active/passive transition behaviour at potentials below ?1.5 V (sce), a distinct dip in stationary passive current at potentials up to about ?1.0 V (sce), and regular passive behaviour at higher potentials. At these higher potentials, perturbations give transients which are in accordance with the Cabrera-Mott equation for currents controlled by site (kink) and electric field dependent transfer of Al(III) ions at the metal/oxide interface. At lower potentials, such transients yield information also on electron transfer reactions (hydrogen evolution, oxygen reduction) at oxide covered aluminium. Implications to the corrosion of aluminium are considered.     

The electrochemical behaviour of copper in alkaline solutions containing sodium sulphide

The electrochemical behaviour of copper in NaHCO₃ solution (pH 9) and NaOH solutions (pH 14) in the presence of sodium sulphide (10^–3 to 5×10^–2m) was investigated by using rotating disc electrode and rotating ring-disc techniques, triangular potential scanning voltammetry and potentiostatic steps. When the potential increases from –1.2V upwards, copper sulphide layers are firstly formed at potentials close to the equilibrium potentials of the Cu/Cu₂S and Cu/CuS reversible electrodes. When the potential exceeds 0.0 V (NHE), the copper oxide layer is electroformed. Pitting corrosion of copper is observed at potentials greater than –0.3 V. The charateristics of copper pitting are also determined through SEM optical microscopy and EDAX analysis. There are two main effects in the presence of sodium sulphide, namely, the delay in the cuprous oxide formation by the presence of the previously formed copper sulphide layer and the remarkable increase of copper electrodissolution when the potential exceeds the cuprous oxide electroformation threshold potential. These results are interpreted on the basis of a complex structured anodic sulphide layer and on a weakening of the metal-metal bond by the presence of adsorbed sulphur on the copper surface.     

On the behaviour of copper in oxalic acid solutions

The behaviour of copper in oxalic acid solutions was studied both by cyclic voltammetry and electrochemical impedance technique. It was shown that, due to the easy complexation of copper with oxalate ions, the species present in the solution depend on the pH. During the dissolution, it was shown that the electrode was blocked by the precipitation of copper oxalate. On the other hand, a model was proposed to explain the copper deposition in concentrated oxalic acid solution. We found a rather good agreement between the experimental and calculated impedances.     

Electrochemical reduction of nitrate in weakly alkaline solutions

The electrocatalytic activity of several materials for the nitrate reduction reaction was studied by cyclic voltammetry on a rotating ring disc electrode in solutions with different concentrations of sodium bicarbonate. Copper exhibited highest catalytic activity among the materials studied. Nitrate reduction on copper was characterized by two cathodic shoulders on the polarization curve in the potential region of the commencement of hydrogen evolution. In this potential range an anodic current response was observed on the Pt ring electrode identified as nitrite to nitrate oxidation. This indicates that nitrite is an intermediate product during nitrate reduction. These conclusions were verified by batch electrolysis using a plate electrode electrochemical cell. Copper and nickel, materials representing the opposite ends of the electrocatalytic activity spectra, were used in batch electrolysis testing.     

Anodic behaviour of alkaline solutions containing copper cyanide and sulfite on the graphite anode

Sulfite may be added to copper cyanide solutions to reduce cyanide oxidation at the anode during copper electrowinning. Anodic sulfite oxidation is enhanced in the presence of copper cyanide. Sulfite also suppresses the oxidation of copper cyanide. The effect of sulfite on the oxidation of copper cyanide decreases with increasing mole ratio of cyanide to copper. This is related to the shift in the discharged species from Cu(CN)₃ ^2– to Cu(CN)₄ ^3– with increasing mole ratio of cyanide to copper. Sulfite is oxidized to sulfate. At [Cu⁺] = around 1 M, CN:Cu = 3.0–3.2, [OH^–] = 0.05–0.25 M, [SO₃ ^2–] = 0.4–0.6 M and the temperature = 50–60 °C, the anodic current efficiency of sulfite reached 80–90%. With further increase in sulfite concentration beyond 0.6 M, the current efficiency of sulfite oxidation will not be increased significantly. Further increase in CN:Cu mole ratio will result in decrease in the anodic current efficiency.     

The potentiodynamic behaviour of iron in alkaline solutions

The potentiodynamic behaviour of iron in alkaline solutions under carefully controlled perturbation conditions reveals that the overall electrochemical process is more involved than was thought earlier. The electrochemical characteristics of the systems are explained through a series of successive conjugated redox couples principally involving Fe(OH), Fe(OH)₂ and FeOOH as limiting stoichiometric species. The yield of soluble species such as either FeO^2?₂ or HFeO^?₂ increases with the pH. Ageing effects of reactants and products are also distinguished through the potentiodynamic E/I records.     

关于钢在酸性溶液中的钝化

在酸性溶液中,钢是活泼的,随着阳极过程的进行而发生腐蚀．但是,由于一些因素的作用,使阳级过程受到强烈的阻滞,同样可进入钝化状态．微酸性除铜钝化工艺是利用在亚硝酸钠－柠檬酸溶液中,钢的钝化区间扩大,钝化电流变小,钝化电位负移,同时铜溶解产生的强氧化电流使钢的阳极过程受到阻滞,从而进入钝化状态．     

The mechanism of oxidation of copper in alkaline solutions

The formation of Cu₂O by the oxidation of Cu in alkaline solutions under various controlled potential conditions has been studied by potentiodynamic methods, the rotating ring disc technique and by employing colloidal Cu(OH)₂ electrodes supported on vitreous carbon.The kinetics of the electrochemical reactions, both anodic and cathodic, are interpreted in terms of a complex reaction mechanism involving various intermediates participating in the phase oxide formation, (e.g. adsorbed OH, soluble Cu(I) and metal sites of different activity).Besides the electrochemical reactions the model includes various ageing and surface restructuring processes. The growth mechanism is envisaged to depend on the conditions of oxidation.     

The electrochemical oxidation of alkaline copper cyanide solutions

A systematic investigation of the electrochemical oxidation of copper cyanide was carried out. At low pH, cyanide destruction is believed to be catalyzed by the heterogeneous reaction involving adsorbed [Cu(CN)₃]²⁻ and possibly [Cu(CN)₄]³⁻. At high pH, rapid oxidation of cyanide was observed around 0.75 V versus Hg/HgO with the formation of a black copper oxide film. This enhanced electrocatalytic activity is believed to be related to the formation of an active copper(III) species. The transition point between low and high pH as a function of cyanide and copper concentrations is discussed. Bulk electrolysis of a copper cyanide solution at 0.90 V oxidized most of the cyanide to cyanate. Prolonged electrolysis further oxidized the cyanate to nitrate. The copper oxide film is found to be catalytic, capable of electro-oxidizing hydroxide to oxygen and cyanate to nitrate.     

The potentiodynamic behaviour of copper in NaOH solutions

The potentiodynamic behaviour of Cu in different NaOH solutions at 25° C is studied paying particular attention to the anodic formation and cathodic reduction of the Cu(I) and Cu(II) surface species occurring during the electrochemical processes. The potentiodynamic response of the electrochemical interface is strongly dependent on the perturbation conditions and it reveals the complexity of the electrochemical reactions occurring there as well as the inter-relation of the processes taking place at different potentials. A reaction pathway to interpret the corresponding behaviour is advanced.     

The initial behaviour of freshly etched copper in moderately acid, aerated chloride solutions

When freshly etched samples of various types of copper were exposed in moderately acid, aerated chloride solutions, two phenomena were observed. First the corrosion potential and the pH of the solution decreased over a shorter time, then the potential increased over a long period (600-1500 min), following an s-shaped pattern. Increase in pH during the second stage was avoided using a pH-stat. The corrosion rate increased little or not at all over the entire period. A tentative interpretation of the short-term behaviour is presented with some reservation. The long-term development of the potential suggests phase formation or transformation following the Avrami pattern. By suitable derivations it was possible to fit the development of potentials to the Avrami equation. Subsequent examinations by Auger spectroscopy proved the presence of thin layers of Cu₂O on the copper surfaces, increasing in thickness with exposure time. The dissolution kinetics can be described in terms of two parallel electrochemical reactions and a simultaneous non-electrochemical dissolution reaction.     

EIS investigation of passive film formation on mild steel in oxalic acid solution

The properties of oxide films on mild steel formed anodically in aqueous solution containing oxalic acid were investigated by means of electrochemical impedance spectroscopy (EIS). The morphology of the passive layers at different stages of film formation was investigated using scanning electron microscopy (SEM). Passivation of mild steel in oxalic acid solution is a multi-stage process that occurs before passive layer breakdown. The influence of potential on the electrochemical behavior of the passive layer was also investigated. Depending on the passive layer potential, the EIS spectra obtained exhibited a one- or two-time constant system. This suggests the formation of either a one layer or two layer oxide film on the mild steel surface, depending on the passivation potential.     

Electrochemical behaviour of iron in alkaline sulphate solutions

The electrochemical behaviour of pure iron in alkaline sulphate solutions was studied using cyclic voltammetry atT = 298 K. It has been found that the results depend on the polarization pre-treatment of the electrode and the activation state of its surface. At starting potentials in the range of hydrogen evolution and at maximum activation conditions, the voltammograms show three anodic current maxima and two cathodic ones. A correlation between these different maxima is given. The electrode processes, which may take place at these maxima, are discussed in terms of possible iron dissolution mechanisms.     

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions has been investigated. The results of hydrogen collection experiments showed that aluminium has a lower corrosion rate in alkaline methanol solutions compared to water based solutions and that the corrosion rate increases with increasing water content of the solution. Polarization and galvanostatic discharge experiments showed that there is a wide potential window of electrochemical activity and a better discharge performance in the alkaline methanol solutions with a certain amount of water. Scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) showed that the passivation in the later stages of discharge in alkaline methanol solutions at relatively high current densities is due to the formation of a dense Al(OH)₃ layer on the surface of the anode.     

Electrochemical behaviour of iron in alkaline sulphide solutions

Reactions of the SH^? anion on iron electrodes in deoxygenate alkaline solutions were studied using constant potential, constant current and acyclic voltammetric techniques.In the absence of sulphide, a dual oxide layer is formed. When sulphide is present, oxide growth is inhibited. At sufficiently anodic potentials the oxide layer is attacked, leading to a breakdown of passivity and the formation of sodium ferric sulphide, identified by its characteristic visible spectrum. Oxidation of the sulphide ion leads to repassivation of the electrode by a deposit of elemental sulphur. The effectiveness of this repassivation is dependent on the sulphide concentration since excess sulphide dissolves the passivating sulphur deposit to form polysulphides. Polysulphides were identified by colour and by their uv — visible spectrum.The voltammetric data were analysed according to the theory for film formation under Ohmic control. Agreement with this theory suggests that sulphide attacks the oxide in a similar manner to Cl^? (ie pitting attack) and that the repassivation is due to the local plugging of these pits with electrochemically deposited sulphur.     

Electrode processes of bismuth in weakly acidic,neutral and alkaline solutions

The electrode process Bi(III)+ 3e = Bi(Hg) has been investigated in weakly acidic, neutral and alkaline solutions by means of amalgam polarography and cyclic voltammetry and chronopotentiometry with the use of hanging amalgam drop electrode. It has been found that in weakly acidic and neutral solutions, oxidation of bismuth amalgam goes through aquocomplexes of bismuth. In the cathodic step on cyclic voltammetry curves four peaks of reduction are observed, with magnitude and shape strongly dependent on pH of the solution. These peaks have been ascribed to the reduction of Bi³⁺, Bi(OH)⁺₂, Bi(OH)⁺² and Bi(OH)₃.In alkaline solutions, oxidation of bismuth amalgam leads to Bi(OH)₃. In cathodic scan on cyclic voltammetry curves only one peak is observed, which corresponds to Bi(OH)₃ reduction. Formal potentials of electrode process and solubility product of Bi(OH)₃ have been calculated and discussed. The step-wise nature of the overall electrode reaction has been ascertained and discussed. At higher concentrations of OH^? ions, Bi(OH)₃ dissolves.     

The electrochemical behaviour of copper in sodium salicylate aqueous solutions

Cyclic voltammetry has been used to investigate the behaviour of copper surfaces in sodium salicylate aqueous solutions. The observed copper anodic passivation dependence on the presence of salicylate ion in solution is explained. The analysis of the experimental data supports the formation of a complex passivating film formed by a Cu₂O inner layer and a mixed cupric oxide/cupric salicylate outer layer; this film provides a partial passivation of the copper surface and can be completely removed upon excursion to negative potentials values; the composition of the passivating layer depends on the electrolyte nature, i.e. sodium salicylate ion and solution pH, and on the potential programme the copper electrode is subjected to.     

The anodic dissolution of copper in hydrochloric acid solutions

The electrodissolution of copper in hydrochloric acid solutions at the rotating ring-disk electrode was found to be controlled by both mass transfer and reaction in the apparent-Tafel region in HCl concentrations of between 0.1 and 1.0 M. The proposed mechanism describes the adsorption of CuCl_ads on the corroding copper surface and the diffusion of CuCl⁻₂ from the copper surface. The reaction in the limiting-current region was found to be controlled by the diffusion of Cl⁻ to the copper surface through a porous CuCl layer that forms on the surface. The thickness of this porous layer is dependent on the stirring conditions, and independent of the Cl⁻ concentration. Cu²⁺ is also produced at the Cu surface during electrodissolution. A mechanism describing the formation of a porous film of CuCl on the surface, the diffusion of Cl⁻ through this film and the formation of Cu²⁺ has been proposed.