Observations of anomalies in the anodic behaviour of microcrystalline aluminium

The anodic behaviour of sputtered microcrystalline Al (mc-Al) was investigated in neutral Na₂SO₄ electrolyte under varied conditions. Our results revealed that Cl⁻ addition led to a reduction in the anodic current density, which we considered unusual. Mott-Schottky analysis showed that Cl⁻ introduction altered the semiconducting property of the passive film from n-type to p-type, implying that the p-type film can possessed a relative higher stability. Immersion of mc-Al in other electrolytes yielded films with n-type, p-type and positive p-n junction structure. The results also indicate that the p-type film was most stable and the positive p-n junction film least stable.     

Effect of solution concentration on semiconducting properties of passive films formed on austenitic stainless steels

The semiconductor properties of passive films formed on AISI 316L in three acidic solutions were studied by employing Mott-Schottky analysis in conjunction with the point defect model (PDM). Based on PDM, the key parameters for passive film growth are the diffusivity and density of the defects within the film. The results indicated that donor densities are in the range 1-4 × 10²¹ cm⁻³ and increased with solution concentration. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficient of the donors in three acidic solutions are calculated to be approximately 1-5 × 10⁻¹⁶ cm²/s.     

Corrosion behaviour of aluminium in copper containing environment

Corrosion behaviour of pure aluminium galvanically connected to metallic copper or in the presence of Cu²⁺ ions was investigated by electrochemical measurements in Na₂SO₄ and Na₂SO₄ + NaCl test solutions. It has been found that in aerated Cl⁻ ion containing solutions pitting corrosion of aluminium emerged immediately, while in the absence of oxygen this process was less violent. Effect of passivating pre-treatment of aluminium surface on corrosion behaviour Cu-Al bimetallic system is also demonstrated.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

The effect of various halide ions on the passivity of Cu, Zn and Cu-xZn alloys in borate buffer

Cu and Zn metals and four of their alloys, Cu-10Zn, Cu-20Zn, Cu-30Zn and Cu-40Zn, were studied in borate buffer, pH = 9.2, in the presence of the halides, NaF, NaCl, NaBr and NaI. Electrochemical polarization and SEM/EDS methods were used in the study. The mechanism of corrosion of copper and copper alloys induced by iodide ions differed fundamentally from that induced by other halide ions. Iodide ions promote the general type of corrosion during which very slowly soluble CuI is formed. F⁻, Cl⁻ and Br⁻ ions promote localized corrosion attack. All halide ions induce localized corrosion on the passive layer on zinc. The breakdown potentials of Cu, Zn and four Cu-xZn alloys were measured as a function of concentration and type of halide ions.     

Semiconducting and passive film properties of nitrogen-containing type 316LN stainless steels

The semiconducting property of passive films of nitrogen-containing type 316LN stainless steels in different electrolytic media (0.5 M NaCl, borate buffer and borate buffer + 0.5 M NaCl) was investigated by electrochemical impedance spectroscopy (EIS). The nitrogen effect on the chemical composition of the passive films was investigated using X-ray photoelectron spectroscopy, (XPS). Based on capacitance results, the semiconducting parameters obtained from the Mott-Schottky plots indicated a decrease in the donor and acceptor density (N_D and N_A) with increase in nitrogen content, and variation in the flat band potential (E_FB), depending on the electrolytic media. Thus indicating that the oxide layers of the passive film are modified by nitrogen addition. The presence of nitrogen and in the passive film was confirmed by the XPS analysis of the passive film. Cyclic polarization for pitting and repassivation corrosion studies indicated a decrease in hysteresis loop with increase in nitrogen content in 0.5 M NaCl solution. In the highest nitrogen-containing alloy (0.556 wt.% N), the hysteresis loop was small and negligible indicating that the pit initiation is minimum in this alloy. Based on the results obtained, an attempt was made to correlate the semiconducting nature of the passive films with pitting corrosion resistance.     

Passivation oxide film on Nd-Fe-B permanent magnets in borate buffer solution by ellipsometry

The passivation of Nd-Fe-B permanent magnet was investigated in neutral borate solution at pH 8.4. The thickness of the passive oxide film on the magnets was measured by ellipsometry and the composition was estimated by glow discharge optical emission spectroscopy (GD-OES).The passivation of the magnets takes place in the potential range between −0.2 and 1.0 V vs. Ag/AgCl/Sat. KCl. In the potential range, current density decays to the lower than 10⁻⁶ A cm⁻² after potentiostatic oxidation for 1800 s. The passive oxide film growth is assumed to be optically simulated from a model with a homogeneous film with complex refractive index, N = 2.1 − j0.086. The thickness estimated from the refractive index linearly increases with potential from 3.6 nm at −0.2 V to 7.8 nm at 1.0 V. The passive film growth follows the ionic migration model under high electric field, i.e., the Cabrera-Mott growth model. The ionic conductivity estimated from the model is about κ = 1.7 × 10⁻¹⁶ Ω⁻¹ cm⁻¹. The passive oxide film is preferentially composed of iron oxide/hydroxide. Boron and neodymium are, respectively, concentrated at the surface of the oxide film and at the inner layer in the oxide film.     

Effect of underpotential deposition of lead on polarization behavior of nickel in acidic perchlorate solutions at room temperature

The effect of Pb²⁺ on polarization behavior of nickel has been investigated in 0.1 M NaClO₄ + 10⁻² M HClO₄ + x M PbO solutions (x = 0, 10⁻⁵, 10⁻⁴, 10⁻³) at room temperature. The cyclic voltammogram has suggested that Pb²⁺ degrades the stability of the passive film on Ni. The corrosion potential of Ni shifted to the more noble direction and the anodic current peak of Ni dissolution decreased with increasing Pb²⁺ concentration in solution, indicating that Pb²⁺ suppresses significantly the anodic dissolution. The underpotential deposition (UPD) of lead on Ni in the potential range more noble than −0.215 V (SHE) corresponding to the equilibrium potential of the Pb²⁺ (10⁻³ M)/Pb electrode was confirmed by XPS and GDOES analyses. The anodic Tafel slope, b⁺, of Ni dissolution changed from b⁺ = 40 mV decade⁻¹ in the absence of Pb²⁺ to b⁺ = 17 mV decade⁻¹ in the presence of 10⁻⁴ or 10⁻³ M Pb²⁺, which was ascribed to the increase in active sites of Ni surface emerged as a result of electrodesorption of Pb adatoms. The roles of Pb adatoms in active dissolution and active/passive transition of Ni were discussed from the above results.     

Generation mechanism of microdischarges during plasma electrolytic oxidation of Al in aqueous solutions

The generation mechanism of microdischarges during plasma electrolytic oxidation (PEO) of aluminium was investigated at constant current densities in aqueous alkaline solutions. Microdischarges were generated only in weakly alkaline solutions under high applied voltages. The breakdown voltage of the anodic oxide film was not dependent on the applied anodic current density while it was strongly dependent on the OH⁻ concentration in the solution. The size and density of microdischarges became larger and lower, respectively, with increasing PEO treatment time. The mechanism of microdischarge generation could be explained based on the movement of ions in the oxide film and resistive property of the oxide film.     

Prevention of passive film breakdown and corrosion of iron in 0.1 M KClO4 with and without Cl by covering with an ultrathin two-dimensional polymer coating and healing treatment in 0.1 M NaNO3

A two-dimensional polymer coating, the self-assembled monolayer of 16-hydroxy hexadecanoate ion HO(CH₂)₁₅ modified with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃ was prepared on the passivated iron electrode and further, the passive film was healed by additional treatment in 0.1 M NaNO₃. This electrode was immersed in oxygenated 0.1 M KClO₄ solutions with and without 1 × 10⁻⁴ to 1 × 10⁻² M of Cl⁻. Protection of passive film against breakdown by covering the electrode with the polymer coating was examined by monitoring the open-circuit potential during immersion in the solutions for many hours to determine the time for passive film breakdown, t_bd. Repeated polarization measurements were carried out during immersion in these solutions for obtaining the protective efficiency, P. The t_bd value of the passivated, polymer-coated and healed electrode in 0.1 M KClO₄ solutions with and without Cl⁻ increased with a decrease in the concentration of Cl⁻. No breakdown occurred on the electrode during immersion in 0.1 M KClO₄ solutions with and without 1 × 10⁻⁴ of Cl⁻ for 360 h. The P values were extremely high, more than 99.9% before t_bd, indicating complete protection of iron from corrosion. The effect of healing treatment in 0.1 M NaNO₃ on passive film breakdown was investigated by electron-probe microanalysis.     

Effect of common water contaminants on the corrosion of aluminium alloys in ethylene glycol-water solution

The effects of common water contaminants of chloride (Cl⁻), cupric (Cu²⁺) and ferric (Fe³⁺) ions, in four different mixture combination of Fe³⁺ + Cu²⁺, Cl⁻ + Fe³⁺, Cl⁻ + Cu²⁺ and Cl⁻ + Fe³⁺ + Cu²⁺, were examined on the corrosion behaviour of aluminium alloys in ethylene glycol-water solution, using mass loss technique. The highest material losses were recorded for the two alloys in ethylene glycol solution containing the combination of the chloride and the two heavy metal ions. The corrosivity of the solution in the presence of the combination of ions was in the order of Cl⁻ + Fe³⁺ + Cu²⁺ > Cl⁻ + Cu²⁺ > Cl⁻ + Fe³⁺ > Fe³⁺ + Cu²⁺. The results gave first-order kinetics with respect to aluminium in ethylene glycol solution-ion systems. Alloy 3SR exhibits maximum corrosion in all the solutions. It is concluded that the two commercial alloys in the solution polluted with all the three ions would not be able to survive for reasonable period of time without corrosion inhibitor.     

Stress generated porosity in anodic alumina formed in sulphuric acid electrolyte

The generation of pores is investigated in anodic films formed at 5 mA cm⁻² on aluminium in 0.4 M sulphuric acid electrolyte at 293 K. The study follows the behaviour of a fine tungsten tracer layer, initially located in the aluminium, during anodizing. Significantly, the tungsten is incorporated into the anodic film with negligible loss of the tracer to the electrolyte. The findings indicate that pores develop primarily due to flow of film material in the barrier layer under the influences of the stresses of film growth. The flow of material from beneath pores toward the cell walls is accommodated by the increased thickness of the anodic film relative to that of the oxidized metal by a factor of about 1.35.     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

Protective properties of hexacyanoferrate containing polypyrrole films on stainless steel

The protective properties of polypyrrole coatings containing hexacyanoferrate anions are presented. The composite coatings were electrodeposited galvanostatically on stainless steel from the aqueous solutions containing pyrrole monomer and potassium hexacyanoferrate(II) together with traces (10 μmol dm⁻³) of free CN⁻ ions. The protective properties of the resulting coatings were investigated in the highly aggressive medium of 0.1 mol dm⁻³ HCl and 0.4 mol dm⁻³ NaCl. Diagnostic experiments included measurements of open circuit potential with time of exposure, chronoamperometric (current-time) examination of the steel samples at a selected constant potential of 0.5 V and potentiodynamic measurements. To get insight into the dynamics of the steel dissolution at open circuit potential and at the selected anodic potential, changes of concentrations of iron and chromium in the test solution as a function of exposure time were determined using atomic absorption spectrometry. Morphology of the composite film was examined using scanning electron microscopy.     

Corrosion behavior of glassy Ni55Co5Nb20Ti10Zr10 alloy in 1 N HCl solution studied by potentiostatic polarization and XPS

Corrosion resistance of glassy Ni₅₅Co₅Nb₂₀Ti₁₀Zr₁₀ (at.%) alloy in 1 N HCl solution was investigated with respect to the electrochemical behavior and the compositions of the passive film and the underlying alloy surface just below the passive film. The potentiostatic polarization curve indicated that the alloy was spontaneously passivated with a low passive current density of the order of 10⁻³ A m⁻². The quantitative X-ray photo-electron spectroscopy (XPS) analysis revealed that the thickness of the surface film increased linearly with an anodizing ratio of 1.5 nm V⁻¹. The high corrosion resistance of the glassy alloy was due to the formation of niobium, titanium and zirconium-enriched passive film. The growth mechanism of the passive films is also discussed.     

Formation, structure and composition of anodic films on AM60 magnesium alloy obtained by DC plasma anodising

Anodising of AM60 magnesium alloy (6% Al + 0.27% Mn) was studied in a solution containing 1.5 M KOH + 0.5 M KF + 0.25 M Na₂HPO₄ · 12H₂O with addition of various NaAlO₂ concentrations. The experiments were carried out in DC current galvanostatic mode. Observations of phenomena occurring at the sample surface plus voltage monitoring revealed three stages: traditional anodising, followed by microarc anodising and finally arcing. The film was porous and cracked, with poor bonding to the substrate. It was composed of magnesium and aluminium oxide, and contained all the elements present in the electrolyte. The aluminium concentration in the film was dependent on the concentration of aluminate ions in the electrolyte. The transition from microarc to arcing stage took place when the alloy surface was completely covered by the anodic film.     

Current distribution during galvanic corrosion of carbon steel welded with type-309 stainless steel in NaCl solution

Galvanic corrosion of carbon steel welded with type-309 stainless steel in NaCl solution was tentatively evaluated with a newly developed multi-channel electrode technique in which the welded specimen was divided into nine working electrodes (WEs), reconstructed in resin, and connected individually to an imaginary ground level of an electric circuit via relay switches. This allows the WEs to join a galvanic couple and simultaneous measurement of participating current or open circuit potential of each WE. WEs were immersed together in 5.1 × 10² mol dm⁻³ or 2.1 × 10⁻⁴ mol dm⁻³ NaCl solutions, and spatial distribution of participating currents and open circuit potentials were monitored as a function of immersion time. The WE of the weldment acted as a cathode throughout the immersion period, while the other WEs of base steel became anodes or cathodes depending on their location, immersion time and concentration of the electrolyte solution. The ability of zinc-rich paint to protect the welded specimen as sacrificial anode was also investigated.     

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.     

Anodic behaviour of a model second phase: Al-20at.%Mg-20at.%Cu

The anodic behaviour of sputtering-deposited Al-20at.%Mg-20at.%Cu alloy is investigated during anodizing and potentiodynamic polarization treatments using transmission electron microscopy, X-ray photoelectron spectroscopy and medium energy ion scattering. The composition of the alloy is close to that of the S-phase in 2024 aluminium alloy. The anodizing behaviour in both 0.1 M ammonium pentaborate and 0.1 M sodium hydroxide electrolytes follows the behaviour of more dilute, solid-solution, aluminium alloys, with enrichment of copper developing in the alloy during the growth of an alumina-based initial oxide containing incorporated magnesium species. Oxygen gas is generated following sufficient enrichment of copper for its oxidation to proceed and hence, for copper species to enter the oxide film. The generation of oxygen gas causes extensive damage to the film, which limits the voltage to relatively low values. Potentiodynamic polarization in 0.1 M sodium hydroxide electrolyte revealed mainly passive behaviour following an initial period of corrosion during which the passive film is developed. In this initial period, copper enriches in the alloy, beneath an oxide film containing aluminium and magnesium species. The magnesium species migrate faster through the film than the aluminium species and form a surface layer of MgO/Mg(OH)₂, which protects against losses of aluminium species to solution and permits the establishment of the passive film. The steady open-circuit potential of the passivated alloy in the hydroxide solution is about −550 mV (SCE), compared with about −1940 mV (SCE) for aluminium.     

Enrichment of alloying elements in anodized magnesium alloys

The possibility of enrichment of alloying elements in magnesium alloys as a consequence of growth of an anodic film has been investigated for sputtering-deposited Mg-0.4 at.% W and Mg-1.0 at.% W alloys. The alloys were anodized at 10 mA cm⁻² to various voltages, up to 150 V, in 3 M ammonium hydroxide/0.05 M ammonium phosphate electrolyte at 293 K. The alloys revealed enrichments of tungsten to at least 1.7×10¹⁵ and 2.9×10¹⁵ W atoms cm⁻² for the Mg-0.4 at.% W and Mg-1.0 at.% W alloys respectively. The enrichment behaviour appears to be similar to that in dilute aluminium alloys, which occurs for alloying elements with oxides having Gibbs free energies per equivalent for formation exceeding that for formation of alumina.