Modelling the anodizing behaviour of aluminium alloys in sulphuric acid through alloy analogues

Anodic film morphologies on aluminium aerospace alloys are strongly influenced by alloying elements. The present study uses model alloys to interpret the early stages of anodizing of AA2024-T3 and AA7075-T6 aluminium alloys in 0.4 M sulphuric acid electrolyte. Further, coupled model alloys, representative of matrix and second phase regions, are employed as alloy analogues. The findings enable assignment of transient anodic currents during potentiodynamic polarization of the commercial alloys to oxidation of Al₂CuMg phase at 0 V SCE and of Al₂Cu, Al₇Cu₂Fe and Al–Cu–Fe phases at 5–6 V SCE. The phases that oxidize at the latter potential also cause voltage arrests during galvanostatic anodizing.     

Cathodic reduction of the duplex oxide films formed on copper in air with high relative humidity at 60 °C

The cathodic reduction of duplex air-formed oxide film on copper was performed at a constant current density of i_c = −50 μA cm⁻² in deaerated 0.1 M KCl solution to investigate the sequence of cathodic reduction of each oxide layer and its mechanism. The single-phase thick CuO film on copper was also cathodically reduced at i_c = −50 μA cm⁻² or −2.5 mA cm⁻². The surface characterizations of the air-formed oxide film and single-phase CuO film before cathodic reduction and after partial or complete cathodic reduction were performed by XPS and X-ray diffraction, respectively.The two plateau regions appeared in the potential vs. time curve during cathodic reduction of the duplex air-formed oxide film on copper, while one plateau region was observed in the potential-time curve during cathodic reduction of the single-phase CuO film on copper. The potential in the first plateau region for the air-formed film coincided with that in the plateau region for the CuO film. The results of XPS and X-ray diffraction suggested that in the first plateau region, the outer CuO layer is directly reduced to metallic Cu, while in the second plateau region, the inner Cu₂O layer is reduced to metallic Cu.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Discontinuities in the porous anodic film formed on AA2099-T8 aluminium alloy

The anodizing behaviour of constituent particles (Al–Fe–Mn–Cu) and dispersoids (Al–Cu–Mn–Li and β′(Al₃Zr)) in AA2099-T8 has been investigated. Low-copper-containing Al–Fe–Mn–Cu particles anodized more slowly than the alloy matrix, forming a highly porous anodic oxide film. Medium- and high-copper-containing Al–Fe–Mn–Cu particles were rapidly dissolved, resulting in defects in the anodic film. The anodizing of Al–Cu–Mn–Li dispersoids is slightly slower than the alloy matrix, forming a less regular anodic oxide film. β′(Al₃Zr) dispersoids anodized at a similar rate to the alloy matrix. Further, the potential impact of the discontinuities in the resultant anodic films on the performance of the filmed alloy is discussed.     

Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu2O multilayer film

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, i_O^2-, and of Cu²⁺ dissolution through the passive film, i_Cu²⁺. As a result, in the case of the pure Cu film, the anodic current density was mainly due to i_Cu²⁺, while in the case of the multilayer film, i_Cu²⁺ was almost equal to i_O^2-. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

Oxidation behaviour of Ti–Al–C films composed mainly of a Ti2AlC phase

This paper addresses the oxidation behaviour of Ti–Al–C films composed mainly of a Ti₂AlC phase. The films exhibited rather low oxidation rates at 600 and 700 °C, with an oxygen-rich zone or a thin oxide layer appearing on the film surfaces. Much faster oxidation rates were observed at 800 and 900 °C. The Ti₂AlC phase was quickly consumed by oxidation. From the film surface to the inner zone, TiO₂-rich layer, Al₂O₃-rich layer, and TiO₂ + Al₂O₃ mixed layer was observed, respectively. The oxidation mechanism of the Ti–Al–C film is discussed based on the experimental results.     

Surface characterization and corrosion behavior of a novel gold-imitation copper alloy with high tarnish resistance in salt spray environment

A novel gold-imitation copper alloy (CuZnAlNiSnBRe) was designed and its corrosion behavior in salt spray environment was investigated. The new alloy has better tarnish resistance and corrosion resistance than the current coinage alloy used in China (H7211). A multi-layer film formed on the surface of the new alloy after a period of exposure to salt spray was responsible for the good resistance of the alloy. The corrosion products were a mixture of CuO, Cu₂O, ZnO, Al₂O₃ and Al(OH)₃, with the transition from Cu₂O to CuO occurring during the corrosion process.     

Study of the intermetallic growth in copper-clad aluminum wires after thermal aging

Study of the solid-state diffusion between copper and aluminum was carried out in the temperature range [573–673] K in order to better understand the aging mechanisms which occur in copper-clad aluminum thin wires. A complete microscopic analysis was performed to evaluate the interface composition and corresponding microstructure. The intermetallic phases developed during annealing identified by TEM and X-Ray diffraction analysis are respectively Al₂Cu, AlCu, and Al₄Cu₉. A fine layer containing nanometric copper grains was also depicted and identified as a diffusion-induced recrystallization region. These results agree with EDXS analysis and nanoindentation measurements. The effective heat of formation model was used to evaluate the first phase(s) which happens in the interface and the sequence formation of intermetallic compounds during annealing. This model finely describes the metallurgical aging of copper-clad aluminum wires and explains the presence of only three intermetallic compounds in the interface between copper and aluminum.     

Anodic oxidation and dielectric behaviour of aluminium-niobium alloys

The anodizing behaviour of sputtering-deposited Al-Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb₂O₅ and Al₂O₃ in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb⁵⁺, Al³⁺ and O²⁻ species, with cation transport numbers, in the range 0.31-0.35, and formation ratios, in the range 1.35-1.64 nm V⁻¹, intermediate between those of anodic alumina and anodic niobia. Al³⁺ ions migrate slightly faster than Nb⁵⁺ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb₂O₅ into anodic alumina increases the dielectric constant from about 9 to the range 11-22 for the investigated alloys.     

Effect of artificial aging on intergranular corrosion of extruded AlMgSi alloy with small Cu content

The effect of artificial aging parameters on the corrosion performance of air cooled AlMgSi(Cu) model alloy extrusions was investigated. Accelerated corrosion test revealed that the extrusions were highly susceptible to intergranular corrosion (IGC) in the naturally aged condition. However, IGC susceptibility was reduced, and finally eliminated, by artificial aging. Overaging introduced slight pitting susceptibility. EDS X-ray mapping in FE-TEM revealed Mg₂Si and Q-phase (Al₄Cu₂Mg₈Si₇) grain boundary precipitates and a continuous Cu-enriched grain boundary film. IGC susceptibility was related to the Cu-enriched grain boundary film. Increased IGC resistance was caused by coarsening of the grain boundary film by aging. Pitting susceptibility by over aging evolved due to coarsening of the Q-phase particles in the grain bodies.     

Role of alloyed copper on corrosion resistance of austenitic stainless steel in H2S–Cl− environment

Corrosion test, surface analysis and thermodynamic calculation were carried out in the H₂S–Cl⁻ environments to clarify the role of alloyed Cu on the corrosion resistance of austenitic alloys. The alloyed Cu improved pitting corrosion resistance in the H₂S–Cl⁻ environment. The surface film of Cu-containing alloy indicated double layer consists of copper sulfide and chromium oxide, and the copper sulfide was able to exist stably compared to iron sulfide and nickel sulfide. It is concluded that the copper sulfide would enhance the formation of chromium oxide film which improve the pitting corrosion resistance in the H₂S–Cl⁻ environment.     

Influence of phase constituent and proportion in initial Al–Cu alloys on formation of monolithic nanoporous copper through chemical dealloying in an alkaline solution

An alkaline medium has been used to fabricate monolithic nanoporous copper (NPC) ribbons through chemical dealloying of melt-spun Al–Cu alloys with 33–50 at.% Cu. The results show that phase constituent and proportion in the initial alloys have a key influence on the formation of NPC. The alloy ribbons comprising one or a combination of Al₂Cu and AlCu can be fully dealloyed in alkali solution only when there is no or a minor AlCu in the initial alloys. Additionally, the length scales of ligaments/pores in NPC can be broadly modulated by simply changing the amount of AlCu in the initial alloys.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

Anomalous high-temperature oxidation in the zirconium-copper system

The high-temperature oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu in oxygen is characterized by selective oxidation of zirconium while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₈Cu₅ phase. The oxidation of Zr₂Cu at elevated temperatures shows an anomalous decrease of the oxygen consumption rate in the temperature range 890-975 °C. The oxide layer consists of monoclinic ZrO₂ mainly, with preferentially oriented crystallites in depth region at 900 °C and tetragonal ZrO₂ on the surface below 600 °C, and small amounts of CuO and Cu₂O. The reaction kinetics obeys a parabolic rate law. The activation energy of 117.5 and 54.4 kJ/mol has been estimated for the oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu, respectively.     

Heat-transfer corrosion behaviour of cast Al alloy

Heat-transfer corrosion behaviour of an ISO 2379 cast Al alloy was studied in antifreeze radiator coolant under heat-rejecting condition. Extensive analyses of microstructures and corroded surfaces were carried out under the optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X-ray diffractometer. Heat-rejecting condition led to a cavitation process and cavities were observed within the α-Al matrix. Crevice corrosion was predominant at oxygen depleted regions in heat-transfer corrosion cell. Al₂Cu, Al₁₅(Fe,Mn)₃Si₂ dendrites, Al₄Cu₂Mg₈Si₇ and Si phases served as the effective cathodes resulting microgalvanic corrosion at the anodic site of α-Al matrix.     

Assessment of the surface state behaviour of Al71Cu10Fe9Cr10 and Al3Mg2 complex metallic alloys in sliding contacts

Electrochemical measurements and friction measurements during continuous and intermittent unidirectional sliding tests are used to monitor and to evaluate the surface characteristics of two types of metallic materials characterized by a huge unit cell, namely Al₇₁Cu₁₀Fe₉Cr₁₀ and Al₃Mg₂. The modification of the surface characteristics results from the periodic mechanical removal of a surface film during sliding, and the subsequent (electro)chemical re-growth of a surface film in-between successive sliding contacts. Al₇₁Cu₁₀Fe₉Cr₁₀ and Al₃Mg₂ materials were tested in a phosphate buffer solution pH 7 at 25 °C to compare their depassivation and subsequent repassivation behaviour. The Al₃Mg₂ material was also tested in a 0.1 M KOH solution pH 13 and 25 °C to reveal the role of constituting metallic elements on the surface film formation. The effect of film formation and removal on the coefficient of friction recorded during unidirectional sliding is discussed.     

The effect of oxidation on the corrosion resistance and mechanical properties of a Zr-based metallic glass

The oxidation kinetics of the Zr₆₄Cu₁₆Ni₁₀Al₁₀ bulk metallic glass (BMG) roughly follows a two-stage rate law at both 433 and 593 K in air. An oxide film of 940 nm can be formed by oxidation at 593 K, which is ZrO₂-enriched but Cu-depleted on the outer surface. The oxide film leads to a superior passivity in 0.5 M NaCl and great corrosion resistance improvements in other solutions. The oxidation effect on mechanical properties were characterized by nanoindentation, wedge indentation and compression tests. The Zr-based BMG still keeps the amorphous nature and its good mechanical properties are retained after oxidation.     

Electrochemical behavior of multicomponent amorphous and nanocrystalline Zr-based alloys in different environments

Electrochemical behavior of multicomponent melt spun Zr-based amorphous as well as nanocrystalline alloys have been studied in three different corrosive media (neutral NaCl, basic NaOH and acidic H₂SO₄ solutions). Due to the presence of strong passivating elements, (Zr and Ti) melt spun ternary alloys (Zr₅₅Ti₂₅Ni₂₀) have shown complete passivation in NaCl solution even as they contain a small amount of crystallinity. Amorphous multicomponent alloys containing Cu (Zr₅₈Cu₂₈Al₁₀Ti₄ and Zr₆₅Cu_7.5Al_7.5Ni₁₀Pd₁₀) show active nature in NaCl solution and this is strongly related to selective dissolution of base metal (Zr) and enrichment of Cu in the pit region. In NaOH and H₂SO₄ solutions, all the alloys have shown complete passivation irrespective of the alloy composition.     

Comparison of the corrosion of Cu50Pd50 and Cu50Pd44M6 (M = Y,Mg, or Al) hydrogen separation membrane alloys in simulated syngas containing H2S

The resistance to sulfidation was examined by exposing coupons including a Cu–Pd binary alloy and three ternary alloys with 6 at% Y, Mg, or Al to simulated syngas containing varying amounts of H₂S. The mass change of the samples was determined and the exposed surfaces were characterized by SEM/EDS and XRD. The best corrosion resistance of the ternary alloys was observed in the Cu₅₀Pd₄₄Al₆ alloy. A slight decrease was observed when Mg was added, but both the Al and Mg alloys were roughly on par with the binary composition. The Y addition resulted in a degradation in the corrosion resistance by forming extensive internal Y₂O₃ and Cu_1−xY_xS and external Pd_4−xCu_xS and Pd ₁₃Cu₃S₇.