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.     

Effect of high temperature heat treatment on intergranular corrosion of AlMgSi(Cu) model alloy

Copper containing 6000-series aluminium alloys may become susceptible to intergranular corrosion (IGC) as a result of improper thermomechanical processing. Effect of cooling rate after solution heat treatment on the corrosion behaviour of a model AlMgSi(Cu) alloy of nominal composition (wt%) 0.6 Mg, 0.6 Si, 0.2 Fe, 0.2 Mn and 0.1 Cu was investigated. Slow cooling rates were simulated by isothermal treatment for predetermined times in lower temperature baths immediately after solution heat treatment. Treatment for 10-100 s at temperatures below 400 °C introduced susceptibility to IGC. Longer heat treatment at the same temperatures introduced susceptibility to pitting. A corrosion resistant time zone was found between the zones of IGC and pitting at temperatures lower than 350 °C. Quenching in water after solution heat treatment prevented IGC. IGC was related to microgalvanic coupling between the noble Q-phase (Al₄Mg₈Si₇Cu₂) grain boundary precipitates and the adjacent depleted zone. Pitting was attributed to coarse particles in the matrix. Possible mechanisms causing the corrosion resistant intermediate zone are discussed. The results indicate possible methods for obtaining increased corrosion resistance of similar alloys by proper thermal processing.     

Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al_xCrFe_1.5MnNi_0.5 alloys, obtained in H₂SO₄ and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al_xCrFe_1.5MnNi_0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.     

Electrochemical corrosion characteristics of ZrNi5−xCox alloys

The electrochemical corrosion behaviour of a series of ZrNi_5−xCo_x alloys with x=0-4 has been tested using potentiokinetic polarisation technique. The polarisation curves were measured in deaerated 0.5 M sulphate solutions with pH=0.2-7 and in strong alkaline solution of KOH (pH=15). It is shown that the presence of greater amounts of Co in the alloy (x?2) worsens the passivating properties of the alloy in acidified sulphate solutions. On the other hand, in strong alkaline solutions, both low- and high-cobalt alloys undergo stable passivation. The degree of Ni substitution by Co in the alloys does not generally affect the shape of cathodic polarisation curves.     

Corrosion characterisation of alumina-magnesium metal matrix composites

Alumina magnesium metal matrix composites are a precious alternative for aerospace and automotive applications because of their high stiffness-to-weight ratio. A considerable deal of investigations had been devoted to their processing and mechanical properties, while the corrosion behaviour is still uncertain. In this study, the corrosion behaviour of Al₂O₃ fibres strengthened magnesium AS41 composite, in aqueous solutions containing various concentrations of NaCl at different pH values, was studied and compared with the behaviour of pure AS41 magnesium matrix alloy using electrochemical techniques, hydrogen evolution test, optical microscopy, and scanning electron microscopy (SEM) coupled with EDX and WDX capabilities. The results showed that the corrosion behaviour of the composite was comparable to its pure matrix alloy, yet a reduction in the corrosion resistance was observed in the composite at higher chloride concentrations. The corrosion mechanism involved, as well as the corrosion characteristics, was extensively discussed in terms of the effect of Al₂O₃ fibres. Also, an appropriate model describing the corrosion mechanism was proposed.     

Corrosion behaviour of die-cast AZ91D magnesium alloy in aqueous sulphate solutions

The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48 h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl > MgSO₄ > Na₂SO₄. The main corrosion products were Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solutions and the product film was compact. Precipitation of MgAl₂(SO₄)₄·22H₂O required a threshold immersion time.     

Localized dissolution initiated at single and clustered intermetallic particles during immersion of Al–Cu–Mg alloy in sodium chloride solution

Aiming at understanding how intermetallic phases response when AA2024-T3 aluminium alloy is exposed to chloride-containing aqueous medium, scanning electron microscopy was employed to provide morphological information on alloy surface before and after corrosion testing. Energy dispersive X-ray spectroscopy was carried out to determine compositional change in intermetallic particles. Atomic force microscopy was used to examine topographical variation introduced by the reactions of intermetallic phases. Transmission electron microscopy combined with ultramicrotomy was carried out on dealloyed Al₂CuMg particles and their periphery region. It is found that dealloyed Al₂CuMg particles exhibited porous, polycrystalline structure comprised of body-centred cubic copper particles with sizes of 5 to 20 nm. Aluminium matrix started to trench in the periphery of Al₂CuMg particles at the early stage of dealloying. Development of trenching in Al–Cu–Fe–Mn–(Si) particle's periphery was not uniform and took longer time to initiate than Al₂CuMg dealloying. Localized corrosion at a cluster of Al₂CuMg and Al₂Cu particles was mainly associated with Al₂CuMg particles.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

Identifying the role of nanoscale heterogeneities in pitting behaviour of Al-based metallic glass

To clarify the correlation of nanoscale heterogeneity with corrosion in Al-based metallic glasses, three model alloys with a single nanoscale α-Al, Al₃Ni or Al₁₁Ce₃ phase embedded in amorphous Al-Ni-Ce alloy matrix were obtained directly by melt quenching. The results indicated that the high pitting corrosion resistance of AM alloys was not deteriorated by nanocrystalline α-Al precipitation; whereas the pitting potential was slightly decreased and considerably reduced relative to their amorphous state due to the precipitation of nanocrystalline Al₃Ni or Al₁₁Ce₃ respectively. Such a pitting sensitivity of different types of heterogeneities attributes to the nano-scale pit initiation events.     

Corrosion resistance of dilute CuMg alloys at elevated temperature

In comparison with CuAl (Al: 0.2 and 0.5 wt.%) alloys, corrosion resistance (CR) of CuMg (Mg: 0.12 and 0.34 wt.%) alloys was studied at 673-1173 K in atmospheric O₂. All the samples were pre-annealed at 873 K in atmospheric H₂. The CR of CuMg alloys at 673-973 K is improved in contrast to a pure Cu but much poorer than that of CuAl alloys, while the improvement can hardly be observed for CuMg alloys at and above 1073 K, which is similar to CuAl alloys. The poorer CR of CuMg alloys compared with that of CuAl alloys at 673-973 K is largely attributed to the incorporation of Cu in the MgO surface layer and the low Pilling-Bedworth ratio of CuMg-O system smaller than unity, and the vanishing of CR for CuMg alloys at and above 1073 K is ascribed to the instability of the MgO layer at the Cu₂O/CuMg interface.     

Corrosion behaviour of several aluminium alloys in contact with a thermal storage phase change material based on Glauber’s salt

This article presents the results of a study about the corrosion behaviour of four aluminium alloys (EN AW 2024, 3003, 6063, and 1050) in contact with a commercial thermal storage material based in the Glauber’s salt (Na₂SO₄·10 H₂O). Results indicate that the Al 2024 alloy is not compatible with this material due to the extense formation of NaAlCO₃·(OH)₂ in contact with air. The aluminium alloys 3003, 6063 and 1050 showed to be fully compatible with the material.     

Corrosion behaviour of intermetallic aluminide coatings on nitrogen-containing austenitic stainless steels

The present work reports the effect of aluminide layers on the aqueous corrosion behaviour of four different 316L stainless steels containing various nitrogen contents (0.015%, 0.1%, 0.2% and 0.56% N). Diffusion annealed aluminide layers are generated over the surface by heat treatment of the aluminium precoated alloys at 750 °C for 25 h in nitrogen atmosphere. X-ray diffraction patterns of the surface modified samples showed the presence of AlN, Al₁₃Fe₄ and FeAl₂ phases. Diffusion of aluminum into the alloy, and the formation of AlN by the reaction of aluminium with matrix nitrogen, was identified using secondary ion mass spectrometry (SIMS). The nitrogen peak in the diffused layer was found to increase with increasing nitrogen content of the base alloy. SEM observation of cross-sectionally mounted alloys showed the presence of spherical AlN phase in addition to iron aluminide intermetallic phases. The role of such a composite surface layer containing intermetallic aluminides and nitride on the corrosion resistance of austenitic stainless steels in 0.5 M NaCl and 0.5 M sulphuric acid is discussed in greater detail based on open circuit potential (OCP)–time measurements, potentiodynamic polarisation studies and electrochemical impedance spectroscopy (EIS) investigations. The aluminide layered alloy with 0.1% N content showed better corrosion performance. The presence of nitrogen was found to have a positive effect in enhancing the hardness of the composite layer. Role of matrix nitrogen on the microstructure and microchemical distribution at the surface, and its role on corrosion resistance in acidic and chloride media are discussed in detail.     

Corrosion evaluation of microarc oxidation coatings formed on 2024 aluminium alloy

Dense alumina ceramic coatings of 7 μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2-6 h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24 h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96 h): corrosion process was controlled by the diffusion of corrosion products.     

Corrosion behaviour of dc magnetron sputtered Fe1−xMgx alloy films in 3 wt% NaCl solution

Fe_1−xMg_x alloy films (with x ? 43.4 at.% Mg) were deposited by dc magnetron sputtering onto glass slide substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for application as new friendly environmentally sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied prior to electrochemical experiments, and then the degraded surfaces were analysed to determine the composition and nature of corrosion products. Alloys with <25  at.% Mg were single-phase body-centred cubic (bcc) with enlarged lattice parameters, whereas for magnesium contents above 25 at.%, amorphisation occurred. The reactivity of the alloys in saline solution is strongly dependent on the Mg content and the alloy structure. The incorporation of magnesium leads to an open circuit potential shift of the alloy towards more negative values, that confers an attractive interest of these alloys as sacrificial coatings. A transition in corrosion activity is observed at 25 at.% Mg from which the reactivity decreases with the magnesium content increase. The evolution of the alloy corrosion behaviour is discussed in terms of structural and corrosion products evolution versus magnesium content.     

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.     

Localized corrosion of 2024 T351 aluminium alloy in chloride media

The corrosion behaviour of 2024 T351 alloy was studied in chloride solutions. The polarization curve of this alloy showed two breakdown potentials. The first was related to the dissolution of the coarse intermetallic Al₂CuMg particles. A severe Mg dealloying of the particles resulted in Cu-rich remnants. The matrix surrounding the particles was systematically found to be dissolved. Indeed, the particles are surrounded by a dispersoid-free zone which is anodic in comparison to both the rest of the matrix and the S phase particles. The second breakdown potential corresponded to the matrix breakdown potential. Pits but also intergranular corrosion developed. Grain boundaries were preferentially attacked because they are anodic in comparison to the grain body and the previous dissolution of the intergranular precipitates made them fragile.     

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.     

Limitations in the use of the potentiodynamic polarisation curves to investigate the effect of Zn on the corrosion behaviour of as-extruded Mg–Zn binary alloy

The effects of Zn on corrosion behaviour of as-extruded Mg-(1-4)Zn alloys were investigated using an immersion test, a zero resistance ammeter technique, and a potentiodynamic polarisation test. As a result, it was revealed that the solutionised Zn enhanced protectiveness of the passive film, and accelerated the H₂ evolution rate of the Mg–Zn binary alloys. The acceleration of the H₂ evolution rate by addition of Zn leads to an increase in the net corrosion rate of the Mg–Zn alloy. In this research, the polarisation test was found to have some limitations for evaluating the true corrosion behaviour of passive Mg–Zn alloy.     

Simulation study on function mechanism of some precipitates in localized corrosion of Al alloys

The function mechanism of different types of aging precipitates in localized corrosion of Al alloys was studied. The function mechanism of the precipitates of θ (Al₂Cu) and η (MgZn₂) is validated. The precipitate of θ containing noble element Cu is cathodic to the alloy base, resulting in the anodic dissolution and corrosion of the alloy base at its adjacent periphery. The precipitate of η containing active element Mg is anodic to the alloy base, anodic dissolution and corrosion occur on its surface. Meanwhile, a localized corrosion mechanism conversion associated with the precipitate of T1 (Al₂CuLi) is advanced, which contains noble element Cu and active element Li simultaneously. The precipitate of T1 is anodic to the alloy base and corrosion occurs on its surface at the beginning. However, during its corrosion process, the preferential dissolution of Li and the enrichment of noble element Cu make its potential move to a positive direction. As a result, the corroded T1 precipitate becomes cathodic to the alloy base at a later stage, leading to the anodic dissolution and corrosion of the alloy base at its adjacent periphery.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.