Effect of iron-containing intermetallic particles on the corrosion behaviour of aluminium

The effect of heat treatment on the corrosion behaviour of binary Al-Fe alloys containing iron at levels between 0.04 and 0.42 wt.% was investigated by electrochemical measurements in both acidic and alkaline chloride solutions. Comparing solution heat-treated and quenched materials with samples that had been subsequently annealed to promote precipitation of Al₃Fe intermetallic particles, it was found that annealing increases both the cathodic and anodic reactivity. The increased cathodic reactivity is believed to be directly related to the increased available surface area of the iron-containing intermetallic particles acting as preferential sites for oxygen reduction and hydrogen evolution. These particles also act as pit initiation sites. Heat treatment also causes depletion in the solute content of the matrix, increasing its anodic reactivity. When breakdown occurs, crystallographic pits are formed with {1 0 0} facets, and are observed to contain numerous intermetallic particles. Fine facetted filaments also radiate out from the periphery of pits. The results demonstrate that the corrosion of aluminium is thus influenced by the presence of low levels of iron, which is one of the main impurities, and its electrochemical behaviour can be controlled by heat treatment.     

Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al₂CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al₂CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current-potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys.     

Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution

This work is focused on the role of intermetallics in pitting corrosion of Al2219 alloy. Second phase particles were characterized by AES, SAM and EDX. Their behaviour in a solution of NaCl was investigated as a function of exposure time. The results confirmed the cathodic nature of the intermetallics with respect to the aluminium matrix. Corrosion products rich in aluminium and oxygen were found to progressively accumulate around the particles and iron was dissolved from the intermetallic, followed by back-deposition. Copper and manganese did not show any major activity. After 32 h of exposure the larger intermetallics were completely covered.     

Bi-modal trend in the long-term corrosion of aluminium alloys

A wide range of literature data including in situ immersion, tidal, coastal atmospheric and industrial exposures is used to show that the trend for longer term corrosion of aluminium alloys is nearly always more consistent with a bi-modal model than with the classical power-law function. It is proposed the bi-modal characteristic results from the accumulation of corrosion products causing localised anoxic conditions. These permit a change from predominantly cathodic oxygen reduction to hydrogen ion reduction under anoxic autocatalytic conditions within pits. This mechanism is consistent with established theory for pitting corrosion in aluminium.     

The study of intergranular corrosion in aircraft aluminium alloys using X-ray tomography

Atmospheric corrosion is one of the leading causes of structural damage to aircraft. Of particular importance is pitting and intergranular corrosion, which can develop into fatigue cracks, stress corrosion cracks, or exfoliation. Therefore it is of interest to the Australian Defence Force (ADF) to understand how corrosion ensues in susceptible aircraft aluminium alloys, such as AA2024-T351 and 7050-T7451. However, there are many difficulties in measuring the extent of intergranular corrosion, since it is predominantly hidden below the surface. Traditionally, cross-sectioning has been used to view and measure the depth of attack. In the present work, 2 mm diameter pin specimens were contaminated with a droplet of 3.5% NaCl and exposed to constant humidity that resulted in intergranular corrosion. X-ray computed tomography was then used to non-destructively assess the depth and volume of corrosion both as a function of time in 97% relative humidity, and as a function of relative humidity after 168 h exposure. Both corrosion depth and volume increased with time, but there was evidence for a limiting depth in AA2024. Depth and volume also increased with relative humidity of the environment, for which the time-of-wetness and oxygen concentration of the droplets were considered the important factors in driving the corrosion process.     

Electrochemical characterization and CFD simulation of flow-assisted corrosion of aluminum alloy in ethylene glycol-water solution

An impingement jet system was used to study flow-assisted corrosion (FAC) of 3003 aluminum (Al) alloy in ethylene glycol-water solutions that simulates the automotive coolant by corrosion potential and electrochemical impedance spectroscopy (EIS) measurements as well as computational fluid dynamics (CFD) simulation. The effects of solution pH and fluid impact angle on Al FAC were determined. An increase of solution pH enhances the activity of Al due to dissolution of Al oxide film in alkaline environment. Moreover, Al activity decreases with the increasing fluid impact angle to the specimen. A CFD simulation shows that, with the increase of impact angle, the electrode area under high-velocity flow field decreases and that under low-velocity flow field increases. Consequently, the shear stress generated on electrode surface and the resultant mechanical effect on electrode activity decreases. Therefore, the electrode is more stable than that impacted at a smaller impact angle. There is an essential role of fluid hydrodynamics in corrosion of Al electrode, which is confirmed by corrosion potential and EIS measurements as well as CFD analysis.     

Applicability of constant dew point corrosion tests for evaluating atmospheric corrosion of aluminium alloys

Field exposure tests of aluminium alloys were conducted at three sites in Japan. Meteorological data indicated that the dew point of the ambient atmosphere and aluminium panels remained constant for the short-term. Constant dew point corrosion tests were employed to reproduce atmospheric corrosion of aluminium alloys in the laboratory. The corrosion rates, corrosion morphology and corrosion product composition after 7 days of tests in the laboratory were similar to those formed after 3 months of exposure at coastal sites. Not only did the constant dew point corrosion test effectively reproduce the atmospheric corrosion of aluminium alloys, it also accelerated it.     

Influence of cerium molybdate containers on the corrosion performance of epoxy coated aluminium alloys 2024-T3

Cerium molybdate containers loaded with 2-mercaptobenzothiazole were incorporated into epoxy coatings onto aluminium alloys 2024-T3 and investigated with respect to the corrosion protection of the metallic surfaces. The coatings were deposited via the dip-coating process. The morphology of the coatings was examined by Scanning Electron Microscopy. Their composition and structure were investigated by Fourier Transform Infrared Spectroscopy and Energy Dispersive X-ray Analysis. The corrosion resistance of these coatings was investigated by using electrochemical impedance spectroscopy and open circuit potential. After exposure to 0.05 M NaCl solution for 28 days, the coatings with the loaded containers exhibit improved corrosion performance.     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

The corrosion of two aluminium sacrificial anode alloys in SRB-containing sea mud

Corrosion of two sacrificial anodes in marine sediment with and without sulphate-reducing bacteria (SRB) was performed. Weight loss experiments indicated that the corrosion rate of Al–Zn–In–Sn was 2–3 times higher than that of Al–Zn–In–Mg–Ti in the SRB-containing sediment. Electrochemical analysis suggested that the corrosion rates of the two anodes were enhanced substantially by SRB. Surface analysis revealed the localised corrosion of the two sacrificial anodes in abiotic and biotic sediments. The concentration of Al³⁺ in the surface of the samples immersed with SRB was lower than that of the samples without SRB.     

The role of chelating agents on the corrosion mechanisms of aluminium in alkaline aqueous solutions

The influence of 1,2-diaminoethane (DAE) on the mechanism of aluminium corrosion in KOH solutions at pH 13 was investigated by combining time-resolved inductively coupled plasma optical emission spectrocopy, open-circuit potential measurements and X-ray photoelectron spectroscopy. In pure KOH solutions, a very slow corrosion rate is initially observed, corresponding to the dissolution of the native oxide layer. Following this incubation stage, the corrosion rate is increasing due to the formation and oxidation of Al hydride, until a steady state is reached. DAE behaves as a strong initial corrosion accelerator, due to synergistic effects with hydroxyl ions and a dissolution mechanism in three successive steps has been proposed: (i) a rapid initial dissolution induced by the formation and detachment from the surface of bidentate (chelate) Al-DAE metal bound surface complexes; (ii) a slower step ascribed to the formation and release of monodentate Al-DAE metal bound surface complexes and (iii) a final step dominated by direct oxidation of surface aluminium hydride by hydroxyl species as in pure KOH.     

In situ X-ray tomography of intergranular corrosion of 2024 and 7050 aluminium alloys

In situ synchrotron X-ray tomography was used to monitor the development of corrosion in service-like conditions. Deliquescence of sodium chloride contaminants formed a droplet environment that caused intergranular corrosion in 2024-T351 and 7050-T7451 alloys. For the 2024 alloy, corrosion grew at comparable rates in the longitudinal and long-transverse directions. Initiation occurred at an intermetallic particle, which was surrounded by a cluster of four other particles within a 20 μm radius. For the 7050 alloy, corrosion grew fastest in the long-transverse direction, intermediate in the longitudinal direction, and slowest in the short-transverse direction. Corrosion growth of one fissure was constrained due to corrosion growth in another fissure, which may indicate that the two fissures were competing for current in the cathodically limited conditions.     

Electrochemical aspects of exfoliation corrosion of aluminium alloys: The effects of heat treatment

Electrochemical approaches are used to investigate the exfoliation corrosion (EFC) of a 7XXX series aluminium alloy that has undergone different tempering treatments. EFC was produced under an artificial crevice at open circuit potential in neutral chloride solutions, and is found to be associated to current and potential transients. EFC was also produced under galvanostatic control conditions. Observations made through Scanning Electron Microscopy (SEM) suggest that these transients result from the progression of inter-granular cracks. Last, over-ageing heat treatments that are known to decrease both metal hardness and EFC sensitivity were found to decrease the number of transients.     

Electrochemical corrosion parameters of as-cast Al-Fe alloys in a NaCl solution

The aim of this study is to evaluate the general electrochemical corrosion resistance of Al-Fe alloys in the range of hypoeutectic compositions, Al-0.5 wt.% Fe and Al-1.5 wt.% Fe alloy. EIS plots, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters in a 0.5 M NaCl solution at 25 °C. It is shown that for an Al-0.5 wt.% Fe alloy, coarse cells tend to improve the corrosion resistance mainly due to the reduction in cellular boundaries and for an Al-1.5 wt.% Fe alloy, an opposite trend has been detected.     

The influence of some triphenylmethane compounds on the corrosion inhibition of aluminium

Potentiostatic and potentiodynamic polarization, and scanning electron microscopy were used to investigate the effect of some basic and acid triphenylmethane derivatives, on the anodizing of pure aluminium in neutral ammonium borate electrolyte containing traces of chloride ions. The absence of sulfonic groups as substituents in these compounds restricts their action. The presence and position of sulfonic groups as substituents in their molecules influence the extent to which the molecular structures of these compounds approach coplanarity and their electronic configuration. As a result, a differentiation of their influence on the field assisted adsorption of chloride ions from solution on the hydrated oxide surface, and on the subsequent localized autocatalytic dissolution of the barrier oxide film by them is observed.     

Inhibition by tetradecylpyridinium bromide of the corrosion of aluminium in hydrochloric acid solution

The inhibition effect of tetradecylpyridinium bromide (TDPB) on the corrosion of aluminium in 1.0 M HCl solution was studied by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) methods. The results show that TDPB is a very good inhibitor at little concentrations, and the adsorption of TDPB on aluminium surface obeys Langmuir adsorption isotherm. Polarization curves reveal that TDPB behaves as a cathodic inhibitor. EIS spectra consist of large capacitive loop at high frequencies followed by a large inductive one at low frequency values, and confirm the inhibitive ability.     

Improving the corrosion protection of aluminium alloys using reactive magnetron sputtering

Most high strength aluminium alloys used in the aircraft industries are susceptible to corrosion. Up to now hexavalent chromium is the conventional corrosion inhibitor. Because chromium in hexavalent state is carcinogenic, it is necessary to develop effective alternative inhibitor systems. We investigated magnetron sputtered substoichiometric and stoichiometric aluminium nitride (AlN_x with x ? 1) coatings for corrosion protection of aluminium alloys 2024-T3, 6061-T4 and 7075-T6. The corrosion behaviour of the treated surfaces was tested by anodic polarization scanning and salt spray testing.From the polarization curves it can be concluded that magnetron sputter coating with AlN_x leads to a higher pitting potential of the three aluminium alloys investigated. The salt spray tested samples also confirm the protective effect of the coatings. In addition we found that AlN_x layers with high nitrogen content lead to a stronger shifting in pitting potential than those with low nitrogen content. Anyway, the results of the salt spray testing show that particularly nitrogen-rich AlN_x layers are less stable towards NaCl electrolyte.     

Effect of solution treatment on the corrosion behaviour of aluminium alloy AA7150: Optimisation for corrosion resistance

Studies for Al-alloys normally employ specimens subject to a fixed solution heat treatment (SHT), and hence the specific role of solutionising is overlooked. It is revealed that SHT has a major role in corrosion of AA7150 as judged by electrochemical, microscopic and profilometry studies. SHT dictates the constituent particle type and population remaining in the alloy for subsequent processing, and is thus not only a principal factor in corrosion, but a factor that can be engineered in developing more damage tolerant AA7150. Effect of an optimised SHT minimising the residual MgZn₂ and Al₂MgCu content on corrosion resistance is demonstrated.     

Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005 wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12 wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al₄Mg₈Si₇Cu₂ (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).     

In situ monitoring of the microstructural corrosion mechanisms of zinc–magnesium–aluminium alloys using time lapse microscopy

A novel technique has been developed that enables in situ monitoring of the microstructural wet corrosion mechanisms of zinc–(1–2 wt.% magnesium)–(1–2 wt.% aluminium) galvanising alloys using time lapse optical microscopy. The technique enabled the imaging of the progression of anodic attack, the development of corrosion product rings radially to the anode and pH gradients between anodes and cathodes using an indicator. It was found that corrosion initiated in the binary and ternary eutectic regions within the microstructure of the alloy with preferential de-alloying of MgZn₂ lamellae. After eutectic dissolution, anodic attack proceeded on the primary zinc rich dendrites.