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.     

Anodising as pre-treatment before organic coating of extruded and cast aluminium alloys

Anodising has been investigated as a replacement for chromating as pre-treatment prior to organic coating of aluminium. Both AC and DC anodising gave filiform corrosion properties that were comparable to chromated samples. A correlation between filiform corrosion performance and cathodic properties of converted surface was found. Chromating and hot AC anodising gave significant reduction in cathodic current density. The reduction is due to chromium oxide covering intermetallic particles after chromating, while iron rich intermetallics normally acting as cathodic sites were removed by hot AC anodising.     

Combined anodic/cathodic transient currents within nucleating pits on Al–Fe alloy surfaces

Pit nucleation and metastable growth on microelectrodes of aluminium and some Al alloys in chloride solution below the pitting potential are described by recurring current transients. Pit nucleation is a sharp, violent event and is observed repeatedly on all materials tested: pure aluminium, titanium and iron binary aluminium alloys, and alloy 1050A. At the imposed electrode potential, current transients due to pit nucleation recorded for pure aluminium and a solid solution titanium binary alloy are exclusively anodic. Those from alloys containing iron intermetallics sometimes exhibit both anodic and cathodic wings. The cathodic wing is due to transient hydrogen evolution at sites which border iron-rich intermetallic particles: these are preferred pit nucleation sites.     

Influence of the surface activation and local pitting susceptibility on the AC-electrograining of aluminium alloys

AC electrograining of aluminium is strongly influenced by the surface microstructure. The mechanical and electrochemical properties of the sub-surface present in aluminium alloys affect the electrochemical reactions that prevail during electrograining. Etching pre-treatment of aluminium removes intermetallics and rolled-in oxides; as a result, the attack on the aluminium substrate starts with the initial cycles of the electrograining process. Local electrochemical investigations show differences in corrosion and passivation properties between rolled-in oxides and clean surfaces. The interface between rolled oxides and aluminium matrix acts as a weak point for pit initiation.     

Pitting corrosion of rheocast A356 aluminium alloy in 3.5 wt.% NaCl solution

In this study, the microstructure and corrosion behaviour of rheocast and gravity-cast A356 aluminium alloys were examined and compared. Scanning Kelvin probe force microscopy (SKPFM) results proved that large potential differences between iron-containing intermetallics and the α-Al matrix were responsible for the initiation of the attack at the intermetallics/α-Al interfaces. For longer immersion times, corrosion attack proceeded through the eutectic areas. Semisolid processing refined the eutectic silicon and iron-intermetallics and reduced the potential difference between secondary phases and the matrix. This resulted in improved pitting corrosion resistance of the rheocast A356 aluminium alloy.     

Adsorption of sodium dodecyl sulfate and sodium dodecyl phosphate at the surface of aluminium oxide studied with AFM

Within the framework of water-based aluminium pigments the difference in inhibition offered by the adsorption of sodium dodecyl sulfate (SDS) and sodium dodecyl phosphate (SDP) was studied by atomic force microscopy (AFM). While SDS did not show strong adsorption on aluminium oxide, SDP adsorbed strongly forming bilayers which patches did not change noteworthy over several hours of SDP exposure, indicating irreversible binding to the surface. Quantization of the surface coverage of the adsorbed SDP revealed that only 63% coverage was achieved, which shows surprisingly that a complete coverage is not necessary to inhibit oxidation of aluminium pigment particles.     

Corrosion properties of laser beam joints of aluminium with zinc-coated steel

This paper presents results of accelerated corrosion tests in a salt spray chamber as well as microelectrochemical measurements of thermally joint steel-aluminium mixed materials. The focus was set on analysing the corrosion behaviour of the different metallic materials (brazed seam, intermetallic phases, aluminium and steel sheet) in or within the vicinity of the brazed seam.Both corrosion tests show that the joining zone has the most negative corrosion potential and is the first to corrode. The degree of corrosive deterioration depends on the cathodic behaviour of the adjacent metal. Next to effective cathodes, such as steel or Fe-containing intermetallics, the attack is the most. However, contact to an aluminium alloy (AA6016) with its insulating oxide layer does not affect the corrosion properties of the respective filler materials.     

Galvanic coupling between copper and aluminium in a thin-layer cell

The Al/Cu coupling was investigated in a thin-layer cell formed by a large Cu electrode and an Al microelectrode embedded in an insulator placed above the Cu electrode. By using a scanning electrochemical microscope (SECM) the thickness of the thin layer was perfectly controlled with a precision in the micrometer range. A copper deposit on an electrochemical quartz crystal microbalance (EQCM) was also used as SECM substrate to quantify the copper dissolution rate. It was shown that such an experimental set-up allows to mimic the galvanic corrosion of intermetallic particles embedded in the aluminium matrix of the 2XXX series aluminium alloys. The combination of the SECM and the EQCM permitted the evaluation of the corrosion rate of copper at the corrosion potential of the 2024 Al alloy, whereas cyclic voltammetry performed on the SECM tip indicated the enrichment in Cu²⁺ ions in the thin electrolyte layer.     

In-situ investigation on the pitting corrosion behavior of friction stir welded joint of AA2024-T3 aluminium alloy

The surface corrosion behavior of an AA2024-T3 aluminium alloy sheet after friction stir welding was investigated by using an “in-situ observation” method. SEM observations showed that the density and degree of the pitting corrosion in the shoulder active zone were slightly larger compared to the other regions on the top surface. The origins of the pitting corrosion were in the regions between the S phase particles and the adjacent aluminium base. The effect of Al-Cu-Fe-Mn-(Si) intermetallic compounds on the pitting corrosion was attributed to their high self-corrosion potential which induced the anodic dissolution of the surrounding aluminium matrix.     

Corrosion behaviour of Al/Al3Ti and Al/Al3Zr functionally graded materials produced by centrifugal solid-particle method: Influence of the intermetallics volume fraction

Intermetallic particles, Al₃Ti and Al₃Zr were formed in Al–5mass%Ti and Al–5mass%Zr alloys, respectively, by centrifugal casting, in order to create functionally graded materials (FGMs). At present, no information is available on the influence of the amount of intermetallics on the electrochemical properties of these alloys.In this paper, the corrosion resistance of Al/Al₃Ti and Al/Al₃Zr FGMs was investigated by open-circuit measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Results suggests that the corrosion resistance of the FGMs is affected by galvanic effects between the intermetallic particles and the metallic matrix. Lower centrifugal forces resulted in an improvement of the electrochemical properties.     

Using EIS to analyse samples of Al–Mg alloy AA5083 treated by thermal activation in cerium salt baths

This paper describes a study undertaken of the morphological and anticorrosive characteristics of surface layers formed on the Al–Mg alloy AA5083 from solutions of Ce(III), by means of various heat treatments while immersed in baths of cerium salts. SEM/EDS studies have demonstrated the existence of a heterogeneous layer formed by a film of aluminium oxide/hydroxide on the matrix and a series of dispersed islands of cerium deposited on the cathodic intermetallics. With the object of evaluating the degree of protection provided by the layers formed and of characterising the particular contribution of the electrochemical response of the system in NaCl, the results obtained by means of EIS are presented and discussed.     

FIB/SEM study of AA2024 corrosion under a seawater drop: Part I

The role of metallic microstructure in 0.5 μl seawater droplet corrosion of aluminium alloy 2024 (AA2024) has been investigated. Focussed ion beam/scanning electron microscopy (FIB/SEM) was used to determine the relationships between the corrosion products formed at specific sites at the surface and the underlying attack mechanisms. Dealloying of S-phase particles, matrix/particle interfacial attack and grain boundary attack were the predominant attack modes. Cooperative behaviour between IM particles was made possible by networks of etched grain boundaries, which provided a connecting path.     

The role of combustion synthesis in the formation of slurry aluminization

Slurry processes have been investigated for several years as an alternative technique to conventional CVD-derived aluminizing to achieve similar diffusion coatings. This study investigates the coating formation mechanisms during heat treatment processes on pure nickel using slurries, which contain high amounts of micro-sized aluminium particles. At temperatures in the range of 550 °C–1000 °C, aluminium diffuses into the nickel substrate, promoting the formation of intermetallic nickel–aluminide layers. In order to control this process, it is important to understand the mechanisms that occur in the initial stages, when the metallic aluminium powder melts and reacts in contact with nickel. While a conversion of closely pressed nickel–aluminium into aluminide by combustion synthesis is well known, DTA measurements were undertaken to investigate if and when such processes occur in loosely packed powders. Two compositions of nickel with aluminium or eutectic aluminium–silicon alloyed particles were used in order to reveal a potential influence of the melting point of the aluminium alloy particles. The influence of the atmosphere was studied by comparing results during exposure in argon and air. Subsequently, for comparison with the more complex mechanism of slurry aluminizing, both powders were applied to pure nickel substrate and the coating formation during heat treatment at 600 °C, 650 °C, and 700 °C was investigated. The results clearly show the importance of combustion synthesis on the formation of slurry coatings on nickel. Based on the observations, four steps were identified, which lead to the formation of aluminides and the subsequent growth of the aluminide layer: After melting of the aluminium powder, a network of molten aluminium forms within and between the particles, followed by dissolution of nickel in the aluminium melt. If enough Al is available, combustion synthesis between Ni and Al takes place. After this highly exothermic reaction, solid state diffusion controls the further formation of slurry coatings on nickel. Finally, the mechanism was verified by coating industrially used superalloys with the Al-based slurry in air and argon.     

Corrosion in artificial defects. I: Development of corrosion

Artificial defects (slots) were milled through a polyurethane topcoat and chromate-inhibited epoxy polyamide primer to the underlying aluminium alloy 2024-T3. The slots were then exposed to neutral salt spray (NSS) for up to 16 days. Prior to and after exposure, the slots were examined using scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDS) and Raman spectroscopy. The milling process generated features on the surface not seen on polished surfaces, including smearing of the matrix alloy and fragmentation of Cu-Fe-Mn-Al intermetallic particles. It was found that the smears and S-phase particles acted as sites for the initiation of corrosion, which eventually developed more generally across the surface.     

Effect of heat treatment on anodic activation of aluminium by trace element indium

The presence of trace elements in Group IIIA-VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000 ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1 h at 300 °C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1 h heat treatment at 600 °C, also activated by surface segregation of In on alloy containing 1000 ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20 ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.     

Emeraldine base as corrosion protective layer on aluminium alloy AA5182, effect of the surface microstructure

AA5182 aluminium alloy cold rolled samples were coated by thin films of emeraldine base (EB) obtained from a 5% solution in N-methylpyrrolidinone. Accelerated corrosion tests prove this coating very effective for corrosion protection of aluminium alloys in neutral environment. This study underlines the prominent role of surface cathodic intermetallic particles in pit initiation and coating break down in enhanced corrosion conditions and suggest that, beside the EB barrier properties, the enhanced corrosion resistance observed on the EB coated samples could partly arise from two other mains factors:

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a weak redox activity of the polymer which passivate the metal,

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a proton involving self-healing process taking place at the polymer-metal interface, which contributes to delay local acidification in first steps of corrosion on EB coated aluminium surfaces.

     

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.     

Self-propagating high-temperature synthesis of complex nitrides of intermetallics

In our previous papers, the Self-Propagating High-Temperature Synthesis (SHS) of aluminium and titanium nitrides was studied. In this paper the synthesis of the complex nitrides of intermetallics, based on titanium, aluminium and nickel, was carried out. The samples were produced by the direct SHS of nickel–aluminium and titanium–aluminium mixtures in a nitrogen atmosphere, as well as by the SHS synthesis of nickel–aluminium and titanium–aluminium intermetallics followed by pulsed plasma ion nitriding. Composition of products was studied by chemical analysis, as well as by X-ray diffraction and X-ray photoelectron spectroscopy. Experiments revealed that the highest extent of nitridation was achieved during direct one-stage SHS synthesis of the complex intermetallic nitrides.     

Formation and characterisation of a chromate conversion coating on AA6060 aluminium

AA6060-T6, an AlMg0.5Si0.4 model alloy and α-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150-200 nm, while significantly thinner films were formed on the α-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg₂Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium oxide film on the α-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.     

Corrosion of aluminium in copper-aluminium couples under a marine environment: Influence of polyaniline deposited onto copper

In this study, we examined how aluminium corrosion in Al-Cu/PANI galvanic couples in a marine environment is influenced by deposition of polyaniline (PANI) on copper. Polarization curves and immersion assays in 0.1 M NaCl were performed. The morphologies of etched Al and corrosion products were observed by SEM, and the Al ions in solution were quantified by atomic absorption spectroscopy. A reduction in aluminium damage due to galvanic corrosion was observed as a result of decreased effective area for the oxygen reduction reaction on Cu/PANI electrode. Furthermore, an electrochemical reduction of PANI from leucoemeraldine to emeraldine base is proposed.