Influence of heat treatment and surface conditioning on filiform corrosion of aluminium alloys AA3005 and AA5754

The effect of high-temperature heat treatment combined with different cleaning and pre-treatment practices on filiform corrosion resistance has been investigated for aluminium alloys AA3005 and AA5754. High-temperature heat treatment severely reduces the filiform corrosion resistance of alloy AA3005, while the corrosion properties of alloy 5754 are only moderately affected. The drastic loss of filiform corrosion resistance of alloy AA3005 after high-temperature heat treatment is attributed to preferential microstructural changes in a heavily deformed, micrograined surface layer caused by large surface shear strain during rolling. The enhanced deformation of the near-surface region promotes precipitation of a fine dispersion of intermetallic particles during subsequent heat treatment. The higher density of intermetallic particles combined with lower supersaturated solid solution levels of manganese in the surface layer as compared to the bulk structure makes the heat-treated AA3005 material susceptible to superficial corrosion attacks and results in poor filiform corrosion resistance. Application of a commercial acid cleaning/chromating pre-treatment did not restore the filiform corrosion resistance of heat-treated alloy AA3005. Alloy AA5754, containing lower levels of manganese and iron than AA3005, did not undergo similar preferential microstructural changes during heat treatment. A moderate increase in the amount of filiform corrosion of heat-treated AA5754 samples is attributed to poor protective properties of the thick, magnesium enriched, thermally formed surface oxide on this alloy. Any cleaning/pre-treatment practice that removes the thermally formed oxide on this alloy results in a very high filiform corrosion resistance.     

Filiform corrosion of AA3005 aluminium analogue model alloys

The effect of the metal substrate microstructure on filiform corrosion (FFC) susceptibility was investigated for super purity based model alloys with compositions based on the specifications of AA3005. Variations in alloying levels of the elements iron, silicon and copper were investigated. Alloys with high silicon content were more susceptible to FFC than alloys with low silicon content. The iron content, at the levels investigated, did not strongly affect FFC properties. The apparent detrimental effect of a high silicon content is attributed to the influence of silicon on secondary intermetallic particle precipitation. Given the same thermo-mechanical treatment, alloys with high silicon content underwent more extensive secondary precipitation of manganese containing intermetallic particles than those alloys with a low silicon content. The resulting microstructure is characterised by a higher density of finely dispersed intermetallic particles and a lower content of manganese in the adjacent supersaturated solid solution. These conditions provide a large number of potential corrosion initiation sites on the surface and also enhance microgalvanic coupling between intermetallic particles and the surrounding aluminium rich matrix, thus promoting the propagation of filamental corrosion attacks. Additions of copper had a detrimental effect on the FFC resistance. The role of copper in promoting FFC is attributed to preferential dissolution phenomena during the corrosion process, whereby copper is locally enriched on the corroding surface. This copper enrichment provides additional area for cathodic reaction, thus enhancing the corrosion process.     

Effect of heat treatment on electrochemical behaviour of aluminium alloy AA3005

The effect of heat treatment on the electrochemical properties of bare metal samples of aluminium alloy 3005 in an acidified sodium chloride solution was investigated. Annealing at increasing temperatures in the range 150-500°C causes negative potential transients of increasing magnitude and longevity for samples exposed at open circuit. Electrochemical characterisation shows the potential transients to be caused by an increased anodic reaction rate of the annealed material surfaces as compared to the cold rolled, hard temper substrate. It is suggested that this increased anodic activity during short time exposure of the bare metal samples may be attributed to changes in the protective properties of the surface oxide as a result of the high temperature heat treatment. The mechanisms controlling short term electrochemical properties of bare metal samples may not be identical to those controlling the filiform corrosion susceptibility of coated products and a cautious approach is recommended when using electrochemical techniques for predicting corrosion properties.     

Effect of trace elements on electrochemical properties and corrosion of aluminium alloy AA3102

The purpose of this work is to study the effect of heat treatment and chemical processing on the electrochemical behaviour of aluminium alloy AA3102. Aluminium alloy 3102 was electrochemically activated in chloride solution as a result of heat treatment for periods exceeding 10 min at temperatures higher than 400 °C. The electrochemical activation was determined by the presence of deep negative potential transients when exposed to an acidified chloride solution. Furthermore, the anodic current densities became large at a given potential relative to the as-extruded surface as a result of high temperature heat treatment. This activation phenomenon was attributed to enrichment of the surface by lead, which was present in the material as a trace element. Enrichment of lead at the metal-oxide interface was ascertained by GD-OES depth profiling. Chemical and structural changes occurring in the oxide as a result of heat treatment did not have a direct role in the activation process. It was also shown that enrichment of the surface by lead had a sacrificial effect in protecting the surface against pitting corrosion.     

A morphological study of filiform corrosive attack on cerated AA2024-T351 aluminium alloy

SEM and EDS studies were carried out to characterise filiform attack on a cerated AA2024-T351 aluminium alloy with a polyurethane topcoat. The filiforms developed on AA2024-T351 were sectioned, stripped of corrosion product and etched to reveal the grain structure. Examination of sections through the filaments and the filaments themselves, revealed severe local attack in the form of pitting resulting in grain etch out, grain boundary attack and subsurface etch out. Chloride ions were detected deep within pits and the subsurface etch out. The observations were similar to those found with filiform corrosion on chromated and coated surfaces. The observations led to development of a filiform corrosion model naming the volume expansion of the corrosion product as the principal cause for delamination.     

Galvanic corrosion of laser weldments of AA6061 aluminium alloy

Galvanic corrosion of laser welded AA6061 aluminium alloy, arising from the varying rest potentials of the various weldment regions, was examined. The weld fusion zone is found to be the most cathodic region of the weldment while the base material is the most anodic region. The rate of galvanic corrosion, controlled by the cathodic process at the weld fusion zone, increases with time until a steady state maximum is reached. On galvanic corrosion the corrosion potential of the weld fusion zone shifts in the positive direction and the free corrosion current increases. It is proposed that the cathodic process at the weld fusion zone causes a local increase in pH that in turn causes dissolution of the surface film resulting in the loss of Al to solution and the increase of intermetallic phases. The increase in galvanic corrosion may result from either the build up of the intermetallic phases in the surface layer and/or significant increase in surface area of the weld fusion zone due to the porous nature of the surface layer.     

The influence of surface treatment on filiform corrosion resistance of painted aluminium alloy sheet

Filiform corrosion of AA 5005 H14 aluminium alloy sheet has been investigated. Painted and scribed panels, with different surface treatments, were inoculated in HCl and exposed in a constant humidity cabinet maintained at 40 °C and 75-85% RH for 1000 h. After exposure, the panels were examined by optical and electron microscopy. It is evident that filiform corrosion susceptibility is determined largely by the near-surface microstructure. Heavily deformed layers, comprising oxide-decorated fine grains and dispersoids on as-rolled and mechanically ground alloy surfaces, are readily susceptible to filiform corrosion. Removal of these deformed layers, by caustic etching and acid cleaning, results in a high filiform corrosion resistance. On such surfaces, underfilm corrosion progresses by localized corrosion of the substrate; with comparatively slowly growing filaments propagating by repeated blistering of the overlying lacquer.     

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.     

Precipitation in an AA6111 aluminium alloy and cosmetic corrosion

The near-surface deformed layer on AA6111 automotive closure sheet alloy, generated by mechanical grinding during rectification, has an ultrafine grain microstructure, of 50–150 nm diameter, and a sharp transition with the underlying bulk alloy microstructure. Grinding and heat treatment to simulate rectification and paint baking processes result in the nucleation and growth of ∼20 nm diameter precipitates at grain boundaries within the near-surface deformed layer. High-resolution transmission electron microscopy has shown Q phase precipitates in the deformed layer, giving dramatically increased corrosion susceptibility compared with the bulk microstructure, and this is responsible for the rapid-onset filiform corrosion. Transmission electron microscopy of the corrosion attack showed directly that the mode of corrosion was intergranular and that the Q phase precipitates were preserved after the passage of the corrosion front.     

A morphological study of filiform corrosive attack on chromated and alkaline-cleaned AA2024-T351 aluminium alloy

In order to characterise filiform corrosion on a commercial AA2024-T351 aluminium alloy, a detailed microscopical study using SEM and EDS was performed. One set of AA2024-T351 aluminium alloy samples was alkaline-cleaned and deoxidised and chromate conversion coated. Another set was alkaline-cleaned only. Both samples were similarly spray coated with a 42 μm clear polyurethane topcoat. Filaments were subjected to a range of specimen preparation techniques. Sections and top views examined by SEM revealed varying degrees of attack ranging from generalised etching without local attack to severe local attack in the form of pitting, resulting in grain etchout, grain boundary attack and subsurface etchout. EDS revealed the presence of chloride deep into the pits and the subsurface etchout.     

Crystallographic defects induced localised corrosion in AA2099-T8 aluminium alloy

Abstract

A correlation has been revealed between the localised corrosion susceptibility and the grain stored energy in AA2099-T8 aluminium alloy, with grains of higher stored energy being relatively more susceptible to corrosion attack. The grains of high stored energy contain increased crystallographic defects, which could consequently decrease their corrosion resistance. Additionally, since crystallographic defects are preferential sites for the precipitation of T₁ (Al₂CuLi) phase, it is suggested that grains of high stored energy could also contain increased volume fraction of T₁ phase after aging, which might further increase corrosion susceptibility due to the electrochemically high activity of lithium rich T₁ phase.     

Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy

The 1,2,4-triazole, 3-amino-1,2,4-triazole, benzotriazole and 2-mercaptobenzothiazole were evaluated in the present work as corrosion inhibitors for protection of the 2024 aluminium alloy in neutral chloride solutions. The corrosion protection performance was investigated by means of DC polarization and electrochemical impedance spectroscopy (EIS). Scanning Kelvin probe force microscopy (SKPFM) and atomic force microscopy (AFM) were used to study the evolution of the Volta potential distribution and the surface topography during corrosion tests.The results show that all inhibitors under study confer corrosion protection to the AA2024 alloy forming a thin organic layer on the substrate surface. Benzotriazole and 2-mercaptobenzothiazole offer better corrosion protection in comparison with the other two. The inhibitors studied act decreasing the rate of both the anodic and cathodic processes. In the latter case the dealloying of the copper-reach particles is hindered, slowing down the oxygen reduction.     

The role of silicon in the corrosion of AA6061 aluminium alloy laser weldments

The galvanic corrosion temporal increase observed on examination of the weld fusion zone (WFZ) of AA6061 laser weldments in 3.5 wt.% NaCl solution cannot be attributed to electron tunnelling as the surface oxide layer is too thick, or the presence of Cl⁻ within the surface layer as this element was not found to be present. Aluminium alloy and WFZ galvanic and surface analyses indicate that the cathodic WFZ corrosion characteristics are due to increases in silicate concentrations in the surface oxide layer, leading to increased ionic and/or p-type semi-conductor conductivity, intermetallic concentrations and surface area.     

Pitting corrosion of A357 aluminium alloy obtained by semisolid processing

This paper studies the pitting corrosion of a structural component of A357 aluminium alloy obtained by a semisolid metal forming process. The mechanical properties of the A357 alloy were improved by applying standard heat treatments T5 and T6. Impedance measurements were conducted at the rest potential and polarisation curves were plotted using a 3% NaCl test solution. After polarisation experiments the specimens were analysed by scanning electron microscopy. The corrosion process is favoured through the eutectic regions. The results show that T6 heat treatment improved the corrosion resistance of the A357 aluminium alloy.     

Microstructural characterization and corrosion behavior of multipass friction stir processed AA2219 aluminium alloy

Multipass friction stir processing of AA 2219-T87 aluminium alloy to a depth of 2 mm in a 5 mm plate resulted in fine α-Al grains, reduction and dissolution of both eutectic phase (CuAl₂) and the strengthening precipitates (CuAl₂). Anodic polarization and electrochemical impedance tests in 3.5% NaCl showed an improved corrosion resistance of the processed alloy, which increased with the number of passes. Salt spray and immersion tests also showed improved resistance to corrosion. The increased resistance to corrosion is attributed to the dissolution of CuAl₂ particles, which was established by XRD and DSC studies.     

Vanadate post-treatments of anodised aluminium and AA 2024 T3 alloy for corrosion protection

Porous anodic films formed on aluminium in sulphuric acid were post-treated using various double-dip processes that incorporate a sparingly-soluble, vanadate, corrosion inhibitor. The processes were optimised by investigating the effects of the double-dip order, the rinsing step, the immersion time and the double-dip method. The optimised processes were applied to AA 2024-T3 alloy, which was then exposed to salt spray. V-Ce(III) and V-Zn processes provided corrosion protection similar to that of sealed, sulphuric acid-formed films. A V-Ni process provided increased protection, similar to that of hot water sealed films formed in chromic acid.     

Thermal data processing to characterise quasi-unidirectional solidification of cast aluminium alloy AA 354

Abstract

In this work the authors show how to build a semi-industrial scale macrothermal analysis experimental apparatus for low pressure aluminium casting AA 354 with quasi-unidirectional solidification. Several thermocouples were connected to a multichannel electronic device allowing a sampling rate up to 10 Hz; the thermocouples were installed in the mould at different locations to acquire the discontinuous cooling curves at those same locations.

With this type of experiment and appropriate mathematical procedures it was possible to build a reasonable response surface T?=?f(x, t) and the respective derivatives: ?T/?t and ?T/?x. Exponential polynomials were applied for modelling the curves and linear interpolation to relate the several cooling curves. Mathematical tools applied to the modelled curves allowed the authors to identify different solidification events and correlate them with the specific thermal gradient, cooling conditions and solidification fronts phenomena such as columnar to equiaxed transition of α aluminium grains. Growth velocity of proeutectic aluminium α and eutectic fronts were also determined.     

Electroplastic effect on AA1050 aluminium alloy formability

The electroplastic effect on AA1050 aluminium alloy sheets was investigated with the aim of decoupling the electroplastic and thermal behaviour due to Joule effect in electrical-assisted forming. Different stress triaxiality conditions, obtained using smooth, notched and single shear specimens, were tested applying a range of Direct Current densities to demonstrate that the material formability can be enhanced just thanks to the electroplastic effect. The obtained results were supported by TEM analysis of the dislocation density.     

Influence of the microstructure and laser shock processing (LSP) on the corrosion behaviour of the AA2050-T8 aluminium alloy

The corrosion behaviour of AA2050-T8 was studied after polishing and after laser shock processing (LSP) treatment using the electrochemical microcell technique and the SVET. After polishing, pitting at constituent particles and intergranular corrosion were observed. By contrast, no intergranular corrosion developed after LSP. Microcell measurements revealed that LSP increases the pitting potential. SVET measurements revealed that local anodic currents are systematically lower on LSP-treated surfaces than on polished ones. The current density on the LSP-treated surface remains constant around 50 μA cm⁻² up to 123 min after immersion, while on the polished surface it reaches 200 μA cm⁻².     

Influence of magnesium nitrate on the corrosion performance of sol-gel coated AA2024-T3 aluminium alloy

Traditional anticorrosion technology has relied heavily on using reducible metal species, predominantly hexavalent chromium (Cr(VI)), for protecting reactive metal alloys such as aluminium which is extensively used in the aerospace sector. However, the impending changes in the use of Cr(VI) in Europe and the United States have forced aerospace manufacturers to examine alternative materials for protecting aluminium. One of the most promising alternatives being investigated are organosilane based sol-gels containing anticorrosion additives. In this work the anticorrosion properties of magnesium (II) nitrate (Mg(NO₃)₂) as an inhibitor were investigated at different concentrations (0.1%-1.0 wt.%) in a methyltrimethoxysilane (MTEOS) sol-gel on the aluminium alloy AA 2024-T3 and compared to Alodine^TM 1200 (the established Cr(VI) pre-treatment). Electrochemical evaluation of the coating system by electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) measurements correlated strongly with results obtained from Neutral Salt Spray (NSS) exposure data. The surface morphology of the coating was studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM) and Energy Dispersive X-ray Spectrometry (EDS). The results indicated that the optimum performance was achieved with a Mg (NO₃)₂ level of 0.7% w/w. It is proposed that the superior anticorrosion properties of the Mg²⁺ rich sol-gel are due to the pore blocking mechanism of insoluble magnesium precipitates formed during the hydrolysis process.