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The influence of room temperature (RT) storage on the microstructure and intergranular corrosion (IGC) behaviour of AA6082-T4 alloy is investigated by the combination of electron microscopy, electrochemical measurement and immersion testing. After 10 months storage at RT, precipitation of β’ phase and segregation of Mg had occurred at high angle grain boundaries (HAGBs) in the alloy that was in T4 condition, leading to an increased IGC susceptibility, which is even higher than that for the alloy in T6 condition.

A series of nano PdFe alloy film electrodes with different ratios were prepared by high-vacuum magnetron sputtering under the action of high-strength magnetic field. Among them, the Pd3Fe alloy fil...

A combination of X-ray computed tomography (CT) and focused ion beam - scanning electron microscopy (FIB-SEM) has been employed to investigate substrate and related surface defects in a niobium coated superconducting radio frequency (SRF) copper cavity. The cavity was manufactured by spinning, with subsequent application of a sputtering-deposited niobium coating (≈40 μm thick) on the internal surface. Before coating, the copper surface was pre-treated in several stages, ending with chemical polishing. CT and FIB-SEM identified furrow defects (≈20 μm deep) in the copper beneath the coating, which originated from the spinning process. The furrows were filled with niobium and contained voids at the Nb/Cu interface that extended a few microns into the niobium coating. The presence of the defects led to similar furrows at the niobium surface. The study revealed the importance of pre-treatment of the cavity internal surface and control of the spinning process to avoid defects that may have deleterious influence on the Q slope and durability of the niobium coating.

Active anti-corrosion pigments, such as strontium chromate, are essential components of many corrosion protective organic coatings, since these leach out to provide active inhibition to the metallic substrate at the defective areas of the coatings arising during service from mechanical and/or environmental damages. Currently, formulators use empirical tests to determine effective inhibitor concentration, because the factors that determine leaching behaviour are poorly understood. In this study, we present insights into leaching mechanisms by correlating the microstructure of model coatings pigmented with strontium aluminium polyphosphate hydrate (SAPH) to the transport of different species. It is found that diffusion and transport of active species through the polymeric matrix does not significantly contribute to the leaching kinetics. Thus, leaching starts when inhibitor pigments are in direct contact with the environment via surface-breaking defects in the coating, and continues as long as the cluster of connected inhibitor pigments is in direct contact with the environment, until 3-dimensional connectivity is lost. Therefore, the extent, shape and size of the clusters of connected inhibitor pigments, as well as the solubility and dissolution rates of individual pigments, play important roles in the leaching process. Additionally, the 3-dimensional percolation threshold (Pc) for pigment connectivity is proposed as a critical parameter that has significant influence on the leaching rate as well as the barrier properties of corrosion protective coatings.

The presence of alloying elements in aluminium alloys has a significant impact on the anodizing behaviour and results in the formation of porous anodic films with different chemical composition and morphology compared with those generated on pure aluminium. In this work, the effect of alloy element enrichment at the alloy/film interface and of cell diameter on the incorporation and distribution of alloying element species in porous anodic films is considered. It is proposed that above a critical cell diameter, Dcrit, the critical alloy enrichment sufficient for oxidation of the alloying element and its incorporation into the film can be maintained across the alloy/film interface. Below Dcrit, only a sub-critical enrichment can be maintained and the alloying element is then incorporated into the film at the cell boundaries. Dcrit depends on the concentration of the alloying element in solid solution and on the critical enrichment. The proposed role of Dcrit is supported by alloying element distributions from literature data for model Al-Au and Al-W alloys and new results for anodic films on AA 2024-T3 alloy.

AuIr alloy nanoparticles were successfully prepared without using surfactants for the first time despite Au and Ir being immiscible according to phase diagrams. The lattice parameters of the AuIr alloy can be adjusted arbitrarily. The oxygen reduction reaction (ORR) activity of Au5Ir5 alloy is better than that of Au or Ir, and the oxygen evolution reaction (OER) activity of the Au5Ir5 alloy is as good as that of Ir in alkaline solution.

An Al–Cu–Li aerospace alloy has been investigated to determine the order in which corrosion at different types of sites occurs in AA2099-T83. Specifically, the sequence of galvanic attack on intermetallic (IM) particles and other sites of AA2099-T83 was determined as a function of time, in 0.1 M NaCl, through the use of scanning electron microscopy and electron backscatter diffraction characterization techniques. The earliest attack occurred at isolated grains and grain boundaries and on Li-containing dispersoids. Similarly, some constituent IM particles showed evidence of trenching in the surrounding alloy matrix. These IM particles included Al7Cu2Fe and another group of unidentified particles which displayed complete trenching within the first 10 min of exposure. Al13(Fe, Mn)4 were next most active followed by Al37Fe12Cu2 with Al6(Fe,Mn) and large TiB2 particles being the least active.

It is known that typical porous-type anodic films formed on aluminum and its alloys consist of two layers, i.e. a thick porous layer and thin barrier layer. However, the effect of the two layers on corrosion resistance of the anodic film is still not clear, which prevents a thorough understanding of the protection mechanism of the anodic film, thereby limiting the potential to further optimize the anodizing treatment. Herein, an electrochemical barrier layer thinning (EBLT) process was employed to reduce the thickness of the barrier layer of the porous-type anodic films formed on an AA2055 Al–Cu–Li alloy and pure aluminum. Then the structure and composition of the anodic films before and after immersion in a NaCl solution were studied in a comparison manner. It was found that the EBLT process evidently decreased the corrosion resistance of the anodic films formed on AA2055 alloy and pure aluminum. Thus, it was conclusively validated that the barrier layer of the porous-type anodic film played a decisive role in controlling the corrosion resistance of anodized aluminum and aluminum alloys prior to post sealing.

Joining of dissimilar light metals by friction stir welding (FSW) is of interest to reduce weight and fuel consumption in the transport sector. Such coupled metals may need protective surface treatments, e.g. against wear or corrosion, for some applications. In this work, the formation of plasma electrolytic oxidation (PEO) coatings in a silicate-based electrolyte for corrosion protection of FSW AZ31B magnesium alloy-titanium joints has been studied. The joints, if unprotected, may be susceptible to severe galvanic corrosion in chloride-containing environments. The coatings were characterized by scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. Mg2SiO4 and MgO were identified in the coating on the AZ31B alloy and rutile and anatase on the titanium. Immersion of the joints in 3.5 wt% sodium chloride solution for 24 h resulted in severe corrosion of the AZ31B alloy in an uncoated joint; in contrast, corrosion of the AZ31B alloy was localized following PEO owing to the barrier protection provided by the coating. Corrosion of the titanium was negligible. The severe corrosion of the unprotected AZ31B alloy led to precipitation of a large amount of Mg(OH)2 from the sodium chloride solution by the end of the test. The weight of precipitate was reduced by a factor of ≈8 by the application of the PEO coating.

In the present study, correlative corrosion testing and electron microscopy, including quasi-in-situ electron microscopy, are employed to investigate the corrosion behaviour of 2A97-T8 Al-Cu-Li alloy extrusion in 3.5 wt. % NaCl solution. It is found that the corrosion behaviour of the alloy is influenced by synergetic effect of the plastic deformation introduced by the thermomechanical processing during alloy fabrication and the precipitation of T1 phase during T8 treatment. Localized corrosion initiates and propagates preferentially within non-recrystallized regions of the partially recrystallized alloy. Corrosion initiated at coarse constituent intermetallic particles is confined to its periphery, which doesn’t develop further when the particles are disconnected from alloy matrix.

To understand Si precipitation and its influence on mechanical property improvement in high pressure die casting (HPDC) Al9Si3CuFe alloy under direct ageing heat treatment, the precipitates in Al9Si3CuFe alloy containing in-situ MgAl2O4 particles is analyzed via atomic-resolution scanning transmission electron microscopy. It is shown that lath-like Si precipitates form in an average size of 50 nm in length, and a good orientation relationship of {010}Al//{111}Si is observed between Si precipitate and matrix. Interestingly, Cu and Zn are observed to segregate onto Si/Al interface, which has a coarsening resisting effect on precipitate growth. The presence of these Si precipitates is the main factor contributing to the further improvement of mechanical properties of Al9Si3CuFe alloy, which results in an increase of 70 MPa in yield strength compared to the alloy without Si precipitates.

An epoxy amine-based seawater ballast tank coating was characterized after 18 years of exposure to service environment. Coating flake specimens were taken from a flat region of the ballast tank, which appeared intact and well attached to the steel substrate. The flakes were examined using analytical electron microscopy and Fourier transform infrared (FTIR) spectroscopy. Significant morphological changes in the pigments and binder in the near surface region were observed. These were related to the hygrothermal cycles due to filling and discharge of sea water in the ballast tank, accompanied by cyclic temperature variations, generating expansion and contraction stresses within the coating. The analyses also reveal for the first time differences in the damage to barrier pigments (i.e. talc and kaolin) compared to the degradation of aluminium flake metal pigment. The former were fractured internally while the latter showed failure at the pigment/binder interface. Since pigments play a significant role on the degradation of the coating, their orientation towards the surface was also found to be of high importance. Regions of coating with pigments orientated nearly vertically to the surface showed a more severe degradation than regions with pigments orientated nearly parallel to the surface.

Thick-section plates made from a recently developed Al–Cu–Mg–Li alloy have been evaluated to understand the influence of microstructure on the anisotropy of tensile strengths after natural and artificial ageing treatment. Pancake-shaped grains with a coarse substructure and strong crystallographic texture with a β-fibre orientation at the mid-thickness position are observed. In addition, an inhomogeneous distribution of T1 precipitates through the plate thickness has been revealed with the volume fraction of intragranular precipitates ∼40% higher at the plate centre than the ¼ thickness position. Altogether these microstructural features contribute to the in-plane anisotropy of tensile strengths that is ∼5% higher at the mid-thickness position than the ¼ thickness position. The variation of ageing-induced T1 precipitates through the plate thickness further contributes to the through-thickness anisotropy that is ∼3% higher in T8 temper as compared to T3 temper.

The effects of applied current density, anodizing time and electrolyte temperature on the cell and pore morphology of anodic films and the voltage-time response obtained during galvanostatic anodizing of AA2024-T3 alloy in sulphuric acid electrolytes have been studied. Scanning electron microscopy was employed to observe the film morphology. Sponge-like porous structure was promoted by anodizing at relatively low current density and high electrolyte temperature. In contrast, linear porous structure was favoured under the converse conditions. Intermediate conditions resulted in films containing either sequential layers of the two morphologies or a morphology incorporating features of the two types; such conditions were associated with anodizing voltages in the range 25 to 35 V. The reasons for the morphological differences are proposed to be due to interactions between film growth stresses and stresses arising from oxygen evolution on the development of the alumina cells.

In the present study, the impact of machining on the microstructure of AA7150-T651 aluminium alloy is investigated. It is found that a near-surface deformed layer is formed on the alloy due to severe shear strain during machining operation. The near-surface deformed layer of approximately 500 nm thickness distributes uniformly across the machined alloy surface. Ultrafine equiaxed grains with the dimensions of 50–100 nm are formed within the near-surface deformed layer. Further, the strengthening precipitates (MgZn2) that are initially present in the alloy in T651 temper are dissolved into the alloy matrix within the near-surface region. Subsequently, the dissolved alloying elements segregate at the grain boundaries of the newly formed fine grains in the near-surface deformed layer.

Electron and proton microprobes, along with electron backscatter diffraction (EBSD) analysis were used to study the microstructure of the contemporary Al–Cu–Li alloy AA2099-T8. In electron probe microanalysis, wavelength and energy dispersive X-ray spectrometry were used in parallel with soft X-ray emission spectroscopy (SXES) to characterize the microstructure of AA2099-T8. The electron microprobe was able to identify five unique compositions for constituent intermetallic (IM) particles containing combinations of Al, Cu, Fe, Mn, and Zn. A sixth IM type was found to be rich in Ti and B (suggesting TiB2), and a seventh IM type contained Si. EBSD patterns for the five constituent IM particles containing Al, Cu, Fe, Mn, and Zn indicated that they were isomorphous with four phases in the 2xxx series aluminium alloys including Al6(Fe, Mn), Al13(Fe, Mn)4 (two slightly different compositions), Al37Cu2Fe12 and Al7Cu2Fe. SXES revealed that Li was present in some constituent IM particles. Al SXES mapping revealed an Al-enriched (i.e., Cu, Li-depleted) zone in the grain boundary network. From the EBSD analysis, the kernel average misorientation map showed higher levels of localized misorientation in this region, suggesting greater deformation or stored energy. Proton-induced X-ray emission revealed banding of the TiB2 IM particles and Cu inter-band enrichment.

The initial phase separation and coarsening of the Cr-enriched α′ phase in Fe–Cr alloys were studied by utilizing phase field model. The phase separations of the α′ phase via non-classical nucleation and growth, the transition state from nucleation and growth to spinodal decomposition, and the spinodal decomposition were investigated at 700 K for the alloys containing 25, 28 and 35 at.% Cr. The time exponent of the average radius of α′ phase decreases from the growth and coarsening stage to the steady-state coarsening stage for the nucleation growth mechanism, the slope of number density of α′ phase increases at the steady-state coarsening stage with the increased Cr concentration. The velocity of phase separation in 25 at.% Cr alloy with a nucleation growth mechanism are slower than that in high Cr alloys with a spinodal decomposition mechanism. The results indicate that a low concentration alloy is more stable at high temperature, and the phase separation is fast at the early stage, which supply a reference for the alloy composition selection, and for the analysis of the relationship of property and morphology.

A quantitative investigation of phase decomposition in Fe-35 at.% Cr alloy aged at 773 K was performed by using atom probe tomography (APT) and phase-field simulation. The kinetics of the α′ phase formation via spinodal decomposition was studied by the variation of volume fraction, average particle radius and particle size distribution of α′ phase at 700, 725 and 750 K. The simulated morphology and composition are in good agreement with the APT and transmission electron micrograph (TEM) results, demonstrating that the thermodynamic parameters used in the simulation were optimized. The growth and coarsening of the α′ phase were promoted by the increasing aging temperature. Also, the coarsening rate increases and induces a larger slope for the number density of particles at the steady-state coarsening stage. The time exponent of the average radius of the α′ phase shows a three-stages variation from about 0.31 to 0.20 and larger than 0.33 for the initial phase decomposition, growth and coarsening, and the steady-state coarsening stages, respectively. The particle size distribution of the α′ phase is similar with the Brailsford-Wynblatt's (BW) predication at early stage of phase decomposition, and becomes lower than BW's value at coarsening stage. The kinetics of nanoscale α′ phase from initial decomposition to coarsening is useful for the composition design and properties prediction of Fe–Cr alloys working at high temperature.

The precipitation kinetics of γ phase in the inverse Ni–20.4 at. % Al alloy are studied by utilizing phase-field method, combining with the volume fraction, particles number density, average particle radius, average aspect ratio and particle size distribution (PSD) of γ phase at different aging temperatures. It is found that the decrease rate of particles number density increases with the elevated aging temperature, and the decrease rate in the early coarsening stage are greater than that in the stable coarsening stage. The average particle radius shows a linear relationship with time through kt 1/3 in the stable coarsening stage of the inverse Ni–Al alloy, and the coarsening rate constant k increases with the elevated temperature. As the aging progresses, the peak value of PSD becomes smaller and the normalized radius r / r > moves from 1.1 to 1.0, while the peak value of PSD becomes larger and the corresponding r / r > value moves from 0.9 to 1.0 with the elevated temperature. The peak value of PSD is less than that predicted by Lifshitz-Slyozov-Wagner (LSW) theory at different aging temperatures.

An organic primer pigmented with strontium aluminium polyphosphate for the corrosion protection of zinc alloy coated steel was investigated by electron microscopy and electrochemical impedance spectroscopy after exposure to sodium chloride solution. The development of defects within the organic coating was found to be related to the solubility of the pigment, which resulted in two conflicting effects. On the one hand, the inhibitive species released from the solid pigment reduced the corrosion rate of the metal substrate by formation of precipitated products containing zinc, strontium, aluminium, phosphorus and oxygen. On the other hand, the dissolution of the inhibitive pigment resulted in porosity in the coating that created easy pathways not only for the inhibitors to reach the substrate but also for the aggressive species to migrate inwards to the metal surface and, importantly, for the transport of corrosion products outwards.

T1 (Al2CuLi) phase precipitates, the main strengthening precipitates in third generation aluminum-copper-lithium (Al-Cu-Li) alloys, play a critical role in determining the corrosion behavior of these alloys. Herein, the T1 precipitates, sufficiently large to be visualized by scanning electron microscopy, were intentionally grown in a commercial Al-Cu-Li alloy through a high temperature annealing process. The corrosion and anodizing behavior of the alloy associated with individual T1 precipitate plates was subsequently investigated. It was observed that corrosion initiated instantaneously on T1 precipitate plates when the alloy was exposed to laboratory air. When immersed in NaCl solution, T1 precipitate plates corroded through a dealloying process and then, drove anodic dissolution of the adjacent aluminum alloy matrix by forming copper-rich nanoparticles at the sites of dealloyed T1 precipitates. The T1 phase precipitates were anodized relatively faster than the aluminum matrix in tartaric-sulfuric acid solution under a constant voltage of 14 V. The anodic film formed from T1 precipitates was dissolved quickly by the anodizing electrolyte during anodizing at relatively higher temperatures, resulting in cavities of sizes similar to those of T1 precipitate plates.

A study has been carried out of the effect of an anodizing pre-treatment in a sulphuric acid electrolyte on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding. Lap-shear tests were used to determine the joint strength. Comparisons were made with joints pre-treated using sand blasting. The failed specimens were examined by scanning and transmission electron microscopy. Anodizing improved the strength of the joints by a factor of about 6 compared with sandblasting. For the anodizing pre-treated joints, melted polymer infiltrated deeply within the nanoporous anodic film, forming a strong polymer-film bond. Joint failure occurred by ductile tearing of the polymer at or near the film surface. In contrast, sandblasted joints failed at the alloy/polymer interface.

Silanol type hybrid polymers modified with silver nanoparticles encapsulated with SiO2 for biocorrosion protection of 2024-T3 aluminum alloy were studied through electrochemical characterization and surface analysis techniques. Two different encapsulated silver nanoparticles were synthesized using tetraethoxysilane as a core shell. The hybrid polymer was prepared by the sol–gel technique by mixing tetraethoxysilane and triethyl(octyl)silane in 1-propanol, followed by the incorporation of silver nanoparticles or encapsulated silver nanoparticles. Relatively uniform coatings were observed by a scanning electron microscopy analysis. Transmission electron microscopy and dynamic light scattering results indicated that the diameter of the silver nanoparticles was around 20 nm, whereas the encapsulated silver nanoparticles presented diameters between 24 and 30 nm. The viability results showed that polymers modified with encapsulated nanoparticles exhibit higher antibacterial properties than the polymer modified with free silver nanoparticles. This fact may be associated with a higher hydrophobicity of the coatings modified with silver encapsulated nanoparticles. Additionally, impedance measurements revealed a protective effect of all synthesized coatings for 2024-T3 aluminum alloy in chloride media inoculated with Pseudomonas aeruginosa. Keywords: Hybrid polymers, Silver nanoparticles, SiO2 nanocapsules, Biocorrosion, Pseudomonas aeruginosa

The role of intermetallic particle (IMP)-introduced anodic film defects in the initiation and propagation of localized corrosion in an anodized Al-Cu-Li alloy prior to sealing is investigated. Anodizing of IMPs results in cavity defects of micrometer-scale in the anodic film. Corrosion is preferentially initiated in the regions with anodized IMPs at the film/alloy interface. Localized corrosion then propagates in the alloy substrate beneath the anodic film, which ultimately results in cracking of the bulk anodic film. It is suggested that the barrier and porous layers of the anodic film play different roles in different stages of a localized corrosion event.

A study has been carried out of the effect of plasma electrolytic oxidation (PEO) on the strength of AA 5052 alloy/polypropylene joints prepared using friction stir spot welding (FSSW). The joint strengths were determined using lap-shear tests and failure modes were investigated using scanning electron microscopy. Comparisons were made between control joints prepared with the alloy in the as-rolled condition or the as-rolled condition with a mechanical key and with PEO-treated alloy, with or without a mechanical key. Mechanical keying alone, provided by infiltration of polymer into holes of either 3 or 4.5 mm diameter drilled in the alloy, yielded enhancements of the joint strength by a factor of 1.8 and 3.8, respectively, compared with the as-rolled alloy. In contrast, PEO pre-treatment provided a much greater improvement in strength, by a factor of 21.3, with no significant influence of the presence of a mechanical key. The fracture of PEO-treated joints involved a mixture of de-bonding at the polymer/coating interface, cohesive failure within the coating and ductile failure of the polymer. The last resulted from formation of gas bubbles within the polypropylene due to thermal degradation of the polymer during FSSW. In contrast, in the absence of PEO, the fracture path of the as-rolled and also the as-rolled and mechanically keyed joints passed along the alloy/polypropylene interface and through gas bubbles within the polymer. The enhanced strength of the PEO-treated joints resulted from the strong polypropylene/coating bond resulting from flow of molten polymer into the coating porosity.

The effect of Ag on cathodic activation and corrosion behaviour of Mg-Mn-Ag alloys are investigated in the present work. During corrosion, Mg-Ag intermetallic phase undergoes dealloying to form porous Ag-rich remnant whereas Ag in the alloy matrix is dissolved to form Ag+ ions that subsequently redeposit on the alloy surface via displacement reaction with Mg to form Ag nanoparticles, which lead to significant cathodic activation. The cathodic activation is a self-catalysed process since enhanced cathodic activity promotes anodic dissolution which, consequently, results in further enhancement of cathodic activity. Thus, the corrosion rate of Mg-2 Mn-4Ag alloy increases significantly with immersion time.

In the present study, the behaviour of AA5754 and AA5052 alloys in alkaline solution is investigated using analytical electron microscopy. Significant differences in dissolution rate, change of intermetallic particle population density, compositional and morphological change of various intermetallic particles of the two alloys are observed. The dissolution rate of AA5754 alloy is approximately 1.3 μm/min., which is significantly lower than the dissolution rate of 2.3 μm/min. For AA5052 alloy. After alkaline etching, the population density of coarse intermetallic particles (mainly Mn-rich) on AA5754 alloy surface is significantly reduced whereas the population density of coarse intermetallic particles (mainly Fe-rich) on AA5052 alloy surface remains almost unchanged. Further, the root cause for the different behaviour of the two alloys in alkaline solution is discussed.

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.

A comparison has been made of the influence of two alloy pre-treatments and two coating post-treatments on the formation, composition and corrosion protection of a trivalent chromium conversion coating on AA2024-T351 alloy. The investigation employed analytical electron microscopies, ion beam analysis, X-ray photoelectron spectroscopy (XPS) and electrochemical tests. The pre-treatments used alkaline etching followed by de-oxidizing in either nitric acid or a commercial de-oxidizer. The conversion coatings were formed in SurTec 650 chromitAL and revealed two-layers, comprising an inner aluminium-rich layer and an outer chromium- and zirconium-rich layer, with a Cr:Zr atomic ratio in the range ~ 0.73–0.93. XPS indicated a chromium-enriched near-surface region that contained ~ 2 at.% of Cr(VI) species. Potentiodynamic polarization and electrochemical impedance spectroscopy revealed an improved corrosion protection for a pre-treatment that left copper-rich sponges, probably de-alloyed S phase, and fewer residues of other intermetallic particles on the alloy surface. Post-coating immersion treatments in deionized water at 20 °C or 40 °C resulted in a significant difference in the zirconium species in the region adjacent to the coating surface that is accessible to XPS, with oxide and hydroxide dominating at the respective temperatures.

In order to manifest the influence of specific microstructural component on the development of severe localized corrosion in an AA2099 aluminum–lithium alloy, the corrosion behavior of the alloy subjected to solution heat treatment, cold working and artificial ageing was investigated. Immersion testing and potentiodynamic polarization were employed to introduce localized corrosion; scanning electron microscopy and transmission electron microscopy were used to characterize the alloy microstructure and corrosion morphology. It was found that the susceptibility of the alloy to severe localized corrosion was sensitive to thermomechanical treatments. Additionally, the state of alloying elements influenced the mechanism of localized corrosion propagation. Specifically, the alloy in T8 conditions showed higher susceptibility to severe localized corrosion than that in other conditions. During potentiodynamic polarization, the alloy in solution heat-treated and T3 conditions displayed crystallographic corrosion morphology while the alloy in T6 and T8 conditions exhibited selective attack of grain interiors and grain boundaries in local regions.

Corrosion behaviors of recently developed 2A97-T6 aluminum-copper-lithium alloy in sodium chloride solution are investigated using scanning electron and transmission electron microscopies in conjunction with electron backscatter diffraction. It has been found that corrosion product rings were established on the alloy surface as early as 5 min during immersion in sodium chloride solution. Meanwhile, hydrogen continuously evolved from within the rings. Pitting corrosion is evident with crystallographic dependant corrosion channel facets mainly parallel to {100} planes. Non-uniform distribution of misorientation in the 2A97 aluminum alloy results in a portion of grains of relatively high stored energy. Such grains were preferentially attacked, serving as local anodes, during the development of crystallographic pitting.

The protective film formed in a defect by leaching of lithium oxalate from model organic coatings during neutral salt spray exposure has been investigated. A scribed area of about 1 mm width was introduced on the coated AA2024-T3 aluminium alloy. The scribed area was examined before and after exposure in neutral salt spray environment for 4, 8, 24 and 168 h by scanning and transmission electron microscopies. It was found that the lithium oxalate was able to leach from the organic coating during neutral salt spray exposure and it promoted the formation of a film that provided effective corrosion protection to the alloy. The typical film morphology consists of three different layers, including a relatively compact layer near the alloy substrate, a porous middle layer and a columnar outer layer. Variation of the film morphology was also observed at different locations of the scribed alloy surface, which may be related to the difference of local concentration of lithium species. Electron energy loss spectroscopy detected lithium, aluminium and oxygen in the film. Although the film showed the varied morphologies in different regions of the scribed area, the alloy substrate was protected from corrosion when the film was formed. Copyright © 2016 John Wiley & Sons, Ltd.

A heterogeneous microstructure is intentionally developed in 2A97-T3 aluminium–copper–lithium alloy during solidification and thermomechanical processes to obtain good mechanical properties. As a consequence, the alloy is susceptible to localized corrosion. Electron microscopy was employed to observe intermetallic particles and their periphery and the initiation and development of intermetallic particle induced localized corrosion in 2A97-T3 aluminium–copper–lithium alloy. In-situ optical microscopy monitoring, energy dispersive X-ray spectroscopy and electron backscatter diffraction were also used to provide supportive evidence. Compared with the small number of continuous localized corrosion events, discontinuous localized corrosion event is relatively common. They are associated with corroded Al 2 Cu IM particles and Al–Cu–Fe–Mn-(Si) IM particles, as well as corrosion pits that are formed by particle fall-out due to dissolution of surrounding aluminium matrix. Discontinuous localized corrosion is confined within the shallow near-surface region of aluminium matrix. Triggered immediately after immersion, hydrogen gas evolution developed in form of bubbling at a continuous localized corrosion site which is associated with severe surface etching and sub-surface attack. Intergranular corrosion initiated from the corrosion pit bottom, connects to the corrosion pit via small openings, and developed into the large network buried underneath the alloy surface. T1 phase precipitate remnant and corroded IM particles at grain boundary induced dissolution in the periphery of the particle, drive intergranular corrosion to propagate. Copper is much less oxidized than aluminium and lithium during grain boundary attack, and therefore accumulated at the corrosion product – aluminium matrix interface in the intergranular corrosion filament. Then, the copper enrichment band acts as further local cathode to support reduction reactions.

The effect of anodization temperature and tartaric acid concentration on the morphology and corrosion resistance of the anodic film formed on AA2099-T8 alloy in tartaric-sulfuric acid was investigated. It was found that the dissolution of the anodic film during anodizing led to increased pore size, rod-shaped cavities and grain boundary grooves in the anodic films. The rod-shaped cavities and grain boundary grooves are associated with selective dissolution of the anodic film formed from fine T1 (Al2CuLi) phase precipitates due to the difference in the reactivity of the films formed from different phases. The increased porosity due to dissolution degraded the corrosion resistance of the anodic film. In the temperature range of 22–47°C, with 0.53 M tartaric acid addition, anodizing at 42°C provided the best corrosion performance and a relatively high anodizing efficiency; in the tartaric acid concentration range of 0–0.9 M, at 37°C, anodizing in electrolytes containing 0.7 and 0.9 M tartaric acid provided good corrosion resistance with little decrease of anodizing efficiency. The corrosive medium did not penetrate the anodic film uniformly but preferentially at local sites, resulting in localized corrosion of the anodized alloy.

The caustic etching behaviour of the automotive AA6111 alloy with different aging conditions that simulate the various levels of heat input within the heat affected zone during the welding heating cycles has been investigated. The alloy dissolution rate was found to substantially increase with the pre-aging temperature, which enhances precipitation of the cathodic Q-phase and results in depletion of the alloy solid solution in magnesium and silicon. On the other hand, enhancing the kinetics of precipitation at high temperatures induces a more uniform distribution the Q-phase precipitates, which minimizes the potential difference between the interiors and boundaries of the adjacent grains. This was observed to reduce the severity of the surface topography developed by subsequent caustic etching surface pre-treatment.

High strength Al-Zn-Mg-Cu (7xxx) aluminium alloys obtain the required balance of mechanical and corrosion properties through complex heat treatments developed through empirical experiment. Understanding the influence of these heat treatments on the composition of the strengthening precipitates (η′ and η-phase) is critical to obtain a better mechanistic understanding of their effect. In this work, the composition of strengthening precipitates has been studied at the atomic scale in microstructures of commercially processed AA7010 alloy in the standard overaged temper state. Copper enrichment in the precipitates on over-aging is confirmed for both small (η′) and large (η) particles. It is demonstrated that in addition to entering the precipitates, a copper enriched layer forms at the particle/matrix interface on the broad faces of the precipitate plates. The implications of this structure for the mechanical and environmentally assisted cracking performance of the alloy are discussed.

Pigment distributions have a critical role in the corrosion protection properties of organic paint coatings, but they are difficult to image in 3D over statistically significant volumes and at sufficiently high spatial resolutions required for detailed analysis. Here we report, for the first time, large volume analytical serial sectioning tomography of an organic composite coating using a xenon Plasma Focused Ion Beam (PFIB) combined with secondary electron imaging, energy dispersive X-ray (EDX) spectrum imaging (SI) and electron backscattered diffraction (EBSD). Together these techniques provide a comprehensive quantitative description of the physical orientation and distribution of the pigments within a model marine ballast tank coating, as well as their crystallographic and elemental characterisation. Polymers and organic materials are challenging because of their propensity for ion beam damage and possible beam heating effects. Our novel, optimised block preparation technique permits automated data acquisition with minimal operator intervention, and can have significant applications for the structural and chemical characterisation of a wide range of organic materials. Our results revealed that the paint contained 7.5 vol% aluminium flakes and 25 vol% quartz particles. The aluminium flakes were oriented parallel to the substrate surface, which is beneficial in terms of the corrosion protection capability of the coating.

An experimental protocol was studied to improve the adhesion of a polymeric poly(methyl methacrylate) coating that was modified with silver nanoparticles to an aluminum alloy, AA2024. The nanoparticles were incorporated into the polymeric matrix to add the property of inhibiting biofilm formation to the anticorrosive characteristics of the film, thus also making the coating antibiocorrosive. The protocol consists of functionalizing the surface through a pseudotransesterification treatment using a methyl methacrylate monomer that bonds covalently to the surface and leaves a terminal double bond that promotes and directs the polymerization reaction that takes place in the process that follows immediately after. This results in more compact and thicker poly(methyl methacrylate) (PMMA) coatings than those obtained without pseudotransesterification. The poly(methyl methacrylate) matrix modified with nanoparticles was obtained by incorporating both the nanoparticles and the methyl methacrylate in the reactor. The in situ polymerization involved combining the pretreated AA2024 specimens combined with the methyl methacrylate monomer and AgNps. The antibiofilm capacity of the coating was evaluated against P. aeruginosa, with an excellent response. Not only did the presence of bacteria decrease, but the formation of the exopolymer subunits was 99.99% lower than on the uncoated aluminum alloy or the alloy coated with unmodified poly(methyl methacrylate). As well and significantly, the potentiodynamic polarization measurements indicate that the PMMA-Ag coating has a good anticorrosive property in a 0.1-M NaCl medium.

The interface reaction zone (RZ) in a unidirectional continuous carbon-coated SiC fibre reinforced Ti-17 titanium alloy composite is investigated. Micro-computed tomography (CT), scanning and transmission electron microscopy are employed to characterize the fibre/matrix interface. It is revealed that the interface RZ is a 400 nm thick titanium carbide (TiC) layer which is composed of two sublayers, a 60 nm thick fine-grained sublayer and an approximate 340 nm thick coarse-grained sublayer. The RZ is formed through chemical reaction between carbon coating on the SiC fibre surface and Ti, Zr and Sn in the alloy matrix. The reaction is controlled by atom diffusion occurring at the fibre/matrix interface. However, in the reaction process, Al, Cr and Mo in the matrix are rejected and piled up in front of the RZ on the matrix side. A structure model is proposed to describe the formation mechanism of the interface RZ.

The anodizing behavior of iron-containing intermetallic particles in AA2099 aluminum-lithium alloy and their effect on structure and corrosion resistance of the anodic film were investigated using electrochemical measurements and scanning electron microscopy. High-copper-containing Al-Fe-Mn-Cu particles (HCCPs) dissolved preferentially through dealloying at ∼ 0 V (vs. saturated calomel electrode) in a tartaric-sulfuric acid solution, at 22, 37 and 42◦C, leading to formation of copper-rich nanoparticle of 50–200 nm diameters. They dissolved completely under normal anodizing conditions, resulting in cavity defects of micrometer dimensions in the anodic film and sunken regions in the alloy substrate immediately beneath the dissolved HCCPs. Immersion testing of the anodized alloy in 3.5% NaCl solution for 24 h indicated that localized corrosion of the anodized alloy predominantly developed at the site containing dissolved HCCPs at the film/alloy interface. It is suggested that HCCPs play a critical role in controlling the corrosion resistance of the anodic film formed on AA2099 aluminum-lithium alloy.

In the present study, the influence of precipitate distribution on the corrosion behaviour of 2A97-T6 Al-Cu-Li alloy in 3.5 wt.% NaCl solution is investigated. It is found that localized plastic strain introduced to the alloy during sheet fabrication leads to the formation of dense T1 (Al2CuLi) phase precipitate bands and precipitate-free bands within selected grains during aging. Preferential dissolution of T1 phase in the dense precipitate bands results in intragranular corrosion in the alloy, in the form of corrosion bands. Further, the dissolution of the alloy matrix propagates in the form of crystallographic corrosion pit.

The effect of thermal exposure at 70 and 100 °C on intergranular corrosion (IGC) susceptibility of AA5083 alloy has been studied. At both temperatures, metastable β′ phase has been detected at grain boundaries. The thermal exposure resulted in sensitized microstructure in the alloy and, consequently, increased mass loss values during the nitric acid mass loss test (NAMLT). The sensitization of the alloy is related to the formation of the β′ phase. The as-received O temper alloy showed relatively high resistance to IGC and exhibited a discontinuous distribution of the β′ phase precipitates at grain boundaries. After exposure at 70 °C for 480 h, the size of β′ phase precipitate at the grain boundaries slightly increased, resulting in an increased mass loss. After exposure at 100 °C for 240 h, the alloy became severely sensitized due to the more continuous distribution of β′ phase precipitate along the grain boundaries.

This paper investigates the structure of the copper–enriched layer formed at the alloy/anodic film interface during anodizing of Al–2 wt.% Cu binary alloy using transmission electron microscopy. It was revealed that θ ′ phase was formed within the copper–enriched layer. For the copper–enriched layer formed on {1 0 0} aluminum planes, the interface between the aluminum matrix and the θ ′ phase within the copper-enriched layer is coherent. For the copper–enriched layer formed on {1 1 0} and {1 1 1} aluminum planes, the interfaces between the aluminum matrix and the θ ′ phase within the copper-enriched layer are semi-coherent or incoherent. The interfacial coherency influences the formation of oxygen gas bubbles within the resultant anodic films.

The formation of a trivalent conversion coating on aluminium has been investigated using analytical electron microscopy, atomic force microscopy, ion beam analysis, glow discharge optical emission spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The coating is shown to comprise a chromium- and zirconium-rich outer layer and an aluminium-rich inner layer. Zirconium and chromium are present in chemical states consistent with ZrO 2 , ZrF 4 , Cr(OH) 3 , Cr 2 (SO 4 ) 3 , CrF 3 and CrO 3 or CrO 4 2 − . However, negligible amounts of Cr(VI) species occurred in coatings formed in de-aerated solution. Electrochemical impedance spectroscopy revealed that the inner layer provides the main corrosion protection during short-term tests in 0.1 M sodium sulphate solution at room temperature.

Localized corrosion in an AA2099-T83 aluminum–lithium alloy has been correlated with localized plastic deformation (LPD) introduced by cold working. The high population density of dislocations in the slip bands associated with LPD promoted preferential precipitation of needle-shaped T1 (Al2CuLi) phase during artificial aging. Consequently, preferential attack of the electrochemically active T1 phase in corrosive environment resulted in selective corrosion of the bands, leading to the development of the characteristic morphology of narrow, parallel volume of corrosion. Copyright © 2015 John Wiley & Sons, Ltd.

It was revealed that an AA5083 alloy had been severely sensitized after 40 years in service at ground atmosphere temperature. The degree of sensitization of the various regions through the alloy plate thickness is different, which is associated with the non-uniform distribution of Mg through the plate thickness. The alloy plate exhibits Mg-rich bands that promoted the heterogeneous distribution of Mg-containing precipitates. The composition of these precipitates ranges from that for low Mg content intermetallics to that of the β′ phase. Structure characterization revealed the β′ phase and a cubic Al-Mg phase that has lower magnesium contents compared to the β phase.

The influence of prolonged ambient temperature storage on the subsequent precipitation behavior of the AA6111-T4P automotive alloy during artificial aging has been investigated using hardness testing and high-resolution transmission electron microscopy. The results indicate that growth of atomic co-clusters and GP zones during ambient storage suppress the onset of Q′ and Q precipitation, and retard the formation of elemental silicon particles. However, the overall hardening response of the stored AA6111 T4P alloy to subsequent peak-aging and over-aging treatments is not altered by the significant reduction in the volume fraction of Q′ and Q precipitates due to their lower strengthening effects compared with GP zones and the Q″ phase precipitates.

The corrosion behavior of intermetallic particles and their role in the process of localized corrosion in AA2099-T83 aluminum–lithium alloy has been investigated. It was found that both high- and low-copper containing Al–Fe–Mn–Cu-(Li) particles could result in superficial pits on the alloy, and the high level of lithium in the high-copper-containing particles rendered them electrochemically more active than the low-copper-containing particles. Additionally, severe localized corrosion was found not to be directly related to the distribution of constituent particles in the alloy. The findings are not only relevant to the understanding of corrosion mechanism but also beneficial to the evaluation of thermomechanical treatments of the alloy.

The scanning vibrating electrode technique has been used to characterize the galvanic interactions between AA2024T3 and mild steel at room temperature and elevated solution temperatures up to 60 °C in naturally aerated 3.5% NaCl solution. The work shows that from a solution temperature of 43 °C and above, highly localised anodic activities with high intensities occur on the mild steel, compared with the anodic activities occurring on the AA2024T3 alloy in a couple system of the two alloys. The highly localised activities on the mild steel were not observed at 34±1 °C with the surface showing no distinct corrosion activities and AA2024T3 alloy was completely the bulk anodic material at this solution temperature. At room temperature, AA2024T3 was also the bulk anode, although mild anodic activities were detected on the surface of the mild steel alloy. The local corrosion activities revealed by the scanning vibrating electrode technique were in agreement with the micrographs of the surfaces of the alloys after the immersion tests.