Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3

This study consists of two parts. In the first part, the corrosion of 2024-T3 aluminum alloy (AA 2024-T3) was studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the anodic S phase (Al₂CuMg) particles dealloyed Al and Mg during the 3.5 h of immersion in a neutral 0.6 M sodium chloride (NaCl) solution; with the dealloying of Mg being the most severe. Simultaneously, a heavy dissolution was also observed for the surrounding Al matrix of the S phase particles. This Al dissolution is likely to be caused by a local alkalization resulting from the coupled cathodic reaction (water and/or oxygen reduction). Such corrosion in AA 2024-T3, however, can be inhibited efficiently after the treatment of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). The associated studies on bis-sulfur silane treated AA 2024-T3 will be presented in the second part.     

Corrosion of AA2024-T3 Part III: Propagation

Optical and electron microscopies and EBSD were used to study the early stages of corrosion propagation during stable pit formation on AA2024-T3. Polished AA2024-T3 developed large scale rings of corrosion product, typically a few hundred microns in diameter, within 2 h of exposure to 0.1 M NaCl at room temperature. These features were sectioned using diamond ultramicrotomy and substantial subsurface attack, in the form of intergranular corrosion was observed beneath these sites with virtually no grain etchout. A model is proposed for the mechanism of stable pit progression which involves extensive grain boundary attack, followed by grain etchout leading to open pit formation.     

Analytical characterisation and corrosion behaviour of bis-[triethoxysilylpropyl]tetrasulphide pre-treated AA2024-T3

This work aims at studying the corrosion behaviour of AA2024-T3 pre-treated with bis-[triethoxysilylpropyl]tetrasulphide. Simultaneously, the work investigates the influence of the Cu-rich intermetallic particles on the formation of the silane film. The analytical characterisation of the silane films was performed by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The corrosion performance of the pre-treated substrates was evaluated by electrochemical impedance spectroscopy. Atomic force microscopy associated with Kelvin probe was also used to determine the influence of the silane film on the Volta potential distribution on the alloy surface. The results show that copper present in the intermetallics plays an important role on the film formation.     

Corrosion Protection of AA2024-T3 by Cerium Malate and Cerium Malate-Doped Sol-Gel Coatings

Cerium malate (CeMal) was tested as a corrosion inhibitor for AA2024-T3 in this work. Corrosion inhibition on bare AA2024-T3 indicated that the inhibiting effect was a result of the synergistic effect of cerium cations and maleic anions. The corrosion of AA2024-T3 was stagnated by greatly reducing the corrosion current when CeMal was present in NaCl solutions. CeMal was adsorbed on the surface of AA2024-T3 forming a protective film in the initial stage. Then, cerium cations transformed to cerium oxide/hydroxides, precipitating on the cathode sites to inhibit the further corrosion. The electrochemical impedance spectra results of the sol-gel coatings proved that CeMal was an effective corrosion inhibitor in the sol-gel coatings to provide corrosion protection for AA2024-T3.     

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.     

2024-T3铝合金在硫酸-硼酸-磷酸中的阳极氧化和腐蚀行为

在含有10%硫酸、5%硼酸和2%磷酸的混合电解液中,对2024-T3铝合金进行阳极氧化处理,以提高其耐腐蚀性能。使用电化学阻抗频谱分析研究阳极氧化处理后合金的腐蚀行为。利用塔菲尔图和盐水喷雾技术进行对比发现,与只用磷酸或硫酸和硼酸的电解液相比,使用含有10%硫酸、5%硼酸和2%磷酸的混合电解液阳极氧化处理后的2024-T3铝合金,具有更好的耐腐蚀性和持久性。该电解液可以替代普遍用于阳极氧化铝合金的铬酸盐浴。     

Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings

The processing variables that contributed to the formation of subsurface crevices under cerium-based conversion coatings on AA 2024-T3 were investigated. Focused ion beam milling revealed the presence of subsurface crevices underneath a small fraction (∼ 10%) of coated areas, typically in areas with large cracks through the coatings. A solution of sodium chloride and H₂O₂ etched AA 2024-T3 and produced features similar to subsurface crevices, which confirmed that crevices formed during deposition due to the composition of the coating solution. Using sodium nitrate in place of sodium chloride resulted in no etching of the substrate. Thus, coatings free of subsurface crevices could be produced by using cerium nitrate instead of cerium chloride in the coating solution. Electrodeposited coatings, even those deposited from solutions containing chloride ions and H₂O₂, were also free of subsurface crevices. As a result, subsurface crevices are not inherent to cerium-based conversion coatings, but rather were formed due to certain process parameters, specifically the presence of chloride ions and hydrogen peroxide in the coating solution.     

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.     

Statistical evaluation of EIS and ENM data collected for monitoring corrosion barrier properties of organic coatings on Al-2024-T3

This paper describes the statistical treatment of electrochemical noise (electrochemical noise methods, ENM) and impedance (electrochemical impedance spectroscopy, EIS) data collected for corrosion monitoring of epoxy coated aluminum 2024-T3. The epoxy was applied using electrodeposition to Al-2024-T3 panels treated with one of three surface pretreatments. Six coating systems were prepared by varying the electrodeposition parameters and the pretreatment. Linear regression analysis of the data was used to detect the individual contributions relating to the analysis technique, topcoat application and pretreatment type. As a result of the statistical treatment it was found that EIS data has significantly less variance compared to ENM data, that EIS data can be used to differentiate the effects generated from the coating and pretreatment, and that ENM data cannot be used to differentiate the effects generated from the coating and pretreatment due to the high variance in data values associated with ENM. None of this information could be quantified without the statistical treatment.     

Effect of gelatin additions on the corrosion resistance of cerium based conversion coatings spray deposited on Al 2024-T3

The corrosion resistance of cerium based conversion coatings on Al 2024-T3 was improved by the addition of a water soluble gelatin to the coating solution. Auger electron spectroscopy depth profiling showed that coatings deposited from solutions containing 800-3200 ppm gelatin were ~ 400 nm thick, while coatings deposited from solutions with 0-200 ppm gelatin were ~ 850 nm thick. The thinner coatings exhibited reduced surface cracking and spalling. Open circuit potential measurements during deposition showed that adding gelatin to the coating solution resulted in a more negative and stable potential with increasing gelatin concentrations. Visually, increasing gelatin concentrations promoted the formation of stable bubbles that covered panel surfaces, which limited transport of cerium species to the surface and decreased the deposition rate. X-ray diffraction analysis revealed that only coatings deposited from solutions containing 400-3200 ppm gelatin could be converted to CePO₄H₂O during post-treatment, potentially improving the corrosion resistance compared to coatings deposited from solutions without gelatin.     

Synergistic effect of pH and oxalate concentration on corrosion of aluminium alloy 2024-T3

This work aims at obtaining comprehensive information on the corrosion behaviour of AA2024-T3 in oxalic acid solutions under different concentrations (0.007–0.2?M) and pH (2–6). Various tests to characterise the corrosion behaviour are performed, including weight loss and electrochemical tests. Coupon surfaces are examined using infinite-focus microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy. Corrosion rates, anodic and cathodic reaction kinetics are detailed to provide a fundamental understanding of the electrochemical behaviour of AA2024-T3 as a function of pH and oxalate concentration. The synergistic effect of pH and oxalate concentration on corrosion of AA2024-T3 is evaluated for both AA2024-T3 matrix and main intermetallics (i.e. Al–Cu–Mg and Al–Cu–Fe–Mn) and the electrochemical behaviour of each constituent element in AA2024-T3 is systematically studied.     

The corrosion resistance of hot dip galvanised steel and AA2024-T3 pre-treated with bis-[triethoxysilylpropyl] tetrasulfide solutions doped with Ce(NO3)3

The present work aims at evaluating the anti-corrosion behaviour of a novel pre-treatment based on bis-[triethoxysilylpropyl] tetrasulfide (BTESPT) doped with cerium nitrate for application on hot dip galvanised steel and AA2024-T3 substrates. The corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) and by the scanning vibrating electrode technique (SVET), during immersion in NaCl solutions. The electrochemical results showed that the pre-treatment provides excellent corrosion protection to the substrates. Furthermore, the results evidenced improved protection comparatively to the use of undoped BTESPT pre-treatments, both for galvanised steel and AA2024-T3. This improvement is most likely due to enhanced barrier properties of the film and additional active corrosion protection originated from the inhibiting action of the cerium-based inhibitor impregnated in the silane matrix.     

Influence of pre-treatments in cerium conversion treatment of AA2024-T3 and 7075-T6 alloys

The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO₃)₃ at 85 °C with H₂O₂ accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of  300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements.     

Improved corrosion resistance of AA2024 alloys through hybrid organic-inorganic sol-gel coatings produced from sols with controlled polymerisation

In this work we present the development of a nanocomposite material composed by silica nanoparticles in a hybrid organic-inorganic sol-gel matrix for corrosion protection of aluminium alloys. The sol-gel matrix was produced from an inorganic precursor, tetraethoxysilane (TEOS), a hybrid precursor organically functionalized with CC groups, 3-metacryloxypropyltrimethoxysilane (MPS), and an organic bi-functional monomer, ethyleneglycol-dimethacrylate (EGDMA) used to increase the cross-linking network. Silica nanoparticles, on the other side, increase the density and provide a major mechanical performance through the reinforcement of the coating. The evolution of the sol, mainly the chemical structure, during the processes of hydrolytic condensation and organic polymerisation was studied as a function of the sol concentration through Fourier transformed infrared spectroscopy (FTIR), rheometry, laser diffraction analysis and contact angle. Mono and multilayer coatings were deposited by dipping onto AA 2024 substrates and characterised by profilometry. The corrosion behaviour was followed through potentiodynamic tests and Electrochemical Impedance Spectroscopy (EIS).     

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in sodium chloride solution.: Part 2: mechanism for corrosion protection

The corrosion protection of AA 2024-T3 by films of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane) was studied in a neutral 0.6 M NaCl solution using potential transient, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results showed that a highly crosslinked or dense interfacial layer that developed between the silane film and the aluminum oxide is the major contribution to the corrosion protection of AA 2024-T3. The formation of this interfacial layer heavily restricts pit growth underneath via retarding the transport of corrosion products, as well as effectively blocks a number of cathodic sites available for cathodic reactions.     

A study of the corrosion of aluminum alloy 2024-T3 under thin electrolyte layers

The corrosion of aluminum alloy 2024-T3 (AA2024-T3) under thin electrolyte layers was studied in 3.0 wt% sodium chloride solutions by cathodic polarization and electrochemical impedance spectroscopy (EIS) method. The cathodic polarization measurements show that, when the electrolyte layer is thicker than 200 μm, the oxygen reduction current is close to that of the bulk solution. But in the range of 200-100 μm, the oxygen reduction current is inversely proportional to the layer thickness, which shows that the oxygen diffusion through the electrolyte layer is the rate-determining step for the oxygen reduction process. In the range of 100 μm to about 58 μm, the oxygen reduction current is slightly decreased probably due to the formation of aluminum hydroxide or the change of the diffusion pattern from 2-dimensional diffusion to one-dimensional diffusion. The further decrease in electrolyte layer thickness increase the oxygen reduction current to some extent again, because the diffusion of oxygen plays more important role in thin electrolyte layers.The EIS measurements show that the corrosion is controlled by the cathodic oxygen reduction at the initial stage, showing the largest corrosion rate at the electrolyte layer thickness of 105 μm. But at the later stage of corrosion, the anodic process begin to affect the corrosion rates and the corrosion rates show a maximum at 170 μm, which may be the thickness where the corrosion changes from cathodic control to anodic control. The corrosion rate under the very thin electrolyte layer (62 μm in this study) is even smaller than that in bulk solution, this is due to that the anodic process is strongly inhibited.     

Electrochemical studies on the corrosion inhibition of AA2024 aluminium alloy by rare earth ammonium nitrates in 3.5% NaCl solutions

The inhibitive effect of Ce(III) and Ce(IV) ammonium nitrate inhibitors on the corrosion behavior of aluminium alloy AA2024 in 3.5% NaCl solution has been investigated. Four different concentrations (10^?2, 10^?3, 10^?4, and 10^?5 M) of the inhibitors were studied. Ce³⁺ and Ce⁴⁺ ions are introduced into the corrosive medium with similar anionic compositions – (NH₄)₂Ce(NO₃)₅ and (NH₄)₂Ce(NO₃)₆. The inhibition efficiency (IE%) of the ions has been evaluated by using electrochemical methods – linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS). Moreover, the inhibitive ability has been investigated at different duration after addition of these salts into the corrosive media. It is established that the pH of the solutions plays an important role on the inhibition process for both cations. The results revealed that at relatively similar conditions, the cerium (III) ions showed higher inhibition efficiency than cerium (IV) ions, when they were compared to the case without inhibitor.     

Comparative study of protection of AA 2024-T3 exposed to rare earth salts solutions

The inhibitive effects of the dibutyl phosphate anion in cerium dibutylphosphate [Ce(dbp)₃] on AA 2024-T351 immersed in 0·05M NaCl were evaluated by comparison with immersion in cerium chloride (CeCl₃) in 0·05M NaCl as well as 0·05M NaCl alone. Scanning electron microscopy and energy dispersive X-ray hyperspectral mapping were used to examine the surfaces after exposure to the different inhibitor solutions. It was found that the dibutylphosphate anion produced additional inhibition for the matrix and the S phase intermetallic particles. The most significant mode of corrosion in the absence of inhibitors was in the form of rings of corrosion product. CeCl₃ largely suppressed the formation of the rings of corrosion product, and the Ce(dbp)₃ entirely eliminated them (for the time scales investigated here), indicating the additional inhibitive action of the dibutylphosphate anion.     

Corrosion of AA2024-T3 Part II: Co-operative corrosion

Polished specimens of AA2024-T3 alloy were immersed for up to 120 min in 0.1 M NaCl. The development of corrosion was monitored using scanning electron microscopy with energy dispersive X-ray spectroscopy (EDXS) and particle induced X-ray emission spectroscopy (PIXE). Both techniques revealed the intermetallic (IM) particle distributions and attack sites as distinguished by detection of chloride species. The earliest stages of attack involved localized attack around isolated IM particles as reported in Part I. Additionally attack occurred on a larger scale developing rapidly with rings of corrosion product surrounding clusters of IM particles. There were significantly higher numbers of IM particles within the corrosion rings, indicating that local clustering played an important role in co-operative corrosion.     

Corrosion behaviour of solar reflector coatings on AA 2024T3 - an electrochemical impedance spectroscopy study

Electrochemical impedance spectroscopy in the 100 kHz-5 mHz frequency range was applied to the study of corrosion behaviour of solar reflector coatings on AA 2024 exposed to 3.5% NaCl solution. Solar reflector coatings were obtained by sulphuric acid anodization of the alloy in presence of oxo-anions of molybdenum or vanadium. Corrosion behaviour of the oxide films was evaluated by determining the film resistance and capacitance values with exposure time. Comparison of the results reveals that, vanadium addition confers better barrier properties and corrosion resistance than molybdenum additions. Analysis using damage function based on the impedance at 100 mHz shows that solar reflector coatings have improved corrosion behaviour than normal sulfuric acid anodized oxide films on AA 2024.