The high throughput assessment of aluminium alloy corrosion using fluorometric methods. Part II - A combinatorial study of corrosion inhibitors and synergistic combinations

The discovery of new corrosion inhibition systems remains dependent on experimental methods for screening and characterization. This paper examines a new method to rapidly assess aluminium alloy corrosion and inhibitor performance through quantification of aluminium ion concentration using fluorometric probes. The fluorometric probes, lumogallion and morin, were used in conjunction with a plate reader to rapidly assess fourteen corrosion inhibitors and their combinations for AA2024-T3. AA2024-T3 wire electrodes were exposed to 3.4 mM total inhibitor concentration in 0.6 M NaCl adjusted to pH 2, 4, 7, 10, and 12 for 1-7 days. Corrosion inhibition provided by mixtures of lanthanum and molybdate, cerium and molybdate, and metavanadate with phosphate was found to be superior to the equivalent concentration of sodium chromate.     

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.     

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 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.     

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.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

The effect of inhibitor structure on the corrosion of AA2024 and AA7075

A range of structurally-related compounds were tested for their capacity to inhibit corrosion on aluminium alloys AA2024-T3 and AA7075-T6 in 0.1 M NaCl solution. It was found that the thiol group, positions para- and ortho- to a carboxylate, and substitution of N for C in certain positions strongly inhibited corrosion. The hydroxyl group was slightly inhibitive, while the carboxylate group provided little or no corrosion inhibition on its own. In several cases, different activities were found on the different alloys, with some compounds (particularly thiol-containing compounds) being more effective on AA2024 than on AA7075.     

Corrosion protection of aluminium alloy 2024-T3 in 0.05 M NaCl by cerium cinnamate

The corrosion protection of AA 2024-T3 in 0.05 M NaCl by cerium cinnamate has been studied. Polarization measurements demonstrate that cerium cinnamate is markedly effective for suppressing anodic process of the alloy corrosion during the initial 72 h of immersion. The protection mechanism during the immersion period appears to involve two stages: the deposition of cerium cinnamate, and then hydrolysis of cerium ions forming a cerium oxide/hydroxide, prevailing over the foregoing deposition. This study also elucidates the inhibition effect of cerium cinnamate on the early corrosion attack occurred from the second phase particles by X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Corrosion of AA2024-T3 Part I: Localised corrosion of isolated IM particles

Polished specimens of AA2024-T3 were immersed for various times up to 120 min in 0.1 M NaCl. The development of corrosion around isolated intermetallic particles was monitored using scanning electron microscopy with energy dispersive X-ray spectroscopy (EDXS). The earliest stages of attack started with localised corrosion of the S-phase particles resulting in dealloying which was followed by trenching around these particles. Subsequently, trenching was observed around cathodic particles where trenching started with AlCuFeMn particles with Cu/Fe ratios typically around 2.5 and then progressed to AlCuFeMnSi particles. This latter category of particles had a much lower Cu/Fe ratio, typically 0.5.     

Localized coating failure of epoxy-coated aluminium alloy 2024-T3 in 0.5 M NaCl solutions: Correlation between coating degradation, blister formation and local chemistry within blisters

The role of pH on the nature and rate of the degradation of epoxy coatings on AA2024-T3 panels and subsequent corrosion of the substrate during immersion in NaCl solutions was investigated. In acidic solutions both blister formation and growth are rapid. Blisters become very large (≈1 cm) and new blisters appear to form for a certain time after exposure. Often very small (∼0.1 mm) clear blisters surround these large blisters. Enhanced blister formation is due to irreversibly increased permeability of the coating for chloride ions and protons, the formation of more defect sites within the coating, and the weakening/dissolution of the oxide layer in low pH environments. In neutral pH solutions, coatings fail by forming one, or at most two, active blisters (red in color) within a few days of immersion with the time-to-failure dependent upon coating quality and thickness. Blister growth is a very slow process, and blister diameters rarely exceed a few millimeters even after several weeks. The accumulation of corrosion product within the blister slows down the corrosion rate and blister growth. The chloride concentration in the occluded solutions within the blister is significantly increased over the bulk concentration, and the pH is often in the acidic range. From electrochemical measurements it can be concluded that the anodic and cathodic reactions are confined to the blister and its immediate surroundings, rather than involving more of the surface over which the coating is intact. Based on corrosion morphology it is concluded that replated copper contributes to the overall cathodic reaction.     

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

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

Co-operative corrosion phenomena

The corrosion of aluminium alloy 2024-T3 (AA2024-T3) was studied as a function of immersion time from 2.5 to 120 min in 0.1 M aqueous NaCl solution. At immersion times as short as 5 min, rings of corrosion product of 100 to 200 μm diameter, containing smaller domes of corrosion product, were observed using SEM. The domes of corrosion product had greater chloride concentrations than elsewhere on the surface and represented sites of anodic attack. As the immersion time was increased, significant grain boundary attack was observed within the rings of corrosion product. Analyses of Particle Induced X-ray Emission (PIXE) maps of the corroded surfaces showed a significantly higher number of IM particles around the chloride attack sites than the average particle density for the maps, indicating clustering of IM particles. These results suggest a co-operative corrosion effect as a result of clustering of the IM particles. A mechanism for the generation of the corrosion rings is discussed.     

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.     

The high throughput assessment of aluminium alloy corrosion using fluorometric methods. Part I - Development of a fluorometric method to quantify aluminium ion concentration

This paper examines a new method developed for the rapid assessment of corrosion inhibitors and their combinations for aluminium alloy 2024-T3 (AA2024-T3). Two fluorometric probes, lumogallion and morin, were used to rapidly assess the concentration of aluminium ions in solution through the use of a spectrophotometric plate reader. Fourteen potential inhibitor compounds and other assay constituents were screened for fluoroprobe interference: sodium chromate, sodium metavanadate, cerium chloride, lanthanum chloride, europium chloride, gadolinium chloride, neodymium chloride, yttrium chloride, barium metaborate, sodium metatungstate, potassium phosphate, sodium metasilicate, sodium phosphate, and sodium molybdate. Calibration curves were established for the two probes.     

Preparation and electrochemical characterization of amoxicillin-doped cellulose acetate films for AA2024-T3 aluminum alloy coatings

Cellulose acetate films doped with amoxicillin were deposited onto AA2024-T3 aluminum alloy and the corrosion protection in 0.05 M NaCl was evaluated by Electrochemical Impedance Spectroscopy (EIS) and the Scanning Vibrating Electrode Technique (SVET). The doping of the cellulose acetate film with amoxicillin resulted in a significant increase in the high frequency resistance and a decrease in the capacitance of the material. The protective effect could be observed under anodic polarization by way of a marked decrease in the anodic current. These results show the promising potential for the deposition of cellulose acetate films doped with amoxicillin onto AA2024-T3.     

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.     

Corrosion inhibition of 7000 series aluminium alloys with cerium diphenyl phosphate

Cerium diphenyl phosphate (Ce(dpp)₃) has previously been shown to be a strong corrosion inhibitor for aluminium-copper magnesium alloy AA2024-T3 and AA7075 in chloride solutions. Surface characterisation including SEM and ToF-SIMS coupled with electrochemical impedance spectroscopy (EIS) measurements are used to propose a mechanism of corrosion inhibition which appears to involve the formation of a complex oxide film of aluminium and cerium also incorporating the organophosphate component. The formation of a thin complex film consisting of hydrolysis products of the Ce(dpp)₃ compound and aluminium oxide is proposed to lead to the observed inhibition. SEM analysis shows that some intermetallics favour the creation of thicker deposits predominantly containing cerium oxide compounds.     

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 2024-T3 aluminium alloy by electro-polymerized 3-amino 1,2,4-triazole in sulphate solution containing chloride

Electro-oxidation of 3-amino 1,2,4-triazole (ATA) on Al 2024-T3 alloy carried out simultaneously to the anodizing process in methanol acetate or in aqueous sulphate medium, leads to the formation of Cu^I-ATA complex upon Cu-rich particles. The anodizing film grows, in both media, according to high field-assisted migration mechanism and the potential field is in the order of 10⁶ V cm⁻¹. Significant protective effect of films towards localized corrosion was observed due to Cu^I-ATA and poly(ATA) formations on Cu-rich particles. These complex layers hinder also the oxygen reduction reaction leading to a lower corrosion rate at the open circuit conditions.     

High-throughput channel arrays for inhibitor testing: Proof of concept for AA2024-T3

A high-throughput test has been developed for screening aqueous corrosion inhibitors on aluminium AA2024-T3. The method adapts basic microfluidic technology to create multiple channels in polydimethylsiloxane, which allow solutions to flow over the surface of the alloy, causing severe corrosion within hours if no inhibitor is present. In three-channel experiments under various channel conditions, corrosion rates were accelerated up to 15 times when compared to standard immersion tests. In addition, 10-channel experiments were conducted to simultaneously test 10 different inhibitors, and the results were compared visually and to actual corrosion results obtained quantitatively via solution analyses.