Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al2CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al2CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current–potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys

Statistical Study of the Corrosion Behavior of Al2CuMg Intermetallics in AA2024-T351 by SKPFM

A statistical study combining atomic force microscopy, scanning Kelvin probe force microscopy (SKPFM), and energy-dispersive spectroscopy was carried out on more than 300 Al2CuMg intermetallic particles of AA2024 alloy to determine their corrosion behavior in chloride-containing solutions. The combination of these three techniques allowed the correlation of the dissolution depth of the S-phase particles to their SKPFM potential and their chemical composition. This study also revealed that SKPFM measurements must be carried out with many precautions, but it is a powerful tool for the study of localized corrosion

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

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

Combination of AFM, SKPFM, and SIMS to Study the Corrosion Behavior of S-phase particles in AA2024-T351

The dissolution mechanism of S-phase particles in 2024-T351 aluminum alloy at open-circuit potential in chloride-containing sulfate solutions was investigated using atomic force microscopy (AFM), scanning Kelvin probe force microscopy (SKPFM), and secondary ion mass spectroscopy (SIMS). The combination of the three techniques allowed the correlation between SKPFM measurements and the corrosion behavior of AA2024 to be confirmed, leading to a better understanding of the electrochemical behavior of S-phase particles. A three-step mechanism for the dissolution and accompanying processes occurring near S particles was proposed: (i) preferential aluminum and magnesium dissolution, (ii) galvanic coupling between the copper-enriched particles and the surrounding matrix, leading to an increased passivity of the matrix around the particles, and (iii) copper deposition around the corroded particles

Density functional theoretical study of Cun, Aln (n = 4–31) and copper doped aluminum clusters: Electronic properties and reactivity with atomic oxygen

A DFT study of the electronic properties of copper doped aluminum clusters and their reactivity with atomic oxygen is reported. Firstly we performed calculations for the pure Cun and Aln (n = 4, 9, 10, 13, 25 and 31) clusters and we determined their atomization energy for some frozen conformations at the B3PW91 level. The calculated work functions and M–M (M = Cu, Al) bond energies of the largest clusters are comparable with experimental data. Secondly, we focused our attention on the change of the electronic properties of the systems upon the substitution of an Al atom by a Cu one. This latter stabilizes the system as the atomization energy of the 31- atoms cluster increases of 0.31 eV when the substitution is done on the surface and of 1.18 eV when it is done inside the cluster. We show that the electronic transfer from the Al cluster to the Cu atom located at the surface is large (equal to 0.7 e) while it is negligible when Cu is inserted in the Aln cluster. Moreover, the DOS of the Al31 and Al30Cu systems are compared. Finally, the chemisorption energies of atomic oxygen in threefold sites of the Al31, Cu31 and Al30Cu clusters are calculated and discussed. We show that the chemisorption energy of O is decreasing on the bimetallic systems compared to the pure aluminum cluster

Electrochemical Behavior of Magnetron-Sputtered Al–Cu Alloy Films in Sulfate Solutions

Model alloys, generated by magnetron sputtering, have been employed to understand the role of copper on the corrosion behavior of aluminum alloys. Binary Al–Cu alloys, with copper contents between 0 and 100 atom %, were synthesized with well-controlled compositions, embracing single-phase alpha and theta alloys together with multiphase alloys. Electrochemical measurements confirmed the stability of the thin alloy films and revealed that the corrosion behavior of the alpha, theta, and eta2 phases differed strongly in the cathodic region. Further, in the anodic region, phases of high copper content suffered pitting in sulfate solutions, while the alpha phase remained passive

Empirical propagation laws of intergranular corrosion defects affecting 2024 T351 alloy in chloride solutions

In the present work, a first attempt was made to determine propagation laws of intergranular corrosion defects for Al 2024 T351 in various NaCl solutions as a first step for future predictive modeling of 2024 alloy. In a first step, the effect of chloride concentration on the susceptibility to intergranular corrosion of 2024 alloy was studied using current–potential curves. In a second step, conventional immersion tests were performed in chloride-containing solutions and statistical analysis was carried out to determine the depth of the intergranular corrosion defects, depending on the chloride concentration and on the immersion time. The results were compared to those obtained by measuring the load to failure of precorroded tensile specimens versus preimmersion time in a chloride solution. The latter method was selected to measure the depth of the intergranular defects even though results showed that it was not possible to use it for chloride concentrations higher than 3 M and immersion times longer than 1200 h, considering the chloride concentrations and the durations of immersion studied in this work. Thus, empirical propagation laws are proposed for chloride contents as high as 3 M and immersion times as long as 1200 h

Intergranular Corrosion of 2024 Alloy in Chloride Solutions

Experiments were performed to determine the propagation kinetics of intergranular corrosion on 2024 aluminum alloy immersed in 1 and 3 M chloride solutions. Tests consisting of immersion in a corrosive solution followed by optical observations on sectioned samples were carried out. This method was found to be time consuming and led to a lack of reproducibility due to the random nature of the corrosion attacks. Another method proved to be more efficient; it consisted of measuring the load to failure on precorroded tensile specimens vs preimmersion time in an aggressive environment. This method was found to allow the mean depth of the corrosion defects to be determined. Further, in 1 and 3 M chloride solution, intergranular corrosion led to the formation of a nonbearing zone, the thickness of which was equal to the mean depth of the corrosion defects. This corroded zone explained the premature failure of the specimens when a uniaxial tensile stress was applied

Localized approach to galvanic coupling in an aluminum–magnesium system

The corrosion behavior of a pure aluminum/pure magnesium couple in a weakly conductive sodium sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method algorithm. Magnesium acted as the anode of the system while oxygen and water were reduced on aluminum. Calculations predicted a large current peak at the Al/Mg interface related to a local increase in both Mg dissolution and oxygen and water reduction on aluminum, leading to a local pH increase. Optical and scanning electron microscope observations confirmed the strong dissolution of magnesium concomitantly with depassivation of aluminum at the Al/Mg interface. Local electrochemical impedance spectroscopy showed the detrimental effects of the galvanic coupling both on aluminum and magnesium

Galvanic Coupling Between Pure Copper and Pure Aluminum Experimental Approach and Mathematical Model

The corrosion behavior of a pure aluminum/pure copper couple in a weakly conductive sulfate solution was investigated. Potential and current distributions on the surface of the model couple at the beginning of immersion were obtained by solving the Laplace equation using a finite element method (FEM) algorithm. The potential distribution predicted by the calculations was checked using a Ag/AgCl microreference electrode. A good agreement was found between experimental and theoretical results. It was shown that the reaction occurring at the copper electrode was oxygen reduction, while aluminum remote from the Al/Cu interface remained in the passive state. Moreover, calculations predicted a large cathodic current, related to an increase in oxygen reduction, restricted to copper at the Al/Cu interface. This led to a local pH increase reaching values higher than 9, allowing the dissolution of aluminum to occur close to the interface. Combining these data with optical and scanning electron microscope observations after 24 h of immersion in the sodium sulfate solution allowed a three-step mechanism to be proposed to explain the corrosion damage, and particularly the presence of a copper deposit on the aluminum surface, some distance from the Al/Cu interface, a phenomenon currently observed in commercial copper-rich aluminum alloys