Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

This work presents a theoretical assessment of galvanic (relative) nobility of four intermetallic particles (IMPs), Al 2 Cu, Al 2 CuMg, Mg 2 Si and MgZn 2 , in aluminum alloys through work function calculation based on density functional theory (DFT). The concepts of work function, Volta potential and relative nobility are discussed with respect to the IMPs and aluminum matrix. The calculated Volta potentials are compared with reported experimental Volta potentials measured by scanning Kelvin probe force microscopy (SKPFM). Various crystal faces and terminal types are examined in the DFT calculation, showing that these two factors have a significant effect on the work function value. Considering the large divergence in the reported experimental data, the comparison shows a general agreement between the calculated and experimental Volta potential data for the investigated IMPs. The DFT calculations provide theoretical explanations for several experimental phenomena. The results demonstrate that DFT calculation is a valuable theoretical approach for assessment of the relative nobility of different phases in the alloys, providing complementary information to experimental data from SKFPM. Moreover, the implications of the calculated Volta potentials are discussed with respect to the corrosion potentials. The addition of various alloying elements into the aluminum (Al) matrix to improve mechanical performance of Al alloys may simultaneously increase their susceptibility to localized corrosion. This is mainly due formation of intermetallic particles (IMPs) that may trigger micro-galvanic corrosion along the interface between the IMPs and the alloy matrix.1 The potential differences between the IMPs and the matrix may result in localized dissolution of the matrix or dealloying of the IMP. 2The most common IMPs present in 2xxx and 7xxx series Al alloys include Al 2 CuMg (S-phase), Al 2 Cu (θ-phase), Al 7 Cu 2 Fe (β-phase), MgZn 2 (η-phase) and Mg 2 Si.3-5 IMPs containing Cu and Fe, such as Al 2 Cu. 6 and Al 7 Cu 2 Fe, 7 are considered to be cathodic with respect to the matrix, acting as local cathodes and thus promoting cathodic reactions such as oxygen reduction. In contrast, Mg-rich IMPs, such as Al 2 CuMg, are considered anodic relative to the matrix, resulting in selective dissolution and concomitant Cu-enrichment of the IMP.8 These localized corrosion events may eventually trigger other corrosion forms, such as stress corrosion cracking or intergranular corrosion.9 Therefore, micro-galvanic corrosion effects between IMPs and the Al matrix is considered to be of utmost importance for the overall service performance of Al alloys.In 1987, Stratmann started to use the Kelvin probe to investigate corrosion properties of metals under thin electrolyte films.10 His measurements revealed a linear relationship between Volta potential and corrosion potential of Cu, Zn, Fe and other metals. Later, the scanning Kelvin microprobe was developed and used for Volta potential measurements of several metals in humid atmospheres.11 Du...

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“…47 MgZn 2 in 0.5M NaCl, relative to AlZn5Mg1 alloy. 49 The difference in electrode potential between the IMP and metal matrix, E …”

Section: Resultsmentioning

confidence: 99%

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

Jin

Liu

Zhang

et al. 2017

J. Electrochem. Soc.

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“…Moreover, equilibrium η-MgZn 2 can form coarse particles at the grain boundaries, which are mainly responsible for intergranular and exfoliation attack. [2,3,16,17] Depending on the copper content of the alloy and the heat treatment, the composition of the η-phase can vary nominally from MgZn 2 to MgAlCu. With changing the composition of this phase, the electrochemical properties such as the breakdown potential change, as well.…”

Section: Introductionmentioning

confidence: 99%

Detection of nanoscale η‐MgZn₂ phase dissolution from an Al‐Zn‐Mg‐Cu alloy by electrochemical microtransients

Wloka

Virtanen

2008

Surface & Interface Analysis

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In this article we demonstrate how the dissolution of nanosize intermetallic particles present in Al alloys can be detected using microelectrochemical techniques. The local electrochemical properties of a high-strength Al-Zn-Mg-Cu alloy, which contains nanoscale η-MgZn 2 phases, were investigated with a microcapillary cell. At the open-circuit potential (OCP) with the sample surface in the range of 1000 µm 2 , potential fluctuations (microtransients) can be observed. Under polarization of such small areas, current transients are detected in the passive range of the alloy. An estimation of the size of dissolution events from the charge passed during the current transient leads to the conclusion that the current transients could stem from the dissolution of η-phase particles, with diameters in the order of 100 nm. This size scale corresponds well with the size of the grain-boundary MgZn 2 . Transmission electron microscopy (TEM) and high-resolution SEM images provide further insights for possible mechanisms leading either to potential or to current microtransients.

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“…Quantitative TEM analysis using electron dispersive spectroscopy (EDS) showed that these particles have high contents of Zn and Mg, suggesting that they are η or ή. It was reported that η or ή is more active than the matrix in AA7xxx [27,28]. It is possible that the fine particles in T6 grain boundaries resulted in a faster sharp IGC fissure growth rate and the absence of the small grain boundary particles in the as-received and T7 resulted in a slower sharp IGC fissure growth rate.…”

Section: Temper Effectsmentioning

confidence: 99%

Kinetics of sharp intergranular corrosion fissures in AA7178

Huang

Frankel

2007

Corrosion Science

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During service in structural aircraft applications, AA7178 has been found to develop sharp intergranular corrosion (IGC) fissures, a special form of attack. A new laboratory approach to quantify the kinetics of sharp IGC fissure growth was developed in this work. Sharp IGC fissures, similar to those formed in service, grew in samples that were given an electrochemical pre-treatment and then exposed in a humid environment. The time for the first sharp IGC fissure to penetrate the thin sample was determined by visual observation of the back side of the sample. The depth of the sharp IGC fissure was determined by serial sectioning. The kinetics measured by this approach did not represent the fastest rates, but rather the rates of long sharp IGC fissures. Relative humidity had no measurable effect on fissure kinetics, indicating that there was little connection of the local sharp IGC fissure region with the outside environment. AA7178 in the as-received and T7 conditions exhibited slower sharp IGC fissure rates than in the T6 condition. The sharp IGC fissures were found to be filled with corrosion products, which possibly exert a stress that could play an important role in driving fissure growth. It is suggested that the sharp IGC fissure could be a form of SCC.

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Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

Cited by 29 publications

References 6 publications

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

Detection of nanoscale η‐MgZn₂ phase dissolution from an Al‐Zn‐Mg‐Cu alloy by electrochemical microtransients

Kinetics of sharp intergranular corrosion fissures in AA7178

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Mechanism of Exfoliation (Layer Corrosion) of AI-5%Zn-1%Mg

Cited by 29 publications

References 6 publications

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

First-Principle Calculation of Volta Potential of Intermetallic Particles in Aluminum Alloys and Practical Implications

Detection of nanoscale η‐MgZn2 phase dissolution from an Al‐Zn‐Mg‐Cu alloy by electrochemical microtransients

Kinetics of sharp intergranular corrosion fissures in AA7178

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Detection of nanoscale η‐MgZn₂ phase dissolution from an Al‐Zn‐Mg‐Cu alloy by electrochemical microtransients