Volta Potential Evolution of Intermetallics in Aluminum Alloy Microstructure Under Thin Aqueous Adlayers: A combined DFT and Experimental Study

Abstract

In this work, first-principle density functional theory (DFT) was used to calculate the work function and Volta potential differences between aluminum alloy matrix and two intermetallic phases (Mg 2 Si and Al 2 Cu) with varying surface terminations as a function of adhering monolayers (ML) of water. The calculated data were compared with experimental local Volta potential data obtained by the scanning Kelvin probe force microscopy (SKPFM) on a commercial aluminum alloy AA6063-T5 in atmospheric environments with varying relative humidity (RH). The calculations suggest that the surface termination has a major effect on the magnitude and polarity of the Volta potential of both intermetallic phases (IMP's). The Volta potential difference between the IMP's and the aluminum matrix decreases when the surface is gradually covered by water molecules, and may further change as a function of adhering ML's of water. This can lead to nobility inversions of the IMP's relative to the aluminum matrix. The measured Volta potential difference between both IMP's and their neighboring matrix is dependent on RH. Natural oxidation in ambient indoor air for 2 months led to a nobility inversion of the IMP's with respect to the aluminum matrix, with the intermetallics showing anodic nature already in dry condition. The anodic nature of Al 2 Cu remained with the introduction of RH, whereas Mg 2 Si became cathodic at high RH, presumably due to de-alloying of Mg and oxide dissolution. The DFT calculations predicted an anodic character of both IMP's in reference to the oxidized aluminum matrix, being in good agreement with the SKPFM data. The DFT and SKPFM data were discussed in light of understanding localized corrosion of aluminum alloys under conditions akin to atmospheric exposure.

Passive film characterisation of duplex stainless steel using scanning Kelvin probe force microscopy in combination with electrochemical measurements

“…The naturally formed passive film over the ferrite is different than that of the austenite, giving rise to a typical Volta potential difference of 40-70 mV of numerous DSS's. 12,13,19,25 Laurent et al 24 reported a chromium-rich oxide, in the form of chromia, that became further enriched in the passive film upon passivation in nitric acid, with the thickness, however, not been majorly altered. This may explain the reason why the Volta potential of ferrite and austenite of the DSS were similar after immersion in nitric acid.…”

“…Scanning Kelvin probe force microscopy (SKPFM), instead, is one technique that can provide electronic information in nanometre scale which in turn provide insightful information into electrochemical processes, being helpful in assessing the likelihood of local corrosion events such as micro-galvanic corrosion of multi-phase alloys. [10][11][12][13] It measures the Volta potential with a sensitivity of 1 mV and a spatial resolution of~15 nm. 12,13 The Volta potential is an intrinsic property of a metal and forms with the surface potential the Galvani potential, which describes the real electrode potential of the metal.…”

“…[10][11][12][13] It measures the Volta potential with a sensitivity of 1 mV and a spatial resolution of~15 nm. 12,13 The Volta potential is an intrinsic property of a metal and forms with the surface potential the Galvani potential, which describes the real electrode potential of the metal. [14][15][16][17] The Volta potential is intrinsic as such that it is always present in any condition or environment, and it is a natural, inherent occurrence or phenomenon.…”

“…There is fairly reasonable correlation between calculated Volta potentials by density functional theory and experimentally measured Volta potentials in the presence of oxides as well as adhering monolayers of water, demonstrating its usefulness in assessing the nobility of complex microstructures. 12 In this work, SKPFM was used to assess the intrinsic (electrochemical) nobility, also known as practical or relative nobility, 10,12,18,19 of grade 2507 super duplex stainless steel (DSS) exposed to different electrochemical conditions in combination with EIS and potentiodynamic polarisation analyses performed in similar environments. Information obtained from conventional electrochemical measurements can be correlated with the Volta potential, if reported in as-measured values (e.g.…”

Toward Improving Ambient Volta Potential Measurements with SKPFM for Corrosion Studies

“…Employed under either ambient, inert, or electrolytic environments, SKPFM can spatially resolve features at the nanoscale, [3][4][5] and has there fore found use in a wide range of diverse applications, including semiconductor [4][5][6][7] and electrical device characterization, [8][9][10][11][12] as well as corrosion studies. [13][14][15][16][17][18][19][20] SKPFM produces nanoscale maps of Volta potential differ ences (VPD or ψ), which can be calculated theoretically from the difference in work function between the sample surface and the probe (which acts as a pseudo reference), as shown below in 3, [21][22][23] Equation 1.…”