Imaging and quantifying the chemical communication between single particles in metal alloy

Abstract

The communication within particle agglomerates in industrial alloys can have significant impact on the macroscopic reactivity, putting a high demand on the adaptation of wide-field methodologies to clarify this phenomenon. In this work, we report the application of correlated optical microscopies probing operando both local pH and local surface chemical transformation correlated with identical location scanning electron microscopy to quantify in-situ the structure-reactivity of particle agglomerates of foreign elements in Al alloy. The optical operando analyses allow (i) to reveal and quantify the local production of OH- from proton and oxygen reduction at individual Si- or Fe-rich microparticles and (ii) to quantify (and model) the chemical communication between these active sites, within a few µm range, on the local chemical transformation of the material. Wide-field image analysis highlights the statistical importance of the chemical communication that may introduce a new conceptual framework for the understanding the mechanisms in related fields of charge transfer, electrocatalysis and corrosion.

Unsupervised Analysis of Optical Imaging Data for the Discovery of Reactivity Patterns in Metal Alloy

“…The absolute value of the OHflux kinetics of the Al(OH)3 was not measured in this work, but values were chosen within the range reported in the literature and summarized in ref. [37] in order to qualitatively match the observed phenomenon.…”

“…EDX analysis shows that the particles are rich in Si and are composed of the SiO2 phase, as reported in our previous work. [37] It should be noted that Fe-rich particles were also observed in some areas of the sample (SI-2). However, since there was no significant evolution of surface films over Fe-rich particles within the time frame of the experiment (as shown in SI-2 and previous work [37] ), only the reactivity over Si-rich particles will be discussed in the main text.…”

“…Si-rich particles are galvanically coupled to the less noble Al matrix and serve as centers for the generation of OHdue to ORR and HER (Figure 1b) (as reported in ref. [37] using fluorescence microscopy). The pH gradients control the dynamics of surface film evolution, as illustrated from the Al(III)-pH diagram in Figure 1c ).…”

“…Our studies on Al6061 alloy (herein and ref. [37] ) showed that a distance increase from 2 µm to 6 µm was sufficient to inhibit interparticle communication and promote intraparticle communication. Automated image analysis was crucial in detecting this trend, as it may have gone unnoticed using traditional manual methods.…”

“…[34][35][36] When exposed to an electrolytic solution, these particles initiate multiple localized electrochemical reactions across a large surface area, making Al alloys a suitable model system for the development of pattern recognition algorithms. In our recent study, [37] utilizing correlative techniques such as optical (reflectivity, fluorescence) microscopies, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX), we observed that all particles within an Al6061 alloy drive pH basification through the reduction of oxygen (ORR) and evolution of hydrogen (HER), leading to localized dissolution and precipitation of films. These film evolution patterns are primarily defined by the overlapping of OHdiffusion fields between adjacent particles, also referred to as interparticle chemical communication.…”