3.1.1. SEM/EDS

Figure 2 gives an overview of a section through the primer. In this section, the primer had a thickness in the vicinity of 35 ± 5 μm, contained a high level of solids and was applied to an anodised layer that was around 2–3 μm, as described in the Experimental section. The primer itself had a high level of inorganics (PVC was approximately 30%), which is reflected in the high density of particles in Figure 2. The brightest particles in the backscatter image are BaSO4, which are the easiest inorganic components to identify. They are generally angular with a range of sizes (slightly less than 1 μm to over 10 μm, which is consistent with around 90% of the particle sizes for this additive (Table 2)), and an aspect ratio slightly larger than one (Figure 3b). There were another group of particles with very little contrast difference from the polyurethane containing Mg, which were assumed to be a mixture of Magnesium oxides and hydroxides, and will be referred to as Mg-(hydr)oxide in the rest of the paper. In many instances, they appeared to have a layered structure where the layers had a thickness typically 250 nm and lengths with a minimum size of around 1 μm, and typically 5–10 μm (Figure 3c), which was again consistent with the particle size distribution determined from the dispersed particles (Table 2). Mg-(hydr)oxide particles without this structure were assumed to be rotated so that the layers were viewed from the top (Figure 3a). The TiO2 was not easily distinguished on the basis of backscatter contrast, as it was similar to the smaller particles of BaSO4. Finally, there are dark particles (indicated in Figure 3a) in the film that show C and O peaks, but no significant levels of Ba (from BaSO4), Mg (from Mg(hydr)oxide) or Ti (from TiO2), implying that they are probably the Li2CO3 particles (Li cannot be detected in standard EDS). The sizes of these particles were similar to those of the free particle size distribution for the Li2CO3 particles (Table 2). This last category of particles has similar greyscale contrast to voids in the coating, making it difficult to distinguish the two without closer examination.

**Figure 2.** Backscattered electron images of sections of the primer prior to exposure to NSS. The mounting medium is at the top of the image, the primer is in the centre, and the AA2024-T3 is at the bottom of all images.

**Table 2.** Inorganic additive size distribution presented as the percentage of particles at a particular size in microns.


Positive identification of each of the inorganic phases using EDS alone is not straightforward. The inorganic particles sizes ranged from less than a micron up to 10 μm for larger particles, which meant that only the large particles could be sampled using EDS, with some certainty that interaction volume effects had been minimised. This can be seen in their respective spectra, where each type

of particle typically contains some signal from other particles due to the interaction volume effect (Figure 4). This effect is largest for the smallest particles, which are the TiO2 particles. Compositions (expressed as ratios of major elements) for the larger BaSO4 and Mg-(hydr)oxide particles are presented in Table 3. The analyses indicate for BaSO4 that the composition is close to stoichiometric, with perhaps a small underestimation of O. For Mg-(hydroxyl)oxide, the data indicates a mixture of MgO and Mg(OH)2. Only C and O were detected in any significant amount for the particles thought to be Li2CO3, but, given that the samples were carbon-coated prior to analysis, it was not possible to conclude anything definitive from the quantitative analyses of these particles. It was not possible to determine the composition of the TiO2 particles because of their small size (Figure 3d). This was not just due to the sampling volume containing some of the polymer matrix, but it might also contain other subsurface inorganic particles (see Appendix A).

**Figure 3.** Examples of inorganic particles that make up the sample configurations for measuring Li depletion. Examples of (**a**) MgO and Li2CO3 partticles, (**b**) BaSO4 particles, (**c**) Mg-containing particles and (**d**) mixed TiO2 and BaSO4 particles. The dark areas spots show where point analyses have been performed.

**Figure 4.** Typical X-ray spectra for the different inorganic components of the primer system. The arrows either point to that indicated the elements positions or sit above them. It can be seen that for each compound, there is some level of "contamination" due to sampling volume effects or small particles. This is most evident for the small TiO2 particles that show significant Mg levels as well as S, Ba and a little Al.


**Table 3.** Composition (at %) from EDS analysis of inorganic particles in the primer. Each datum is an average of five determination on large particles. (Data overaged over four analyses).

Therefore, as discussed in the appendix, quantitative mapping derived from standardless fitting of the EDS spectra from hyperspectral data was used to generate elemental maps (Figure 5). The backscatter electron contrast shows several different types of particles in the primer cross section in Figure 5a, and the phases are identified in Figure 5b, which is a four-colour map of O (red) Mg (blue), Ba (green) and S (yellow). Figure 5c shows the Ti-containing particles (pink), the BaSO4 particles, and highlights the Mg-(hydr)oxide particles. In both Figure 5c,d, there are particles containing O, but none of Ti, Mg or Ba; these particles are attributed to Li2CO3. Figure 5a–d all show an oxide at the interface, which is the anodised coating. S was detected in this layer, presumably due to the incorporation of SO4 <sup>2</sup><sup>−</sup> ions from the anodising process (Figure 5c) [57].

**Figure 5.** EDS mapping results for sample without exposure to neutral salt spray (NSS). (**a**) Backscattered electron image and maps derived from quantitative analysis, and composite maps for (**b**) O-Mg-Ba-S with phase labelling (**c**) O-Mg-Ti, (**d**) O-Ba-S and (**e**) Cl-Ti map. (N.B. colour mixing is not the same as three-colour mixing). The Li2CO3 was assigned on the basis that only O and C were detected at any significant levels. The arrow tip in (**e**) indicates where a very low amount of chlorine was detected.

From Figure 5c, it can be seen that there was a homogeneous distribution of Mg-(hydr)oxide particles in the coating, with larger particles appearing to be randomly distributed throughout the coating. The smaller Mg-(hydr)oxide particles also appear homogenously distributed within the coating. Similarly, Figure 5c suggests a homogeneous distribution of TiO2 particles. The large BaSO4 particles tend to be present as small clusters of two or three particles, which are randomly distributed throughout the coating, whereas the smaller BaSO4 particles appear more evenly distributed. Finally, Figure 5e is a map showing the Ti and Cl distribution. There is only one region where a very small

Cl signal was detected (in the vicinity of the tip of the white arrow in Figure 5e at the periphery of a BaSO4 particle). The rest of the contrast is due to the presence of Ti. This image is included for later comparison with the samples that had undergone 500 h exposure to NSS, and is discussed later.
