3.1.2. PIXE/PIGE

As reported in the experimental section for the PIXE and PIGE, data analysis begins with the summed spectrum for the complete mapped region. In PIXE, maps are generated by fitting the X-ray spectrum, removing the background, and mapping the net counts under the peaks for the elements of interest. In PIGE, elemental maps were generated by determining the net counts under respective peaks after a local linear background subtraction.

A typical X-ray spectrum (PIXE) extracted for the primer is presented in Figure 6a. The position of the X-ray peaks are the same as in normal EDS, since they involve normal K- and L-series lines; however, the lines are generated by proton interaction rather than electron interaction as in normal EDS. The PIXE spectrum of the AA2024-T3 is shown in Figure 6b. In the spectrum from the primer (Figure 6a), the major peaks are Ti, Ba, Fe, Cu, Zn and Zr. Since the primer includes additives such as TiO2, BaSO4 (and SrSO4 as impurity) and Li2CO3, the Ti, Sr and Ba peaks can be attributed to these species. The Zr may arise from a coating applied to the TiO2, since Al and Zr compounds are used to stabilise the TiO2 particles (Table 1). While the Ba and Ti signals overlap in EDS spectra and maps from the SEM, this effect is considerably reduced in PIXE, because the Ba Kα lines dictate the intensity in the Ba Lα lines in the 5.0–7.5 keV region of the spectrum. Thus, there is only a small residual signal of Ba in the Ti map arising from residual fitting errors.

**Figure 6.** PIXE (X-ray) spectra of the (**a**) primer and (**b**) AA2024-T3. Corresponding PIGE (γ-ray) spectra of the (**c**) primer and (**d**) AA2024-T3. The red lines in (a) and (**b**) are the fitted curves to the spectra. The dashed lines in (**a**) and (**b**) are backgrounds used in the fitting.

Individual PIGE spectra for the primer and the AA2024-T3 can be extracted from the maps, and typical examples are shown in Figure 6c,d, respectively. The Υ-ray spectrum (PIGE) shows Li (peak positions), Al and Mg. Peaks labelled "back" arise from laboratory background signals and are not part of the sample. The Li peak at 429 keV was used for the determination of the Li distribution. For the AA2024, Figure 6d only shows the Al and Mg signals.

The combined PIXE and PIGE maps for a region of a sample prior to leaching is shown in Figure 7. The Li, Ba, Sr and Ti maps clearly show that these elements are present in the coating. Sr is an impurity in the BaSO4, and is probably present as SrSO4 (Table 1). It should be pointed out that some of these elements are present in very low levels, and it is only through the sensitivity of PIXE that they are detected at all.

**Figure 7.** Combined PIXE and PIGE maps for a sample that has not been exposed to NSS. The PIGE maps (Li-PIGE and Al-PIGE) are labelled as such, and the rest are PIXE maps. The schematic on the top shows the sample configuration and indicates the direction and interface from which leaching has occurred. The colour scale is thermal, with warm colours representing higher concentrations.

With respect to the AA2024-T3, the PIXE spectrum of the AA2024-T3 substrate (Figure 6b) shows Al, Cu, Mn, Fe, Zn, Ga and Zr. The Zr may be an additive used in the formation of ZrAl3 used for grain refining [58]. Cu and Mn were detected both in the matrix and constituent IM particles, and Fe only in the constituent particles [59–67]. The presence of Cu and Mn in the matrix can be explained by a small but significant solubility of Cu in Al, as well as Cu and Mn being present in a number of IM particles (hardening precipitates (Cu) and dispersoids (Al20Mn3Cu2)), which are much smaller than the resolution of the technique [68]. Elements such as Ga have been reported before when using Rutherford Backscattering spectroscopy (RBS) to examine aluminium alloys [69]. In some Al-alloys, Zn is used for precipitate hardening using the η-phase (Zn2Mg) in 7xxx series alloys [70] but, again, it is not expected as an alloy addition here, even though Zn is detected in the AA2024-T3 sheet product [60]. In this study, it is associated with Cu-containing constituent particles, and may be present as an impurity from a mixed stock starting material used to manufacture the AA2024.

Figure 8 shows three-colour maps of the primer region, where Li is in red and Ba is in blue for all these maps, and green reflects the changing element. The Li-Cu-Ba map indicates the distribution of the Li2CO3 (red) and BaSO4 (green) particle distributions within the primer, and the Cu (green) reveals relationship of the primer to the AA2024-T3 substrate. The dark band separating the AA2024-T3 from the primer in the Cu map coincides with a purple strip in the Al map on top of the metal. In the middle and top maps, blue is the anodised layer. In the Li-Sr-Ba map, Ba-containing particles are light blue, indicating a mixing of the colours associated with the Sr (green) with the Ba (blue), which confirms the presence of SrSO4 in the BaSO4. From these maps, it is clear that there are regions that are rich and poor in Li2CO3 particles. These regions can be as deep as the coating itself (e.g., point A in Figure 8a) and 20–30 μm wide. There was no suggestion of layering in these maps.

**Figure 8.** Three-colour maps. (**a**) Li-Cu-Ba, (**b**) Li-Ti-Ba and (**c**) Li-Sr-Ba. In all cases, the Li is red, the barium is blue, the middle element is green; i.e., green changes from top to bottom as Cu, Ti and Sr.

## *3.2. NSS Exposure and Li Depletion in the Primer*
