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 BaSO
4, 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 TiO
2 was not easily distinguished on the basis of backscatter contrast, as it was similar to the smaller particles of BaSO
4. 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 BaSO
4), Mg (from Mg(hydr)oxide) or Ti (from TiO
2), implying that they are probably the Li
2CO
3 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 Li
2CO
3 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.
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 TiO
2 particles. Compositions (expressed as ratios of major elements) for the larger BaSO
4 and Mg-(hydr)oxide particles are presented in
Table 3. The analyses indicate for BaSO
4 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 Li
2CO
3, 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 TiO
2 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).
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 BaSO
4 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 Li
2CO
3.
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 SO
42− ions from the anodising process (
Figure 5c) [
57].
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 TiO
2 particles. The large BaSO
4 particles tend to be present as small clusters of two or three particles, which are randomly distributed throughout the coating, whereas the smaller BaSO
4 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 BaSO
4 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.
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 TiO
2, BaSO
4 (and SrSO
4 as impurity) and Li
2CO
3, the Ti, Sr and Ba peaks can be attributed to these species. The Zr may arise from a coating applied to the TiO
2, since Al and Zr compounds are used to stabilise the TiO
2 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.
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 BaSO
4, and is probably present as SrSO
4 (
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.
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 ZrAl
3 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,
60,
61,
62,
63,
64,
65,
66,
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 (Al
20Mn
3Cu
2)), 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 ƞ-phase (Zn
2Mg) 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 Li
2CO
3 (red) and BaSO
4 (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 SrSO
4 in the BaSO
4. From these maps, it is clear that there are regions that are rich and poor in Li
2CO
3 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.