*3.3. Measurement of Li-Depletion*

The above data show that Li depletion manifests itself in several separate ways, meaning that there isn't a single metric to describe it. Thus, the depletion depth needs to be determined by inference from the different types of data collected here. In this section, three different approaches to measuring depletion depth are presented.

The first method is based on elemental line profiles across the coating using traverse profiles by analysis of the PIXE/PIGE data as displayed in Figure 13. These are constructed by first choosing a region for the profile, as shown by the green box in Figure 13a. The average counts along a number of lines (e.g., the red line) at fixed spacing are then determined to produce a point (red point) in the line profile. Thus integrated elemental profiles for Ti, Ba, Li and Al across sections of the primer are generated and presented in Figure 13b for the sample without exposure to NSS and that exposed for 500 h (Figure 13c). In this case, Ba and Ti profiles were used as indicators of the coating thickness based on the assumption that their distributions were unaffected by NSS exposure. (The thicknesses determined using the Ti maps tended to be slightly larger than those determined from the Ba maps, which may be due to the Ti particles being finer and closer to the surface of the film). The width of the profiles shown in Figure 13 are larger than would be obtained from individual line profiles, because the traverse method averages along a line (red line) at a particular depth through the cross section of the primer, and thus incorporates surface variation, such as roughness. Figure 13b shows Ti, Li, Ba and Al traverse profiles for the sample without NSS exposure. The average thickness was 33.7 ± 2.4 μm, which was determined from five measurements across the coating. (Note: This is different from the SEM measurement since it is a different region of the sample.) In repeated measurements, the Li profiles showed that there was a region near the surface where there was a lower concentration of Li for the sample without NSS exposure. Such a region is indicated in Figure 13b and labelled the "skin layer". This may be due to fewer smaller Li2CO3 particles compared to the other additives (Table 2). The thickness of this zone of lower concentration (skin layer) appeared to be in the vicinity of 3–5 μm, as determined from line scans (not the traverse method).

The thickness for the sample exposed for 500 h to NSS was 45.4 ± 6.5 μm, which was determined in the same fashion as the sample without NSS exposure. In this instance, the depleted zone near the surface was in the vicinity of 11 μm (Table 4). As stated in the experimental section, the actual Li concentrations are difficult to determine in a complex matrix such as the polyurethane with a heterogeneous distribution of inorganic additives. However, in Figure 13c the Al PIGE profiles have been adjusted to have the same level of counts in the metal, which allows a *qualitative* comparison of the Li profiles. The position of the surface is indicated for both profiles, remembering that the two samples have different coating thicknesses. It can be seen that there is depletion of Li from a greater depth into the primer for the coating exposed to NSS for 500 h. Moreover, there is significant Li depletion from the body of the coating to around 30% of the level in the sample without NSS, indicating a depletion of Li from within the coating. This is probably due to dissolution of the surface on Li2CO3 particles, which are deeper in the coating.


**Table 4.** Characteristic thickness of Li-depletion zones in microns.

A second method for determining the depletion depth was applied based on the local absence of Li from the PIGE Li map of the cross section of the film. In this method, the local depletion of Li was assessed compared to the thickness of the primer coating, as shown in Figure 14a (white dashed line). A drawback of this method is that the Li2CO3 distribution prior to the NSS exposure is unknown, so, while the absence Li2CO3 particles in any particular region of the primer coating is assumed to be due to inhibitor loss, in reality it will include regions where there were no Li2CO3 particles to start with. As expected, the depletion measured using this method shows a larger zone of depletion for the 500 h NSS exposure sample than the sample without exposure to NSS ("SEM 500 h NSS" under "Deepest Depletion" Table 4). The absence of Li at the top of the coating with 500 h NSS exposure is inferred from the EDS measurements presented in Figure 10b. This provides the second method for depletion thickness determination being in the region 9–12 μm, which is similar to that measured using the PIGE traverse approach.

**Figure 13.** Traverse elemental profiles. (**a**) Example of a traverse profile where a region is chosen for the profile (green box) and individual elemental intensities are averaged along a line (red line) to produce a point on a line profile (red point); (**b**) Profiles for Ti (PIXE), Ba (PIXE), Li (PIGE) and Al (PIGE) across the without NSS. Ti and Ba signals are multiplied by five to show the skin layer. (**c**) PIGE profiles for Al and Li without salt spray and after 500 h NSS. Note the thickness of the coatings are different and the surface is indicated by markers. The 500 h NSS results have been adjusted so that the Al intensity from both conditions is the same, thus allowing *qualitative* comparison between the two Li profiles.

A third approach is to use SEM to determine the deepest point where there are voids (assumed to be due to Li2CO3 dissolution) in the coating. An example of void distribution for the sample exposed for 500 h to NSS is shown in Figure 14b, and voids can also be seen in Figure 10a. Summary depletion depths for the deepest depletion are presented in Table 4. Figure 14c is a higher magnification image of the region near the surface showing the extent of interconnection between the voids, which indicate a cluster formation, as previously reported by Hughes et al. for chromate clusters in an epoxy-based coating [72–74]. These data show that local depletion within the coating can be considerably deeper than the homogeneous depletion depth.

**Figure 14.** (**a**) Example of the local dissolution front on the three-colour map of the sample exposed to NSS for 500 h; (**b**) Secondary electron image of the distribution of voids within a region of the sample exposed to NSS for 500 h; (**c**) Enlargement of the region within the square in (**b**) showing the connectivity between the voids. The voids are assumed to be due to the dissolution and loss of Li2CO3 particles.
