**4. Discussion**

In this paper, the combination of SEM/EDS and PIXE/PIGE have been used to investigate the distribution and chemical composition of inorganic components in a polyurethane coating prior to and after NSS exposure. The SEM/EDS results show that particle sizes observed in the coating are similar to those measured on free particles. They also show that the smaller particles of each type tend to be more homogenously dispersed than the larger particles. In the case of Li2CO3, which has fewer small particles, this appears to result in a "skin layer" near the surface where there are far more smaller TiO2, BaSO4 and Mg(hydr)oxide particles.

It was shown that (semi-) quantitative analysis can be used to separate the distribution of TiO2 from BaSO4 in SEM/EDS, and in PIXE these are separated using the Ba Kα line. PIXE was also able to identify the presence of Sr in BaSO4. Upon exposure to NSS, SEM/EDS was able to provide useful information on the loss of Li2CO3, even though Li could not be directly detected. This was achieved using a combination of PIXE and PIGE to show that there was loss of Li accompanied by a change in the morphology of the coating through the formation of voids via dissolution of Li2CO3 particles. Moreover, there were other changes in the coating, such as the change in the Mg:O ratio of the Mg-(hydr)oxide upon NSS exposure.

The results presented here show that the determination of a Li2CO3 depletion depth in organic coatings is complicated by the types of technique available to detect Li, as well as the morphology of the depletion itself. In the former case, none of the techniques presented here can be individually be used to determine the level of leaching. In the case of the PIGE, while it is possible to map the Li distribution and show that there is a region of homogeneous depletion as well as possible local depletion, this data needs to be supplemented by SEM and PIXE. In the case of SEM, it is necessary to ensure for regions of apparent depletion that voids, which indicate the depletion, have been created through the dissolution of Li2CO3 particles. This is to distinguish leaching phenomena from inhomogeneous distributions of Li2CO3 particles. PIXE is also required to supplement the PIGE results, since other inorganics in the coating act as markers for the coating thickness. Examples include the Ba and Ti distributions, which have been used here to determine the coating thickness.

The combination of PIGE/PIXE and SEM/EDS reveal that depletion of Li from the Li2CO3 loaded primer after 500 h NSS exposure is a complex process. First, there is the development of a uniform depletion zone from the surface. The changes to Mg-(hydr)oxide may lead to additional pathways for the release of Li2CO3 that has not been released up to that point. The presence of local depletion of Li2CO3 penetrating further into the coating below this zone, rather than uniform depletion, suggests that clusters of Li2CO3 particles are involved in the release process. Release from a cluster involves (i) direct connection of the cluster to the external electrolyte and (ii) gradual dissolution from the cluster. The gradual dissolution need not necessarily move as a "front" through the cluster (i.e., particles nearer the surface must completely dissolve before the next particles can dissolve), but may occur through the simultaneous dissolution of particles at different depths in the clusters, which is dictated by transport of the inhibitor through the electrolyte in the cluster/void structure. The creation of these voids generates a fractal network that acts a porous medium. Small voids and delamination from the polyurethane observed around Li2CO3 particles support this hypothesis. The detailed dynamics of the release would need to be determined as a function of time and cannot be revealed here, since only two times were examined. These concepts are summarised in Figure 15. It should be noted that in the chromate case, these pathways were important because the size of the chromate ion meant that it could not diffuse through the epoxy, only through channels created by the dissolution of the chromate particles. In the case of Li2CO3 dissolution, while it is possible that the Li ion might be small enough to diffuse through the polyurethane by itself, it is much more likely to diffuse through channels connected to the external electrolyte and created by the dissolution of clusters of Li2CO3 particles themselves.

Finally, Figures 5e and 10e show a Ti-Cl and a Ti-Cl-Na map for 0 h NSS and 500 h NSS, respectively. Neither Na nor Cl was detected in the primer coating without exposure to NSS; however, both were detected in the sample exposed to NSS for 500 h (Figure 10). The intensity of Na and Cl varied across the sample exposed to 500 h NSS, which suggested separate pathways for the diffusion of each of these ions. It was not significant in any of the larger inorganic particles, and only appeared in the polyurethane matrix. Significantly, it was not observed near channels created by the dissolution of the Li2CO3. In a model where the leaching is via transport paths created by the dissolution of the Li2CO3 particles, the role of the external electrolyte needs to be considered. The voids created by the interfacial interaction of inhibitor particles or their complete dissolution appear to be clean in the SEM studies presented here. This suggests that there must be liquid in the voids that is lost during the sample preparation process, for if there were precipitates in these voids, the preparation procedure would capture it (non-polar solvents were used for preparation, so dissolution of precipitates is unlikely). Na and Cl were only detected away from the Li2CO3 particles, either in the polyurethane matrix or in other pathways originating from interfaces between the polyurethane and the non-inhibitor inorganic components. This suggests that only water without the salt components (Na or Cl) diffuses into the inhibitor pathways. This is probably because these pathways, generated by Li2CO3 dissolution,

quickly become saturated with ions from the inhibitor phase, providing an ionic barrier to ions of the external electrolyte. It could also be concluded from these observations that the transport of electrolytes through the coating is complex, with multiple and separate pathways for the external electrolytes and the "internal electrolytes" (water with inhibitor ions): an area that warrants further investigation.

**Figure 15.** Model of leaching from the Li2O3-inhibited primer based on the observations for 500 h exposure to NSS. Leaching appears to occur both uniformly as well as locally. The uniform depletion appears to be associated with changes in coating, whereas the local depletion appears to be associated with selective removal of Li2O3 particles.
