**5. Conclusions**

PIGE, PIXE and SEM/EDS have been used to study the distribution and chemistry of inorganic components in a polyurethane coating prior to and after 500 h NSS exposure. Prior to NSS exposure, the PIGE results revealed that there was a zone (3–4 μm deep) near the surface of the primer that appeared to have less Li2CO3, thus forming a type of "skin" layer. This "skin" layer had smaller particles of TiO2 and BaSO4. After NSS exposure, the PIGE results indicated that there was a homogeneously depleted zone extending from the electrolyte/primer interface into the primer (in this case around 11 μm), plus local depletion penetrating much deeper into the primer with a maximum measured depth of around 25 μm. The deeper local penetration was confirmed using SEM, where voids created by partial and complete dissolution of Li2CO3 particles were observed extending deep into the primer towards the metal/primer interface. Magnesium hydroxide/oxide particles also appeared to undergo some change with exposure to NSS, with the Mg:O ratio moving closer to 1. The other inorganic particles (TiO2 and BaSO4) appeared unchanged upon NSS exposure. There was some evidence of chloride penetration into the polyurethane component of the primer, but not within the channels created around the Li2CO3 particles.

**Author Contributions:** Peter Visser, Arjan Mol and Herman Terryn conceived and designed the experiments. Peter Visser prepared samples and performed the NSS testing. Herman Terryn, Arjan Mol, Peter Visser and Tony Hughes were involved in the interpretation of data. Jamie Laird performed the nuclear microprobe experiments and was assisted by Tony Hughes and Chris Ryan in data analysis and interpretation. Tony Hughes performed all SEM experiments and interpreted the data. Tony Hughes wrote the paper with input from all other authors.

**Conflicts of Interest:** There are no conflicts of interest.

## **Appendix A**

The influence of subsurface particles can be seen in Figure A1, which shows images of the same region collected at different accelerating voltages. The ellipses, highlighted using dashed lines in Figure A1a, show two regions where, with increasing voltage, subsurface BaSO4 particles become evident. Thus, the contribution from subsurface particles may distort the analysis of small particles.

EDS maps for a section of the primer are displayed in Figure A2. These maps highlight the difficulty of using EDS mapping alone to study the distributions (and redistribution after leaching) of all the phases that comprise the primer coating. Elements such as Cu and Fe result from changes in the background level in the spectral region of the Kα lines of these transition metals. Of course, the presence of an element in a particular region of the map can and should be checked using EDS spectra where a change in background is easily distinguished from a peak. However, overlapping lines are a different issue. This is important from the perspective of following the changes in primer additives, such as with Ti and Ba, where overlapping lines give misleading information on the distribution of these particles. In this case, the overlapping lines show a co-incidence of Ti- and Ba-containing particles on their respective maps (Figure A2). The overlap in the X-ray lines themselves is clearly seen in Figure 4, where the Ti Kα lines overlap with the Ba L-series lines, and the Si Kα overlaps with the Sr Kα lines. This can only be resolved using a fitting of the EDS spectra, such as that which is achieved when quantifying the spectra. This approach has been used in this paper.

**Figure A1.** Backscatter electron images collected at (**a**) 10 kV, (**b**) 15 kV, (**c**) 20 kV and (**d**) 25 kV. Dashed ellipses highlight regions where the backscatter contrast changes significantly with accelerating voltage.

**Figure A2.** Elemental maps collected from the section of the primer prior to exposure to NSS.
