*2.2. NSS Exposure*

The primer was exposed to neutral salt spray (NSS) for 500 h in a test chamber operated according to ASTM B117 [52].

## *2.3. Particle Induced* Υ *(PIGE) and X-ray Emission (PIXE)*

PIXE and PIGE were performed on the CSIRO beamline attached to a pelletron at the University of Melbourne [53]. Three MeV protons were focussed at the target plane to around 2 μm using a separated quintuplet lens designed for optimal balance between high spatial resolution and maximum beam current. For this work, beam currents were typically in the 0.5–1.0 nA range. A large area Ge(Li) γ-ray detector was placed approximately 5 mm directly behind the sample for a maximum acceptance solid angle. A LiF crystal and pure Al were used for the calibration of the γ-ray detector energy axis. For PIXE, the 100 mm<sup>2</sup> Ge(Li) detector was mounted at 45◦ to the incident proton beam, and around 3 to 4 mm from the sample. A 100 μm thick pure Al filter was placed in front of the detector to accommodate trace level heavy element detection limits. Scan areas chosen for analysis varied, but generally ranged from 10 to 50 μm × 200 μm. The analysis depth was approximately 10–20 μm for both methods. A schematic for data collection using MicroDAQ [54] is presented in Figure 1. For data collection, the sample is moved in a grid of points under the proton beam. At each point, PIGE and PIXE spectra were collected, forming a pixel in a map and making a hyperspectral data set.

After collection, further data analysis was performed using GeoPIXE [55] where regions of interest (ROI) such as the primer, aluminium alloy or depletion zones were examined in more detail by extracting spectra from each of these ROIs. Both the Li 429 and 478 keV lines were considered for PIGE Li analysis, but only the 429 keV peak was employed due to its greater surface sensitivity [47]. In this case, inelastic proton scattering from the nucleus (written 7Li(p, p', γ) generates a clear 429 keV γ-ray signature, making PIGE an excellent technique for following changes in the Li distribution, since Li comes only from the primer in this study [56]. For PIXE, the signature K and L-series X-ray emission lines were used for element identification.

**Figure 1.** (**a**) Flowchart and schematic representation of the collection and analysis of X-ray emission (PIXE) and γ-ray emission (PIGE) data. The sample is scanned under the beam, and at each step a pixel is generated containing a PIGE and PIXE spectrum, thus forming a hyperspectral dataset; (**b**) After the end of the data collection, a total spectrum for the sample must be analysed to extract the regions of interest (ROI), from which quantitative analysis can be performed if standards have been collected. Examples of maps, ROI in plan view and PIGE and PIXE spectra from the green region within the primer; (**c**) Samples can be examined in section or plan view, depending on how they are mounted.
