**1. Introduction**

Over recent decades, there has been a widespread search for alternatives to chromate inhibitors in paints for many applications where corrosion is a threat to aesthetic and structural quality. Many new inhibitor systems have been the subject of extensive research, including rare earths [1–14], vanadates [15,16], organic compounds [17–23], sacrificial particles or functional properties in coatings [24–28], double-layered hydroxides containing inhibitors [29–41] and, in many cases, combinations of these. The search for alternatives is probably most intense for aerospace applications, where chromate inhibitors have been the mainstay of corrosion prevention for many decades. This is because chromate has a proven performance, particularly in the parts of aerostructures that are difficult to access, where many years may pass between inspections. It is in these applications that chromate has demonstrated its reliability [42].

Recently, Visser et al. reported on the promising potential of Li-based inhibitors as chromate replacements for application to aluminium alloys used in the aerospace industry [43]. The successful inhibition of Al, exposed at defects through a primer by Li inhibitors, was proposed to be due to the formation of a hydrated aluminium oxide incorporating Li [44–46]. The source of the Li was from Li2CO3 particles added as a leachable inhibitor to the paint system. The successful application of Li-based inhibitors in coatings, however, requires a detailed knowledge of how to incorporate the inhibitor into a primer formulation, as well as an understanding of the mechanism of leaching. Thus, detailed characterisation of the coating in the as-formulated state, as well as after exposure to conditions where inhibitor leaching occurs, is required.

The objective of this work is to characterise a polyurethane primer, particularly its inorganic components (Li2CO3 inhibitor, TiO2, BaSO4 and Mg-(hydr)oxide) in the as-formulated state, as well as after neutral salt spray (NSS) exposure. Comparing the primer without NSS exposure to that after NSS exposure will provide important insights into changes in the primer chemistry, including changes to the inorganic components, particularly the Li2CO3 inhibitor, resulting from NSS exposure. From a characterisation perspective, Li is one of the more difficult elements to detect in the periodic table. This is due both to the small number of electrons as well as the low interaction cross section for techniques based on electron or photon interaction. On the other hand, the Li nucleus has a relatively high cross section for proton interaction (depending on the proton energy) resulting in γ-ray emission. The method that utilises this interaction is called particle-induced γ-ray emission (PIGE), and a preceding paper on the application of this technique on the Li2CO3-loaded primer studied here has recently been reported [47]. Both PIGE and X-ray emission (PIXE) occur using proton beams, with the X-ray emission resulting from the same transitions as can be seen using energy-dispersive X-ray spectroscopy, but with proton excitation rather than electron excitation. The use of these techniques is not widespread in the corrosion and coatings communities, since nuclear microprobes using MeV ion beams from particle accelerators are rare compared to standard instruments available to research and industry. However, PIXE has been used for corrosion studies [48,49], as well as studies into inhibitor depletion at defects in coatings [50]. In this study, PIGE, supported by PIXE and SEM/EDS has been used to characterise the inorganic components of the primer system, including the Li2CO3, Mg-(hydr)oxides, BaSO4 and TiO2 particles. Of course, after NSS, the focus is on the change in the Li distribution, since this is the leachable component of the system.
