*2.1. Materials and Sample Preparation*

The primer (coating) was a high-solids formulation based on a polyurethane resin with polyisocynate crosslinker, and formulated to a pigment volume concentration (PVC) of 30%, as described elsewhere [43]. The inorganic pigments included Li2CO3, Mg-(hydr)oxide and BaSO4 fillers and TiO2. Trace element analyses of the various inorganic components of the paint indicated that the BaSO4 contained 0.9%m/m Sr, and small amounts of Si, Ca, Al and Ti. The MgO contained 1500–1900 ppm by weight of Ca, 400–700 ppm by weight K and lesser amounts of other elements (Table 1). The Li2CO3 contained alkali metals (Na, K) in the range 400–800 ppm by weight. Particle size distributions for these additives were determined by dispersing in a solvent, which was methylethylketone for Mg-(hydr)oxide, TiO2 and Li2CO3, where water was used for the BaSO4. The BaSO4 particles had the largest particles (up to 50 μm) and the largest spread in particles size. The TiO2 particles were the smallest (up to 14 μm) and slightly smaller than the Mg-(hydr)oxide. The Li2CO3 had the largest size at the lowest end of the distribution and ranged up to 18 μm.

AA2024-T3 was used as a substrate for coating; typical breakdown for this alloy is reported elsewhere [51]. The AA2024-T3 was prepared by standard anodising according to aerospace requirements (AIPI 02-01-003) at Premium AEROTEC, Bremen Germany. This included the following steps: degrease, alkaline clean, acid desmutting followed by anodising in tartaric-sulphuric acid to produce a 2–3 μm thick oxide layer. Subsequently, the primer was applied by spraying using a high-volume, low-pressure (HVLP) spray gun in a single pass to achieve a dry film thickness of approximately 30 μm. In practice, the coating was typically 30 to 40 μm. Finally, the primer was cured for 16 h at 23 ◦C/55% RH, followed by a 30 min baking cycle at 80 ◦C.


**Table 1.** Chemical composition of inorganic additives used in this study.
