3.2.1. SEM/EDS

As can be seen from Figure 9, exposure to NSS for 500 h resulted in the generation of voids in the primer, which is assumed to be due to the loss of Li2CO3 particles. At low magnification, it is difficult to determine the depth of depletion due to the contrast similarity between the Li2CO3 particles and voids created by their dissolution. On closer examination of individual sites, however, it was clear that the depth of Li2CO3 depletion varied considerably from place to place along the section of the primer. In some places, the removal of Li2CO3 particles was from quite deep into the primer to near the metal/primer interface. Examples in Figure 9a shows the presence of voids due to partial/complete dissolution of particles, and is magnified in Figure 9c. These voids appear approximately halfway through the depth of the coating. Figure 9d shows the development of interfacial voids between the Li2CO3 particles and the polyurethane matrix, in this instance near the primer/anodised/metal interface. Moreover, channels were often observed at the base of some of these voids, suggesting that they are not isolated, but connected to other voids. The delamination between the inhibitor particles and the polyurethane indicates the possible development of further transport networks within the coating system, as well as changes in chemistry at the interface between these particles and the polyurethane.

Figure 10a shows a backscatter image of a section of the primer with the various inorganic additives as discussed above. The quantitative maps in Figure 10b and c show that there is an absence of large particles near the surface, which only have an oxygen (and carbon) signal and inferred to be Li2CO3. The Mg-(hydr)oxide, TiO2 and BaSO4 distributions appear to be similar to those of the primer without NSS. However, the quantitative analyses show that S and O decrease after NSS exposure of the BaSO4, perhaps suggesting a loss of sulphate ions (the loss of S is roughly 25% the reduction of O) (Table 2). The Mg-(hydr)oxide data shows that the Mg:O ratio has decreased from 1:1.6 to 1:1, suggesting that a mixture of MgO and Mg(OH)2, present prior to NSS, may have been transformed to MgO after NSS exposure. The origin of this transformation is unclear, since MgO is more soluble than Mg(OH)2 under a range of conditions [70], and it would be expected that exposure to the electrolyte would result in an increase in the hydroxide. Lastly, it is worth noting that Cl was detected in the

coating after NSS exposure, whereas it was not detected prior to NSS (Figure 11). In the sample after NSS exposure, the chloride appeared to be confined to the polyurethane and was not in either the voids left by the dissolution of the Li2CO3 particles or delamination around them (Figure 11). As, an example the spectrum from the region indicated by the circle in Figure 10a is shown in Figure 11. The implications of these results will be discussed in more detail below.

**Figure 9.** (**a**) Secondary and (**b**) backscattered electron images of voids resulting from the dissolution of Li2CO3 inhibitor particles after 500 h NSS exposure; (**c**) Magnification of (**b**); (**d**) Interfacial voids between the primer and the Li2CO3 inhibitor particles deep in the primer near the primer/anodised layer interface. Dashed box in (**a**) indicates the region in (**c**).

**Figure 10.** EDS mapping results for sample with 500 h exposure to NSS. (**a**) Backscattered electron image and maps derived from quantitative analysis and composite maps for (**b**) O-Mg-Ba-S with phase labelling (**c**) O-Mg-Ti, (**d**) O-S-Ba and (**e**) Na-Ti-Cl map. (N.B. colour mixing is not the same as three-colour mixing).

**Figure 11.** Sample spectra from X-sections of the polyurethane coatings without exposure to (from Figure 5) and 500 h exposure to NSS (Figure 10), respectively. The points from which the spectra have been taken are indicated in each figure by the dashed circle.
