# Porcelain Enamel Coatings for Building Facades
## 3 Results and Discussion
### Accelerated Weather Tests  
In the architectural sector, materials are frequently in contact with alkaline concrete matter and concrete products. Enamel coatings suffer in the presence of a basic pH [38], even if adjustments in the type and/or amount of mill additives were shown to improve the resistance of this type of material toward an alkaline pH [39; 40]. The EN 12206-1 standard [28] was thus followed to test the resistance of the enamel coatings to basic concrete by evaluating variations in the color and gloss of the samples after contact with concrete matter. The contact with concrete did not alter the gloss of the enamel coatings, whereas it had a considerable impact on the gloss of the painted reference samples (Figure 4a). Both paints experienced a dramatic loss reduction already after the first application of concrete, with a decrease of about 20 gloss units. Then, gloss values stabilized for the red paint, while the white paint had a further reduction in gloss during the following two concrete applications. Similar results were obtained in terms of color variation (Figure 4b). The color stability of a coated surface is an essential aesthetic parameter in addition to gloss. According to the CIELab color space [30], color is identified by three parameters: (1) L* is the lightness and ranges from 0 (black) to 100 (white); (2) a* indicates the red-green scale (negative \(\Delta\)a* values mean a shift toward green, while positive \(\mathrm{a}\)* variations mean a shift toward red); (3) b* indicates the yellow-blue scale (for positive \(\Delta\)b* values, a shift toward  
Figure 3: Optical micrographs of (**a**) red enamel, (**b**) white enamel, (**c**) red paint, and (**d**) white paint after the impact test. yellow is observed, while negative b* variations are observed for shifts toward blue). The total color variation (\(\Delta\)E*) is calculated as:  
\[\Delta\mathrm{E}=[(\Delta\mathrm{L}^{*})^{2}+(\Delta\mathrm{a}^{*})^{2}+(\Delta \mathrm{b}^{*})^{2}]^{1/2}. \tag{1}\]  
The enamel coatings demonstrated exceptional color stability, exhibiting \(\Delta\)E values below 1 point. On the other side, the tested organic coatings showed a significant color variation, with the red paint showing more pronounced changes compared to the white paint. At the end of the test, the former had indeed a \(\Delta\)E close to 20 points, while the latter had lower \(\Delta\)E values. The different results for paints and enames can be attributed to the varying degrees of permeability exhibited by the two coating typologies. Enamels do indeed tend to have a lower permeability than paints, which restricts the diffusion of alkaline substances and thus minimizes chemical interactions, which may eventually lead to color changes. On the other side, paints have higher permeability. Accordingly, in addition to the significant color variation, by the end of the test, the surface of the painted panels was covered with blisters. The salt spray test was carried out to investigate the adhesion and resistance of the coatings in corrosive and aggressive environments [31]. The enamel coatings showed excellent resistance. Indeed, after 1440 h of exposure, the enameled surfaces were in good condition without the presence of defects (Figure 5a). On the other side, after one week of salt spray exposure, blisters started to appear around the scratch of the painted samples, with a subsequent spread over the surface. Painted panels were exposed to salt spray just for 668 h, and the coating was severely damaged after that time. In fact, the surface appeared to be characterized by swelling and completely covered by blisters (Figure 5b). Moreover, the adhesion between the coating and the steel substrate was investigated according to the UNI EN ISO 4628-8 standard [32]. Delamination was computed as:  
\[\mathrm{d}=(\mathrm{d}_{1}-\mathrm{w})/2, \tag{2}\]  
where \(\mathrm{d}_{1}\) is the mean width of delamination, and \(\mathrm{w}\) is the width of the original scribe.