Extract from S. Rossi, “Enamel and design. The potential of enamelled materials”, Fausto Lupetti Editore, 2011
The steel armour
The enamel is a coating that protects the metal substrate (steel, cast iron, aluminium) acting as a barrier that separates the metal from the aggressive environment. We can say that the layer of enamel is a real armour, thanks to its very high protective properties. This aspect is related to some characteristics of this layer. As we said, it is composed of glass and therefore it is unassailable by all “normal” environments. Only strongly alkaline environments, or the presence of hydrofluoric acid can damage the layer, but such environments can be difficult to find. Thanks to its structure, it is impervious to water and aggressive ions such as chlorides, and therefore one of the elements necessary for the corrosion reactions fails. The only chance of the substrate corrosion depends on the presence or formation of defects for impact or large deformations, which connect locally the steel to the aggressive environment. But even in this case the enamel behaves very well thanks to its excellent adhesion with the substrate; other protective systems such as paints, as discussed in Chapter 24, do not have the same behaviour. If we try to expose enamelled panels in salt spray chamber (one of the most common accelerated test, which consists in exposure to a fog produced by a 5% solution of sodium chloride at 35 °C – ASTM B117), as for the samples reported in the figure, we can safely say that the enamel is protective for thousands of hours of exposure. If we try to compare it with paints, the first competitor of the enamel thanks to their malleability and colour possibilities, we observe the clear superiority of enamels, especially in very aggressive working condition. As reported in the two red examples (often in buildings both enamelled and painted panels are used, due to their versatility, ease of installation and the possibility of many colours), we can see how, after a short period of time, the paint starts to show signs of time: delamination of the paint and corrosion of the substrate. The problem is accentuated by using a galvanized steel because of the difficulty of painting the galvanized layer. The environment is not particularly aggressive, in fact it is a normal city street. In addition to the reduction or disappearance of the protective properties, also the aesthetic-perceptive properties decrease, as well evident in the figure on the left. Enamel layer still retains its characteristics even after long time of life.
Confrontation of resistance to bad weather, between enamel (on the left) and paint (on the right).