Organic coatings degradation: comparison between natural and artificial weathering

The protective properties of organic coated galvanized steel for outdoor applications are generally evaluated by means of accelerated laboratory tests, including electrochemical techniques. An open question is how these accelerated tests can be correlated with natural exposure in different environments. It is well known that many papers did not found any significant correlation between the natural exposure and laboratory tests. In this paper we propose a different approach. The starting point is the consideration to define the natural weathering by monitoring a few different environmental parameters affecting the organic coatings properties (energy from UV radiation, wet time, temperature, etc.), and not just considering the time of exposure. We studied two different galvanized materials: Zn and Zn–Al coatings, three different primers and one top coat. The samples were exposed for 12 months in three different natural environments: Trento (Italy), alpine region, Marseille (France) by the Mediterranean sea and Daytona (USA), a humid tropical environment. At the same time all the meteorological parameters were collected. The coatings weathering was evaluated in the laboratory after some different accelerated tests (salt spray chamber, UV exposure, cyclic test) and after natural exposure. A correlation between the “natural stresses” and the “artificial stresses” was investigated.