It is the polymer coating applied to rolled metal that is the first to assume all possible environmental influences, be it precipitation, wind, ultraviolet radiation, high and low temperatures, the oxidative effect of the atmosphere, and other chemical and physical effects. In order to study the polymer coatings durability under real operating conditions, actual tests are carried out.
"Aging" of coatings under atmospheric effect is a process at which UV radiation and moisture cause polymer destruction on the surface of the coating, as a result of which the pigment and filler particles remain unprotected. On the surface it manifests itself in the so-called chalking effect of coatings (chalk resistance), thus, the degree of chalking is a measure of weather resistance. In accordance with international practices, the chalking is evaluated on a scale from 1 (strong chalking) to 10 points (no chalking), while in the range of 10–6 points there is no change of color. Russian GOST 9.401 provides an assessment of the coating decorative appearance after tests for resistance to influence of climatic factors from 1 (without change) to 5 points.
Panels are placed on a test bench oriented to the south at an angle of 45°, because it has been experimentally established that this position is equivalent to exposure on a vertical surface, but the test is 2-2.5 times faster. Through the established intervals of time, the coatings are photographed and the degree of chalking (DIN 53159) and the color change (DIN 6174) are determined. Moreover, this is standard practice to determine the following parameters: film coating integrity, color stability, coating resistance to contamination, gloss stability.
Corrosion tests in artificial atmospheres. Salt spray tests
Modern protective polymer coatings are sufficiently resistant to corrosion under normal conditions. To carry out accelerated tests on coating durability, a salt spray test method is used However, the correlation between the salt spray resistance and the expected life of the coating is rather weak, because corrosion is a very difficult process and a lot of other factors may influence it. Nevertheless, salt spray testing is widely used in industry to assess corrosion resistance of surfaces and coated products. The sample is placed in a chamber where a solution of salt is sprayed through the nozzle (usually sodium chloride), less often solutions of acetic acid, copper chloride are used. The time after which oxides (corrosion traces) begin to appear on a sample surface is measured and fixed. The results are usually expressed in hours spent in fog without the appearance of corrosion products.
Determination of resistance to humidity. Procedure for exposing test specimens to an atmosphere of condensation moisture
There are three types of tests:
- Test at constant humidity and temperature (CH - constant-humidity),
- Test at varying temperature and humidity (AHT - alternation of humidity & temperature),
- Test at constant humidity and varying temperature (AT - alternation of air temperature).