Explanation of Damp Heat Test of Constant Temperature and Humidity Machine

A. The constant temperature and humidity machine damp heat test is to place the sample or object coated with the paint film in a constant temperature and humidity machine with controlled temperature and humidity, and observe the changes such as foaming, corrosion and adhesion drop after a certain period of time. Test for resistance to hot and humid steam (sometimes including condensation). Assess the environmental adaptability and corrosion resistance of the coating to humid tropical climates.

B. During the constant temperature and humidity machine wet heat test, the water resistance and water vapor permeability of the coating are faster than the general room temperature water immersion test. The corrosion caused by the damp heat test is not as severe as that caused by the salt spray test.

C. The difference between the constant temperature and humidity machine damp heat test and the salt spray test:

(1) The main difference between the damp heat test and the salt spray test is that the mist droplets of the damp heat test have no salt and are distilled water, although the salt droplets are highly corrosive due to their high conductivity and rich chloride ions;

(2) However, in terms of osmotic pressure, the activity of distilled water is high, the coating is a semi-permeable membrane, and the ability of distilled water to penetrate the paint film is stronger than that of salt solution. Moisture penetrates into the paint film and accumulates between the two paint films, which will reduce the adhesion between the layers and cause blistering. Then it develops further, reaching between the paint film and the bottom plate, reducing the adhesion, causing the paint film to foam, and at the same time, the moisture contacts the metal bottom plate, resulting in electrochemical corrosion;

(3) The absorption of water in the paint film will cause the paint film to expand and generate internal stress, reduce the adhesion and cause foaming. The water absorption rate of the paint film is higher in distilled water than in salt water. In terms of the destructive power of the paint film, the damp heat test is very harsh.