The diffusion (from Latin diffundere 'Spread', 'scatter') describes the proper motion of atoms, molecules or charge carriers - commonly referred to as particles. Due to their thermal energy, the particles move unevenly and randomly in space. From a statistical point of view, in the case of an uneven distribution, there are more materials that are highly concentrated than materials that have a lower concentration. This causes the materials to be transported along the concentration gradient until the materials are completely mixed or the concentration is equalized.

If a surface is not pore-tight due to its material properties, particles can penetrate, which undermine and damage the surface. How diffusion-resistant a material is depends on various factors, including the wall thickness, the molecular density or the bonding properties of the particles. Diffusion-promoting effects are temperature differences, overpressure or underpressure and the temporal factor.

Since plastics have a rather porous surface, they are conditionally diffusion-resistant. Coatings which must have chemical resistance or corrosion protection are therefore subject to special requirements. In the diffusion test the resistance of anti-corrosion coatings is put to the test, e.g. B. using the salt spray test.

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