The diffusion (from Latin diffundere 'Propagate', '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. Statistically, uneven distribution involves more materials that are highly concentrated than materials that have a lower concentration. As a result, a transport of the materials along the Konzetrationsgefälles is obtained until it comes to a complete mixing or concentration compensation of the materials.

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.