Definition - What does Diffusion mean?

Diffusion refers to the spreading out of molecules in contact with each other due to the thermal displacement of molecules and atoms, which cause a random movement in solids, liquids and gases. When the molecules of multiple constituents interact with each other they dissipate heat energy and increase the kinetic energy of the system, which tends to move the molecules in random directions, thus becoming dispersed.

Corrosionpedia explains Diffusion

The principle of diffusion is seen when applying a diffusion coating, where the base metal or alloy substrate is placed in one of the following situations that allow the coating's molecules to spread over the substrate:

  1. The substrate to be coated is first coated with another metal or alloy coating and then placed in a high temperature environment where the molecules of the coated material gain thermal energy. Due to the thermal energy gained by the coating's molecules, they begin a displacement of their own with respect to other nearby molecules and spread over the substrate, forming a firm coat over the substrate's surface.
  2. The substrate to be coated is put into a liquid or gaseous coating medium in which the coating's molecules are already becoming displaced due to internal bombardment within themselves. When an external surface is placed in this medium the displaced molecules diffuse toward the external surface and a layer of coating spreads over the substrate. An example of this type of diffusion coating occurs in the automotive industry, where an auto body is placed in a liquid solution of zinc oxide to prevent corrosion.
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