The pores formed by anodic oxidation of aluminum can be considered as bottom-closed capillary tubes, and these pores are perpendicular to the surface of the base metal.

After the anodizing of aluminum, the micropores are filled with an anodizing solution (such as sulfuric acid) and aluminum salts formed during the anodizing of aluminum. These residual sulfuric acid and aluminum salts in the micropores are not easy to remove, and it is difficult to remove the substances in the micropores by washing the outer surface.

There are two ways of moving substances in solution, namely convection and diffusion. In convection, the entire solution, including the dissolved material, moves together, and agitation is one form of convection. The driving force of the diffusion process is the gradient of the chemical concentration. In the case of residual solution in the micropore, it is completely impossible to stir the solution in the micropore, so the diffusion process is the only way to remove the sulfuric acid and aluminum salt in the micropore. The effect of washing is limited to ensuring that the concentration of sulfuric acid and aluminum salts outside the micropores is as low as possible to increase the concentration gradient driving diffusion in the micropores.

In industrial practice, the water washing after anodic oxidation of aluminum generally adopts multi-stage water cleaning, of which the first stage is flowing water cleaning. Air stirring is usually also used, and the design wash time is at least 5mino in an automated production line. Washing time can be set and guaranteed. But in the manual production line, unfortunately, the washing time is always shorter.