Why is it difficult to form an oxide film in and around the holes of aluminum oxide parts?

There are two main reasons for the formation of oxide films in the holes of aluminum profiles and around them:

(1) The aluminum parts are washed thoroughly after caustic washing. Alkali which enters the pores during caustic washing fails to be washed cleanly. After the oxidation treatment, the caustic soda flows out of the cylinder, causing the oxide film around the pores to be corroded.

(2) There are liveliness around the hole of the aluminum workpiece. Aluminium tapping screw hole is very astringent, the operator often lacquer to lubricate, if the lack of emulsifier alkali in the alkaline wash, it is difficult to dilute.

Solution:

(1) Wash it with gasoline before caustic washing, emulsifier should be added in caustic solution;

(2) The workpiece should be washed after caustic washing.