PURPOSE: To fill the defective parts such as micropores and cavities, to improve the strength of the filled part and further to increase the pressure tightness and strength of an aluminum structure main body.

CONSTITUTION: An aluminum die casting is placed in a treating tank and impregnated with a concd. mixed soln. of chromic acid and phosphoric acid. The treated aluminum casting is taken out, washed with water, dried and heat- treated. The process is repeated at least once. The obtained product is excellent in pressure tightness and strength, and the pressure tightness and strength are improved even with few times of treatments.