Method for forming phosphate film on metallic surface

PURPOSE: To make it possible to form phosphate films without generation of waste water by bringing metallic surfaces into contact with a soln. for a phosphoric acid treatment specified in the ratios of free P 2 O 5 and total P 2 O 5 and Fe concn., adding salt-free water to effluent water and supplying this water to a washing bath and treating bath. CONSTITUTION: The soln. for the phosphoric acid treatment is composed of 0.4 to 30g/L Zn, 4 to 30g/L P 2 O 5 , 5 to 50g/L NO 3 , ≤10g/L Fe(II) and ≤0.3g/L Fe(III) and the weight ratios of the free P 2 O 5 and the total P 2 O 5 are specified to (0.04 to 0.50):1. This bath and the metallic surfaces are brought into contact with each other. The low salt water or preferably the salt-free water is supplied to the final washing bath of a cascade succeeding after the treatment and the effluent liquid is supplied to the washing bath and the phosphoric acid treating bath. The treating bath is replenished with the thickened materials consisting of Zn:NO 3 :P 2 O 5 =(0.80 to 0.30):(0.17 to 0.4):1 to maintain the bath component at the specific point number. Air is supplied during the treatment to maintain the Fe(II) concn. at the prescribed concn. COPYRIGHT: (C)1991,JPO