METHOD FOR CONTROLLING PHOSPHATE TREATING LIQUID

PURPOSE:To form a good phosphate film by chemical conversion on the surface of a metalic material even at an ordinary temp. by controlling automatically a phosphate treating liquid of an ordinary temp. type to adequate pH according to the zinc ion concn. and temp. of said liquid. CONSTITUTION:The optimum pH value at a reference temp. is manually set by referencing the graph shown in the figure from the content of zinc in the above-mentioned treating liquid to a pH setter 6. The output value for the above-mentioned optimum pH value of the setter 6 is put together with the temp. signal of a temp. measuring device 3 in accordance with an electrode 1 for measuring the temp. of a treating liquid tank 2 to a temp. corrector 7 which corrects the optimum pH value at the reference temp. to the optimum pH value at the treating liquid temp. The corrected pH value is put into a setting controller 8. The output pH signal from a pH measuring instrument 5 in accordance with the pH electrode 4 of the tank 2 is also put into the controller 8, by which said signal is compared with the above-mentioned optimum pH value. A replenishing pump 9 is driven by the differential signal thereof to replenish a neuralizer 10 to the tank 2 so as to attain the optimum pH value. The adequate pH value is maintained by such method and the formation of the good phosphate film by the chemical conversion even at an ordinary temp. is made possible.