| 摘要 |
PURPOSE:To stably form the most excellent phosphate film to the surface of a metal, by automatically and continuously measuring the nitrite ion concn. in the phosphate treating solution on the surface of a metal and automatically replenishing a nitrite ion on the basis of the measured value. CONSTITUTION:A steel plate is immersed in the aqueous phosphate solution in a phosphate treating tank 18 to form a phosphate film to the surface of said steel plate. During this operation, a phosphate treating solution is recirculated to a cell 5 equipped with a nitrite ion selective electrode 1, a comparison electrode 7 and a temp. compensating electrode 8 by a pump 20. The potential difference signal between the electrodes 1, 7 detected by a potential difference detecting circuit 12 and the temp. signal from a temp. detecting circuit 13 are inputted to a correction circuit 14 to correct potential difference to the potential difference value at a reference temp. This value is compared with the set value of a setting device 15 by a comparator 16 and the drive circuit 17 of a nitrite ion replenishing pump 19 is driven by the difference signal and a nitrite ion replenishing solution 21 is automatically replenished into a treating tank 18 so as to keep a proper concn. |
