GB1032404A - Process and apparatus for forming chemical coatings on metal - Google Patents

Abstract

Apparatus for applying phosphate and chromate coatings to metal workpieces in which replenishment of at least two liquids used in the coating process is controlled by a signal corresponding to their electrical conductivity, comprises a tunnel 10, Fig. 1A, containing 6 treatment stations through which the workpieces pass to be cleaned, rinsed twice in hot water, phosphate coated and rinsed in cold water and dilute chromic acid. Station 1 comprises a tank 12, a pump 14 which supplies spray nozzles 18 and a conductivity cell 22. Concentrated cleaning fluid is fed to tank 12 when required through a valve 26 from tank 24 containing another conductivity cell C1 arranged to act as a liquid level indicator. Stations 4 and 6 are similar except that there is no level indicator whilst in stations 2 and 3 one cell controls the flow of mains water to both tanks in station 5 the cell varies the flow of mains water since some water continually drains to waste. Each cell comprises an electrically insulating tube 110, Fig. 2, having two cylindrical electrodes 112 in the wall connected to terminals 116. Three way valves 106, 108 allow either the liquid to flow through the tube or when the electrodes are dirty a cleaning fluid may be poured through from a funnel 128 to drain 130. A temperature responsive device T is placed in the circuit before each cell except that acting as a level indicator. The conductivity cells C1-6 and devices T2-T6 are electrically connected to parts a and b of a four part rotating switch 182, Fig. 3, which connects each cell and corresponding device T in turn to a circuit comprising bridges 136, 134 each connected to an A.C. source 154. The output of bridge 136 is fed to a primary transformer winding 158 which induces a voltage in the secondary winding 160 corresponding to the conductivity being measured. Bridge 134 includes a split stator variable capacitor 140 mounted on a rotor 169 which is turned to an equilibrium position by an induction motor 176 in accordance with a signal from an unbalance voltage amplifier 170 passed through an unbalance detector circuit 172 and a push-pull power amplifier 174. Rotor 168 carries a series of arcuate contacts K1-6 corresponding to cells C1-6 and co-operating with fixed wiper contacts W1-6. These are connected to the third switch section and arranged to the position of rotor 168 to open or close relays connected to the fourth switch section which operate the pumps and valves controlling the supply of replenishing fluid to maintain the conductivity within prescribed limits or in the case of cell C1 indicates that the liquid level is within prescribed limits. <PICT:1032404/C6-C7/1> <PICT:1032404/C6-C7/2> <PICT:1032404/C6-C7/3>