US3162207A

US3162207A - Method of controlling chromate conversion coating baths

Info

Publication number: US3162207A
Application number: US184779A
Authority: US
Inventors: William C Jones
Original assignee: Chemical Corp
Current assignee: Chemical Corp
Priority date: 1962-04-03
Filing date: 1962-04-03
Publication date: 1964-12-22
Anticipated expiration: 1981-12-22

Description

Dec. 22, 1964 I w. c. JONES 3,162,207

MELIHOD 0F CONTROLLING CHROMATE CONVERSION COATING BATHS Filed April 5, 1962 Q I ma I 9 i I a [m lr\ g Q 4k: $9M I w U R: IIIII I I N M IIIII I I I w IIIIIIHI I:

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United States 3 162,207. METHOD OF CONTROLLING CHROMATE CON- VERSION COATING BATHS William C. Jones, Thompsonville, Conn., assignor to T he Chemical Corporation, Springfield, Mass., a corporation of Massachusetts Filed Apr. 3, 1962, Ser. No. 184,779

1 Claim. (Cl. 137-5) This invention relates to chromate conversion coating from liquid media and particularly to methods of automatically controlling the effectiveness of the conversion coating bath. v

One of the principal problems in quality control of chromate conversion coating processes is accurate quantitative analysis of the active material in the liquid media. For example, in the control of chromate conversion coating baths it is desirable to measure the amount of active chromium in the bath.

Chromate baths contain hexavalent chromium, from chromic acid, other mineral acids, activator and buffers. Conventionally, measurement of the amount of active material in a bath involves the ascertainment of acidity by titration, the determination of the hexavalent chromium content by titration, or the measurement of the pH of the bath or a combination of the above methods.

As a chromate conversion coating bath is used for coating various articles, the strength of the bath decreases through what is known as drag in of Water and drag out, or removal of the coating material. In addition, the metal being coated dissolves to some extent in the bath gradually increasing in quantity as the bath is used. It has been found that the dissolved metal tends to associate itself with hexavalent or active chromium in solution and in effect renders the chromium less active for coating purposes. It has also been learned that the hexavalent chromium degenerates to a reduced condition known as trivalent chromium which also tends to associate and interfere with the coating effectiveness of hexavalent chromium.

As indicated above the conventional methods of measuring coating effectiveness are based on periodical batch type tests, one of which is a total acidity determination. It has been found that this test measures not only the presence of active acidic components which are effective for coating reaction but is alsoresponsive to the other dissolved metal and trivalent chromium in the bath. Thus, it will be realized that as the solution becomes less effective the titration analysis of total acidity becomes less accurate.

The accuracy of tests which involve determination of the amount of hexavalent chromium in the bath is also adversely affected by the presence of other dissolved metals and trivalent chromium in the solution. The reason for this is that although the hexavalent chromium is present in the bath its effectiveness is diminished proportionately to the amount of other dissolved metals and trivalent chromium present.

The other generally used method of controlling coating baths comprises the measurement of pH by electrometric methods. This is not a sufficiently accurate method of control because pH, which is a log function, requires too large changes in total acidity to produce any significant measurable changes. This is particularly true because chromate baths are operated in such low pH ranges, on the order of about 1.5 or less.

It will be, of course, appreciated th'at the two titration procedures described above do not lend themselves to automatic control of coating bath concentration, atid while pH controllers do not have this drawback their lack of sensitivity in the operating ranges encountered in ice chromium conversion coating also makes them unsuitable for accurate coating control.

It is the principal object of this invention to provide an improved method and means for automatically controlling the coating etfectiveness of chromate conversion coating baths.

The above and other objects and advantages of this invention will be more readily apparent from the following description and with reference to the accompanying drawing which is a circuit diagram showing one means of carrying out this invention.

In accordance with this invention it has been found that electric conductivity, or its reciprocal, resistance, can be accurately and continuously determined and that this determination is an extremely accurate measure of the amount of active material in the chromate bath. It was found unexpectedly that chromate solution conductivity is a function of the amount of available or efiective hexavalent chromium present and not merely a measure of the amount of material dissolved in the bath.

In order to evaluate the effectiveness of automatic solution control the following experiment was conducted. A container of chromating solution was mixed having an operating level of chromium concentration suitable for a yellow iridescent film. The conductivity of this solution was measured and a zinc plated test panel was processed.

Thereafter a quantitycof zinc dust was added to the bath in small increments; the bath did not form a satisfactory coating on the plate. At this point the conductivity of the solution was again measured and a distinct decrease was noted. Thus it was learned that the conductivity variation was responsive to diminution in coating effectiveness.

Following the low conductivity reading additional liquid concentrate of chromating material was introduced into the bath sufiicient in quantity to restore the original conductivity. When the conductivity had been restored, the bath once more became eifectivefor depositing satisfactory films on the metal plate. Zinc dust was again added and the process of reducing and increasing coating effectiveness was repeated. It was noted that successive reductions in effectiveness required less and less zinc and correspondingly less and less concentrate was required to restore the original conductivity. During the performance of these tests there was no appreciable drag out of solution or evolution of gas, and actually the amount 'of dissolved metal in the bath increased rather than decreased due to the substantial quantities of zinc introduced into the bath. Notwithstanding the overall increase in dissolved material, the conductivity decreased rather than increased when the bath became less effective for coating purposes. This then constitutes an effective method for indicating the coating effectiveness of a chromate bath.

in the figure is shown diagrammatically apparatus which may be used in carrying out this invention. This apparatus comprises a conductivity cell 2 which consists of two spaced metal plates or electrodes immersed in the liquid of the bath 6, contained in a tank 7. The electrodes preferably are disposed within an insulating chambcr 4 which isolates a portion of the liquid whereby resistance measurement is independent of the volume and location of the cells within the tank 7. The plates of the ceii are electrically connected by leads 9 to one arm of a bridge-rectifier shown generally at 8. In this way variaticns in the conductivity of the bath may be measured as resistance variations between the plates as compared with a resistance balance established by the bridge.

An alternating current is connected to the bridge by means of leads 10 and 12. The output from the bridge is connected by leads 14 and 16 to a suitable amplifying device which may be a magnetic amplifier 20. The magnetic amplifier is a saturable reactor containing an iron core having a primary winding which .receives the signal from the bridge circuit. The secondary coil receives the AC. voltage. The DC voltage input or control signal increases the flux density of the coil with consequent amplification or increase in the output voltage. The AC. voltage is supplied to the amplifier by means of

leads

22 and 24 connected to leads and 12.

An output circuit from the magnetic amplifier is indicated generally at 26 and asshown is in the form of a rectifier comprising diodes 28 and 30. The cathodes of the diodes are connected together and'the junction thereof is connected to one end of a load resistor 32. The other end of the load resistor is connected to a center tap of the secondary of the magnetic amplifier. The'voltage across the load resistor is rectified and applied across a filter capacitor 34 to a DC. sensing relay 36 which operates switch 38. Switch 38 in turn operates a relay 40 to energize a feed pump motor 42. A pump driven by the motor introduces a chromate concentrate 43 into the tank 7.

In operation the cell 2 is immersed in the coating bath 6 mixed to contain ingredients or amounts which will give optimum coating results. When the bath is in this condition a switch 44 is activated to energize the apparatus including the bridge circuit 8. The outputi of the bridge is balanced or nulled by adjustment of the rheostat arm 46, arms 48 and 50 being the ratio'arms of the bridge. Each arm of the bridge contains a diode 52 arranged so that a rectified voltage output is obtained from the bridge.

When the bridge circuit is inv its null condition no output signal is received by the amplifier 20. However, when the effectiveness of the hexavalent chromium in the bath 6 is reduced, resistance between the plates of the cell 2 changes and the electrical balance of the bridge is disrupted and a potential difference occurs at the output corners of the bridge. As a result a DC. current is introduced to the magnetic amplifier by leads 14 and,16. This voltage generates an output signal from the amplifier 20 which is rectified by circuit 26. The rectified signal operates relay switch 38 which in turn operates relay to start the pump motor 42 and concentrate'is pumped into the tank 7 until the bridge circuit is restored to its null condition. I

' Operating lights 56 and 58 may be provided to indicate when the circuit is energized and when the pump is operating.

Having thus described this invention what is claimed is:

In'the method of chromate conversion coating the steps which comprise forming a chromate conversion coating bath containing hexavalent chromium, the trivalent chrc' mium and the metal dissolved from that being coated,"

electrically sensing the effective hexavalent chromium present in said bath, obtaining an electrical signal which is a function of the conductivity of effective hexavalent chromium content of said bath, comparing said signal with a similar signal of predetermined vvalue, and introducing into said bath in a response to deviation of said signal from said predetermined value a chromate concentrate'whereby said bath is continuously and automatically maintained at optimum coating effectiveness.

References Cited by the Examiner UNITED STATES PATENTS ISADOR WEIL, Primary Examiner. CLARENCE R. GORDON, Examiner.