GB2122754A

GB2122754A - Anodic coating removal monitor

Info

Publication number: GB2122754A
Application number: GB08316526A
Authority: GB
Inventors: Donald Gordon Stewart; Philip Ian Brittain
Original assignee: Brent Chemicals International PLC
Current assignee: Chemetall Ltd
Priority date: 1982-06-17
Filing date: 1983-06-17
Publication date: 1984-01-18
Also published as: GB8316526D0

Abstract

A device for monitoring the removal of an anodic coating from a titanium workpiece (3) comprises a first electrode for making electrical contact with the titanium workpiece (3), a bath (1) for containing an acidic etchant (2), and a second electrode (4) for making electrical contact with the acidic etchant (2). Means are also provided for monitoring an electrical property, such as a semiconductor property, of the surface of the titanium workpiece (3) in relationship to the acidic etchant (2) and the second electrode (4). An indicator (18) is arranged to provide an indication when the difference between currents in opposite senses between the first and second electrodes (3) show that the anodic coating has been removed to a predetermined extent. Alternatively an e.m.f. may be measured between the first electrode and a hydrogen-sensitive second electrode (4) such as platinum black in hydrogen gas. <IMAGE>

Description

SPECIFICATION Anodic coating removal monitor Workpieces made from titanium such as parts for aircraft engines are frequently inspected to determine their crystal structure as part of routine quality control checks. To enable the workpieces to have their crystal structure observed by visual inspection under a microscope a blue anodised surface coating has to be largely but not completely etched away using a concentrated acid etchant. The extent of the removal of the blue anodic layer is of critical importance since sufficient of the coating has to be removed to expose surfaces of the crystals of titanium in the surface of the workpiece whilst sufficient of the anodic coating has to remain at the boundaries to reveal the shape and extent of the titanium crystals.

At present the removal of the blue anodised coating is performed manually and controlled entirely by the operator who judges by eye when the acidic etch has reached the required stage. As the anodic coating is removed the saturation of the blue colour of the coating decreases and so the operator has to judge the progress of the removal of the anodic coating entirely by judging the degree of saturation of the colour of the coating. Naturally this judgement is subjective and varies with the visual acuity of the operator, and also is affected by changes in the intensity and colour of the incident light.

According to this invention a device for monitoring the removal of an anodic coating from a titanium workpiece comprises a first electrode for making electrical contact with the titanium workpiece, a bath for containing an acidic etchant, a second electrode for making electrical contact with the acidic etchant, means for monitoring an electrical property of the surface of the titanium workpiece in relationship to the acidic etchant and the second electrode, and an indicator arranged to provide an indication when the electrical properties of the titanium workpiece show that the anodic coating has been removed to a predetermined extent.

The electrical property of the surface of the titanium workpiece that is monitored may be its acid to metal surface interface potential. In this case, the means for monitoring an electrical property of the surface of the titanium workpiece preferably include means to monitor, as the surface changes, the electromotive force generated between the titanium workpiece and the second electrode. This can be achieved by monitoring the potential between the titanium workpiece and a hydrogen sensitive electrode such as platinum black in hydrogen gas.

It is preferred however, that the electrical property of the surface of the titanium workpiece that is monitored is its semiconductor property. In this case the means for monitoring an electrical property of the surface of the titanium workpiece preferably include an alternating current source connected to the first and second electrodes, means for monitoring both the forward and reverse current flow between the first and second electrodes and hence the forward and reverse current flow through the surface of the titanium workpiece, and means to derive the difference between the current flow in the forward and reverse directions.The indicator is driven in dependence upon the difference between the current flow in the forward and reverse directions which, because the anodic film is a semiconductor, provides an indication of the extent to which the anodic coating has been removed by the acidic etchant.

Preferably the means for monitoring the electrical properties of the surface of the titanium workpiece also include means to integrate the current flow in both the forward and reverse directions with respect to time and produce output signals proportional to the integrated forward and reverse currents, and means to divide the difference between the forward and reverse currents by the forward current ao provide a normalised result that is independent of the area of contact between the surface of the titanium workpiece and the acidic etchant.

The device in accordance with this invention may form part of an apparatus for etching automatically the anodising coating from a titanium workpiece to enable its crystal structure to be examined and, in this case, the apparatus also includes means to discontinue the etching of the titanium workpiece automatically as soon as the means for monitoring an electrical property of the surface of the titanium workpiece shows that the anodic coating has been removed to the predetermined extent. The means to discontinue the etching of the titanium workpiece preferably removes the workpiece from the bath containing the etchant and moves it it a rinsing station but, alternatively the means to discontinue may drain the etchant from the bath.

A particular example of a device in accordance with this invention will now be described with reference to the accompanying drawing which is a block diagram of it.

The device comprises a bath 1 containing 300 g/l of 68% nitric acid and 22 g/l of hydrofluoric acid etchant 2, a titanium workpiece 3, and a platinum or carbon electrode 4.

An alternating current from an alternating current generator 5 is connected between the workpiece 3 and the electrode 4. The workpiece 3 includes an anodised coating which is etched by the etchant 2. The anodised coating conducts electricity preferentially in one direction and accordingly a greater electric current passes from the workpiece 3 through the coating to the acidic etchant 2 and hence to the electrode 4 than in the opposite direction. The difference in the current flow in the two directions depends upon the extent to which the surface of the workpiece 3 is coated and as the coating is removed by the acid etchant 2 this difference reduces.

The electrode 4 is connected to the alternating current generator 5 by a pair of diodes 6 and 7 and serially connected identical resistors 8 and 9 connected in parallel. The diodes 6 and 7 are connected in opposition so that forward current flow from the workpiece 3 to the electrode 4 passes through the diode 6 and resistor 8 whilst reverse current flow from the electrode 4 to the workpiece 3 passes through the diode 7 and resistor 9.

The voltage dropped across the resistors 8 and 9 is dependent upon the current passing through them and these voltages are red to buffer amplifiers 10 and 11, respectively. The outputs from the buffer amplifiers 10 and 11 are fed to smoothing circuits 12 and 13. The output from the smoothing circuits 12 and 13 are analogue voltages the magnitude of which represent the forward and reverse current flow between the workpiece 3 and electrode 4.

The outputs from the smoothing circuits 12 and 1 3 are fed to differential inputs of a differential operational amplifier 14. The output from the operational amplifier 14 is thus representative of the difference between the forward and reverse current between the workpiece 3 and electrode 4.

These signals are connected to a pair of voltage controlled oscillators 1 5 and 1 6 in which the voltage signals are converted to oscillating signals the frequencies of which are proportional to the forward, and the difference between the forward and reverse currents, respectively. Outputs from the voltage controlled oscillators 1 5 and 1 6 are then fed to a programmed computer 17 which is programmed to divide the signal representative of the difference between the forward and reverse currents by the signal representative of the forward current. The output from the computer 17 is fed to an indicating device 1 8. Preferably the indicating device 1 8 is calibrated to give a direct read out of the degree of removal of the anodised coating or, at least, give a read out when the coating has been removed to the required extent.

The outputs from the smoothing circuits 12 and 13 may be fed directly to the voltage controlled oscillators 1 5 and 16, and in this case the programmed computer 1 7 is programmed to compute the difference between the forward and reverse current flow signals obtained from the voltage controlled oscillators 1 5 and 1 6 and then divide the difference value with the forward current signal.

Claims

1. A device for monitoring the removal of an anodic coating from a titanium workpiece, the device comprising a first electrode for making electrical contact with the titanium workpiece, a bath for containing an acidic etchant, a second electrode for making electrical contact with the acidic etchant, means for monitoring an electrical property of the surface of the titanium workpiece in relationship to the acidic etchant and the second electrode, and an indicator arranged to provide an indication when the electrical properties of the titanium workpiece show that the anodic coating has been removed to a predetermined extent.

2. A device according to claim 1 ,wherein the means for monitoring an electrical property of the surface of the titanium workpiece includes means to monitor, as the surface changes, the electromotive force generated between the titanium workpiece and the second electrode.

3. A device according to claim 1, wherein the means for monitoring an electrical property of the surface of the titanium workpiece includes an alternating current source connected to the first and second electrodes, means for monitoring both the forward and reverse current flow between the first and the second electrodes, and means to derive the difference between the current flow in the forward and reverse directions.

4. A device according to claim 3, wherein the means for monitoring an electrical property of the surface of the titanium workpiece further includes means to integrate the current flow in both the forward and reverse directions with respect to time and produce output signals proportional to the integrated forward and reverse currents, and means to divide the difference between the forward and reverse currents by the forward current to provide a normalised result that is independent of the area of contact between the surface of the titanium workpiece and the acidic etchant.

5. A device according to claim 3 or claim 4, wherein the means for monitoring the electrical properties of the surface of the-titanium workpiece includes a programmed computer.

6. A device for monitoring the removal of an anodic coating from a titanium workpiece, substantially as described with reference to the accompanying drawing.

7. Apparatus for etching automatically the anodising coating from a titanium workpiece, the apparatus comprising a device in accordance with any of the preceding claims, and means to discontinue the etching of the titanium workpiece automatically as soon as the means for monitoring an electrical property of the surface of the titanium workpiece shows that the anodic coating has been removed to the predetermined extent.

8. Apparatus according to claim 7, wherein the means to discontinue the etching of the titanium workpiece removes the workpiece from the bath containing the etchant.

9. Apparatus according to claim 7, wherein the means to discontinue the etching of the titanium workpiece drains the etchant from the bath.