WO1993008460A1

WO1993008460A1 - Surface sensing equipment

Info

Publication number: WO1993008460A1
Application number: PCT/GB1992/001934
Authority: WO
Inventors: Terry Robert Johnson; John Griffiths; Graham Robert Johnson
Original assignee: Isi Abt Services Limited
Priority date: 1991-10-22
Filing date: 1992-10-21
Publication date: 1993-04-29
Also published as: GB9122380D0

Abstract

Surface sensing equipment for detecting corrosion activity in metal specimens in an electrolyte has tended to give a qualitative measurement only. The slow speed of data acquisition has further limited the usefullness, corrosion being a dynamic process. Resolution was also limited by poor mechanical design of the equipment and design of the sensing probe. The present invention provides surface sensing equipment able to give in real time a qualitative and a quantitative measurement to the amount of corrosion by manipulating data representative of the precise location of a probe with respect to a surface of the specimen and by using a probe which can detect small current fluctuations with a high spatial resolution.

Description

SURFACE SENSING EQUIPMENT

This invention relates to sensing equipment and more specifically, though not exclusively, to sensing equipment used to measure surface characteristics of materials and in particular to sense variations in surface characteristics associated with corrosion in metals.

One type of surface sensing and corrosion measurement technique involves scanning an electrode across the surface of a specimen.

The specimen is at least partially immersed in an electrolyte and held at a controlled potential with respect to an auxiliary electrode also in the electrolyte using a calomel reference electrode to maintain constant potential via feedback to a potentiostat. Local potential differences are then measured by a twin probe mounted on the scanning electrode.

Corrosion, for example in the form of pitting, may be detected as an increase in potential difference. A picture of the ionic activity adjacent to the surface of the specimen may be built up from different potential differences. In order to obtain a sufficiently detailed picture of the surface the probe must be small. Typically the radius of the tip of the probe is of the order of tens of micrometres. In the past manufacturing such a probe has proved to be a problem. The probe must also be chemically inert so as not to react with either the electrolyte or the specimen. Typically the probe is made from tin and is housed in a glass sleeve. This is particularly difficult to machine.

Also, because on occassions specimens have been assessed "in the field", or in remote environments, the probe needs to be rugged. As the probe has had a glass sleeve this has sometimes fractured and rendered the equipment useless without a replacement probe.

Furthermore, because the scanning probe was so small, in order to obtain good resolution of the surface of the specimen in a relatively short time period, it has been necessary to move the probe relative to the surface very quickly.

Inevitably the above have lead to problems of control and monitoring of the movement of the scanning head on which the probe was supported. To reduce this problem and minimise "judder" in the drive equipment, a cylindrical section of specimen was rotated and the scanning head moved relative to the rotating surface, in a substantially axial direction, so as to scan the curved surface of the cylindrical specimen.

A further problem was that associated with monitoring, sampling, collating and processing the large amount of data obtained during scanning. Considerable manipulation of data was often required. Typically normalisation of the data and standardisation to a reference potential was required so as to yield information which was of any use. Moreover, often images were only of a qualitative nature showing either a black "corroded" region or a white "uncorroded" region. It was difficult, if not impossible, to obtain any quantitive feel for the amount or depth of corrosion which was present.

According to a first aspect of the present invention there is provided apparatus for scanning a surface of a specimen comprising: means for supporting and rotating the specimen, means for supporting a probe, a first actuating means for moving the probe relative to the surface of the specimen in a substantially parallel direction to the axis of rotation of the specimen; and a second actuating means to drive the probe relative to the surface of the specimen in a substantially transverse direction to the axis of rotation of the specimen; such that in use the probe and at least a portion of the surface of the specimen are immersed in an electrolyte; the apparatus further comprising a sensor arranged to sense angular displacement, of the specimen relative to the probe; and means to sense variations in a physical constant.

Preferably variations in the electro-potential present between a first and a second portion of the probe are detected. These may be measured in a region proximal to the surface of the specimen, for example in a region where there is micro galvanic action. Of course other variables could be detected and measured, such as the concentration of anions and/or cations. Micro galvanic action is measured as this requires a relatively simple probe. Furthermore because the associated detector can be made so small, this particular type of detector may have good resolution properties.

Measurement of the micro galvanic action is made between two fine wire tips as described below.

According to a second aspect of the present invention there is provided a probe comprising first and second conductive portions, and a screening medium surrounding at least a portion of the surface of each conductive portion.

Preferably the conductive portions are substantially parallel and are held in this position by a suitable binding medium. The medium may be epoxy resin and may act to insulate each conductive portion from the other.

The conductive portions may be of any metal but are preferably formed from a relatively inert metal such as gold.

The screening medium may comprise braided screening wire. Copper wire may be arranged parallel to or wound around the conductive portions, which are preferably elongate metal wires. The copper wire and braiding bundle is coated in a synthetic plastics sleeving which may be heat shrunken onto the surface of the wire bundle. Further layers may be coated or wrapped around the bundle for purposes of noise insulation and/or electrical insulation and/or water proofing.

Advantageously the first and second conductive portions are of different lengths and a measurement of galvanic action is made therebetween. It may be possible to have a plurality of shorter lengths of conductive portion and to arrange for an "averaging", of galvanic readings associated with each shorter length, to be made.

A proximity sensor may be incorporated on or adjacent the probe head.

According to a third aspect of the present invention there is provided a system for determining corrosion in a specimen comprising: a probe arranged to move relative to a surface of the specimen and adapted to procure an electrical signal indicative of a physical characteristic in the specimen; processing means in electrical communication with said probe, said processing means arranged to sample said electrical signal in conjunction with information derived from the relative movement between the probe and the specimen in order to provide data indicative of a surface characteristic of the specimen.

Screening is important as noise becomes increasingly more difficult to control as the distance between the first and second conductive portions decreases. Preferably the distance between the portions is less than 15 micrometres. However, different size probes may be used in environments requiring different sensitivity. A mercury contact can be used in order to reduce noise.

Means may also be provided to output signals representative of the data obtained from the sensors. Suitable monitoring of displacement signals may be made so that relative velocity vectors between the probe and the specimen may be obtained.

Preferably processing apparatus is arranged to monitor, and manipulate the aforesaid data and to display the manipulated data in a meaningful manner.

Preferably the probe is the cathode and the specimen is the anode.

The probe may be mounted on a threaded member and driven relative to the member by a rotary actuator which may be a stepper motor. Because the probe may be driven in a transverse sense relative to the surface of the specimen under test, and because such minute resolution is possible, the probe may measure microscopic potential difference in the electrolyte adjacent to the surface of the specimen under test. Accordingly it may track any corrosion activity at the micro-scopic level. Such an arrangement permits a true evaluation of the surface of the specimen and avoids erroneous readings of a high potential difference, arising as a result of mechanical imperfections or cracks, which readings could otherwise be falsely interpreted as corrosion. Gear trains can be coated with polytetrafluorethane to reduce "chatter" and further improve resolution.

An embodiment of the invention will now be described, by way of example only, and with reference to the Figures in which:

Figure 1A is a diagramatical representation of a surface corrosion evaluation apparatus;

Figure IB is a digrammatical view of a noise free electrical contract;

Figure 1C is a diagrammatical view of a probe support;

Figure 2 is a block-diagram representing steps involved in sampling and data processing signals received from the apparatus in Figure 1A; and

Figure 3 shows a diagrammatical section through a probe and probe head.

Figure 1A shows a support 10 containing a eurocard rack supporting a hinged gantry 11. The support 10 and the gantry 11 are formed from stainless steel sheets and are bolted, riveted or welded together. The support 10 has a recessed base for receiving a glass bowl 12. The gantry 11 supports end stops 13 and 14 between which three shafts 15A, 15B and 16A are mounted. Shaft 15B is a threaded shaft. A mobile housing 17 is mounted on the three shafts 15A, 15B and 16A and is able to move in a lateral direction indicated by the double headed arrow X-X', between the end bearings 13 and 14.

A second housing 18A is supported by the gantry 11. A motor 18 which is connected to a specimen 19 under examination by way of a shaft 19A, a gear train 28 and secured in a chuck 19B. The specimen 19 rotates about the axis of the shaft 19A, as shown by the arrow B. The speed of the motor 18A is controlled by pulses from a micro computer on input line 15.

Housing of motor 18A is insulated from gantry 11 and electrical contact to the shaft 19A is effected by a contact 31. To ensure a "noise" free contact mercury may be used in this application. The contact 31 is secured on a contact holder 31E. Contact pressure is maintained by a spring 3ID.

A mercury contact 31A is made of a mercury impervious but electrically conducting material.

A mercury bead 31C is retained in a recess at the end of shaft 19A by a seal plug 31A. The recess in the shaft 19A is chemically treated so as to render the shaft impervious to mercury attach. The arrangement is shown in Figure IB.

A probe support 20 supports a re-am lifier upon which is mounted an elongate arm 21 and a detachable twin gold probe 24 held by screw fitting 21A. The probe support 20 is connected to the housing 17. The probe support 20 has a drive stepper motor 20A which is capable of moving the probe support 20 in the direction of double headed arrow Z-Z^rvia a worm drive 20B at the end of the motor shaft 20C. This arrangement is shown in detail in Figure lC. Because the probe support 20 is supported by the housing 17 it is also capable of moving in the direction of double headed arrow X-X' being driven by stepper motor 25 connected to threaded shaft 15B.

A third housing 22 is mounted on the opposite side of the gantry 11 to housing 18. Housing 22 supports a saturated calomel reference electrode 23. In use an electric potential difference is set up between an auxilliary carbon electrode 26, and the specimen 19. The potential difference may be varied by altering the voltage appearing across pairs II and L2; and II and LI respectively. This may be done remotely or automatically, as described below. An inductive proximity sensor 27 provides an output signal L5 which allows synchronism with a data display 44. Micro-galvanic action adjacent the specimen surface will give rise to a potential difference between the twin gold probes 24 as measured across lines L3 and L4.

These signals are amplified by pre-amplifier 29 and fed to a micro-processor 36 and used together with other signals, as described below, to compute quantitative information in respect of the surface corrosion characteristics of the specimen 19.

As will be appreciated a portion of the surface of the specimen to be examined must, in use, be immersed in a suitable electrolyte. Typically the level of the surface of the electrolyte is shown by the dotted line at A-A'. Further information is output on lines LI, L2, L3, L4 as described below.

The relative potential measuring micro galvanic action is measured along lines L3 and L4 respectively. Reference potential is fed via line LI to a potentiostat 60 which maintains a constant potential between the sample 19 and the auxiliary electrode 26 during the course of the assessment of the surface characteristic. Further information may be retrieved indicative of such variables as the concentration of anions and/or cations in the electrolyte. Similarly other variables related to the type of metal under test may be input during the scanning process. Such variables may be introducted into the program directly from a key board 42 as indicated in Figure 2. Clearly remote temperature sensing could be performed automatically by using a suitable transducer (not shown) .

Movement of the probe support 20 along the X and Z axes is performed by way of stepper motors 25 and 20A respectively. The stepper motor 25 is arranged to move relative to a threaded portion of the shaft 15B. Accordingly controlled movement may be achieved quickly and with a resolution of one micrometre.

Control of the relative movement of the probe 24 with respect to the surface of the specimen 19 is achieved by microcomputer control of stepper motors 18A, 25 and 20A using information obtained from the potential difference between L3 and L4 respectively.

Figure 2 shows how information retrieved from the electrodes of theprobe 21 is stored in memory 36 of a computer. Selective data is sent along information links 31 where it is stored in memory 36. At predetermined sampling intervals information is sent along information bus 34 to a processor in the computer 36 where, using systems software stored in the memory, control signals are computed which are input into a control program. Voltage control signals are then output along the lines L2 and 12 to ensure that the correct potential differences exist within the electrolyte. Control signals input to control the speed^' of motors 25, 18 and 20A together with an automatic checking procedure in the software, ensure that the probe 24 approaches the surface of the specimen 19 at the correct velocity. Safety features are also built into the equipment and software such that at start-up the probe does not knock into the specimen 19.

The system described above enables data retrieved to be corrected and normalised so that subsequent to the control aspect of the sensing equipment data is transferred along information bus 34 via the computer 36A along information bus 38 to a data control and manipulation program from where it is assessed in accordance with input signals from key board 42 or mouse 41. Processed data then passes to a graphics program 40 from where it is output to a suitable output terminal such as VDU 44 or plotter (not shown) . Because the raw data has been processed and is normalised the visual signal displayed on the VDU 44 can be readily arranged to show the amount of actual corrosion which has taken place instead of mere variations in surface characteristics of the specimen.

The equipment may require some standardisation in the form of calibration prior to measurement. This is because measurements are made relative to a nominal datum. Scanning of the specimen is checked after each revolution as a suitable marker line may be made on the specimen. Detection of this line may be used as a synchronising pulse for resetting and/or checking the flyback time of a rasta scanner at the end of a line of data.

Furthermore because the probe 24 is a gold or other precious metal probe it is able to produce higher and better resolution as it does not react with the electrolyte or specimen 19.

It will be appreciated that the above description is by way of example only and variation to the embodiment may be made without departing from the scope of the invention. For example the device could be incorporated into a smaller more robust and portable surface scanning device for use for example in a nuclear reactor environment or on the surface of submarines or ships etc. Similarly an optical distance measurement probe may be incorporated at or near the tip of probe 24 so as to sense the proximity of the probe 24 to the surface of the specimen 19. This could comprise an optical fibre suitably arranged to feed back a signal to control the movement of the housing 17 along the x-axis.

Claims

1. Apparatus for scanning a surface of a specimen comprising: means for supporting and rotating the specimen, means for supporting a probe, a first actuating means for moving the probe relative to the surface of the specimen in a substantially parallel direction to the axis of rotation of the specimen; and a second actuating means to drive the probe relative to the surface of the specimen in a substantially transverse direction to the axis of rotation of the specimen; such that in use the probe and at least a portion of the surface of the specimen are immersed in an electrolyte; the apparatus further comprising a sensor arranged to sense angular displacement, of the specimen relative to the probe; and means to sense variations in a physical constant.

2. Apparatus according to claim 1 wherein a first and a second portion of the probe is adapted to detect variations in electro-potential between the surface of the specimen and respective first and second portions.

3. Apparatus according to claim 1 or 2 wherein the concentration of anions and/or cations is detected.

4. Apparatus according to claim 1 or 2 wherein micro-galvanic action is measured.

5. Apparatus according to any preceding claim wherein the probe comprises first and second conductive portions, and a screening medium surrounding at least a portion of the surface of each conductive portion.

6. Apparatus according to any preceding claim wherein the conductive portions are substantially parallel and are held in this position by a suitable binding medium.

7. Apparatus according to claim 6 wherein the binding medium is epoxy resin.

8. Apparatus according to claim 5, 6 or 7 wherein the conductive portions are metallic.

9. Apparatus according to claim 8 wherein the conductive portions are formed from a relatively inert metal such as gold.

10. Apparatus according to any of claims 5 to 9 wherein the screening medium comprises braided screening wire.

11. Apparatus according to any of claims 5 to 10 wherein copper wire is arranged parallel to or wound around the conductive portions.

12. Apparatus according to claim 10 wherein the copper wire and braided bundle is coated in a synthetic plastics sleeving.

13. Apparatus according to any preceding claim wherein the first and second conductive portions are of different lengths.

14. Apparatus according to any preceding claim wherein a proximity sensor may be incorporated on or adjacent the probe head.

15. Apparatus according to claim 1 which is adapted to receive different size probes.

16. Apparatus according to any preceding claim adapted to provide output signals representative of the data obtained from the sensors. Suitable monitoring of displacement signals may be made so that relative velocity vectors between the probe and the specimen may be obtained.

17. Apparatus according to claim 15 wherein a computer and monitor is arranged to manipulate the aforesaid data and to display the manipulated data in a meaningful manner.

18. Apparatus according to claim 1 wherein the probe is the cathode and the specimen is the anode.

19. Apparatus according to any preceding claim wherein the drive means is connected to the specimen via a gear train coated in polytetrafluorethane.

20. Apparatus according to any preceding claim wherein a contact to the drive means is by a mercury pool connection.

21. A probe comprising first and second conductive portions and a screening medium surrounding at least a portion of the surface of each conductive portion.

22. A system for determining corrosion in a specimen comprising: a probe arranged to move relative to a surface of the specimen and adapted to procure an electrical signal indicative of a physical characteristic in the specimen; processing means in electrical communication with said probe, said processing means arranged to sample said electrical signal in conjunction with information derived from the relative movement between the probe and the specimen in order to provide data indicative of a surface characteristic of the specimen.