GB2190747A

GB2190747A - A method of marking a detected defect

Info

Publication number: GB2190747A
Application number: GB08710561A
Authority: GB
Inventors: Dr Andrew Thomas Chadwick; Dr Francis Alan Wedgwood
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-05-23
Filing date: 1987-05-05
Publication date: 1987-11-25
Anticipated expiration: 2007-05-05
Also published as: GB8710561D0; GB2190747B; GB8612669D0

Abstract

A method of marking a location of a sub-surface defect on a surface of an object, while scanning a probe over the surface, in particular an ultrasonic probe, involves coating the surface with a couplant, such as a starch gel, and subjecting the couplant to a stimulus from the probe so as to change its appearance. The gel might contain iodide ions; an electrical stimulus would generate iodine and hence a dark blue starch-iodine dye. Several examples are detailed using a redox indicator, or a pH indicator or an electrochromic material. The object may be conductive or non-conductive.

Description

SPECIFICATION A method of marking This invention relates to a method of marking a location on a surface of an object, while scanning a probe over the surface, for example during ultrasonic testing to detect subsurface defects.

Ultrasonic techniques may be used to locate and size defects or discontinuities within an object, and in one such technique the surface of the object is coated with a couplant and an ultrasonic probe is scanned over the surface.

In this technique it would be advantageous to be able to mark the surface either to indicate those parts of the surface over which the probe has been passed, or to indicate the locations at which a defect or discontinuity was detected.

According to the present invention there is provided a method of marking a location on a surface of an object while scanning a probe over the surface, comprising coating the surface with an adherent couplant, and subjecting the couplant to a stimulus from the probe, the couplant heing such as to alter in appearance as a consequence of the stimulus.

The alteration in appearance is desirably a change of colour, and the stimulus might be optical (for example ultra-violet) or, preferably, electrical. It is also desiraole if the colour change is localised within the portion of the couplant adjacent to the surface rather than throughout the whole thickness of the couplant so that movement of the probe will not smear out the portion of the couplan; which has changed its colour.

Where the object is of an electrically conducting material or is coated with a conducting material an applying a potential difference between the object or the coating and an electrode on the probe, or by inducing such a potential difference. Where the object is of an electrically insulating material or the surface is electrically insulating then an electrical stimulus may be provided between two spaced apart electrodes on the probe.

In the former case the couplant may be uniform throughout its thickness and the colour change can be localised next to the surface by an appropriate choice of polarity for the potential difference.

In the latter case the surface may be coated with a first layer of couplant within which the colour change is to occur, and a second layer of coupiant within which a mobile species is generated by the electrical stimulus applied between the two spaced-apart electrodes, the mobile species diffusing into the first layer to cause the change of colour.

The couplant might be a grease, or a thick water-borne colioid or gel, and may include an electrolyte to ensure adequate electrical conductivity.

The invention also provides an ultrasonic couplant for use in the method.

The invention will now be further described by way of example only, as follows: Example 1 A couplant is made by boiling an aqueous starch solution also containing potassium iodide, to produce a thick gel. The preferred composition is with a starch concentration of 10 w/o (i.e. percentage by weight), and with a potassium iodide (KI) concentration of 0.03 molar, as this has a viscosity suitable for use as ultrasonic couplant, is initially colourless, and will give an intense blue colour if electrolysed. For KI concentrations below about 0.02 molar the colour obtainable is faint, but no increase has been observed for concentrations above 0.03 molar; much higher concentrations can lead to problems of corrosion. The optimum starch concentration is between 6 and 14 w/o; at lower concentrations the gel is too runny, while at higher concentrations it tends to set.

The gel is used to coat a clean surface of a steel specimen, producing a layer about 1 mm thick.

An ultrasonic probe is then scanned over the surface, the contact face through which the ultrasonic waves pass being held next to the surface so that the gel coating couples ultrasonic waves from the probe into the specimen. The probe is provided with an electrode adjacent to its contact face in such a position that the electrode too is in contact with the gel coating. A low voltage (about 15 or 20 V) d.c. constant current supply is connected between the electrode on the probe and the steel specimen, the polarity of the supply being such that the specimen is the anode. The d.c. supply is energised when a defect or discontinuity is detected, producing a current density of about 0.2 A cm-2.

The electric current flowing in the gel sandwiched between the electrode on the probe and the adjacent portion of the surface electrolyses the potassium iodide in the gel to form iodine next to the surface of the specimen. The iodine reacts with the starch to produce a blue dye, which remains localised next to the surface, the intensity of the colour depending on the current density. Thus the the location of any defect or discontinuity is clearly indicated by a dark blue coloration in the gel coating.

Example 2(a) A second specimen is of anodised aluminium and so is covered by an electrically insulating layer of oxide. Consequently the technique described in relation to Example 1 is not applicable.

The aluminium specimen is coated with a mixture of 75 to 80 w/o of the starchIKl gel of Example 1, with 25 to 20 w/o of water based emulsion paint (such as a vinyl matt white paint). When this coating has dried for about an hour (or been dried with a hot air blower for a few minutes) it is then coated with a layer ahout 1 mm thick of a viscous polyethene oxide gel containing cobalt(ll) sulphate. This gel is prepared by mixing 10 w/o polyethene oxide to a 0.1 molar aqueous solution of cobalt(ll) sulphate.

An ultrasonic probe is then scanned over the surface as described above, the gel coating coupling the ultrasonic waves from probe into the specimen. In this case the probe is provided with two electrodes adjacent to the contact face, spaced about 1 mm apart and electrically insulated from each other. When a defect is detected an electrical pulse is supplied between these two electrodes.

The electrical pulse oxidises cobalt(ll) ions to cobalt(lil) ions adjacent the anode. Cobalt(lll) ions then diffuse through the gel coating into the starch/KI/paint layer, where they oxidise iodide ions to generate iodine and so in combination with the starch create a dark blue dye. The location of any below-surface defect is therefore indicated by dark blue coloration, clearly visible in contrast to the white colour of the dry coating.

Example 2(b) In a modification to the technique of Example 2 (a), the gel layer contains cerium(lll) nitrate in place of cobalt(ll) sulphate. The concentration of cerium(lil) is desirably between 0.02 molar and 0.5 molar, preferably 0.1 molar. This gel is used in the same way as described above, the electrical pulse oxidising cerium(lll) ions to cerium(lV) ions adjacent the anode. Cerium(lV) ions then diffuse through the gel into the starch/Kl/paint layer to generate iodine and so create a blue coloration.

This gel has been found to give better colour formation than the cobalt(ll) gel of Example 2(a), and to allow higher scan speeds, up to 2 or 3 cm/sec.

Example 2(c) In a further modification, the gel layer contains cerium(lll) ions (as in Example 2(b)) and the dry coating contains a reducing agent to increase the threshold concentration of cerium(lV) for colour formation. This makes possible the use of higher concentrations of ceri um(lil) in the gel and so a higher scan speed without any change in the width of the coloured loci. For example a starch gel may be made by mixing 10 g of 0.1 molar KI solution, 0.5 g of 0.1 molar sodium thiosulphate solution, 16.5 g of water and 3 g of starch; boiling this mixture, and allowing it to cool. 8 g of this gel is mixed with 2 g of white emulsion paint, coated on to the specimen, and allowed to dry. The dry coating is then coated with a polyethene oxide gel containing 0.5 molar cerium(lll) nitrate. This enables a scan speed of 5 cm/sec to be achieved.

It will be appreciated that the method of Examples 2(a), (b), or (c) is applicable to objects of any material, whether conducting or insulating.

Example 3 A third specimen is of steel with a clean rust-free surface, but in this case it is desired not to make any electrical contact to the steel itself. The couplant used is as in Example 2(b), consisting of a first layer of 75 w/o of starch gel mixed with 25 w/o of white emulsion paint; dried; and then coated with 10 w/o polyethene oxide gel containing 0.1 molar cerium(lil) nitrate.

An ultrasonic probe is then scanned over the surface, the ultrasonic waves from the probe being coupled into the specimen by the gel couplant. The probe is provided with a first electrode adjacent to its contact face which is also in contact with the gel; a second electrode is held in contact with the gel about 50 mm away from the probe. Whenever it is desired to mark a location on the surface, an electrical voltage is applied between the electrodes such that the first electrode is the anode and the second electrode is the cathode.

The electrical voltage induces the portion of the steel specimen directly under the second electrode to be an anode, and the portion directly under the first electrode to be a cathode. Electric current flows from the first electrode through the thickness of the gel to the steel, through the steel, and then through the thickness of the gel to the second electrode.

This leads to a blue coloration within the paint layer underneath the first electrode, but very little if any coloration in the vicinity of the second electrode.

It will be appreciated that a wide range of different materials may be used to provide the desired change of appearance of the surface of the sample. In each of the above examples a colour change is brought about utilizing starch/iodine as a redox indicator; it will be understood that other redox indicators may be used instead, such as methylene blue or diphenylamine sulphonic acid. Alternatively a colour change may be provided by utilizing a pH indicator in an aqueous gel, such as phenolphthalein (which gives a purple coloration in an alkaline solution) or methyl orange (which changes from yellow to red if the solution becomes acidic). The local pH change is created by evolving oxygen, or hydrogen, from water at the electrode, which makes the solution locally acid, or alkaline, respectively.

Example 4(a) A surface of a perspex object is coated with a mixture of diphenylamine sulphonic acid in white emulsion paint (in the proportion of a few drops to 2 g of paint), and this layer is dried. This is then coated with a polyethene oxide gel containing 0.1 molar cerium(lll) ni trate. A two-electrode ultrasonic probe is then scanned over the surface, as described in relation to Example 2(a). When the electrodes are energised, a green coloration appears in the gel coating, as cerium(lV) ions created by electrolysis at the anode react with molecules of the indicator which have diffused from the paint layer into the gel.

Example 4(b) As with Example 4(a), except that the paint layer contains sodium diphenylamine sulpho nate. In this case, when the electrodes are energised, a brown coloration appears in both the paint layer and the gel.

Example 4(c) Methylene blue dissolved in water is reduced to a colourless form by reaction with tin(ll) chloride. A 10 w/o starch mixture is then made with the colourless solution and boiled to produce a gel. This gel is then mixed with white emulsion paint (75 w/o of gel), and coated onto the surface as in Example 4(a). After drying, the paint layer is coated with a polyethene oxide gel containing 0.1 molar cerium(lll) nitrate. On energising the electrodes of a two-electrode probe in contact with the gel, dark blue coloration appears in the paint layer. However if left for some hours, blue coloration is found to appear throughout both the paint layer and the gel, possibly due to oxidation of the methylene blue by atmospheric oxygen.

Example 4(d) A mixture of phenolphthalein in white emulsion paint is used to coat a surface of a metal object, and is then dried (the proportions being 10 drops of 1% phenolphthalein in propan-2-ol, to 3 g of paint). This is coated with a polyethene oxide gel containing 0.1 molar sodium sulphate to ensure adequate electrical conductivity. A two-electrode probe is scanned over the surface, and when the d.c.

supply is energised, a pink coloration is rapidly formed adjacent to the cathode in the paint layer. This coloration however is found to fade within 5 minutes.

Yet again there are a wide range of electrochromic materials which may be used. For example a metal object might be coated with iridium oxide, or a tungsten or molybdenum bronze, or a methoxyfluorene, or a phthalocyanine, and this coating covered by a layer of polyethylene oxide gel as a couplant.

One such electrochromic material is Prussian blue (potassium ferric ferrocyanide) which has a dark blue colour due to the complex iron(lll) hexacyanoferrate(ll); this is made use of in the first two of the following examples.

Example 5(a) A dark blue paint layer consisting of a mixture of equal quantities by weight of 0.1 mo lar iron(lil) sulphate, 0.1 molar potassium hex acyanoferrate(ll), and white emulsion paint is coated onto a surface of a metal object. This is dried and then coated with a 10 w/o poly ethene oxide gel containing 0.1 molar sodium sulphate; and a two-electrode probe scanned over the surface. When the d.c. supply is en ergised, the paint layer becomes white adja cent to the cathode on the probe, if sufficient pressure is exerted on the probe.

Example 5(b) A pale yellow gel is made by boiling a 10 w/o mixture of starch with 0.04 molar potassium hexacyanoferrate(ll). When cool, this is coated onto a surface of a metal object, and acts as a couplant for a one-electrode ultrasonic probe, with the metal object connected to be the anode (as in Example 1). When the d.c. supply is energised, the gel ecomes dark blue at the surface of the object adjacent to the probe.

Example 5(c) A clean surface of a steel object is coated by sublimation in a vacuum with a coating of lutetium diphthalocyanine (a lanthanide diphthalocyanine which is readily sublimahle), and this is covered by a 1 mm thick layer of polyethylene oxide gel containing sodium chloride as an electrolyte. At very low potentials the coating exists in a neutral form which is blue-green in colour. At the voltage of a saturated calomel electrode (S.C.E.) the coating is green in colour due to a green radical cation, while at IV versus S.C.E. a red dication is formed. It can be cycled up to a hundred thousand times, being most stable in the presence of chloride ions and deteriorating at high pH. Colour changes can be brought about (as in Example 1 above) by applying a suitable electrical voltage between an electrode on the probe and the steel object itself, the colour change occurring in the portion of the coating immediately adjacent to the probe.

Other electrochromic materials which may be used are alkylated derivatives of p-phenylenediamine, which may be sprayed onto the surface as a non-aqueous solution, and dried; when electrochemically oxidised these form Wurster salts containing coloured cation radicals, which are generally stable in the pH range 3.5 to 6.

Claims

1. A method of marking a location on a surface of an object while scanning a probe over the surface, comprising coating the surface with an adherent couplant, and subjecting the couplant to a stimulus from the probe, the couplant being such as to alter in appearance as a consequence of the stimulus.

2. A method as claimed in claim 1 wherein the stimulus is electrical.

3. A method as claimed in Claim 2 wherein the couplant is substantially uniform, and a change of colour occurs within the couplant as a result of the electrical stimulus.

4. A method as claimed in Claim 2 wherein the couplant comprises a relatively stiff first layer coated directly onto the surface, and a second layer of an electrically conducting, ad herent, viscous, couplant, and wherein a change of colour occurs within the first layer as a result of the electrical stimulus.

5. A method as claimed in Claim 3 or Claim 4 wherein the probe incorporates a single electrode in contact with the second layer, and the stimulus is a potential difference between that electrode and the surface of the object.

6. A method as claimed in Claim 5 wherein an electrical contact is made to the object in order to apply the potential difference.

7. A method as claimed in Claim 4 wherein the probe incorporates two spaced-apart electrodes in contact with the second layer, and the stimulus is a potential difference applied between those two electrodes to generate a mobile species within the second layer, diffusion of the mobile species into the first layer then causing the change of colour.

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8. A method as claimed in Claim 7 wherein the species which undergoes the colour change is localised within the first layer.

9. A method as claimed in any one of Claims 3 to 8 wherein the species undergoing the colour change is either a redox indicator, a pH indicator, or an electrochromic material.

10. An ultrasonic couplant comprising an adherent, viscous material and including a species which undergoes a change of appearance as a result of an external stimulus.

11. A couplant as claimed in Claim 10 wherein the species undergoes a change of colour as a result of an electrical stimulus.

12. A couplant as claimed in Claim 11 comprising a first material to be coated directly onto a surface of an object to form a relatively stiff first layer, and a second, adherent, viscous, electrically conducting material to be coated onto the first layer, the first material incorporating the colour change species.

13. A couplant as claimed in Claim 12 wherein application of an electrical stimulus to the second material generates a mobile species, diffusion of which into the first material generates the colour change.

14. A couplant as claimed in Claim 13 wherein the colour change species comprises a redox indicator, and the mobile species comprises cobalt(lil) or cerium(lV).

15. An ultrasonic couplant substantially as hereinbefore described with reference to any one of the examples.

16. A method of marking a location on a surface of an object while scanning a probe over the surface, substantially as hereinbefore described with reference to any one of the examples.