ELECTROCHEMICAL SURFACE TREATMENT

Abstract

A method and apparatus for the electrochemical removal of material from a surface in which two or more fluid jets or flows are arranged to impinge on the surface of the object and an electrical current flows through one fluid flow path, through the object, and then through a second fluid flow path.

Claims

1-33. (canceled)

34. An apparatus for the electrochemical removal of material from a surface of a conducting metallic object characterized in that two or more fluid jets or laminar flows are arranged to impinge on the surface and an electrical current flows to the metallic object through one fluid flow path in at least one jet or flow, through the object material, and away from the metallic object through a second fluid flow path in at least one second jet or laminar flow.

35. The apparatus according to claim 34, in which two or more coherent fluid jets or laminar flows are arranged to impinge on a common surface of a metallic electrically conductive object material.

36. The apparatus according to claim 34, in which the material contains radionuclides.

37. The apparatus according to claim 34, in which the depth of material removal from the surface is between 0.001 millimeter and 10 millimeters inclusive.

38. The apparatus according to claim 34, wherein the treatment of a surface larger than the cross-sectional area of the fluid path is achieved by moving the contact point of one or more of the jets across the surface.

39. The apparatus according to claim 38, wherein an average rate of movement of an impact point on the surface is between 0.01 and 10 times, inclusive, a diameter of the fluid path per second.

40. The apparatus according to claim 34, where the applied electric current is in the form of DC biased AC waveform.

41. The apparatus according to claim 40, in which the current density in the fluid jet is between one hundred and one hundred thousand amps per square meter

42. The apparatus according to claim 34, wherein the trajectories of the coherent fluid jets fluid flow paths are divergent.

43. The apparatus according to claim 34, where the electrochemical material removal takes place in a series of sequential treatments.

44. The apparatus according to claim 34, wherein the fluid is contained in a volume adjacent to the object by means of a movable seal.

45. The apparatus according to claim 34, wherein the electrical current is passed from the secondary of an isolating transformer and the electrical current flows through one fluid flow path in at least one jet, through the object material, and to return through a second fluid flow path in at least one second jet and returns to a secondary transformer.

46. The apparatus according to claim 34, wherein a fluid in the jets or laminar flows is contained adjacent to the surface after impinging the surface by means of a porous material.

47. The apparatus according to claim 34, wherein the velocity of the fluid forming the coherent jet is between 0.15 m/s and 50 m/s.

48. The apparatus according to claim 34, characterized in that the electrical resistivity of the electrolyte is less than 1 Ωmeter.

49. The apparatus according to claim 34, further including flexible and electrically insulating material conduits to supply the electrolyte flows to outlets close to the surface and electrodes in the conduits or outlets to introduce current to the electrolyte.

50. The apparatus according to claim 49, in which the exposed surface area of the electrodes in contact with the fluid is greater than or equal to 5% of the cross-section area of the fluid jet.

51. The apparatus according to claim 49, in which at least one electrode is constructed of a perforated surface or mesh.

52. The apparatus according to claim 51, in which the electrodes are supported in an electrically insulating housing and the insulating housing contains means of fluid flow conditioning, multiple parallel tubes, or perforated plates, which form part or all the electrode.

53. The apparatus according to claim 34, in which one or more of the coherent fluid jets is formed by an orifice plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 illustrates schematically one illustrative embodiment of the invention configured for the surface treatment of the interior of a vessel;

[0030] FIG. 2 is a schematic cross section of internal surface of a vessel being decontaminated internally by a methods and apparatus according to the invention; and

[0031] FIG. 3 illustrates an exploded view of one outlet and connecting ducts to project a jet of electrolyte towards a contaminated surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] In FIG. 1 two coherent fluid jets or laminar flows 1A and 1B of electrolyte (more than two may be provided but the additional jets are omitted for simplicity) are arranged to impinge at different points 1C on a radionuclide contaminated surface 10 of an object 11 comprising an electrically conducting material. The jets emanate from outlets 7 from ducts 2 which are connected through pipes 3 to a source of electrolyte, the electrolyte being pumped through the pipes at a sufficient pressure to maintain a suitable fluid flow. In the figure, outlets 7 comprise nozzles but they may be orifice plates or slots. In one implementation the outlet was an orifice plate with a circular shape with a radius of between 5 mm and 100 mm. In another implementation, coherent fluid jets were formed by an annular slot or partial annular slot in each outlet 7.

[0033] An isolating transformer 4 is arranged to supply electrical power with a current path from the secondary winding 5 of the transformer through one jet 1A fluid flow path, through the object 11, returning through a second the second jet 1B and back to the secondary winding 5.

[0034] No part of the secondary circuit through winding 5 is grounded resulting in the transformer 4 being isolating. This contrasts with common existing practice, where one side of the transformer winding 5 would be connected to the object 11. In contrast, in this invention, only electrical connection to the object 11 is through the jets 1A and 1B through surface 10. As a result, with no possible current path from the object 11 other than through the jets 1A and 1B, there is no possibility of unwanted electrochemical effects taking place at remote locations away from the intended working area.

[0035] The use of impinging jet 1A and 1B for the purpose of electrochemically material removal from a metallic surface finds application in various sectors including for the purpose of the removal of radioactive contamination from the surface layers of nuclear plant.

[0036] Sponges 8 or other porous media may be used to control the flow of the fluid streams in the vicinity of the impingement points and reduce unwanted splashing and distribution of the liquid away from working area of the surface 10 and to localize the flow in the vicinity of the working area.

[0037] Flexible seals 9 may be placed partially or completely around working area of the surface 10 to contain splashes, at least partly, and to help direct and collect the electrolyte after it impinges the surface 10.

[0038] Shaped openings such as annular partial annular slots may also be used to localize flows around features.

[0039] Electrolyte that has impinged on the surface 10 of object 11 will flow to a suitable collection point 12 either at the lowest point of an internal vessel or else along a pipe.

[0040] The electric current may be introduced into the jets 1A and 1B by various means including by conducting nozzles 7 or by electrodes within the ducts 2 or by electrodes in the electrolyte jet once it has left the nozzle, in any case with said electrode connected by wires 6 to each side of the secondary coil 5. The material that imparts the electric current to the electrolyte flow, whether part of the nozzle or pipe structure or external to the nozzle or pipe structure, is preferably of a material that will not be consumed by an electrochemical process.

[0041] The ducts 2 and piping 3 that carry electrolyte to the outlets 7 is of an insulating material such as plastic and the piping 3 is of sufficient length that the electrical resistance of the flow path from one liquid stream to another along the pipework is considerably greater than the electrical resistance through the jet flow paths and through the work piece.

[0042] The ducts 2 and outlets 7 may be of a flexible electrically insulating material such as rubber so that contact of the ducts and nozzles with the object does not cause a problem. In this instance the electrodes that impart current to the electrolyte flow must be within the ducting 2.

[0043] In the embodiment described the material removal from the metallic surface is typically between 0.001 millimeter and 10 millimeters inclusive. This can be done in a single pass or in more than one pass.

[0044] Typically, the time averaged rate of movement of the impact point of the jets on the surface is between 0.01 and 10 times inclusive the diameter of the fluid path per second.

[0045] The applied waveform was an DC biased AC waveform whose frequency was between 5 and 2000 Hz inclusive. The preferred current density in the fluid jet is between one hundred and one hundred thousand amps per square meter inclusive.

[0046] The transformer 4 is isolating which is to say that no part of the secondary circuit is grounded, as is existing common practice. The arrangement obviates the need for any electrical connection to be made to the object 11 other than through the liquid jets and also means that there is no possible current path from the object 11 other than through the fluid jets and therefore that there is no possibility of unwanted electrochemical effects taking place at remote locations away from the intended working area.

[0047] The distance between the points of impingement of the several jets or laminar flows on the surface 10 of the object 11 will be arranged so that the electrical resistance of the object 11 between those points is such that the surface treatment rate meets the operational requirement. Typically this means that the electrical resistance of the object 11 between the points of impingement of the electrolyte jets or flows is less than the electrical resistance of the electrolyte liquid present at the surface of the metal between those two points but this is not necessarily always the case. It may be for example that for the treatment of particularly inaccessible locations the electrical efficiency of the process is sacrificed in favor of geometrical convenience of access and it is accepted that additional power is lost due to conduction along the electrolyte on the surface 10 of the object.

[0048] The spatial orientation of the electrolyte flows 1A and 1B and points of impingement 1C may be optimized for the geometry of the object being treated. Arrays of jets or extended jets may be used. For the treatment of the exterior of pipes the jets may be arranged in an annular or part annular shape for example. For the treatment of the interior of a vessel jets may be arranged in radiating shapes emanating in multiple directions for example.

[0049] The movement of the points of impingement 1C of the jets or laminar flows 1A and 1B over the surface 10 of the object 11 may be programmed in advance to obtain a pre-determined profile of surface treatment. A program that controls the movement may have regard both for the level of contamination or depth of surface removal required and for the effects of geometry of application on the intensity of treatment. The automatically controlled movement of the electrolyte jets or laminar flows 1A and 1B may also be controlled by measurements of a property of the surface being treated that are made in a survey before the treatment is carried out or in real-time, such as the level of radioactivity present or its reflectivity of light or another suitable measurable property.

[0050] The movement of the multiple jets or laminar flows 1A and 1B over the surface being treated may be controlled in such a way that a predetermined level of surface treatment or removal is achieved. This level may have been determined by prior radiological assessment of the substrate in question.

[0051] High pressure electrolyte jets or laminar flows or jets may be advantageously used to provide a mechanical surface treatment effect in addition to the electrochemical effect described, constituting in effect a pressure-washing. This is useful when there is surface contamination such as oil or grease or particulate matter or paint or other substances that need to be removed. Solid particulate matter may optionally be dispersed in the liquid electrolyte to provide an additional abrasive cleaning effect.

[0052] The production of a continuous and coherent fluid path from the electrode contained in an electrode housing (in the illustrated embodiment the housing is the ducting 2) to the surface 10 of the object 11 is essential for the operation of the system. The fluid path resistance will vary significantly if the jet or flow is broken or forms droplets between outlet 7 and the surface 10 being treated. The generation of coherent flows is achieved using flow conditioning in the electrode housing (duct 2) and reduces the velocity variation from the conduit. This is preferably achieved by expansion of the cross section of duct 2 to reduce velocity of the flow, controlled pressure drops and flow straitening prior to the outlet 7 which is an orifice plate, nozzle or slot which generates a fluid jet with a low variation in velocity. The coherent jets preferably have a continuous path for over 1 meter in free space, but practical voltage limitations limit preferred operating distances to below 0.5 meters.

[0053] The electrodes are preferably a stable material with good electrical conductivity. Suitable are a carbon-based conductor or a metallic conductor or metallic conductor with an applied surface coating. Preferably the metal conductor or metal surface coating comprises a metal selected from the group comprising platinum, gold, stainless steel, chromium, nickel, tantalum, osmium, iridium, palladium.

[0054] The exposed surface area of the electrodes in contact with the fluid is ideally greater than or equal to 5% of the cross-section area of the fluid jet. The electrode is in contact with the electrolyte before the fluid leaves the outlet 7 and is preferably sized to reduce localized current density in the housing (duct 2). Practical arrangements for the electrodes include a perforated surface or mesh mounted in an electrically insulating housing (which in the illustrated embodiment is also the duct 2). An electrode forming the part of the edge of the flow path such as a ring in an electrically insulating housing. An electrode inserted into the flow path such as a tube or rod in an electrically insulating housing. An electrode orifice plate at or near the jet exit which also defines the flow. A combined electrode and flow conditioning arrangement made of a material which also acts as an electrode.

[0055] The electrically insulating housing can contain means of fluid flow conditioning such as multiple parallel tubes, or perforated plates. The parallel tubes and/or perforated plates in the electrically insulating housing can also form part or all of the electrode in that housing.

[0056] The electrode is in contact with the electrolyte before the fluid leaves the outlet 7 and is preferably sized to reduce localized current density in the housing (duct 2). The electrode may be shaped as a ring or cylinder or preferably as a mesh or perforated plate located in the electrode housing (duct 2) or the outlets 7 can also be the electrodes.

[0057] The method and apparatus described is applicable for threating a wide range of geometries of contaminated surface. This includes the interior and exterior of pipes, the interior and exterior of vessels, structures of various sorts including valves, pipe manifolds, support structures, discrete components or any surface that needs surface treatment. The method is suited for the treatment of localized hot spots of contamination.

[0058] FIG. 2 illustrates the treatment of a cylindrical vessel 21 using two electrolyte jets 22 directed at the inside surface of the vessel 21. The two electrolyte jets, 22, are shown as arrows and have divergent paths. The nozzles are shown as 23 (in this embodiment these perform the function of the outlets 7 in FIG. 1), and the electrolyte supply to the nozzles as 24 (this performs the function of duct 2 in FIG. 1).

[0059] In order to treat the surface of a component that is larger than the cross section, the impact points 1C of the fluid jet on the surface are moved. The rate of movement is proportional to the size of the area of the jet impact, the current density applied and indirectly proportional to the depth of material to be removed. The movement could be continuous, step wise or in a raster pattern depending on the features of the surface and the control methodology employed.

[0060] Although two jets or laminar flows 1A and 1B, and 22 are shown in FIGS. 1 and 2 respectively, multiple jets or laminar flows can be used.

[0061] In FIG. 1 the jets or laminar flows 1A and 1B are shown as bring parallel but the inventors have found that divergent jets as shown in FIG. 2 speeds the surface when the jets are directed at concave surfaces such as the inside of cylindrical vessels and ducts. Likewise, convergent jets may speed cleaning of convex surfaces such as the outside of cylindrical vessels.

[0062] It has been found that the best fluid velocity of the fluid forming the coherent jet or laminar flow is between 0.15 m/s and 50 m/s.

[0063] The electrolyte should be a conductive fluid, increased electrical conductivity reduces the voltage required. The fluid electrical resistivity should be less than 1 Ωmeter and preferably less than 0.2 S2 meter.

[0064] It is preferred that dissolved metals should have some solubility in the electrolyte to minimize the fluid volumes required to treat a given area and to prevent precipitation of removed metals.

[0065] It is preferred that the electrolyte should not chemically dissolve or cause localized damage to the substrate to any great extent and that the corrosion should be minimal over the total electrochemical treatment time. For stainless steels and most nickel alloys, nitric acid is the preferred choice as the chemical corrosion rate is not high and the acid is suitably conductive.

[0066] For the treatment of radioactive contamination from the nuclear industry, it is advantageous to use nitric acid or nitrate salts as many radionucleotides are soluble and it is often compatible with known waste treatment routes.

[0067] FIG. 3 illustrates one arrangement for the outlet 7 (in FIG. 1).

[0068] In FIG. 3, fluid enters through a conduit 31 (for example the piping 3 of FIG. 1) into an electrically insulated housing 32 (for example, the duct 2 of FIG. 1). Towards the exit face of the electrically insulated housing is an array of flow conditioning tubes 33. The electrolyte fluid then passes through a perforated plate electrode 34. This is connected to the electrical supply through electrical connection 35. Fluid then passes through a second electrically insulated housing 36, before leaving the outlet 7 through a hole in an orifice plate 37. The orifice plate also has an optional connection 38 to an electrical supply.

[0069] Examples of use of the use of the Illustrated Embodiment

Example 1

[0070] A sample of 304 stainless steel sheet was treated using two jets which were located 90 mm apart and 50 mm away from the sheet sample surface. Nitric acid (30% w/w) was pumped through each 25 mm diameter nozzle to form the jets at a rate of 32001/hour. When electrodes in each nozzle were energized at 160V by an isolated power supply, a current of 25 A flowed through the circuit made of the two jets with current passing through the stainless steel object, without any direct contact to the sample from the isolated power supply and without any current passing to ground if the stainless steel sample was earthed. The sample was treated for 15 minutes and in that time lost 3.2 g of mass from the two jet/sample contact areas. This corresponds to a current efficiency for metal dissolution of 50%.

Example 2

[0071] A sample of 304 stainless steel sheet was treated using two jets which were located 50 mm away from the sheet sample surface. Nitric acid (30% w/w) was pumped through each 25 mm diameter nozzle to form the jets at a rate of 32001/hour. When electrodes in each nozzle were energized at 160V by an isolated power supply, a current of 25 A flowed through the circuit made of the two jets with current passing through the stainless steel object, without any direct contact to the sample from the isolated power supply and without any current passing to ground if the stainless steel sample was earthed. One of the jets was traversed horizontally across the surface at a fixed rate removing 20 microns of material from the surface, along the path which the jet transited across the surface.