WO1998022231A1

WO1998022231A1 - Method and apparatus for the treatment of surfaces of large metal objects

Info

Publication number: WO1998022231A1
Application number: PCT/IL1997/000360
Authority: WO
Inventors: Michael Borochin
Original assignee: Omerco Ltd.
Priority date: 1996-11-18
Filing date: 1997-11-10
Publication date: 1998-05-28
Also published as: AU4793497A; EP0942790A1; IL119634A; IL119634A0

Abstract

A method of treatment of metallic surfaces, particularly for the purpose of cleaning or descaling surfaces of large, metallic bodies (10), which comprises the steps of: selecting an area (21) of the surface of the metallic body (10) as the area to be treated; defining a treatment space having said area as part of its boundary and being sealed from the environment; creating a subatmospheric pressure within said treatment space; and generating an arc confined in said treatment space, wherein said metallic body (10) is the cathode.

Description

METHOD AND APPARATUS FOR THE TREATMENT OF SURFACES OF LARGE METAL OBJECTS

Field of the Invention

This invention relates to the treatment by electric arc discharge in vacuum of metal surfaces, particularly for cleaning purposes, such as the removal of rust, oxidized particles and other contaminants, and more particularly, to the treatment of relatively large metal bodies, e.g., metal constructions such as bridges, large industrial apparatus such as reactors or reservoirs, and like metal objects.

Background of the Invention

It is know^τn to treat metal surfaces for cleaning purposes by vacuum arc discharge in vacuum, wherein the metallic object to be cleaned constitutes the cathode and an anode is provided in the cleaning apparatus.

EP 0 560 526 Bl contains a description of the prior art, which is incorporated herein by reference and comprises USP 4,534,921, GB 2 086 788, a paper by V.E. Bulat and M.Kh. Esterlis in Fizika i Khimiya Obrabotki Materialov, Vol. 21, No. 3, 1987, pp. 49-53, and a paper by I. Nikoshevitz, in Nauka Technico, Minsk, 1988, as well as the book "Vacuum Arcs: Theory and Application," J.M. Lafferty, Wiley, 1980. EPA 0 560 526 Bl discloses and claims a method for the surface treatment of metal objects wherein a vacuum arc discharge is generated between a major exposed portion of an anode and successive restricted areas of the workpiece, which act as a cathode, the arc discharge having an arc current that is not substantially less than 50 A and having a positive voltage -current gradient wherein the maximal work pressure is generally not more than 100 Pa; and an apparatus for carrying out said method, which comprises a chamber, means for generating a vacuum in the chamber, means for controlled introduction of gas into the chamber, at least one anode, means for displaceably supporting the workpiece constituting a cathode in said chamber, and means for applying an arc generating voltage between the anode and the cathode, means for restricting the arc discharge to successive restricted areas of a surface of the workpiece, means for ensuring a substantial uniform arc distribution on the anode and means for effecting relative displacement between the workpiece and the anode in a given direction. The said method and apparatus are ecologically satisfactory and adequately efficient, but are not suitable for the treatment of all metals, as the treatment of most metals and their alloys by electric discharge in vacuum at pressures below 100 Pa leads to a considerable erosion of the treated surface, and also the said method and apparatus are not suitable for the treatment of large metal objects, such as metal constructions or parts thereof, ship hulls, reservoirs, reactors and other large industrial apparatus, because they require that the metal object which constitutes the cathode be introduced into the chamber wherein a vacuum is generated, even though the arc discharge is restricted to successive restricted areas of the workpiece.

It is a purpose of this invention to overcome the drawbacks of the prior art methods and apparatus, and in particular, the aforesaid drawback of the apparatus of EP 0 560 526 Bl. It is another purpose of this invention to provide a method and an apparatus for the treatment, in particular the cleaning, of metal surfaces of large dimensions by means of an arc discharge, under subatmospheric pressure conditions, without the need of providing vacuum chambers in which the workpiece is introduced.

It is a further purpose of the invention to provide a method and apparatus which enable the predetermining of the resulting parameters of the surface which is treated.

It is still a further purpose of this invention to provide such a method and apparatus which are useful for the treatment of surfaces that need not be plane, but may have any geometric configurations, such as angles, corners, curved surfaces having any radius of curvature, and irregular surfaces in general.

It is a still further purpose of the invention to provide an apparatus which achieves the aforesaid purposes and yet is simple, easy to use and economical.

It is a still further purpose of this invention to provide such an apparatus that permit to treat metal surfaces by increments, viz. by treating portions thereof one after the other, or continuously, in order to treat at any time only limited portions of surfaces. Other purposes and advantages of the invention will appear as the description proceeds.

Summary of the Invention

The method of treatment of metallic surfaces, particularly for the purpose of cleaning or descaling surfaces of metallic bodies, according to the invention, comprises the steps of: selecting an area of the surface of the metallic body as the area to be treated; defining a treatment space having said area as part of its boundary and being sealed from the environment; creating a subatmospheric pressure within said treatment space; and generating an arc confined in said treatment space, wherein said metallic body is the cathode.

In order to apply the method to a surface of the metallic body larger than the selected area to be treated, the method may be carried out stepwise, (or, it might be said, incrementally) or continuously.

To carry it out incrementally or stepwise, once the selected area has been treated, the arc is discontinued, ambient pressure is reestablished in the treatment space, another area to be treated is selected and the aforesaid steps of the method are carried out with respect to said other selected area. The same operations are repeated successively for successive selected areas until all the surface of the metallic body, that it is desired to treat, has been treated. To carry out the method continuously, the selected area is changed continuously, the treatment space is correspondingly changed continuously, the subatmospheric pressure is maintained within the changing treatment space and the arc is continuously displaced to maintain it in the same or substantially the same relative position to the changing selected area and treatment space.

The method can also be carried out in a mixed stepwise-continuous manner, by selecting a first domain of the surface of the metallic body, larger than any selected area, carrying out the method continuously within said domain, then selecting one or more other domains and carrying out the method stepwise from one domain to another.

The treatment space is sealed from the environment, so as to make it possible to maintain therein a pressure substantially lower than the ambient pressure, at or near the boundary of the selected area. The pressure in the treatment space is subatmospheric and may vary from 5.32xl0⁴ Pa to 1.33xl0"² Pa (400 to 10^~4 Torr).

It will be appreciated that the expression "large bodies" is relative. In many cases, as in the case of reservoirs having a large volume, or of a ship hull, or of bodies of comparable size, and in general of bodies the surfaces of which have large areas, there is no doubt that that expression applies. Furthermore, the term "large bodies" may also refer to bodies having only one large dimension, such as rolled or drawed bodies having for example, an area of a cross section of 10 cm² and a length of 3 m or more. In other cases, the body treated according to the invention, may be limited enough in size that it could be treated by the methods and apparatus of the prior art, or at any rate by introducing it entirely into a treatment chamber. However, the invention is apphcable^" with advantage to such bodies as well, since it is advantageous to use a treatment apparatus of more limited size than would be necessary according to the prior art, and further, the invention, permitting to treat different portions of a workpiece successively and/or under different process conditions, such as process parameters, has advantages in addition to the reduction of the treatment apparatus size. Therefore it will be understood that, while the invention is particularly suitable for the treatment of large bodies, it is not limited to it.

It will also be appreciated that the invention is apphcable to any shape of surfaces of a workpiece. Said surfaces may be plane, but they may be curved or polyhedral, concave or convex, or have projections or depressions and in general any irregularity whatsoever.

The invention also provides an apparatus for the treatment of surfaces of metallic bodies, which comprises, in combination with a power source and with suction means: a cup-like body or casing; at least one anode; electrically insulating and heat resistant means provided at the periphery of said cup-like body; sealing means provided at the periphery of said cup-like body if needed; said cup-like body having a periphery so matching the surface to be treated that when it is engaged with said surface a sealed treatment space is defined therebetween, and having openings therein for connecting said treatment space to said suction means whereby to generate in said space a subatmospheric pressure; and means for electrically connecting said power source to said anode and to the metallic body to be treated as the cathode, whereby to generate an arc therebetween.

Trigger electrode means or other means for triggering the arc, which may be constituted by means for contacting anode and cathode and subsequently distancing them from one another, also included or provided in the apparatus. Arc triggering means are well known per se and widely used in the art and need not be described, as any such means may be used in carrying out the invention.

The anode is connected to and supported by the cup-like body. It may be a separate component mounted in said cup-like body and electrically insulated from it, if needed, or it may be constituted by a portion of a surface, particularly the inner surface, of said body.

In a form of the invention, the anode is slidably mounted in said cup-like body to control the distance between it and the surface to be treated. A plurality of anodes may be provided and they may be arranged in such a way, and the cup-like body may be correspondingly shaped, as to match any desired shape of the surface to be treated. Also preferably, the electrically insulating and heat resistant means and correspondingly the seahng means are so dimensioned as to create a gap between the lower face of the cup-like body and the surface to be treated when said body is engaged with said surface. Said gap is in an order of magnitude from a few tenths ( e.g. one half) of a millimeter to a few millimeters.

In certain forms of the invention, the cup-like body is made of a conductive material, preferably of metal, and is more preferably a solid body, provided with insulation where required and as will be better explained hereinafter. Preferably, in said forms of the invention, the inner surface of said body constitutes the anode. Said body may also be covered with an insulating layer on the outside and/or on the surfaces- through which it is touched or handled. It may also be provided with inner channels for circulating a cooling medium, as well as with handles or other parts that permit an operator conveniently to hold and use it. In other forms of the invention, the cup-like body is made of a dielectric material and supports a separate anode or a number of anodes.

In still another form of the invention the cup-like body is made of a flexible material.

The power means and its connection to the anode and the metallic body are not different from those known in the art and therefore need not be described in much detail. However, the means for electrically connecting the power source to the metallic body to be treated as the cathode are preferably located out of the treatment space. Likewise, the suction means for creating a subatmospheric pressure may be conventional. It will be clear that the apparatus of the invention automatically solves the problem of confining the arc to a limited area of the workpiece, a problem which the prior art has attempted, not always successfully, to solve by more complicated means.

Brief Description of the Drawings

In the drawings:

Fig. 1 is a perspective view, partly broken off, of an apparatus according to an embodiment of the invention;

Fig. 2 is a vertical cross-section showing at an enlarged scale a detail of the apparatus of Fig. 1;

Figs. 3 is a vertical cross-sections, similar to Fig. 2, showing at an enlarged scale detail of an apparatus according to another embodiment of the invention;

Figs. 4(a), 4(b), and 4(c) schematically illustrate, in vertical cross- sections, differently shaped apparatus according to embodiments of the invention. Figs. 4(d) and 4 (d') schematically illustrate two cross-sections of still differently shaped apparatus according to an embodiment of the invention;

Figs. 5, 6 and 7 are perspective views, partly broken off, of apparatus according to further embodiments of the invention;

Fig. 8(a) and 8(b) show two cross-sectional views of an apparatus according to a further embodiment of the invention;

Fig. 9 is a schematic illustration of one way of carrying out the method of the invention; and

Fig. 10 is an isometric view illustrating the operation of the device of Figs. 4(d) and 4 (d'). Detailed Description of Preferred Embodiments

With reference now to the drawings, Figs. 1 and 2 illustrate a first embodiment of the invention, however in two different stages of the process, as hereinafter explained. Numeral 10 designates a portion of the metal surface to be treated - the workpiece. In the drawings and the description thereof, the workpiece surface to be treated, if flat, is assumed to be horizontal, but in practice it may have any orientation and therefore the terms "horizontal" and "vertical" actually signify parallel and perpendicular to the workpiece surface. Numeral 11 generally designates a treatment apparatus according to said first embodiment of the invention. Apparatus 11 comprises a cup-like body or casing 12, which is shown in Fig. 1 as broken off to evidence its structure. Said casing 12 is made of a conductive material, e.g. copper or another metal or metals, and may be provided, if desired, with an insulating outer layer, not shown in the drawing. An insert 13, made of an electrically insulating and heat-resistant material, is provided on the lower face of casing 12 and has a height, e.g., of a few millimeters. Said insert should preferably have a low friction coefficient in order to ease moving the cup along the surface. Insert 13 forms a projection 14 which has a bottom surface 17' (see Fig. 2). Insert 13 also has a surface 17, which is the lower face of the cup-like body and is located in the treated region of the workpiece. Casing 12 is also provided, if desired, with a vacuum seal 18, which is made of a compressible material, such as rubber. Seal 18 is so dimensioned that, when cup-like casing 12 is placed on the surface of workpiece 10, said seal extends downwardly from the level of said surface 17' by a distance of, e.g., at least 1 mm, and a narrow gap of, e.g., about 1.5 mm exists between the outer edge of said casing and the surface of the workpiece, as better shown at 16 in Fig. 2. When suction is applied and a subatmospheric pressure is created in the space between the inner surface of the cup -like body and the workpiece (hereinafter "the treatment space"), the atmospheric pressure forces said body down onto the workpiece, and seal 18 is compressed to the extent that surface 17' (see Fig. 2) comes into contact with the workpiece, as seen in Fig. 1. In that condition, a gap exists between aforementioned surface 17 and the treated surface of the workpiece, which gap, as has been stated, is in the order of magnitude from a few tenths of a millimeter to a few millimeters.

An opening 19 is provided in one of the walls of cup 12, through which air is drawn from the treatment space by means not illustrated, as they may be conventional. The inner surface of cup-like body 12 constitutes, in this embodiment, the anode, and the workpiece 10 the cathode. The means for triggering the arc are not illustrated, and they could consist in a trigger electrode, e.g. mounted on body 12 and insulated therefrom, or in any other suitable means.

It will be apparent that the "selected area", viz. the area of the workpiece that is treated at any one time, is that indicated at 21 (in the drawing, the unshaded area), which, in the embodiment illustrated, is slightly smaller than the projection of the top of the casing on the workpiece, since the arc is restricted to the treatment space, viz. the space circumscribed by inner surface of the casing 12 and the selected area of the workpiece. The electrical connections to the anode and the workpiece are conventional and are schematically illustrated in Fig. 9, hereinafter described. Optionally, cooling means may be provided in the casing, as shown in Fig. 3, wherein numeral 30 designates the workpiece and numeral 31 designates the lateral wall of the casing, which is generally similar to casing 12 of Fig. 1. On the lower portion of wall 31 there is provided a groove 27, within which a cooling fluid is circulated by means not shown and which may be conventional. The vacuum seal, corresponding to vacuum seal 18 of Fig. 1 and herein indicated at 28, is V-shaped in this embodiment, as shown in the drawing, and seals the groove 27. Wall 31 projects into groove 27, at the bottom thereof, forming a projection 29 for the purpose of preventing the pressure of the cooling fluid, circulating in groove 27, from expelling the seal 28, when the cup-like casing is not resting on the workpiece. Heat resistant and insulating insert 32 corresponds to insert 13 of Fig. 1. When the casing is placed on the workpiece, the seal 28 is urged against the upper surface of the workpiece by the pressure of the cooling fluid which flows through the groove 27.

Figs. 4(a), 4(b) 4(c) and 4(d) illustrate four cross-sections of the cup-like casing for use over surfaces having different profiles. In Fig. 4(a), casing 11a is adapted for use on a workpiece 10a having a V-shaped cross- section. Insulators 30 and seal 31 match the slant of the branches of the workpiece V-surface. In Fig. 4(b), workpiece 10b has an inverted-V cross- section and the casing lib is correspondingly shaped and provided with corresponding insulator 32 and seal 33 (when exists). In Fig. 4(c), the workpiece 10c is arc-shaped and the casing lie is correspondingly shaped, with insulator 34 and seal 35 (if exists) having correspondingly curved undersides. Finally, Figs. 4(d), 4(d') (showing a cross-section made along line A-A of Fig. 4(d)), and Fig. 10 illustrate the operation of a device according to still another embodiment of the invention. As shown, the workpiece 10(d) is a surface of a rolled shaped body, and two opposite walls 36 and 37 of the casing 11(d) correspondingly have shaped openings with insulators 38 and 39, and seals, if needed (not shown).

Fig. 5 illustrates another embodiment of the invention. The workpiece is indicated once again by 10. The casing is generally indicated at 40. This embodiment is particularly useful when the treatment of the workpiece surface must be more intensive, for instance, if the workpiece is covered with a thick layer of rust or oxidized material, or if the metal of which the workpiece is made has particular characteristics which require a more intensive treatment. In this embodiment, the cup-like casing is provided with insulating insert 41 and seal 42, which operate like insert 13 and seal 18 of the embodiment of Figs. 1 and 2. The gap between the edge of the casing and the upper surface of the workpiece 10 is essentially the same as in said embodiment. Numeral 19 designates once again the opening through which air is drawn from the treatment space. Here however the cup-like casing does not constitute the anode, but a separate anode 43 is suspended from two conductor rods 44 connected to insulator

45. Rods 44 are slidable in guides or supports 49 and are urged upwards by compression springs 46. A stopper 47, threadedly mounted on casing 40 and therefore vertically adjustable, limits the upwardly motion of anode 43. Friction means or other suitable, conventional means, not shown, are provided in guides 49 to slow down and amortize the upwardly motion of the rods 44 caused by the elastic reactions of springs

46. If, as is preferred, the casing 40 is made of metal, rods 44 are insulated from it and sealed by means not shown as they may be conventional. Numeral 48 indicates the selected area. In this embodiment a trigger is not needed, though it could be provided if desired. The arc can be triggered by pressing down on handle 45 until anode 43 contact the workpiece and interrupting the contact between the electrodes. Thereafter the handle 45 will be released, thereby causing anode 43 to rise slowly as far as permitted by stopper 47, which has been adjusted to determine the optimal or the final interelectrode gap.

The embodiment of Fig. 6 is particularly useful for treating surfaces or portions of surfaces that are particularly elongated and narrow, or, as one might say, rib-shaped. For this purpose, the casing, indicated herein by 50, has an elongated shape. It is provided with insulator insert 56 and vacuum seal 57 and with opening 19, through which air is drawn from the treatment space. A number of electrodes 52, generally three or more, are placed side by sid*³ with a narrow spacing therebetween and each is attached to a conductor rod 54, sealed, insulated from casing 50 and vertically slideable. All the anodes are connected to a single of current, by means not shown, so that one or more of them may be connected to said source concurrently or according to a predetermined succession, in order successively to treat longitudinally adjacent portions of the surface indicated at 55 in the drawing. Said surface, therefore, may be considered as a selected area or as a domain comprising a number of selected areas, which are the areas treated by a single anode or by that number of anodes that are concurrently connected to the power source, and the method may be considered as being applied continuously within said domain. For treating elongated and/or crooked surfaces, an apparatus such as the one shown in Fig. 7 should be used, the casing 50 of which is made of a flexible, electrically insulating material.

The operation of all the embodiments illustrated is similar. When the method of the invention is carried out in a stepwise or incremental manner, a portion of a workpiece surface (the selected area) to be treated is chosen and the cup-like body or casing is placed on it. Then suction is applied through the appropriate¹ opening to lower the pressure in the treatment space up to the degree of working pressure needed. At that point, t he arc must be triggered, and this is done by lowering the anode or anodes to contact or almost to contact the workpiece surface or by means of a trigger electrode or bj' other means known in the art. The anodes may from the beginning be at the optimum distance from the cathode workpiece for producing the desired arc, or they may be displaced more or less radually, as desired, until they reach a final intcrelectrode gap. The arc current is generally at least between 50 Amps to 1000 Amps, in some applications the current may be below 50 Amps or more than 1000 Amps. For the purpose of providing the predetermining of the resulting parameters of the surface to be treated the treatment may be carried out at constant, predetermined values of the above parameters - viz. pressure, arc current and distance from the anodes to the workpiece - and be discontinued after a period of time that may vary widely, e.g. from a portion of a second to a number of seconds, or, in specific cases, may be even much longer. Optionally, however, the above parameters may have a first value when the arc is generated at the beginning of the treatment, and one or more of them may be continuously changed during the treatment, until they reach a second value. After said period of time has passed, or after said second value of said one or more of the said parameters has been attained, or a combination of parameter values and time elapsed has been reached that is sufficient to effect the desired treatment, the arc is discontinued and ambient pressure is reestablished in the treatment space. The cup-like body is removed from the workpiece, and the above operations are repeated for a different selected area of the workpiece.

It should be noted here that the movement of the cup-like body along the treated surface can be carried out by any conventional means. Those means are not elaborated here for the purpose of brevity. When the method of the invention is carried out continuously, this can be done by displacing the cup-like body over the surface to be treated or by successively activating different anodes.

Figs. 8a and 8b are two cross-sections illustrating an embodiment of the

invention for treating inner surfaces. For this purpose, casing 60, made of

dielectric material, has a periphery 61 which is similar to the shape of the

surface to be treated. A plurality of anodes 62 protrudes from said

periphery. All of the said anodes are electrically connected to the current

source by means going through a slidable pipe 63, which is sealed from

cover 64. Said cover 64, together with cover 65 and the workpiece,

indicated once again as 10, constitute the treatment space (in this case,

the workpiece is an inner surface of a square pipe). All connections

situated within the treatment space are sealed (not shown). The opening, indicated once again by 19, is provided in cover 65, through which air is

drawn from the treatment space. The operation of the anodes may be

carried out by means and succession such as hereinbefore explained for

the device of Fig. 6.

Fig. 9 schematically illustrates an embodiment of the method according to the invention. The workpiece, generally indicated at 80, is illustrated as a large cylindrical body, such as a large vessel of some kind. Numeral 81 indicates the cup-like body according to any one of the embodiments illustrated or any other embodiment of the invention. Cables 82 and 83 connect the cathode, which is the workpiece, and the anode, which is part of the cup-like body 81 or is carried by it, to a source of power such as a welding machine 84. Suction means are schematically indicated at 85 and are connected. by means of a flexible part 86 to the cup-like body 81. Numeral 87 indicates a portion of the workpiece which has already been treated, as the apparatus 81 travels in the direction indicated by the arrow 88.

Elongated or crooked surfaces can also be treated by an apparatus such as the one shown in Fig. 6 or such as the one of Fig. 7. In this case, the anodes are activated in succession, as has been said, thereby treating successive portions of a domain of the workpiece surface without discontinuing the arc. When the whole domain has been treated, if other domains are to be treated likewise, the arc is discontinued, ambient pressure is reestablished in the treatment space, and the cup-like body is shifted to another domain, thereby actuating a mixed continuous- stepwise mode of operation. While a number of embodiments of the invention have been illustrated, it will be clear that the invention is not limited thereto, and that a number of variations, modifications and adaptations can be made therein by

persons skilled in the art without departing from its spirit or exceeding the scope of the claims.

Claims

1. Method of treatment of metallic surfaces, particularly for the purpose of cleaning or descaling surfaces of large, metallic bodies, which comprises the steps of: selecting an area of the surface of the metallic body as the area to be treated; defining a treatment space having said area as part of its boundary and being sealed from the environment; creating a subatmospheric pressure within said treatment space; and generating an arc confined in said treatment space, wherein said metallic body is the cathode.

2. Method according to claim 1, wherein the arc is successively generated between a selected one of a plurahty of anodes and the cathode.

3. Method according to claim 1 wherein the arc is generated once, and then transferred to a next anode without terminating the arc according to a predetermined succession.

4. Method according to claim 3, carried out incrementally by: selecting a first area; treating the same as set forth in any one of claims 1 to 3; thereafter discontinuing the arc and reestabhshing ambient pressure in the treatment space; selecting a second area; treating said second area as set forth in any one of claims 1 to 3; and successively selecting successive areas and treating each one of them as set forth in any one of claims 1 to 3, until all the surface of the metallic body, that it is desired to treat, has been treated.

5. Method according to any one of claims 1 to 3, carried out continuously by continuously changing the selected area, correspondingly changing the treatment space continuously, maintaining subatmospheric pressure within the changing treatment space and continuously displacing the arc by maintaining it in substantially the same relative position to the changing selected area and treatment space.

6. Method according to any one of claims 1 to 3, carried out in a mixed stepwise-continuous manner, by selecting a first domain of the surface of the metalhc body, treating said first domain by selecting a number of areas within it and successively treating said areas by the method of any one of claims 1 to 3, thereafter selecting one or more other domains and treating them successively by selecting in each one a number of areas and successively treating said areas by the method of any one of claims 1 to 3.

7. Method according to claim 1, wherein the treatment space is sealed from the environment at or near the boundary of the selected area.

8. Method according to any one of claims 1 to 3, wherein the subatmospheric pressure varies from 5.32xl0⁴ Pa to 1.33xl0^~2 Pa.

$. Method according to any one of claims 1 to 3, comprising generating and maintaining the arc at predetermined values of the subatmospheric pressure, the current intensity and the distance between the anode and the surface of the metallic body to be treated, and discontinuing the arc after a time sufficient to effect the desired treatment.

10. Method according to claim 9, wherein time sufficient to effect the desired treatment range between portions of a second to tens of seconds.

11. Method according to any one of claims 1 to 3, comprising generating the arc at a first value of the process parameters consisting of subatmospheric pressure, current intensity and distance between the anode and the surface of the metalhc body to be treated, continuously changing at least one of said parameters from said first value to a second value, and discontinuing the arc at said second value of said at least one parameter.

12. Method according to any one of claims 1 to 3, comprising generating the arc at a first value of the process parameters consisting of subatmospheric pressure, current intensity and distance between the anode and the surface of the metallic body to be treated, continuously changing at least one of said parameters from said first value to a second value, and discontinuing the arc until a combination of parameter values and time elapsed has been reached that is sufficient to effect the desired treatment.

13. Method according to claim 1, wherein the metalhc body comprises plane surfaces.

14. Method according to any one of claims 1 to 3, wherein the metallic body comprises surfaces other than plane.

15. Method according to claim 14, wherein the metalhc body comprises surfaces chosen from among surfaces that are curved, polyhedral, concave or convex, or have projections or depressions or any combination thereof.

16. Apparatus for the treatment of surfaces of metaUic bodies, which comprises, in combination with a power source and with suction means: a cup-like body; at least one anode; electrically insulating and heat resistant means provided at the periphery of said cup -like body; sealing means provided at the periphery of said cup-like body; said cup-like body having a periphery so matching the surface to be treated that when it is engaged with said surface a sealed treatment space is defined therebetween, and having openings therein for connecting said treatment space to said suction means whereby to generate in said space a subatmospheric pressure; and means for electrically connecting said power source to said anode and to the metallic body to be treated as the cathode, whereby to generate an arc therebetween.

17. Apparatus according to claim 16, further comprising means for triggering the arc.

18. Apparatus according to claim 16, wherein the at least one anode is a separate component mounted in the cup-like body and electrically insulated from it, if needed.

19. Apparatus according to claim 16, wherein the at least one anode is defined by the inner surface of the cup-like body.

20. Apparatus according to claim 16, wherein the at least one anode is slidably mounted in the cup -like body to control the distance between it and the surface to be treated.

21. Apparatus according to claim 16, comprising a plurahty of anodes arranged in such a way as to match any desired shaped of the surface to be treated and the cup-hke body is correspondingly shaped.

22. Apparatus according to claim 16, wherein the cup-like body is made of a conductive material, provided with insulation where required.

23. Apparatus according to claim 22, wherein the conductive material is a metal.

24. Apparatus according to claim 16, wherein a gap is created between the periphery of the cup-like body and the surface to be treated when said body is engaged with said surface.

25. Apparatus according to claim 16, wherein the cup -like body is provided with means for sliding it over the surface to be treated.

26. Apparatus according to claim 21, for the treatment of elongated surfaces, comprising a plurahty of anodes arranged in longitudinally adjacent relationship and means for successively connecting them to a source of power, successively to activate them and to treat longitudinally adjacent portions of an elongated surface.

27. An apparatus according to claim 16 wherein the means for electrically connecting the power source to the metalhc body to be treated as the cathode is out of the treatment space.

28. An apparatus according to any one of claims 16 to 27 wherein the cup- hke body is flexible.

29. A method according to claim 1 wherein the arc current is in the range of 50 Amps to 1000 Amps.

30. A method according to claim 1 wherein the arc current is less than 50 Amps or more than 1000 Amps.

31. Method according to any one of claims 1 to 3, wherein the subatmospheric pressure is below 5.32xl0⁴ Pa (400 Torr).

32. Method of treatment of metallic surfaces, particularly for the purpose of cleaning or descaling surfaces of large, metallic bodies, substantially as described and illustrated.

33.

for the treatment of surfaces of metallic bodies, particularly for the purpose of cleaning or descaling surfaces of large, metallic bodies, substantially as described and illustrated.