US3900376A

US3900376A - Cleaning of metal surfaces

Info

Publication number: US3900376A
Application number: US413415A
Authority: US
Inventors: Mervyn John Copsey; Brian Hanson Wilby
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1972-11-08
Filing date: 1973-11-06
Publication date: 1975-08-19
Anticipated expiration: 1992-08-19
Also published as: IT999747B; GB1399710A; CH592748A5; SE386691B; FR2213347A1; FR2213347B1; DE2355865A1; DE2355865C2

Abstract

Elongate metal articles such as rods, bars strip and wire are cleaned by passing them through an electrolyte such that a gas e.g. hydrogen is evolved at the metal surface, a high voltage being applied between the article and an inert anode such that the surface of the article in the electrolyte is completely covered by gas and vapour through which a discharge passes, the operation being carried out in the region of the current minimum of the current/voltage characteristic which occurs beyond the normal electrolysers regime as the voltage is increased.

Description

1451 Aug. 19, 1975 [5 CLEANING 0F METAL SURFACES 2,480,845 9/1949 Frager et a1 204 145 R 2,556,017 6/1951 Vonada .1 204/211 [751 lnvemrs= f' P Chester; 2,615,840 10/1952 Chapman... 204 145 R Brlan Hanson y. Leeds. both of 3,507,767 4 1970 Stricker 204/208 England 3.621654 12/1971 Petit 204/141.5

[73] Assignee: The Electricity Council, London,

E d Primary ExaminerT. M. Tufariello Art A t, F Brow Beverid e, 221 Filed: Nov. 6, 1973 g 'ff 5 2 g [21] Appl. No.: 413,415

[57] ABSTRACT [30] Foreign Apphcatlon Pnomy Data Elongate metal articles such as rods, bars strip and I972 United Kingdom 51631/72 wire are cleaned by passing them through an electrolyte such that a gas e.g. hydrogen is evolved at the [52] US. Cl. 204/1415; 204/145 R; 204/211 meml Surface, a high voltage being applied between [5 I I f C23!) 3/02; C2313 U04; C231) 1/06 the article and an inert anode such that the surface of [58] Field of Search 204/145 R, 141.5, 21 1, the article in the electrolyte is complete, covered by 204/210 gas and vapour through which a discharge passes, the operation being carried out in the region of the cur- 156] References C'ted rent minimum of the current/voltage characteristic UNITED STATES PATENTS which occurs beyond the normal electrolysers regime 2,307,928 1/1943 Hogaboom 204 1415 38 the voltage is increased- 2,3I3 422 3/1943 Dimon 204/145 R 2,437,474 3/1948 Orozco 204/1415 6 Clam, 6 Drawmg 11 NORMAL ELEETRIJIJSIS Y1 UNSTABLE REGIME.

R IME. 1011- m y 1 STABLE 1115011111515 1 E UPERATIIII.

CLEANING ELECTROLYTIC I-IEATIIB REGIME REGIME p- 5 M]- a: 5

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I IIIENIEB MISI 9 ISIS CURRENT IN AMPS.

IZI]

5M1] 1 III" 4 NORMAL ELECTRULVSIS UNSTABLE REGIME.

REGIME.

I (STABLE DISCHARGE IE OPERATION.

ELECTROLYTIC HEATING REGIME CLEANING REGIME 2 0 1. 0 u an 100 120 11.0 wen I80 200 APPLIED POTENTIAL m vuus.

PATENTEDAUB 1 91975 500,45;

BAR FEED CLEANING OF METAL SURFACES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the cleaning of metal surfaces and is concerned more particularly with the cleaning of elongate articles such as bars, tubes or wire.

The present invention finds particular application in the cleaning of oxide scales from metal surfaces, particularly iron or steel. It is at present the common practice to use acid pickling processes for removing oxide scales but, with such processes, problems arise in the disposal of the effluents. It is one of the objects of the present invention to provide an improved process for cleaning oxide scales from metal surfaces in which it is possible to have substantially no waste products other than hydrogen and oxygen gas and the material removed from the surface. It is a further object of the present invention to provide a continuous process such as may be used for cleaning elongate articles such as bars, tubes or wire.

Although the invention is particularly applicable for removing oxide scales, the technique employed is not affected by other surface contamination of the article, even exterior contamination for example by paint or grease, such materials being removed by the method of the invention.

2. Prior Art The present invention makes use of an electrolytic process in which the surface area of the article to be cleaned is covered with an electrolyte and an electric voltage is applied between the article and at least one other electrode in contact with the electrolyte. It is known to effect heat treatment of an article in an electrolyte by applying a high voltage such that a layer of gas or vapour covers the surface of the article and an arc discharge occurs through this layer, between the electrolyte and the article. The occurrence of this discharge is a well-known phenomenon and its use for heating of the article is described for example in British Patent Specification No. 1,209,951. Brief reference has also been made to the fact that the process can clean dirty metal surfaces [P. Hoho, Electrical Rev. 104, 185-7 l929): T. Sato and H. Mii, Rep. Govt. Ind. Res. Inst. Nagoya, Japan 5, 415-20 (1956)] and a brief report has been made of Russian work using this principle [P. F. Zuraylev, Masinostroitel 4, 21 (1967)]. The heavy current necessary to effect heating of the article causes the electrolyte to be heated and, in a continuous process in which an electrolyte article is moved through the electrolyte, it is necessary to circulate the electrolyte through a cooling system so that electrolyte flows continuously over the surface of the article to be heated.

We have found that if the electrolyte is static or is made to flow only very slowly, as the voltage is increased towards the high values used for heating, beyond the region where normal electrolysis occurs there is an unstable regime where the discharge starts and where increase of voltage causes a decrease of current. Beyond this unstable region, further increase of voltage causes the current and hence the heating effect to in crease. For heating purposes, because there has to be a high rate of flow of the electrolyte, this unstable regime does not occur. The potentials employed in heating processes are much higher than those for this unstable regime.

SUMMARY OF THE INVENTION The present invention makes use of this unstable regime in that, by having a static or only slowly flowing electrolyte and by operating at a voltage just above that of the unstable regime, it is possible to maintain the layer of gas or vapour covering the surface of the article with the discharge therethrough at a current level which does not cause excessive heating of the article or of the electrolyte but which serves to remove surface contamiants such as oxide scales, paints, and greases from the article.

According to one aspect of the present invention a method of cleaning a surface of an elongate metal article in a continuous process comprises the steps of moving the surface of the article through an electrolyte which does not react chemically with the metal to be cleaned or any surface contaminant on the article so that successive regions of the surface area to be cleaned are covered by the electrolyte and applying an electric voltage between the article and at least one other electrode electrically in contact with the electrolyte, said voltage being sufficiently high that a layer of gas or vapour covers the surface to be cleaned and a discharge occurs through this layer between the electrolyte and the surface, the electrolyte being static or caused to flow sufficiently slowly over said surface that, as the voltage is increased, there is an unstable regime where the current decreases with increase of voltage, the voltage being maintained above the level of this unstable regime.

To minimise heating, the minimum voltage above the unstable regime is employed. This may readily be achieved by increasing the voltage beyond that necessary to initiate stable discharge operation and then reducing the voltage to the minimum required for stable operation. For any given condition, the required voltage may be determined empirically; for example one may determine the voltage for minimum current and then apply this voltage to an article before operation of the cleaning process is commenced. As the article is immersed in the electrolyte, current is drawn and the potential will drop slightly due to the drooping characteristic of the power generator but the discharge will settle down to the cleaning regime. This cleaning regime might, for practical purposes, be considered as being 1 10 volts of the potential for minimum current at the higher voltage end of the unstable regime.

The applied potential may be alternating or direct. If direct, it may be of either polarity but preferably the article to be cleaned is the cathode and a direct voltage is applied between this cathode and one or more anodes which are maintained at a positive potential in respect of the cathode. Typically the voltage will be in excess of I00 volts and the cathodic current density would be made in excess of 5 A cm".

The electrolyte may be made to flow over the surface to be cleaned and then recirculated, solid matter being removed before the electrolyte is passed back over the article. Removal of solid matter may conveniently be by filtration or by settling in a settling tank. The circulation rate however is kept as low as possible since increase of flow rate increases the current density in the cleaning regime and also the potential necessary to maintain it and hence increases the heating. Furthermore, at high flow rates, the unstable regime between normal electrolysis and stable discharge operation does not occur.

The electrolyte is conveniently an aqueous electrolyte and, for descaling steel, an alkaline electrolyte is preferable, such as a w/v aqueous solution of caustic soda or sodium carbonate. The preferred electrolyte for iron and steel is a saturated aqueous solution of mixed potassium and sodium carbonates.

By making the article the cathode, hydrogen is initially generated at the surface to be cleaned. The high voltage and high current density cause substantial heat generation and the surface of the article is covered with a layer containing both hydrogen and steam. The discharge through the gas and vapour layer causes any scale on the article to flake off. Other surface contamination such as greasem paint or rust will also be removed. The speed of descaling is dependent on the voltage applied to the electrodes and the cathodic current density. The voltage must be sufficiently high that normal electrolytic conduction no longer occurs over the whole path between the electrodes; that is to say the voltage must be above that for normal electrolysis. This is because it is necessary that the layer of gas or vapour extends completely over the surface to be cleaned and an electrical discharge occurs through this layer. Operation at a voltage just above the unstable regime meets these requirements.

Any electrolyte which does not react chemically with the metal of the article to be cleaned or the surface contaminant thereon to be removed and which will not plate out material on to the cathode but which results in gas evolution at the surface may be used. It is most convenient to select a high conductivity electrlyte in which the metal to be cleaned is insoluble. Best results are obtained if the material to be cleaned constitutes the cathode; hydrogen evolution then occurs if an aqueous electrolyte is employed.

So far as the cleaning effect at the cathode is concerned, the nature of the anode is not critical. However, with high electrolytic currents, the anode is subjected to severe corrosive attack and it is, for this reason, preferred to use nickel as the anode. Certain stainless steels will also give resistance against corrosion.

The method may be applied as a continuous process by moving the article either continuously or in steps so that successive areas of the article are cleaned. Such an arrangement may be used for cleaning wires, bars, sheet or plate. The article may be passed continuously through a cell. The electrolyte may be pumped continuously through the cell so that it overflows therefrom thereby avoiding any necessity for liquid-tight seal between the article and the cell structure; the pumping rate must be kept low however for the reasons mentioned above.

For cleaning an elongate article, such as a rod, bar or tube, the article may be rotated about its axis as it is moved through the cell. Conveniently it is moved horizontally through the cell and, in this case, it need only be partially immersed in the electrolyte, the rotation being such that any point on the surface of the article makes more than one complete rotation as it passes through the cell.

The invention furthermore includes within its scope an article cleaned by the above-described method.

The invention furthermore includes within its scope apparatus for cleaning an elongate metal article comprising a cell containing an electrolyte, means for moving the article through or over the cell so that at least a part of the surface of the article is in contact with the electrolyte, and means for applying an electric voltage between the article and said electrode, said voltage being sufficiently high that a layer of gas or vapour covers the surface to be cleaned and a discharge occurs through this layer between the electrolyte and the surface, and means causing the electrolyte to flow over said surface, the flow rate being sufficiently slow that, as the voltage is increased, there is an unstable regime where the current decreases with increase of voltage, and the voltage being maintained above the level of this unstable regime.

The article may be a rod, bar or tube which can be moved through the cell either vertically or horizontally, the electrode being coaxial with the article. If the article is moved horizontally, preferably an openstructured electrode is employed to allow the gases and vapour to escape or the electrode is arranged to extend around the underside of the article, the article being rotated as it passes through the cell so that all the surface is cleaned.

If the article is a flat sheet, strip or flat bar to be cleaned on one surface, it may be moved across the top of a cell with the surface to be cleaned in contact with electrolyte in the cell.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical diagram for explaining the invention and illustrating a typical relationship between current and applied potential;

FIG. 2 is a diagram illustrating one form of apparatus for cleaning a metal bar;

FIG. 3 is a diagram illustrating another form of apparatus for cleaning a metal bar;

FIG. 4 is an isometric view of an anode used in the apparatus of FIG. 3',

FIG. 5 is a diagram illustrating yet another form of apparatus for cleaning a metal bar; and

FIG. 6 is a diagram illustrating an apparatus for cleaning a metal sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the relationship between the current and applied potential in a typical case where a steel bar is immersed in a saturated aqueous solution of mixed potassium and sodium carbonates. At low applied potentials, for example at A on the curve, normal electrolysis takes place. The current increases as the potential increases up to B where the potential in this example is about volts, the increase of current with potential being substantially linear over the greater part of this range. When the potential is increased above this level, an unstable regime sets in in which gas and vapour begins to surround the article completely and a discharge takes place through this gaseous layer. As the potential is increased to V1 (about volts in this case), the regime becomes stable at C with a current substantially less than the maximum current achieved at B in the normal electrolysis regime at 120 volts. In this stable regime, the article is now covered completely with a gaseous layer through which a discharge takes place. Increase of potential beyond I50 volts results in increased current but the operation is stable with a discharge through the gaseous layer. If the potential is now decreased, there is a minimum current reached at about I50 volts and further decrease of potential results in an increase of current as shown by the solid line curve from C to D but thereafter the unstable regime takes over and stability is reached again at B when the potential has been reduced to 120 volts.

The present invention makes use of the cleaning regime which is the region between D and E on the curve within about l0 volts on either side of V] where the operation can be stable with a current substantially less than the maximum current for normal electrolysis.

The relationship illustrated in FIG. I is a typical relationship where the electrolyte is static or where the rate of flow of electrolyte in the anode/cathode region is very small. The flow rate however has a strong effect on the characteristic curve. When the flow rate is increased, the power consumption rises rapidly since both the current density in the cleaning regime and the potential necessary to maintain it rise. Furthermore at high flow rates the distinction between normal electrolysis and the stable operation in the cleaning regime and, at higher voltage, the heating regime above E is not as clearly marked as in FIG. 1.

Cleaning occurs at the metal surface but the rate of cleaning does not appear to be a strong function of the surface power density. If the applied potential is increased into the electrolytic heating regime beyond E, it is not possible to obtain the same degree of cleaning by correspondingly decreasing the duration of the exposure time to the discharge. The cleaning action is therefore quite distinct from the electrolytic heating action.

As mentioned earlier, the present invention has particular application to the descaling of black steel bars. FIG. 2 illustrates diagrammatically one form of apparatus for symmetrical descaling of bars of circular hexagon square or other section. In the arrangement of FIG. I, a bar is passed vertically upwardly with a continuous uniform speed by bar feed means indicated diagrammatically at 11. The bar passes through electrical contact rollers 12 which are electrically earthed at 13 and thence through a seal 14 in the bottom of an electrolytic cell 15. This cell 15 is constructed of electrically insulating material and is provided with a cylindrical nickel anode 16 of circular section coaxial with the bar I0. Electrolyte comprising a saturated aqueous solution of a mixture of sodium and potassium carbonates is slowly pumped into the cell through an inlet 17 near the bottom and is allowed to flow over the top of the cell wall into a collecting tray 18 from which it is returned to a storage tank 19 for recirculation by a pump 20. Above the cell, the bar 10 passes through further electrically earthed guide rollers 21. The nickel anode 12 is connected at 22 to the positive terminal of an adjustable direct voltage supply indicated diagrammatically at 23. The negative terminal of the supply is earthed.

In operation the potential applied to the anode is adjusted so that the cell operates in the cleaning regime (between D and E on FIG. 1) as previously explained. The high current density results in hydrogen evolution on the surface of the bar and a gas film completely covers the bar within the cell. A discharge takes place through this gaseous layer. The heat generated results in the production of steam and the layer around the bar thus consists of both hydrogen and steam. The discharge through this gas and vapour layer causes any scale on the bar to flake off, the scale settling in the bottom of the cell 15 or being carried by the flow of electrolyte to settle in the storage tank 19. Positive filtration may however be employed if desired to separate the scale from the electrolyte before it passes into the tank 19. The storage tank 19 also forms a cooling system, the volume of the tank being such that the electrolyte cools before recirculation by the pump 20. As previously explained however the rate of recirculation must be kept low in order to ensure that the cell can operate with a characteristic, such as has been shown in FIG. 1, having a cleaning regime extending for about plus or minus 10 volts of the voltage V] giving minimum current.

Vertical feeding of the bar into a cell may be inconvenient in practice and FIG. 3 illustrates a horizontal feed system. In FIG. 3, by means of a bar feed means 29, a bar 30 is passed horizontally through earthed contact rollers 31 and thence through an end seal 32 into a cell 33. The bar passes out of the cell through a further end seal 34 and earthed support rollers 35. The end seals are shown as roller seals and need not provide a good liquid-tight seal to the bar provided they reduce the rate of leakage of electrolyte at the entry and exit points to a sufi'iciently low level. A collecting tank 36 may be provided underneath the cell to collect electrolyte which leaks out at these points. the electrolyte passing from tank 36 into a collecting tank shown diagrammatically at 37 and thence to a pump 38 for recirculation. A cylindrical nickel anode 40 is arranged coaxially around the bar 30 within the cell 33. This anode is connected as indicated at 41 to the positive terminal of an adjustable direct potential supply source 42, the negative terminal of which is earthed at 43. Electrolyte comprising a saturated solution of a mixture of sodium and potassium carbonate is pumped into the cell through an inlet 42 near the bottom of the cell and overflows from the top of the cell into the aforementioned collecting tank 36. In order to allow free egress of the electrolytically generated gases and vapours. the anode has to be an open structure for example apertured or formed of rods or the like. A preferred form of anode is illustrated in FIG. 4 and comprises a squirrel-cage construction having end rings 45, 46 joined by a series of parallel bars 47.

The cell of FIG. 3 is operated in a similar manner to that of FIG. 2 by applying a suitable voltage to the anode so that the operation is within the cleaning regime of the current voltage characteristic as previously described.

The cell of FIG. 3 may not give a uniform rate of descaling over the whole surface of the bar owing to interference at the top of the bar 30 by rising bubbles of electrolytically generated gases and by thermal convection currents causing a locally enhanced electrolyte flow rate. For many purposes the cell of FIG. 3 will be satisfactory but if a more uniform descaling action is required, the bar may be rotated as it is passed through the cell.

FIG. 5 illustrates diagrammatically a construction of cell in which a bar is rotated. Linear and rotational feed means indicated diagrammatically at 50 feed a bar 51 in such a manner that any point on the bar, in passing through the cell, makes several complete revolutions. Since the bar is rotated, it is no longer necessary to have the anode completely surrounding the bar and in FIG. 5 a semi-cylindrical nickel anode 52 is employed arranged in a cell 53, the anode extending around the undersurface of the bar. The bar is supported by two sets of inclined rollers 54, 55 which are electrically earthed and passes through seals 56, 57 in the end walls 58, 59 of the cell. Electrolyte comprising, a saturated solution of sodium potassium carbonate is pumped into the cell at an inlet 60 and overflows the one of the side walls 61 of the cell 53 at the level of the top of the anode 52 to be collected in a collecting tank and recirculated. For clarity in the drawing, the collecting tray, collecting tank and pump are not illustrated; they may be similar to the corresponding components in FIGS. 2 and 3. The collecting tray also collects any electrolyte which spills through the end seals 56 through which the bar 51 passes into and out of the cell 53. The anode 52 is connected as shown at 62 to the positive terminal of a source 63 of adjustable direct potential, the negative terminal of which is earthed. The supply can be adjusted as before so that the cell operates in the cleaning regime.

With the arrangement of FIG. 5, the axis of the bar is at the level of the top of the edges of the anode 52, that is to say the overflow level of the cell, and hence the bar is half immersed in the cell. The descaling action takes place at the part of the surface of the bar 51 which at any one time is immersed in the electrolyte, that is to say the lower surface. The rotation of the bar however is at such a rate that the whole surface is descaled as the bar passes through the cell. Scale removed from the bar collects in the bottom of the cell 53 and may be removed from time to time.

The following are two examples of descaling operations performed with a cell of the type shown in FIG. 5. The cell length was 60 mm and the anode diameter 240 mm.

EXAMPLE I A mild steel bar of l9.5 mm diameter covered with a loose black oxide scale approximately p. thick was passed through the cell with a linear velocity of 0.6 m/min and with a superimposed rotation of one revolution in mm linear movement. An electrolyte flow rate of IO llmin was used, and the voltage and total current were 150V and 125 A respectively. The bar emerged from the cell as clean grey metal. The energy expenditure directly used in descaling the bar was therefore 8.5 kWh/m of surface area.

EXAMPLE 2 A steel bar of l6 mm diameter covered with a compact well-adhered black scale approximately p thick was passed through the cell with a linear velocity of 0.13 m/min with a superimposed rotation of one revolution in IS mm linear movement. An electrolyte flow rate of IO l/min was used, and the voltage and current were 150V and 75A respectively. The bar emerged from the cell as clean grey metal. The energy expenditure directly used in descaling the bar was therefore 29 kWh/m of surface area.

A higher rate of descaling metal may be simply obtained by increasing the length of the anode and correspondingly increasing the current.

FIG. 6 illustrates a form of cell for descaling a metal sheet. A similar cell may be used for flat strip or flat bars. In FIG. 6, a flat sheet 70 is passed by linear feed means 80 horizontally between two electrical earthing rollers 71 and thence across the top of an electrolytic cell 72 to pass between two further electrical earthing rollers 73. The cell 72 is of generally rectangular form in plan with a rectangular nickel anode 74 extending over the base of the cell and connected at 75 to the positive terminal of an adjustable d.c. supply source 76, the negative terminal of which is earthed. Electrolyte comprising a saturated solution of a mixture of sodium and potassium carbonates is pumped into the cell through an inlet opening 76 to overflow at the top of the cell. The top of the cell is partially closed adjacent two sides by top surfaces 77. Between these top surfaces 77, the end walls extend upwardly as shown at 78 and the plate passes between these two upstanding portions 78 of the end walls. The inner edges of the top surfaces 77 are turned upwardly as shown at 79 to reach to the underside of the plate 70, the electrolyte flowing out through the gap between these edges 79 and the underside of the plate but covering the underside of the plate. A collection tray is provided underneath the cell for collection of the electrolyte which is passed to a collection tank and pump for recirculation but the collector tray, tank and pump are omitted from FIG. 6 for clarity. With this arrabgement, as in the cells of FIGS. 2, 3 and 5, the effective area of the anode is substantially greater than the effective cathode area of the article to be descaled. The operation is similar to that of previous cells, the anode potential being adjusted to the appropriate value so that the cell operates in the cleaning regime of the current/voltage characteristic. By making the clearances between the plate and the cell structure small, only a very low rate of flow of electrolyte is necessary in order to keep the undersurface of the plate wet with electrolyte over the whole region above the top aperture in the cell.

In the above-described constructions, roller-type earthing contacts have been employed. Other types may be used, e.g. sliding contacts but, if it is necessary to eliminate all possibility of contact-arc burn marks on the finished article, immersed electrolyte connections may be used for the earthing contacts.

We claim:

I. A method of cleaning a surface of an elongate metal article in a continuous process comprising the steps of moving the surface of the article through an electrolyte which does not react chemically with the metal to be cleaned or any surface contaminant on the article so that successive regions of the surface area to be cleaned are covered by the electrolyte and applying an electric voltage between the article and at least one other electrode electrically in contact with the electrolyte, said voltage being sufficiently high that a layer of gas or vapour covers the surface to be cleaned and a discharge occurs through this layer between the electrolyte and the surface, the electrolyte being static or caused to flow sufficiently slowly over said surface that, as the voltage is increased, there is an unstable regime where the current decreases with increase of voltage, the voltage being maintained above the level of this unstable regime.

2. A method as claimed in claim 1 wherein the voltage is maintained within t 10 volts of the potential giving a current minimum at the higher voltage end of the unstable regime.

3. A method as claimed in claim I wherein the applied potential is a direct potential with the article to be cleaned as the cathode.

4. A method as claimed in claim I and wherein said other electrode is of substantially greater surface area is fed horizontally and only partially immersed in the electrolyte to leave its upper surface exposed. the speed of rotation being such that any point on the surface of the article makes more than one complete rotation as it passes through a treatment cell.

Claims

1. A METHOD OF CLEANING A SURFACE OF AN ELONGATE METAL ARTICLE IN A CONTINUOUS PROCESS COMPRISING THE STEPS OF MOVING THE SURFACE OF THE ARTICLE THROUGH AN ELECTROLYTE WHICH DOES NOT REACT CHEMICALLY WITH THE METAL TO BE CLEANED OR ANY SURFACE CONTAIMINAT ON THE ARTICLE SO THAT SUCCESSIVE REGIONS OF THE SURFACE AREA TO BE CLEANED ARE COVERED BY THE ELECTROLYTE AND APPLYING AN ELECTRIC VOLTAGE BETWEEN THE ARTICLE AND AT LEAST ONE OTHER ELECTRODE ELECTRICALLY IN CONTACT WITH THE ELECTROLYTE, SAID VOLTAGE BEING SUFFICIENTLY HIGH THAT LAYER OF GAS OR VAPOUR COVERS THE SURFACE TO BE CLEANED AND A DISCHARGE OCCURS THROUGH THIS LAYER BETWEEN THE ELECTROLYTE AND THE SURFACE, THE ELECTROLYTE BEING STATIC OR CAUSED TO FLOW SUFFI-

2. A method as claimed in claim 1 wherein the voltage is maintained within + or - 10 volts of the potential giving a current minimum at the higher voltage end of the unstable regime.

3. A method as claimed in claim 1 wherein the applied potential is a direct potential with the article to be cleaned as the cathode.

4. A method as claimed in claim 1 and wherein said other electrode is of substantially greater surface area than the area of the article immersed in the electrolyte.

5. A method as claimed in claim 1 and for cleaning an elongate article of round or square section wherein the article is rotated about its axis as it is fed through the electrolyte.

6. A method as claimed in claim 5 wherein the article is fed horizontally and only partially immersed in the electrolyte to leave its upper surface exposed, the speed of rotation being such that any point on the surface of the article makes more than one complete rotation as it passes through a treatment cell.