MXPA98007563A - Electrolytic process to clean surfaces electrically duct - Google Patents

Abstract

The present invention relates to: An electrolytic process for simultaneously cleaning and coating the metal of a workpiece surface of an electrically conductive material, said process comprising: (i) supplying an electrolytic cell with a cathode comprising the surface of the workpiece and anode comprising the metal for metal coating of the surface of the workpiece; (ii) introducing an electrolytic in the area created between the anode and the cathode by making it flow under pressure through one or more holes, channels or openings in the anode and in such a way that it comes into contact with the cathode in this way, and (iii) applying a voltage between the anode and the cathode and operating in a regime in which the electric current decreases or it remains substantially constant with the increase in the voltage applied between the anode and the cathode, and in a regime in which discrete gas bubbles and / or vapor are present on the surface of the workpiece during the treatment

Description

ELECTROLYTIC PROCESS TO CLEAN ELECTRICALLY CONDUCTOR SURFACES BACKGROUND OF THE INVENTION The present invention relates to a process for cleaning an electrically conductive surface with, for example, a metal surface. Metals, especially steel in its many forms, usually require cleaning and protection against corrosion before they can be used for their final use. In the state in which it is produced, the steel normally has a film of lamination flakes (black oxide) on its surface that is not uniformly adhesive and renders the underlying material susceptible to galvanic corrosion. The lamination flakes must, therefore, be removed before the steel can be painted, coated or metallized (for example with zinc). Metal can also have other forms of contamination (known in industry as "stains") in its. surface, including oxidation, oil or grease, pigmented lamination compounds, burrs and cutting fluids, as well as polishing compounds »All these elements should normally be removed. Even stainless steel may have an excess of mixed oxides on its surface that must be removed before subsequent use. - Traditional methods for cleaning metallic surfaces * include acid cleaning (becoming less acceptable due to cost problems and environmental problems caused. for the spent acid waste) 5 abrasive jets? polished by drum friction, wet or dry? brushed? 5 incru ation in a salt bath? alkaline incrustation as well as acid cleaning »A multi-stage cleaning operation may include, for example, (i) calcination or solvent removal of organic materials, (ii) application of sand or pellet jets to remove lamination and oxidation flakes; and (iii) electrolytic cleaning with final surface preparation. The cleaned surface must be protected against corrosion by means of metallization, paint or plastic coating, this must be carried out normally rapidly in order to avoid a renewed surface oxidation. A treatment in multiple stages is effective but expensive, both in terms of energy consumption and in terms of processing time. Many of the conventional treatments also present environmental problems. Electrolytic methods are often incorporated to clean metal surfaces in processing lines such as galvanizing and coating of steel strips and sheets » common coatings include zinc, zinc alloy, tin, copper, nickel and chromium. Self-cleaning electrolytic cleaning lines are also used to power numerous downstream operations. li ias to electrolí ica (or "elec fcrul ip? it was") normally involves the use of an alkaline cleaning solution which forms the electrolyte while the workpiece may be either the cell anode elec ica roli, or Well the polarity can alternate. Such processes generally operate at a low voltage (typically 3 3 12 volts) and current densities from i to 15 Amps dm2"Therefore, power consumption ranges from approximately 0.01 to 0.5 k &h m2. The removal of spots is done by generating gas bubbles that release the contaminant from the surface. When the surface of the workpiece is the cathode, the surface may not only be cleaned but also "ac ivarse", giving any subsequent coating an improved adhesion "Electrolytic cleaning is not normally practicable for removing flakes heavy lamination, and this is carried out in a separate operation such as for example cleaning with acid and / or abrasive jets. Conventional electrolytic cleaning and coating processes operate in a low voltage regime where the electric current increases monotonically with the applied voltage (see figure i below in A). Under some conditions, when the voltage rises, a point is reached in which instability occurs and the current begins to decrease with the increase in voltage (see Figure 1, below in E). The unstable regime marks the start of electric discharges on the surface of one or the other of the electrodes. These discharges ("microarrays" or "icraplas as") occur through any suitable non-conductive layer present on the surface, such as a gas or vapor layer. This is due to the fact that the potential gradient in such regions is very high. PREVIOUS TECHNIQUE Document SB-1399710 teaches that a metal surface can be electrolytically cleaned without overheating and. without excessive consumption of energy if the process operates in a just regime beyond the unstable region. The "unstable region" is defined as a region in which the current decreases with increasing voltage. By moving to slightly higher voltages, when the current increases again with the increase in volt and a continuous gas / vapor film is established on the treated surface, an effective cleaning is obtained. However, the energy consumption of this process is high (from 10 to 30 kWh / m2) compared to the energy consumption for cleaning with acid (0.4 to 1.8 kWh / m2 >); SU-A-1599446 discloses a spark erosion cleaning process with the high-voltage crucibles for welding rods that it uses. extremely high corpenbe densities of the order of 1000 A dm2, in a phosphoric acid solution. SU-A-I 244216 discloses a cleaning treatment with microarches for parts of machines operating between 100 and 350 V using anodic treatment. No particular method is taught for the electric d &e handling. Other electrolytic cleaning methods are described in document TB-A-1306337 where a spark erosion step is used in combination with a separate chemical or electrochemical cleaning step to remove scale of rust? in document US-5232563 where inants are removed at low voltages of 1.5 to 2V from discs that are iconducted by the production of gas bubbles on the surface of the discs that give off the contaminants? in EP-A-065756 > 4, where it is taught that normal low-voltage electrolytic cleaning is ineffective in removing grease, but that electrolytically oxidizable metals such as aluminum can be defatted under high-voltage conditions (microarray by acid anodization).

The use of electrolyte jets located near the electrodes in electrolytic cleaning baths to create a high velocity turbulent flow in the area of It piera is taught, for example, in documents JP-A-08003797 and DE- - 4031234 » The elementary cleaning of radioactively contaminated objects by the use of a single electrolyte jet without total immersion of the object is taught in EP-A-0037190 »The cleaned object is medical the voltage used is between 30 50V . Short treatment times of the order of 1 sec are recommended to avoid erosion of the surface and it is considered that the complete removal of the o :. gone is not desirable. The non-immersion is also taught in CA-A-1165271 where the electrolyte is pumped or drained through a box-shaped anode with a set of holes in its base. The purpose of this arrangement is to allow a metal strip to be coated with metal film by electrolysis on one side only and specifically to avoid the use of a consumable anode. DE-A-3715454 describes the cleaning of wires by means of a polar and electrolytic treatment by passing the wire through a first chamber in which the wire is cathodic and through a second chamber in the wire. it is anodic »In the second chamber a plasma layer is formed on the anodic surface of the wire by the ionization of a gas layer q & contains oxygen. The wire is immersed in the electrolyte during the treatment »EP-A-0406417 describes a continuous process for drawing a copper wire from a copper rod where the rod is cleaned by plasma before the drawing operation . The "plasmatran" frame is the anode and the wire is also surrounded by an internal coaxial anode in the form of a perforated U-shaped sleeve. In order to start plasma production, the voltage is maintained at a low but unspecified value, the electrolyte level above the submerged wire is decreased, and the flow rate is decreased in order to stimulate the start of a discharge on the surface of the wire. While low-voltage electrolytic cleaning is widely used to prepare metallic surfaces for electro-coating or other coating treatments, it can not handle thick deposits of?: < gone as for example lamination flakes without an unacceptably high expenditure of energy. These electrolytic cleaning processes should normally be used, therefore, in combination with other cleaning procedures in a multi-stage operation »S We have developed a particularly efficient metal cleaning process that can handle rough oxide flakes »COMPENDIUM OF THE INVENTION Accordingly, in one aspect, the present invention offers an electrolytic process for cleaning the surface of a workpiece and a material electrically. conductor, said process comprises: i) providing an electrolytic cell with a cathode q.s comprises the surface of the workpiece and an inert anode? ii) the introduction of an electrolyte in the area created between the anode and the cathode causing its flow under pressure through one or more orifices, channels or openings in the anode and its coming into contact with the cathode surface, the surface of the cathode is not submerged in another way in the electrolyte? and iii) the application of a voltage between the anode and the cathode and the operation in a regime in which the electric current decreases or remains substantially constant with increased voltage applied between the anode and the cathode, and at a rate in the which discrete bubbles of gas and / or vapor are present on the surface of the workpiece during the treatment. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates the operation regime where the electric current decreases, or does it not increase with the increase in applied voltage? Figure 2a, 2b and 2c illustrate operating parameters where desired operating conditions are achieved? Figure 3 schematically illustrates the process of the present invention? Figure 4 schematically illustrates an apparatus for carrying out the cleaning process of the invention on one side of an object? Figure 5 schematically illustrates an apparatus for performing the cleaning process of the present invention for cleaning both sides of an object? Figure 6 schematically illustrates an apparatus for carrying out the process of the invention for cleaning the two sides of an object at different speeds? and Figure 7 illustrates schematically an installation for cleaning the inner surface of a tube. DETAILED DESCRIPTION OF THE INVENTION The term "inert" as used herein refers to that no material is transferred from the anode to the workpiece. In carrying out the method of the present invention, the workpiece has a surface which forms the cathode in an electrolytic cell. The anode comprises an inert conductive material, such as, for example, a bond. The process is operated in a regime in which the electric current ds muye, n b? ^ N at least not? N >; This is ugly, with an increase in applied voltage between the anode and the cathode. The process of the present invention can be carried out as a continuous or semi-continuous process by arranging for a relative movement of the work piece relative to the anode or to the anodes. Automatically, seasonal articles can be treated in accordance with the process of the invention. The electrolyte is introduced into the working area between the node and the cathode causing its flow or pressure through at least one hole, channel or opg in the anode, so it comes into contact with the cathode (the surface in treatment). Each of these characteristics are described in more detail below. Cathodic arrangement of the surface to be treated The work piece can be of any shape including sheet, plate, tube, wire or rod. The surface of the workpiece that is treated in accordance with the process of the present invention is that of the cathode. For safety reasons, the cathodic work piece is normally grounded. This does not prevent the use of an alternative polarity. The positive voltage applied at the anode can be pulsed. The cathodic processes involved in the treated surface are complex and can > include, among other things, chemical oxide reduction, cavitation, destruction of the crystalline order by shock waves, and implantation of ion ions. Composition at the anode The anode comprises an inert conductor material, such as carbon, for example carbon in the form of one or more blocks, rods, sheets, wires or fibers, or in the form of graphite coated on a suitable substrate. Physical form of the anode The anode will generally have a shape such that its surfaces are at a distance from its constant mass (the "working distance") of the cathode (13 surface to be treated). This distance can be physically approximately 12 mm. Accordingly, if the treated surface is flat, the surface of the anode will also be generally flat, but if said treated surface is curved, the anode can also be curved in order to maintain a substantially constant distance »Non-conductive suies or separators can also be used to maintain working distance in cases where the working distance can not be easily controlled by other means» $ The anode can be of any convnt size, even when areas Large effective anode can be obtained better by using a plurality of anodes »! -» • = > small since this facilitates the flow of electrolyte and 5 waste away from the work area and improves heat dissipation. An essential aspect of the invention is that the electrolyte is introduced into the work area by flow or pressure through the anode having the AIS one and preferably a plurality of holes, channels or opgs for this purpose. Convntly, such holes can be of the order of 1-2 mm in diameter and spaced from 1 to 2 mm. The effect of this method of handling the electrolyte is that the surface of the workpiece to be treated is bombarded with currents, sprays or jets of electrolytes. The electrolyte, together with any residue generated by the cleaning action, is extracted from the workpiece and can be collected, filtered, cooled and recirculated, as necessary. Flow arrangements of Penetration is often used in cng with metal film by electrolysis (see documents US 4405432? US 45294B6? And CA 1165271), but they have not previously been used in the microneedle regime. Any physical form of the anode can be used on condition that allows the electrolyte p fsdz to be handled as described above. Optionally, an electrically isolated screen containing finer ori fi ces than the same node put-to interpose between the Snodo and the work piece »This screen serves to refine the jet or the jets that arise from the anode in finer jets that enter then in contact with the workpiece »Operating regime The process is operated in a regime in which the electric current decreases, or at least does not increase significantly, with an increase in applied voltage between the anode and the cathode »It is region B in figure 1 and was previously mentioned as the" unstable region "in document UK.- -1399710. This regime is a regime in which discrete gas and vapor bubbles are present on the surface of the workpiece being treated, instead of a continuous film or gas layer. This distinguishes the regime used from the regime used in UK "- - 1399710 which clearly teaches that the gas film must be continuous.The successful establishment of the desired" bubble "regime depends on finding an appropriate combination of numerous variables, including the voltage (or the energy consumption), the separation between the electrodes, the flow rate of the electrolyte and the temperature of the electrolyte, as well as external influences known in the art such as ultrasonic irradiation Variable ranges The ranges of the variables Within which useful results can be obtained are the following: Vol e The voltage range used is the range indicated by B in Figure 1 and within which the current decreases or remains substantially constant with an increase in voltage. Actual numerical voltages depend on several variables, but will generally be within the range of 10 V at 250 V, according to the conditions. The start of the unstable region, and therefore the lower end of the voltage range that can be used (indicated Kfsz r "f, can be represented by an equation in the following way.}. C - n (1 / d) (lambda / alpha sigma H) 0.5 where n a numerical constant 1 is the distance between electrodes d is the diameter of the gas / vapor bubbles on the surface lambda is the coefficient of heat transfer of the electral i ta alpha is the temperature coefficient The sigma H heat transmission is the initial electrostatic specific rsconductivity • This equation shows how the critical voltage for the onset of instability depends on some of the variables of the system.In the case of a given electrolyte it can be evaluate, but only if "n" and "d" are known, in such a way that it does not allow a critical voltage prediction ab initis, however it shows how the critical voltage depends on the distance between the electrodes and the properties of the electrolyte solution. 10 Separation between electrodes The separation between anode and cathode, or the working distance, is generally within the range of 3 to m, preferably within the range of 5 to 20 mm. Flow rate of the electrolyte 15 The flow rates can vary quite widely, between 0.O2 and 0.2 liters per minute per square centimeter of anode I / min.cm2). The flow channels through which the electrolyte penetrates the working region between the anode and the work piece are arranged from preference to offer a uniform flow field within this region. An additional flow of electrolyte can be promoted by jets or sprays placed in the vicinity of the anode and the work piece, as is known in the art, such that a certain part (but not all) of the electrolyte pass through the anode itself »T * ^ f Electrolyte temperature The electrolyte temperature also has an important effect on achieving the desired" bubble "regime. Temperatures in the range of 10'C to 855C can be used in a useful way. It will be understood that suitable devices can be offered for the purpose of heating or cooling the electrolyte and consequently maintaining it at the desired operating temperature. Composition of electrolyte 10 The composition of the electrolyte comprises an electrically conductive aqueous solution which does not react chemically with any of the materials with which it comes in contact, such as, for example, a solution of sodium carbonate, potassium carbonate, sodium chloride, sodium nitrate or well another salt of this type. The solute may be present, conveniently, at a concentration of * A to 12%, even when this is provided only by way of example and not to limit the choice of concentration. Optionally, the. electrolyte can include either as one of the components or as the single component, a soluble salt of a suitable metal. In this case, said metal is applied to the work piece during the cleaning process. The concentration of the metal salt, which can, for example, conveniently be 30%, must be maintained by conforming addition is consumed.

Adequate combination of variables It must be clearly understood that the required "bubble" regime can not be achieved with any arbitrary combination of the variables discussed above. The desired regime is achieved only when an appropriate combination of these variables is selected. One of said suitable sets of values can be represented by the curves reproduced in Figures 2a, 2b and 2c which show, by way of example only, some combinations of the variables for which the desired regime is established, using a solution of sodium carbonate to lOíí. Once the anode area, the working distance, the electrolyte flow rate and the electrolyte temperature have been chosen and set, the voltage is increased while the current is measured until the (voltage x current) reach the levels given in figures 2a, 2b and 2c. It will be understood by those skilled in the art that other combinations of variables not specified in Figures 2a, 2b and 2c can be employed to provide the regime of "bubbles" with sapphic results. The process of the present invention can be employed to treat the surface of a workpiece of any desired shape or configuration. In particular, the process can be used to treat a metal in the form of a sheet, either to treat the internal part or the external part of a steel pipe, or to treat the surface of an insulated object. In most known methods of electrolytic cleaning, it is necessary to submerge the surface of the workpiece that must be treated in the electrolyte. We have found that there is a surprisingly large decrease in energy consumption (as compared to the case). submerged) when the process of the present invention is carried out without the treated surface and the anode being submerged in the electrolyte. The method of the present invention is environmentally friendly and efficient from an energy perspective compared to the conventional processes »The cleaned surfaces have a high degree of roughness which facilitates the adhesion of coatings» Furthermore »when the process of the present invention is carried out with the electrolyte including a soluble salt of a suitable metal, the metal coating obtained in this way on the surface penetrates and combines with the metal of the work piece. The process of the invention offers economic advantages compared to the existing cleaning / coating processes. A further feature is that the operation of the process of the invention without immersion, by spraying or aoiicar electrolyte jets through channels, holes or openings in the anode, in such a way that the electrolyte comes in contact with the surface to treat, causes a significant reduction in energy consumption compared to an operation with immersion, which offers an additional commercial advantage. An operation without immersion also frees the process of the limitations imposed by the need to contain the electrolyte and allows the treatment in situ. of isolated objects of various for a. The process of the present invention is further described with reference to Figures 3 to 7 of the accompanying drawings. With reference to these drawings, an apparatus for implementing the process of the present invention is illustrated schematically in Figures 3 and 4. A direct current source 1 has its positive pole connected to the anode 2, which has channels 3 through which an electrolyte is pumped from a supply tank 4 »The work piece 7 is connected to the cathode in the. appliance and apitially connected to earth »The electrolyte coming from the feed tank 4 can be pumped through a distributor 10 to the anode 2 in order to ensure a constant flow of electrolyte through the channels 3 at the anode» A screen 9 electrically isolated, which has finer openings than the channels 3 in the anode, is placed between the anode and the work piece T) 7 or the ob te e pr > Jvocate that ¡s i le t li to r > , > > From 3 o'clock onwards, it is shown in Fig. 3, where the apparatus 5 is equipped with a filter tank 5 py afs.'psr.r lo.- »Esi u del eletrolito» and a * pump ¿> to shoot the electrolyte or filter back to £ -1. What is the electrolyte content? As f IWJI "< -, I, G I in 1 i figure *, s was considered to be -1;;;;;;;;;;;;;;; a cáiaar. of work 8, st ui e as such that the movement 1 opg i tud nií »1 of the p > - > r aba-ti? > v e e the ama to be carried out »La .in.ra 8 i ne also d i | j > ? s? 1 var? to direct the flow the le i l i o h? > . i to the filter block 5 »35 figure 5 illustrate a part of an ipate for clean-up of a μ-work i 7 in 13 > It finds two rings on either side of it. # workpiece 7 both esp-tc i -UUUU-I de anerß equ d i .- > Tafite de 3a p? e, t de tribs o. Figure 6 schematically illustrates a part of a short time pHi li pia the two lacios do "m?? - de raba 7" As the two anodes 2 e-.tap t. "- pc ta a lii. it.fn.-iis dt fen ^ ns de. Your e? fu? > "? of the n?, -.? of work /, thus providing different speeds of li think in the two u erfici »al le» n 311 a e, 1 - * do. There are different lengths (not illustrated), making the treatment time of a moving part differ on both sides. Figure 7 schematically illustrates a part of an apparatus 5 for cleaning the inner surface of a tube el. which forms the work piece 7. In this arrangement, the anode 2 is positioned inside the tube with appropriate arrangements for the supply of the electrolyte to the anode. In carrying out the process of the present invention, conditions are chosen such that discrete bubbles of gas and / or vapor are formed on the surface 11 of the workpiece 7. Electrical discharges through the bubbles of Gas or vapor formed on the surface causes the removal of impurities from the surface during processing and these products are removed by the electrolyte flow and are filtered through the filter block 5. The present invention will be further described with reference to FIG. reference to the following examples EXAMPLE i A hot-rolled steel strip having a 5 micron layer of lamination flakes (black oxide) on its. surface was treated in accordance with the method of the present invention employing a carbon anode »El & The anode was formed by machining rs.nurs.-B on a graphite plate, in two directions, at right angles to provide a work surface having rectangular portions that envelope to increase the surface area. The holes for the electrolyte flow were 2 m in diameter and were formed both in the protruding portions co or in the thinner regions of the plate. The work piece was kept stationary and was not immersed in the electrolyte. The parameters employees were the following; Electrolyte: 10% by weight of an aqueous solution of sodium carbonate Voltage: 120 V Electrode separation: 12 mm 15 anode area: 100 cm2 treated area 80 cm2 Electrolyte flow rate: 9 / min total Electrolyte temperature: oO "C 20 After a cleaning time of 15 seconds and a specific energy consumption of 0.42 kWh / m2, a clean gray metallic surface was obtained that did not show signs of oxide either visually or when examined using an electron microscope. exploration using a x-ray scattering analyzes »The topography.

The surface had numerous deep holes on a microscopic scale, which offers a potential for any subsequent revelation. EXAMPLE 2 The procedure of Example 1 was repeated but using a steel strip with a layer of 15 micrometer thickness of lamination flakes. The cleaning time was 30 seconds and the specific energy consumption was 0.84 kih / m2. EXAMPLE 3: Example of comparison The procedures of examples 1 and 2 were repeated with the workpiece submerged in the electrolyte at a depth of 5 mm. The specific energy consumptions required to carry out the cleaning were the following: 5 micr ? others of lamination flakes: 3.36 kWh / m2 15 micrometers of lamination flakes: 6.83 kWh >; 'm2 It can be seen that the immersion of the work piece had the example of raising the energy consumption by a factor of approximately 8, thus greatly increasing the cost for energy »EXAMPLE 4 The procedure of the Example 1 using a steel strip without lamination flakes, but with an oxidation layer and general stain on its surface. A complete cleaning was obtained in 2 seconds or less with a specific energy consumption of 0.06 k.Wh / m2.

Claims (4)

1. CLAIMS 1. An electrolytic process to clean the surface of a work piece of an electrically conductive material, said process comprises; i) is a cell in the cell treated with a cathode comprising the surface of the workpiece and an inert anode? ii) introducing an electrolyte in the created zone between the anode and the cathode causing its flow under pressure a. Through one or more orifices, channels or openings in the anode and thus coming into contact with the surface of the cathode, the surface of the cathode is not submerged otherwise in the electrolyte? and iii) applying a voltage between the anode and the cathode and ap a in a regime in which the electric current decreases or remains substantially constant with increased voltage applied between the anode and the cathode, and in a regime in which bubbles Discrete gas and / or vapor are present on the surface of the work piece during the treatment.
2. 2. A process according to claim 1 wherein the work piece has a metal surface or an alloy.
3. 3. A process in accordance with claim 2 where the anode is made of carbon.
4. 4. A process according to claim 3 wherein the carbon anode comprises one or several blocks, rods, sheets, wires or carbon fibers, or a coating of and on a substrate on a substrate »5» A process in accordance with any of the above indications where the anode has a plurality of holes, channels or openings. A process according to any of claims 1 to 5 wherein the electrically isolated screen is positioned in the electronic cell adjacent to the anode with the object to refine the electrolyte jets that arise from the anode in finer jets that come into contact with the cathode. 7. A process according to any of the preceding claims wherein a plurality of anodes are employed. 8. A process according to claim 7 wherein at least one anode is placed on one side of a workpiece to be treated and at least one anode is placed on the opposite side of the workpiece to be treated, so that clean the opposite sides of said work piece. 9. A process according to claim 8 wherein the workpiece is in the form of a metal strip, metal foil or metal plate, 10. A process according to any of claims 1 to 7 wherein the workpiece it is a tube »11. A process in accordance with any of the rei indications 1 to 10 where the workpiece is made of stainless steel. 12. A process according to any of the preceding claims wherein the surface of the workpiece is displaced in relation to the anode or anodes during the treatment. 13. A process conforms to any of the preceding claims wherein the electrolyte includes a salt of a metal that is applied to the surface of the work piece during said process. 14 »A metal workpiece cleaned by a process according to claim 1 in any of claims 1 to 12, wherein the surface of the metal is of a rough or porous nature in order to facilitate the mechanical grip of any subsequently applied coating . 15 »A cleaned metal workpiece and a metal 20 coated by means of a process according to claim 13, wherein there is a progressive transition in terms of metal composition of the workpiece to the coating metal. - £ 3