An electrolytic process for cleaning electrically conducting surfaces

IS 2 149 491 T3

DESCRIPTION

Electrolytic procedure for cleaning electroconductive surfaces.

Background of the invention

The present invention relates to a process for cleaning an electrically conductive surface, such as a metal surface.

Metals, especially steel in its many forms, ordinarily have to be cleaned and / or protected against corrosion before final use. As produced, steel normally has a scale film (black oxide) on its surface that is not uniformly adherent and makes the underlying material prone to galvanic corrosion. Scale must therefore be removed before the steel can be painted, coated or metallized (eg with zinc). Metal can also have other forms of contamination (known in the industry as "dirt") on its surfaces, including rust, oil or grease, pigmented drawing compounds, chips and cutting fluid, and polishing and burnishing compounds. All of these should be removed normally. Even stainless steel can have excess oxide mixed on its surface that must be removed before further use.

Traditional methods of cleaning metal surfaces include acid pickling (which is becoming increasingly unacceptable because of the cost and environmental problems caused by the disposal of spent acid); abrasive blasting; drumming wet or dry; brushed; descaling with salt bath; alkaline pickling and acid cleaning. A multi-stage cleaning operation may involve, for example, (i) loss by fire or solvent extraction of organic materials, (ii) sandblasting or shot blasting to remove scale and rust, and (iii) electrolytic cleaning as a preparation. of the final surface. If the cleaned surface is to receive corrosion protection by metallization, painting or plastic coating, this should normally be done quickly to avoid renewed oxidation of the surface. Multi-stage treatment is effective, but expensive, both in terms of energy consumption and process time. Many of the conventional treatments are also environmentally undesirable.

Electrolytic methods of cleaning metal surfaces are frequently incorporated into processing lines such as those for galvanizing and electroplating steel strip and sheet. Common coatings include zinc, zinc alloy, tin, copper, nickel, and chromium. Autonomous electrolytic cleaning lines are also used to feed multiple subsequent operations. Electrolytic cleaning (or “electrolytic cleaning”) normally involves the use of an alkaline cleaning solution that forms the electrolyte, while the part can be the anode or cathode of the electrolytic cell, or the polarity can be alternated in another way. Such processes generally operate at low voltage (typically 3 to 12 volts) and current densities from 1 to 15 amps / dm.². Energy consumptions thus range between approximately 0.01 and 0.5 kWh / m². Dirt extraction is accomplished by generating gas bubbles that lift the contaminant from the surface. When the part surface is the cathode, the surface can not only be cleaned, but also "activated", thereby giving a subsequent coating improved adhesion. Electrolytic cleaning cannot normally be practiced to remove heavy scale, and this is done in a separate operation such as acid pickling and / or abrasive blasting.

Conventional electrolytic cleaning and electrodeposition processes operate in a low voltage regime in which the electric current decreases monotone with the applied voltage (see Figure 1 below in A). Under some conditions, when the voltage rises, a point is reached where instability occurs and the current begins to decrease with increasing voltage (see Figure 1 below in B). The unstable regime marks the start of electric discharges on the surface of one or the other of the electrodes. These discharges ("microarcs" or "microplasmas") occur through any suitable non-conductive layer present on the surface, such as a layer of gas or vapor. This is because the potential gradient in such regions is very high.

Previous technique

GB-A-1399710 describes that a metallic surface can be electrolytically cleaned without excessive heating and without excessive energy consumption if the process is operated in a regime beyond the unstable region, defining the “unstable region” as one in which the current decreases with increasing voltage. By moving to slightly higher voltages, where the current increases again with increasing voltage and a continuous film of gas / vapor is established on the treated surface, effective cleaning is obtained. However, the energy consumption of this process is high (10 to 30 kWh / m²) compared to energy consumption for acid pickling (0.4 to 1.8 kWh / m²).

SU-A-1599446 describes a high voltage electrolytic spark erosion cleaning process for welding rods using extremely high current densities, on the order of 1000 A / dm², in a phosphaoric acid solution.

SU-A-1244216 describes a micro arc cleaning treatment for machine parts operating at 100 to 350 V using anoadic treatment. No particular method of electrolyte handling is described.

Other electrolytic cleaning methods have been described in GB-A-1306337 where an electroerosioan stage is used in combination with a separate chemical or electrochemical cleaning step to remove oxide scale; in US-A-5232563 where contaminants at low voltages of 1.5 to 2 V are removed from semiconductor chips by producing gas bubbles on the surface of the pads that lift the contaminants; in EP-A-0657564, which describes that normal low-voltage electrolytic cleaning is ineffective in removing grease,

ES 2 149 491 T3 but electrolytically oxidizable metals such as aluminum can be successfully degreased under high voltage (micro arc) conditions by acid anodization.

The use of electrolyte jets located near the electrodes in electrolytic cleaning baths to create high speed turbulent flow in the cleaning zone is described for example in JPA-08003797 and DE-A-4031234.

Electrolytic cleaning of radioactively contaminated objects using an electrolyte uanic jet without general immersion of the object is described in EP-A-0037190. The cleaned object is anoadic and the voltage used is between 30 to 50 V. Short treatment times of the order of 1 second are recommended to avoid surface erosion and complete extraction of oxide is considered undesirable. Nor is immersion described in CA-A-1165271 where the electrolyte is pumped or poured through a box-shaped anode with a network of holes at its base. The purpose of this assembly is to allow the electrodeposition of a metal strip 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 bipolar electrolytic treatment by passing the wire through a first chamber in which the wire is cathoadic and a second chamber in which the wire is anaodic. In the second chamber, a plasma layer is formed on the anode surface of the wire by ionization of a layer of oxygen-containing gas. The wire is immersed in the electrolyte throughout its treatment.

EP-A-0406417 describes a continuous process for drawing copper wire from copper rod in which the rod is plasma cleaned prior to the drawing operation. The "plasmatroan" shell is the anode and the wire is also surrounded by an inner coaxial anode in the form of a perforated U-shaped sleeve. To initiate plasma production, the voltage is kept low but unspecified, the electrolyte level above the submerged wire is lowered, and the flow rate is decreased to stimulate the initiation of a discharge at the surface of the wire.

Although low voltage electrolytic cleaning is widely used to prepare metal surfaces for electroplasty or other coating treatments, it cannot treat coarse oxide deposits such as scale without unacceptably high energy expenditure. Such electrolytic cleaning processes should normally be used, therefore, in conjunction with other cleaning procedures in a multi-stage operation.

We have now developed a particularly efficient metal cleaning process that is capable of dealing with thick oxide scale. Summary of the invention

Accordingly, in one aspect the present invention provides an electrolytic process for cleaning the surface of a part of an electrically conductive material, which process includes:

i) providing an electrolytic cell with a cathode that includes the part surface and an inert anode;

ii) insert an electroalite into the area created between the anode and the cathode by causing it to flow under pressure through one or more holes, channels or openings in the anode and thereby impinge on the surface of the cathode, not otherwise submerging the cathode surface in electrolyte; and iii) applying a voltage between the anode and the cathode and operating in a regime in which the electric current decreases or remains substantially constant with the increase of the voltage applied between the anode and the cathode, and in a regime in which discrete bubbles Gas and / or steam will be present on the surface of the piece during the treatment.

Brief description of the drawings

Figure 1 schematically illustrates the operating regime where the electric current decreases, or does not increase with the increase of the applied voltage.

Figures 2a, 2b and 2c illustrate operating parameters where the 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 carrying out the cleaning process of the invention for cleaning the two 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 schematically illustrates a facility for cleaning the internal surface of a tube.

Detailed description of the invention

The term "inert", as used herein, means that no material is transferred from the anode to the part.

In carrying out the method of the present invention, the part has a surface that forms the cathode in an electrolytic cell. The anode includes an inert conductive material, such as carbon. The process operates in a regime in which the electric current decreases, or at least does not increase considerably, with an increase in the voltage applied between the anode and the cathode. The process of the present invention can be carried out as a continuous or semi-continuous process providing that the relative movement of the part takes place in relation to the anode or anodes. Alternatively, stationary articles can be processed following the process of the invention. The electrolyte is introduced into the working zone between the anode and the cathode causing it to flow under pressure through at least one hole, channel or opening in the anode, thereby colliding with the cathode (the surface under treatment).

Each of these features is described in more detail below.

IS 2 149 491 T3

Cathodic arrangement of the surface to be treated

The part can be of any shape or configuration including sheet, plate, tube, pipe, wire or rod. The surface of the piece that is treated according to the process of the invention is that of the caitode. For safety reasons, the cathode part is grounded normally. This does not preclude the use of alternating polarity. The positive voltage applied to the anode can be pulsed.

The cathodic processes involved in the treated surface are complex and can include among other effects: chemical reduction of ioxide; cavitation; destruction of crystalline order by shock waves; and ionic implantation.

Composition of the anode

The anode includes an inert conductive material, such as carbon, for example carbon in the form of one or more blocks, rods, sheets, yarns or fibers, or as a graphite coating on a suitable substrate.

Physical form of the onod

The anode will generally be such that its surface is at a substantially constant distance (the "operating distance") from the caitode (the surface to be treated). This distance can typically be about 12mm. Thus, if the treated surface is flat, the anode surface will also generally be flat, but if the former is curved, the anode can also advantageously be curved to maintain a substantially constant distance. Non-conductive guides or spacers can also be used to maintain operating distance in cases where operating distance cannot be easily controlled by other means.

The anode can be of any convenient size, although large effective anode areas can best be obtained by using a plurality of smaller anodes as this facilitates the removal of electrolyte and debris from the work area and improves heat dissipation.

A key aspect of the invention is that the electrolyte is introduced into the work zone by pressure flow through the anode which is provided with at least one and preferably a plurality of holes, channels or openings for this purpose. Such holes may conveniently be on the order of 1-2 mm in diameter and 1-2 mm apart.

The effect of this electrolyte handling method is that the surface of the part to be treated is bombarded with electrolyte currents, sprays or jets. The electrolyte, together with the waste generated by the cleaning action, leaves the part and can be collected, filtered, cooled and recirculated as necessary. Flow-through assemblies are commonly used in electroplasty (see US 4405432; US 4529486 and CA 1165271), but have not previously been used in the microplasma regimen.

Any physical form of the anode can be used that allows the electrolyte to be manipulated as described above.

Optionally, an electrically insulated sieve containing holes smaller than the iodine itself can be interposed between the iodine and the part. This sieve is used to refine the jet or jets that come out of the iod into finer jets that later collide with the piece. Operating regime

The process operates in a regime in which the electric current decreases, or at least does not increase considerably, with an increase in the voltage applied between the anode and the cyatode. AND^í This is region B in Figure 1 and was previously referred to as the "unstable region" in UK-A-1399710. This regime is one in which there are discrete bubbles of gas and vapor on the surface of the part being treated, rather than a continuous film of gas or layer. This distinguishes the regime employed from that employed in UK-A-1399710 which clearly describes that the gas film must be continuous.

Successful establishment of the desired "bubble" regime depends on finding an appropriate combination of a number of variables, including voltage (or power consumption), electrode gap, electrolyte flow rate, and electrolyte temperature. and external influences known in the art as ultrasonic irradiation.

Variable bands

The bands of the variables within which useful results can be obtained are the following:

Voltage

The voltage band used is designated B in figure 1 and within which the current decreases or remains substantially constant with increasing voltage. Actual numerical voltages depend on several variables, but would generally be on the order of 10 V to 250 V, depending on the conditions. The start of the unstable region, and therefore the lower end of the usable voltage band (designated Vcr), can be represented by an equation of the form:

Vcr = n (l / d) (λ / ασΗ)^0,5where n is a numerical constant;

l is the distance between electrodes;

d is the diameter of the gas / vapor bubbles on the surface;

λ is the heat transfer coefficient of the electrolyte;

α is the heat transfer temperature coefficient;

σ_Η is the initial specific electroconductivity of the electrolyte.

This equation demonstrates how the critical voltage for the onset of instability depends on some of the system variables. With respect to a given electrolyte, it can be evaluated, but only if n and d are known, so that it does not allow a prediction of the critical voltage ab initio. However, it shows how the critical voltage depends on the electrode distance and the properties of the electrolyte solution.

Electrode gap

The anode acid spacing, or operating distance, is generally on the order of 3 to 30 mm, preferably on the order of 5 to 20 mm.

IS 2 149 491 T3

Electrolyte flow rate

Flow rates can vary very widely, between 0.02 and 0.2 liters per minute per square centimeter of anode (l / min.cm²). The flow channels through which electrolyte enters the operating region between the anode and the workpiece are preferably arranged to provide a uniform flow field within this region. The additional flow of electrolyte can be promoted with jets or sprays placed close to the anode and the workpiece, as is known in the art, so that some (but not all) of the electrolyte does not pass through the anode itself.

Electrolyte temperature

Electrolyte temperature also has a considerable effect on achieving the desired "bubble" rate. Temperatures on the order of 10 ° C to 85 ° C can be usefully employed. It will be understood that appropriate means can be provided to heat or cool the electrolyte and keep it at the desired operating temperature. Electrolyte composition

The electrolyte composition includes an electrically conductive aqueous solution that does not react chemically with any of the materials it contacts, such as a solution of sodium carbonate, potassium carbonate, sodium chloride, sodium nitrate, or other salts. The solute may conveniently be present in a concentration of 8% to 12% although this is by way of example only and does not limit the choice of concentration. Optionally, the electrolyte may include a component or the uonic component, a soluble salt of a suitable metal. In this case, said metal is coated on the piece during the cleaning process. The concentration of the metal salt, which may conveniently be, for example, 30%, has to be maintained by addition as it is consumed. Adequate combination of variables

It should be clearly understood that the required “bubble” regime cannot be obtained with any arbitrary combination of the variables explained above. The desired regime is obtained only when an adequate combination of these variables is selected. Such a suitable set of values ​​can be represented by the curves reproduced in Figures 2a, 2b and 2c representing, by way of example only, some combinations of the variables for which the desired regime is established, using a sodium carbonate solution at 10%. Once the anode area, operating distance, electrolyte flow rate, and electrolyte temperature have been chosen and established, the voltage is increased while the current is measured until the wattage (voltage x current) it reaches the levels indicated in figures 2a, 2b and 2c. Those skilled in the art will understand that other combinations of variables not specified in Figures 2a, 2b and 2c can be used to provide the "bubble" regime, with satisfactory results being obtained.

The process of the present invention can be used to treat the surface of a part in any desired shape or configuration. In particular, the process can be used to treat sheet metal, or to treat the inside or outside of a steel tube, or to treat the surface of a self-contained object.

In most of the known electrolytic cleaning methods it is necessary to immerse the surface of the part to be treated in the electrolyte. We have found that there is a large and surprising increase in energy consumption (compared to the case of immersion) when the process of the invention is carried out without immersing the treated surface and the anode in the electrolyte.

The method of the present invention is environmentally friendly and uses energy efficiently compared to conventional processes. The cleaned surfaces have a high degree of roughness that facilitates the adhesion of the coatings applied to them. Furthermore, when the process of the invention is carried out with an electrolyte that includes a soluble salt of a suitable metal, the metallic coating obtained by this on the surface penetrates and binds with the metal of the part.

The process of the invention offers economic advantages over existing cleaning / coating processes. Another characteristic is that the operation of the process of the invention without immersion, dripping or spraying the electrolyte through channels, holes or openings in the anode, so that the electrolyte collides with the surface to be treated, leading to a great reduction in the consumption of energy in relation to the operation with immersion, providing an additional commercial advantage. The non-immersion operation also frees the process from the limitations imposed by the need to contain the electrolyte and allows the in situ treatment of self-contained objects in various ways.

The process of the present invention is better described with reference to Figures 3 to 7 of the accompanying drawings.

With reference to these drawings, an apparatus for carrying out the process of the present invention is schematically illustrated in Figures 3 and 4. A direct current source 1 has its positive pole connected to the anode 2, which has channels 3 arranged through the that an electrolyte is pumped from a feeder tank 4. The part to be coated 7 is connected as the cathode in the apparatus and optionally grounded. The electroolyte from the feeder tank 4 can be pumped through a distributor 10 to the anode 2 to ensure a uniform flow of electrolyte through the channels 3 in the anode. An electrically insulated screen 9, having finer holes than channels 3 in the anode, is placed between anode and part 7 to cause electrolyte sprayed from anode channels 3 to break down into more fine sprays.

As shown schematically in figure 3, the apparatus was provided with a filter tank 5 to separate residues from the electrolyte, and a pump 6 to circulate the filtered electrolyte back to the electrolyte feed tank. It is also contemplated, as represented in figure 4, that the piece 7 passes through a working chamber 8, which is constructed in such a way that the

ES 2 149 491 T3 longitudinal movement of the part through the chamber. Chamber 8 is also provided with means to direct the flow of electrolyte to filter block 5.

Figure 5 schematically illustrates a part of an apparatus for cleaning both sides of a part 7 in which two anodes 2 are placed on both sides of the part 7 and both are equidistantly spaced from the part.

Figure 6 schematically illustrates a part of an apparatus for cleaning the two sides of a part 7. As shown, the two anodes 2 are spaced at different distances from the surfaces of part 7. Alternatively, the two anodes may be of different lengths. different (not shown) causing the treatment time of a movable part to differ on the two sides.

Figure 7 schematically illustrates a part of an apparatus for cleaning the inner surface of a tube that forms part 7. In this assembly the anode 2 is placed inside the tube, providing appropriate assemblies for supplying the electroalite to the anode.

In carrying out the process of the present invention, the conditions are chosen so that discrete bubbles of gas and / or vapor are formed on the surface 11 of part 7. The electric discharge through the gas or vapor bubbles formed. on the surface causes impurities to come off the surface during processing and these products are removed by the electrolyte flow and filtered by the filter block 5.

The present invention will be better described with reference to the following examples.

Example 1

A hot rolled steel strip was treated with a scale layer (black oxide) of 5 microns on its surface following the method of the invention using a carbon anode. The anode was formed by machining grooves in a graphite plate, in two directions at right angles to obtain a work surface with rectangular protrusions to increase the surface area. The holes for electrolyte flow were 2mm in diameter and were formed through both protrusions and the thinned regions of the plate. The piece was kept stationary and was not immersed in the electrolyte. The parameters used were the following:

After a cleaning time of 15 seconds and a specific energy consumption of 0.42 kWh / m², a clean gray metal surface was obtained that will not show signs of oxide visually or when examined using an scanning electron microscope using dispersive X-ray analysis. The surface topography was deeply pitted on a microscope scale, providing potential for bonding to any subsequent coating. .

Example 2

The procedure of Example 1 will be repeated, but using a steel strip with a 15 micron thick scale layer. The cleaning time was 30 seconds and the specific energy consumption was 0.84 kWh / m².

Comparative Example 3

The procedures of Examples 1 and 2 were repeated with the part immersed in the electrolyte to a depth of 5 mm. The specific energy consumptions necessary for the complete cleaning were the following:

<td>5 microns of scale 15 microns of scale</td><td>3.36 kWh / m²6.83 kWh / m²</td>

It is seen that the immersion of the part has the effect of increasing the energy consumption by a factor of about 8, thereby greatly increasing the energy cost.

Example 4

The procedure of Example 1 will be repeated using a steel strip without scale, but with a layer of rust and general dirt on its surface. Complete cleaning was achieved in 2 seconds or less with a specific energy consumption of 0.06 kWh / m².

<td>Electrolyte:</td><td>10% by weight of aqueous sodium carbonate solution</td>

<td>Voltage:</td><td>120 V</td>

<td>Electrode separation:</td><td>12 mm</td>

<td>Anode area:</td><td>100 cm²</td>

<td>Treated area:</td><td>80 cm²</td>

<td>Flow rate</td><td></td>

<td>electrolyte:</td><td>9 l / min total</td>

<td>Temperature of</td><td></td>

<td>electrolyte:</td><td>60 ° C</td>