MXPA00002861A - Electro-plating process - Google Patents

Abstract

An electrolytic process for metal-coating the surface of a workpiece of an electrically conductive material, which process comprises:i) providing an electrolytic cell with a cathode comprising the surface of the workpiece and an anode;ii) introducing an electrolyte comprising a aqueous solution containing one or more water soluble compounds of the metal or metals to be deposited into the zone created between the anode and the cathode in a manner such that the cathode is bathed but not immersed in the said electrolyte;and iii) applying a voltage between the anode and the cathode, characterized in that iv) an electrical plasma arc is maintained between the anode and cathode during the deposition of the metal-coating onto the surface of the cathode.

Description

ELECTRODEPOSIT PROCEDURE _ The present invention relates to a method for the electrodeposition of a coating of metal or of an alloy in a substrate using an electrolytic cell as in the conventional electrodeposition but without using a conventional electrodeposition regime. BACKGROUND OF THE INVENTION Depositing metal coatings or alloys by electrodeposition techniques is well known in the art. The electrodeposition is typically carried out with voltages of 5 to 20 volts DC, current densities of 0.2 to 60 amp / dm2 (more typically 1-10 amp / dm2), temperatures within the range of 10 to 90 ° C and under conditions in which the work piece is completely immersed in the electrolyte solution. It is also common to use sacrificial anodes of the same composition * -that the coating metal for the purpose of maintaining the electrolyte concentration during the process. The electrodeposition process is relatively slow, typically metal is deposited at a rate of 1 to 5 micrometers of thickness / minute. The electrodeposited layer is normally dense (solid and without voids), while its surface finish is normally smooth and reflects light. The thickness of the electrodeposited layers can have almost any value according to the time and density of the current used. Typical thicknesses would be within the range of 10 to 50 micrometers. In "brush" deposit processes such as, for example, the "Sifeo" process (Sifco Industries Inc. of Cleveland, Ohio, United States of America), higher electrodeposition rates of up to 20 microns can be achieved under favorable conditions. of thickness / minute. This procedure uses a porous cushion or "brush" saturated with the electrolyte with high concentrations of metal, the cushion occupying the space between the anode and the cathode (work piece). Therefore the workpiece is not immersed in the electrolyte as in conventional electrodeposition processes. Voltages from 6 to 20 V are generally used. A high-speed electrodeposition can be carried out at very high current densities. For example, a deposition rate of 150 micrometers / minute is reported in the case of iron coating from a solution of Fe (NH2S03) 2 with a current density of 690 amp / dm2 (Electro-plating Engineering Handbook [Ed .LJ Durney], Van Nostrand Reinhold, 1984, 4th edition, pages 767-771). However, these known electrodeposition processes are not suitable for producing intentionally porous or rough coatings which may be desired, for example, to provide a better mechanical grip if an additional coating, eg, paint or plastic, is to be applied. In some circumstances it is also desirable to deposit a metal coating at a much higher rate than is feasible by employing a known electrodeposition process at normal current densities (below 100 amp / dm2). The present invention focuses on these matters. PRIOR ART In PCT Application No. PCT / IB96 / 00876, the present inventors describe a method for cleaning and coating electrically conductive surfaces with metal, which employs one or more elaborated anodes of the metal or metals to be deposited. The method includes the transfer of metal from the anode or anodes to the workpiece that forms the cathode in an electrolytic cell. The process is operated in a regime in which the DC current decreases or remains substantially constant with increasing voltage and where discrete gas bubbles are formed in the workpiece. The electrolyte is a spray or thrown on the surface of the workpiece through one or more holes in the anode. Even though the work piece may be immersed in the electrolyte, it is preferred that it not be the case. In the previous Patent Application, it is not necessary that the electrolyte contains a salt or a soluble compound of the metal being deposited, since the metal deposited on the surface of the workpiece is transferred from the anode or the anodes, which must have the same composition that the metal or metals to deposit. The process described in the present application is distinguished from said previous application insofar as (a) it includes the conventional deposit of metal from the electrolyte solution and (b) it is carried out using an anode that can be made from of any electrically conductive material. The electrodeposition without immersion of the work piece is known in the process of depositing "brush" as mentioned above and is also taught, for example, in document CA-A-1165271 wherein the electrolyte is pumped or emptied through of an anode in the form of a box with a set of holes in its base. such that it comes in contact with the work piece located under the anode box or in movement under the anode box. The benefit of this arrangement is that only one side of the work piece is coated instead of the entire work piece, as would be the case in a bathroom deposit method. Also avoid the use of a consumable anode. As for operation in a plasma regime or sparks discharge, there are numerous patents in which the purpose is only to clean a metal surface. Examples of such patents are GB-A-1399710, SU-A-1599446, SU-A-1244216 and GB-A-1306337. In terms of coating processes, microarco processes were described for depositing oxide and silicate coatings in metals. In these processes, the deposited coatings are non-metallic and are carried out at the anode, not at the cathode as in the case of the present invention. (See, for example, US Patent 3,834999; AV. Timoshenko et al., Protection of Metals, (Vol. 30, No. 2, 1944, pages 175-180). We have now developed an electrodeposition method in which the metal is deposited from an electrolytic solution as in a conventional electrodeposition process, but in the presence of an arc discharge from an electric plasma. SUMMARY OF THE INVENTION In accordance with the present invention there is provided an electrolytic process for the metallic coating of the surface of a workpiece of an electrically conductive material, said process comprising: i) supplying an electrolytic cell with a cathode consisting of the surface of the workpiece and an anode; ii) introducing an electrolyte comprising an aqueous solution containing one or more water-soluble compounds of the metal or metals to be deposited in the area created between the anode and the cathode in such a way that the cathode is bathed but not immersed in said electrolyte; and iii) applying a voltage between the anode and the cathode, characterized in that iv) an electric plasma arc is maintained between the anode and the cathode during deposition of the metallic coating on the surface of the cathode. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates the deposit of zinc as a function of time as described in example 3; and Figure 2 is an electromicrograph of a cross-section of the zinc layer deposited in Example 4. DETAILED DESCRIPTION OF THE INVENTION Although characteristics (i) to (iii) detailed above are known in the electrodeposition technique, characteristic (iv) is considered to be novel both individually and in combination with the characteristics (i) to (iii). In carrying out the process of the present invention, the anode (or several anodes) can be made from any electrically conductive material, and the cell is operated substantially in a DC mode, although a polarity reversal can optionally be employed. during a smaller part of the operating time. To operate the process according to the present invention, a DC voltage is applied between the anode and the cathode (workpiece) and the electrolyte is introduced into the space between the anode and the cathode by any suitable means such as flow, spray, atomization, thrown well drip, either through one or several holes in the anode or otherwise, but without producing the immersion of the workpiece. It is necessary to start the arc and this can be achieved in several ways. An effective way is to set the voltage and electrode spacing and then introduce the electrolyte, initially at a speed (the "flow rate") high enough to avoid arcing between the anode and the cathode. The flow rate is then progressively decreased until a plasma arc occurs, becoming visible to the naked eye, at this point a rapid deposition of the metal or metal alloy coating occurs. The plasma arc does not necessarily fill the space between the anode and the cathode; there is normally considerable dark space on the anode side. The deposition rates can exceed a thickness of 60 micrometers per minute with current densities no greater than 100 amp / dm2 compared to a normal value of approximately 10 micrometers / minute and up to 20 micrometers / minute in the case of conventional electrodeposition processes which are carried out with a comparable current density. The nature of the coating obtained using the method of the invention may be different from the nature of the coating obtained using a conventional electrodeposition process, as described below. As the flow velocity is reduced even beyond the point at which an arc is initiated, the intensity of the plasma arc rises to eventually reach a maximum and begins to fall again. Once the flow velocity is reduced to zero, neither arcing nor depositing occurs. For any fixed value of other material variables (ie voltage, distance between anode and cathode, and electrolyte temperature), there is therefore a range of flow velocities in which the process can operate. This range will depend on the values chosen for the different variables, as well as the electrolyte composition. It will be understood that the method described for starting the arc is only by way of example and that other methods may be employed, such as for example a gradual increase in voltage while keeping other variables constant. Optionally, sacrificial anodes of the metal or metals to be deposited can be placed inside the cell and in contact with the electrolyte to help maintain the electrolyte concentration as is well known in the art. The workpiece can be in any way insofar as the anode can be maintained at a substantially constant distance from the workpiece. For example, the work piece comprises a metal strip, a sheet of metal, a metal plate or a pipe. The surface of the workpiece receiving electrodeposition in accordance with the invention is the surface of the cathode. VARIABLE RANGES The ranges of variables within which useful results can be obtained are the following: VOLTAGES The range of voltages used in the method of the present invention is generally from 50 to 300V DC, which is relatively high and considerably higher than the range of 5 to 20V DC used in conventional electrodeposition processes. CURRENT DENSITY _ The current density can vary within the range of 1 to 200 amps per square decimeter of anode (amp / dm2). ELECTROLYTE FLOW SPEED _ _ Flow rates can vary widely, between 0.2 and 25 liters per minute per square decimeter of anode (1 / min.dm2).

ELECTROLYTE _ _ ___ The electrolyte comprises an aqueous solution of one or more compounds of the metal or metals to be applied. Typical concentrations are within a range of 1% by weight until saturation. Typical examples of solutions that can be used are as follows: for zinc: zinc sulphate or zinc nitrate for aluminum: aluminum sulfate or aluminum nitrate for iron: iron sulfate ammonium for lead: nitrate of lead for copper: copper sulfate. ELECTROLYTE TEMPERATURE Temperatures in the range of 10 to 90 ° C can be usefully employed. It will be understood that appropriate means may be provided in such a manner that the electrolyte is heated or cooled and is therefore maintained at a desired operating temperature. SEPARATION BETWEEN ELECTRODES _ _ _ The separation between the anode and the cathode, or the working distance, is generally within a range of 3 to 30mm, preferably within a range of 5 to 20 mm. It will be understood that arcing can not necessarily be obtained with any combination of variables within the ranges presented above, but only with some combinations. The combinations that will work will depend on the electrolyte composition, the metal to be coated, the shape and mass size of the work piece, and the configuration of the electrolytic cell. In order to establish whether a given combination of variables will initiate and support an arc, all that is referred to is to fix all the named variables except one and then to vary this last variable over its entire range. As indicated above, the easiest parameter to vary is the electrolyte flow rate. THE COATINGS ~ _ The coatings can be of any metal or combination of metals that have soluble salts or soluble salt compounds. Typically, the coating obtained will consist of two layers that are bonded adjacent to the surface of the original workpiece (bottom layer) and a porous layer adjacent to the solid continuous layer (top layer). This solid / porous double structure offers both a strong corrosion barrier and a porous top layer that provides an excellent surface for mechanical grip in the case of subsequent coatings (eg plastic or paint) and provides protection against mechanical damage. The structure of the coating will depend on the precise conditions employed and coatings of a structure different from that described above, for example, fully solid coatings can also be obtained. DEPOSIT SPEEDS The maximum deposit velocity is fast compared to conventional electrodeposition methods at comparable current densities, and can exceed 1 icometer per second (60 microns per minute). The coating thickness can be controlled by, inter alia, traversing the workpiece through the cell (work area) in such a way that it moves relative to the fixed anode but at a constant anode spacing. The time spent in the work zone will be inversely proportional to the crossing speed and the thickness of the coating will be related to that time. The work piece can be passed several times through the same work area, or through several work zones sequentially. _ The high coating speeds available indicate that the process is particularly useful for in-line or continuous metal processing. The present invention will be described in greater detail with reference to the following examples. EXAMPLE 1 A 0.31 cm (0.125 inch) thick preformed fresh metal plate forming the cathode in an electrochemical cell was coated with a mixture of zinc and aluminum under the following conditions. Voltage: 160V Current: 42A Anode: stainless steel, area 48 cm2 with holes of a diameter of 2.4 rom Electrolyte: equal parts of saturated solutions of ZnSo4 and A12 (S0) 3 Flow rate: 2.61 / min at 70 ° C Separation of electrodes: 12 mm After a time of treatment of 80 seconds, a zinc / aluminum layer approximately 27 micrometers thick was deposited at the cathode consisting of a solid (compact) bottom layer adjacent to the steel cathode substrate and a top porous layer. When it was tested to determine the adhesion of the coating on the substrate using a test No. D4541-95"detachment" of ASTM, the average release force was 39 MPa which exceeds industry standards. When tested to determine cathodic separation through industry-standard tests, the separation distance was less than 2 mm after 24 hours and below 3mm after 48 hours, which are values much higher than the average industrial values of 8mm and 12mm, respectively.

EXAMPLE 2 A pre-thin strip of mild steel, 19mm wide and 2. litim long that forms the cathode of an electrochemical cell was coated with a mixture of zinc and aluminum as in Example 1, under the following conditions: Voltage: 150V Current: 10A Anode: stainless steel; area 48 cm2 with holes of 2.4 mm diameter Electrolyte: equal parts of saturated solutions of ZnS? 4 and A12 (S04) 3 _ Flow rate: 2.61 / min at 70 ° C Separation of electrodes: 10 mm The steel strip was passed through the work area of the cell 5 times, each pass taking 36 seconds. This corresponds to a total treatment time of 10 seconds for each cathode area. A zinc / aluminum layer of approximately 6 to 10 micrometers was deposited at the cathode EXAMPLE 3 The general procedure of example 2 was repeated using a sample of 6.2cm wide and 21cm long strip of mild steel and a solution of Zinc sulphate saturated as electrolyte The progressive deposition of zinc, in terms of the thickness of the coating layer, as a function of the treatment time is shown in Figure 1. The deposition rate exceeds 1 micrometer per second EXAMPLE 4 The zinc layer deposited in Example 3 consists of a solid layer (N) of approximately 5 micrometers in thickness adjacent to the steel substrate This structure is illustrated in the scanned and digitized electron micrograph of Figure 2. An EDS analysis energy dispersion) of several areas (N, 0, P &Q) in the cross section showed: N - 2.2% iron in zinc O - 1.0% iron in zinc P - 0.3% iron in zinc Q - 1.5% iron, 1.7% sulfur and a moderate amount of oxygen in zinc. EXAMPLE 5 The procedure of Example 2 was repeated under the following conditions: Voltage: 190V Current: 20A Electrolyte: ZnSo4 (saturated solution) Electrolyte temperature: 73 ° C Treatment time: 125 Electron micrographs of a cross section of the coated steel showed that A solid (non-porous) zinc coating about 10 micrometers thick was deposited on the steel.

Claims (11)

1. CLAIMS An electrolytic process for coating the surface of a workpiece of an electrically conductive material with metal, said method comprising: i) providing an electrolytic cell with a cathode consisting of the surface of the workpiece and an anode; ii) introducing an electrolyte comprising an aqueous solution containing one or more water-soluble compounds of the metal or metals to be deposited in the area created between the anode and the cathode in such a way that the cathode is bathed but not immersed in said electrolyte; and iii) applying a voltage between the anode and the cathode, characterized in that iv) an electric plasma arc is maintained between the anode and the cathode during deposition of the metal coating on the surface of the cathode.
2. A method according to claim 1 wherein several anodes are employed.
3. A method according to claim 2 wherein at least one anode is a sacrificial anode.
4. A method according to any of the preceding claims wherein the workpiece has a metal surface or a metal alloy surface.
5. 5. A method according to any of the preceding claims wherein the workpiece is displaced relative to the anode during the process.
6. 6. A method according to any of the preceding claims 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 the opposite sides of the work piece are coated with metal.
7. 7. A method according to claim 6 wherein the workpiece comprises a metal strip, a metal foil or a metal plate.
8. 8. A method according to any one of claims 1 to 5 wherein the work piece is a pipe.
9. 9. A method according to claim 8 wherein at least one anode is positioned outside the pipe and at least one anode is placed inside the pipe, such that both the inner part and the outer part of the pipe are coated. .
10. 10. A method according to any of the preceding claims wherein the voltage is within a range of 50 to 300V DC.
11. 11. An electrically conductive workpiece that has been coated with a metal through a process according to any of the preceding claims, wherein the coating consists of a solid layer in contact with the workpiece and a porous layer in contact with the solid layer. . "An electrically conductive workpiece coated with metal according to claim 1 wherein the workpiece is formed from carbon steel, and the coating comprises zinc or a zinc / aluminum alloy. electrically conductive workpiece, coated with metal according to claim 11 wherein the solid layer is an alloy containing atoms of the material of the workpiece that are derived from said workpiece.