FR2650304A1 - METHOD FOR ELECTROLYTICALLY COATING A METAL SURFACE, AND ELECTROLYTIC CELL FOR ITS IMPLEMENTATION - Google Patents

Abstract

The subject of the invention is an anode unit of an electrolysis cell for the continuous coating of a metal surface, such as a steel sheet, the cathode of which is formed by the product to be coated, and comprising a membrane cation exchange delimiting an anode compartment and a cathode compartment. This anode unit consists of a housing comprising at least one anode basket ensuring the electrical connection of the anode unit. Means are provided so that said anode basket remains in permanent contact with at least one block of the metal or metals to be deposited on the cathode which dissolves in the electrolyte, and means are provided to allow the introduction of these blocks into the anode basket. A subject of the invention is also a process for the continuous electrolytic coating of a metallic material, said coating comprising a number n of elements, which can be implemented by means of n electrolysis cells comprising a cation exchange membrane, and whose anode units may be of the above type. The invention makes it possible to obtain a stationary state of composition of both the anolyte and of the catholyte, while maintaining constant the distance between the electrodes, in order to ensure the constancy of the composition of the coating of the product. It also allows rapid changes in the nature of the coating to be deposited.

Description

I 2650304

  ELECTROLYTIC COATING PROCESS

  A METAL SURFACE, AND ELECTROLYSIS CELL

FOR ITS IMPLEMENTATION

  The invention relates to electroplating, in particular coating

  electrolytic materials of iron-zinc and nickel-zinc on steel plate. The application for a French Certificate of Utility, published under No. 2,617,509 in the name of the Applicant, describes an electrolytic coating process using a cation exchange membrane. In this process, an anode electrolyte composition (called anolyte) is chosen that is different from that of the cathodic electrolyte (called catholyte) and the cathodic current density is adjusted to a constant value making it possible to keep the faradic efficiency of each of the reactions constant. cathodes, whereby the composition of the anode and cathode electrolytes remains substantially constant,

  as well as that of the elaborate coating.

  However, in the case where one deposits on the cathode a plurality of elements present in the solution, itself fed by one or more soluble anodes, chemical shift phenomena may introduce imbalances in the compositions of the electrolytes. One can not then maintain over a long period, a stationary state of operation of the installation. On the other hand, the progressive consumption of the soluble anodes causes a modification of the geometrical characteristics of the cell, which

influences its operation.

  The object of the invention is to obtain a stationary state of composition of both the anolyte and the catholyte, while maintaining constant the essential geometrical characteristics of the cell, in order to ensure the constancy of the composition of the coating of the product. Another object of the invention is to enable

  rapid changes in the nature of the coating to be deposited.

  With these objectives in view, the object of the invention is an anode unit of an electrolysis cell for the continuous coating of a metal surface, such as a steel plate, the cathode of which consists of the metal surface. and coating, cell of the type comprising a cation exchange membrane delimiting an anode compartment and a cathode compartment, characterized in that it forms the anode compartment of the cell and consists of a housing comprising at least one anode basket , in that said membrane constitutes at least in part the face of the anode unit intended to be placed facing the surface to be coated, said anode basket being made of an insoluble electroconductive material, and ensuring the electrical connection of the anode unit , means being provided so that said anode basket remains in permanent contact with at least one block of the metal or metals to be deposited on the cathode which dissolve in the electrolyte, and means are also provided to allow the introduction of

these blocks in the anode basket.

  Preferably, the anodic basket constitutes the wall

interior of said housing.

  The subject of the invention is also a process for the continuous electrolytic coating of a metallic material that can be implemented using electrolysis cells whose anode units are of the type just described, said coating comprising a number n greater than 1 of elements Mi, with i = 1 to n, characterized in that: - n electrolytic cells each comprising a cation exchange membrane defining an anode compartment and a cathode compartment, and allowing the dissolution in the anode compartment of an element Mi, said cells being electrically connected in parallel with each other, their anode compartments being separately supplied with anolyte, their cathode compartments being supplied in series by the catholyte, their common cathode being constituted by the the product to be coated, and, in order to keep the compositions of the catholyte constant, anolyte, and the coating deposited on the surface. the compositions of the anolyte and of the catholyte and the densities A of current I are selected in each branch of the circuit in such a way that: A-- Mi '= - i mi i = l rMi being the faradic efficiency of deposition on the cathode of the element Mi, and RMi being the migration efficiency of the element Mi across

the cation exchange membrane.

  As will have been understood, the cells according to the invention comprise, on the one hand, a soluble anode ensuring the regeneration of the electrolyte according to a device known under the name of "anodic basket", or "panode", and other part of a cation exchange membrane, which contributes to maintaining the constancy of the composition

of the deposit.

  In an electroplating installation as described in the application for a utility certificate already mentioned, in the case where an electrolyte based on SO4 ions and a soluble anode in zinc or iron is used, problems of chemical shifts: the Zn ++ Fe- + ratio in the electrolyte is unbalanced and the operating pH is impaired. This problem can be solved by combining two cells, one with zinc electrodes, the other with iron electrodes. This two cells are mounted "in parallel" in the electrical circuit. The respective anode compartments of the two cells do not communicate. In contrast, both cells have their

  communicating cathode compartments.

  The invention will be better understood from the description which

  follows, with reference to the appended figures: FIG. 1 schematizes the principle of an electroplating installation according to the invention; FIG. 2 schematizes an electrolysis cell of the type

  preferentially used in the previous installation.

  FIG. 1 represents the schematic diagram of an installation for coating metal objects, such as electroplating plates, of the type comprising two associated cells and allowing deposit on the object to be coated with a mixed layer of iron and

  zinc in constant proportions.

  The cells 1 and 1 'are placed in two chambers connected in parallel in the electrical circuit of the electroplating installation. The total current density in the circuit is denoted J. The cell 1 comprises an iron anode 2, and is fed by a current A density JFe. The cell 1 'comprises a zinc anode 2' A and is fed with a current of density JZn. These two anodes are

A A

  connected to a common terminal 11il, and J = JFe + JZn The cathode of these two cells is common and consists of the part to be coated such as, for example, a steel sheet 3 scrolling. Each of these cells is provided with a cation exchange membrane 4a, 4'a, which divides the cell into two compartments, respectively anodic Sa, 'a, and cathodic 6a, 6a. These cells are not necessarily identical to each other. These cation exchange membranes

  (NEC) are of the same type (for example the commercialized membrane under

  the name NAFION by DUPONT DE NEMOURS) and are characterized by

  the iron and zinc migration yields Re and Rn of the compartment

  Anode 5a, 5'a to the cathode compartment 6a, 6'a of each cell. The Snodic compartments 5a, 5'a are independent of each other, whereas the cathode compartments 6a, 6'a communicate: the catholytes of the two compartments therefore have the same composition. The compositions of the anolytes and the catholyte are, in the general case, different. According to the indications given in the aforementioned Application for a Certificate of Utility, these compositions are chosen in such a way that the composition of the catholyte remains constant. The requirement, for this purpose, is that the faradic deposition efficiencies on the iron cathode, noted rF and zinc noted, ccc rZn (with r + rZ = 1) are respectively equal to the yields of Zn Fe Zn migration RFe and RZn of iron and zinc through ECMs 4a and 4'a respectively. In order for the system to work in a completely stationary state, ie for the composition of the anolyte to remain constant, it is necessary and sufficient that the anodic starting iron and zinc anode yields are respectively equal to the RFe and RZn migration efficiencies. This condition can be achieved by adjusting the feed current densities of the

A A

  two anodes JFe and JZn so that: JF ecRfFe-FeA and Jzn_rcR-Zn -Zn J This equilibrium condition makes it possible to ensure for the given operating conditions, the constancy of the compositions of the anolytes and the catholyte which is necessary to obtain a stable composition of

  deposition of iron and zinc on the product to be coated.

  Another necessary condition for the stability of the composition of the deposit on the cathode is the maintenance at a constant value of the distance between the cathode and the source of dations. In the case of conventional electrolysis cells with soluble anode devoid of MEK, the anode-cathode distance must be kept constant by mechanical means as the consumption of the anode. In the case of a cell with soluble anode equipped with an ECM, it is the distance between the membrane and the cathode which governs the operation of the

  cell, and it is easy to keep it constant by construction.

  The described example can be extended to the case where it is desired to deposit on the cathode product a coating comprising a number n of elements M. It is then necessary to use a number of cells n placed in parallel electrical, whose anode compartments are, in the general independent case and whose compartments communicate with each other. They are placed one after the other in the path of the scrolled product to be coated, and each cell i has an anode in a Ni element. In general, the densities A of current Ji in each branch of the circuit must obey the cn I relation ri with J = J i = l On the other hand, in such an installation, it is interesting to have cells allowing a change fast of the nature of the

  or metals passing in solution in the anode compartment.

  This condition can be fulfilled by using as "anode compartment" an "anode basket" or "panode", as shown in FIG. 2. This anode basket 7 is constituted by a container 8 made of insoluble conductive material, such as titanium, platinum, zirconium, etc. This container contains an A-fed anolyte and an electric current under a current density Ji.A, and forms an anode compartment 5 similar to that of one of the previously described cells. It is separated from the cathode compartment 6b by a cation exchange membrane 4b. The renewal of a deposition element Mi, which migrates through the membrane irreversibly, is ensured by the dissolution of at least one consumable block 9 of this element, acting as a soluble anode deposited in the anodic basket and whose contact with the wall of the container 8 is provided simply by gravity. In the lower part of the basket, is arranged. a lowered bottom receiving the consumable blocks 9, which makes it possible to better ensure the contact between the basket and the blocks

  They can be simulated while isolating them from the membrane.

  Advantageously, the container is coated with an insulating envelope of electricity 10. The upper part of the basket is provided with an openable extension 13, allowing the introduction, continuous or discontinuous, consumable blocks 9. In front of the membrane 4b is arranged a grid 14 made of a rigid material, which makes it possible to protect the membrane during the introduction of the consumable blocks 9. A tilting bottom 15 makes it possible, after draining the anolyte, to remove the blocks 9

  not yet dissolved at the end of treatment. As for the catholic compartment

  6, it is constituted as in the general case, the part in

  scroll to be coated 3 being placed in cathode.

  In this way, it is possible to modify the nature of the metals to be deposited simply by changing the nature of the consumable blocks 9, without changing the structure of the electrode. The heavy manipulations required when the soluble anode is integrated into the structure of the electrode are thus avoided. Likewise, the alternating use of a S0- electrolyte and of a Cl electrolyte, in which there are no chemical shifts between iron and zinc, is possible without requiring modifications of the installation other than

the change of electrolyte.

  The same anode basket may contain consumable blocks of different natures. If chemical shifts are to be avoided, one can, as we saw previously, use several cells - placed in parallel electric, whose baskets anodic

  each contain a different element.

  The invention is not limited to the examples which have just been described and shown. In particular in a cell, the element made of a conductive and insoluble material which is in contact with the block (s) to be dissolved can simply be immersed in the anode compartment without constituting its inner wall, and be formed by a

simple partition or receptacle.

  The invention applies to the continuous electrolytic coating of a metal product such as a steel plate by a plurality of

  of elements, for example by an iron-zinc or nickel-zinc alloy.

Claims (6)

REVENDICATIOUS   1) Anode unit (7) of an electrolysis cell for the continuous coating of a metal surface, such as a steel sheet, whose cathode (3) consists of the metal surface to be coated, cell of the type comprising a cation exchange membrane (4b) delimiting an anode compartment (Sb) and a cathode compartment (6b), characterized in that it forms the anode compartment (5b) of the cell and consists of a stacker comprising at least one anode basket (8), in that said membrane (4b) constitutes at least in part the face of the anode unit intended to be placed facing the surface to be coated (3), said anode basket being in an insoluble electroconductive material, and providing the electrical connection of the anode unit, means being provided so that said anode basket remains in permanent contact with at least one block (9) of the metal or metals to be deposited on the cathode which dissolve in the the electric rolyte, and means are also provided   to allow the introduction of these blocks into the anode basket.   2) anode unit according to claim 1, characterized in that the anode basket (8) constitutes the inner wall. said housing.   3) Anodic unit according to claim 1, characterized in that the electroconductive element providing the electrical connection of the anode unit constitutes a partition housed inside said casing. 4) Anodic unit according to claim 1 characterized in that the means for ensuring a permanent contact of the electroconductive element (8) making the electrical connection of the unit   anodic are constituted by a lowered bottom (12) of said casing.   ) Anodic unit according to claim 1 characterized in that the means for allowing the introduction of the blocks (9) of the metal.disposition on the cathode are formed by an openable extension (13) to the upper part of said boot.   6) Process for electrolytic coating of a metallic material, said coating, comprising a number n greater than 1 of elements Ki, with i = 1 & n, characterized in that: -on uses electrolysis cells comprising each a cation exchange membrane defining an anode compartment and a cathode compartment, and allowing dissolution in the anode compartment of one of the elements! i, the anode compartment being included in an anode unit according to claim 1, said cells being mounted in electrical parallel, their anode compartments being separately supplied with anolyte, their cathode compartments being fed in series by the catholyte, their common cathode being constituted by the product & coating, and, in order to maintain constant the compositions of the catholyte, the anolyte and the coating deposited on the surface, the compositions of the anolyte and the catholyte are selected and the JA current densities in each branch of the circuit such that: Ji A JA. c c n _ r Mi = i, rMi being the faradic efficiency of deposition i = 1 on the cathode of element Mi and RMi being the migration efficiency   of the element Mi through the cation exchange membrane.   7) Method according to claim 6, characterized in that one   uses two electrolysis cells, one of which allows dissolu-   iron and nickel, and the other (1 ') allowing the dissolution of zinc, in order to coat the product with an iron-zinc alloy - or zinc nickel.   8) Plant for carrying out the process according to claim 6 or 7, characterized in that the anode units of the   Electrolysis cells are of the type according to claim 1 or 2.