CS58692A3 - Electroplating process - Google Patents

Abstract

A steel strip (1) is electrolytically coated with a metal layer at least partly of zinc in an electrochemical cell having an insoluble anode (4), wherein the strip acts as cathode. The electrolyte in the cell contains chloride ions and has high conductivity, reducing the voltage required. To avoid production of chlorine gas, hydrogen in the form of a hydrogen gas or gas containing hydrogen is supplied to the anode (4). The anode (4) is selected so that the anode reaction: H2 -> 2H<+> + 2e<-> (V> takes place, this reaction (V) predominating at the anode over the reaction: 2Cl<-> -> Cl2 &uarr& + 2e<-> (IV). <IMAGE>

Description

- 1 -

Electrolytic plating method

Technical field

BACKGROUND OF THE INVENTION The present invention relates to a method for electroplating a strip steel with a layer which at least partially contains zinc in an electrochemical cell equipped with an insoluble anode. steel is usually continuous ·

BACKGROUND OF THE INVENTION In the known electrolytic plating process, a sulphate solution is universally used as an electrolyte. A conventional dimensional stationary rigid anode (DSA) is used as the non-soluble anode, e.g. it dissolves in the re-generator and thus replenishes in the electrolyte. The reactions occurring in this known method are as follows: o * - cathode reaction: Ζηώ + 2 e ~ —- ► Zn (I) - reaction at the anode: 2 HgO — 4 H + + - 4 e “+ OgtClI ). - reaction in regenerator: Zn + 2 Ή -> Zn · + - (III)

If, however, a zinc-coated banded oeel is used for the electrolytic plating of a zinc-coated cell with an insoluble anodelectrolyte that contains chloride, then the reaction (II) undergoes the following reaction: - anode reaction: 2 Cl &lt; IV)

Chlorine gas evolving in reaction (IV) means significant complications for the apparatus and for the efficiency of the process that it is possible to avoid the use of a chloride-containing electrolyte. However, some adequate chloride-containing electrolytes are nevertheless available.

SUMMARY OF THE INVENTION

The invention is directed to solving the above-mentioned problems and provides an improved method for electrolytic plating of zinc-containing strip steel to promote insoluble anode when using an electrolyte containing chloride.

The invention provides a method of electroplating a metal layer sintered steel comprising at least partially zinc in an electrolytic cell having an insoluble anode, wherein the strip steel acts as a cathode and the electrolyte in the cell contains chloride ions and hydrogen in the form of hydrogen gas or gas containing hydrogen is fed to the anode and an anode reaction takes place at the anode: 2 H + + 2 e "(V) said reaction (V) predominates at the anode above the reaction: 2 Cl '—► Clgf + 2 e' (IV) )

As found in reaction (V), which is the resultant reaction when hydrogen is fed to the anode, the reaction (IV) does not occur either at all or only to a small extent. As a result, the chlorine gas is no longer formed or is formed only to a small extent, so that an electrolyte containing chlorine ions can be used for production. The forming hydrogen ions are incorporated into the electrolyte of reactions (V) and can be used in the regenerator to aid in the addition of zinc ions, for example by reaction (III). The preferred reaction rate (V) at the anode is at least three times that of reaction (IV), most preferably the reaction (IV) at the anode is completely suppressed.

As an anode suitable for reaction (V), it is preferable to use a gas anode type gas diffusion gas diffuser, which is anodaphoretic, supported by a catalyst and is equipped to supply hydrogen gas or a hydrogen-containing gas, wherein directed further away from the cathode so that the gas comes into contact with the electrolyte in the pores of the anode and at the gas interface, the electrolyte and anode reacting (V) under the influence of the catalyst. In NL-A-8801511, a so-called gas diffusion anode is proposed for electroplating. This technical document focuses its interest on the displacement of the reaction (II) that occurs on the insoluble anode during the spinning process to increase the limited lifetime of the insoluble anode, which shortens due to the oxygen-forming corrosion. The present invention, which is presented herein, is directed to suppressing the reaction (IV) at the anode. The preferred voltage drop between the anode and the cathode is either 10 V or less than 10 V. The main advantage of the invention is that the chloride ion electrolyte can have a typical conductivity of approximately three times that of the sulfate ion electrolyte. Therefore, when using the chloride ion electrolyte, a voltage drop occurs at one and the same current density, which is about 15V less than using a sulfate ion electrolyte. With zinc coating, about 75 GWh per year is saved in the production of 300,000 tons per year, which means approximately 7,500,000 florins (Dutch currency) each year at the current price. This goes to the cost of the required gas pipeline. The invention described above is for the zinc coating of strip steel. However, the invention can also be used for plating a steel strip with a zinc-containing main component, and some of the zinc is replaced by another metal to improve the properties of the layer. Such an alloy is, for example, an alloy of nickel containing 10% to 15% nickel and a zinc-iron alloy containing 10% to 20% iron. In the case of the zinc-iron alloy, the present invention offers the additional advantage that there is no risk of formation of a FeCOH precipitate with respect to the fact that when chloride ion electrolyte is used in the chloride medium, no Pe ^ + cations are formed, which forms sulphate ions in the electrolyte.

The invention will be further illustrated by the following figures and by way of example only, but not by way of limitation.

Figure 1 shows an apparatus for electrolytically coating a strip steel with a zinc-containing layer according to the present invention, and Figure 2 shows a gas di-fusion anode in detail.

Referring to Figure 1, a zinc-plated sheet metal (1) is shown to be produced in an electrochemical cell (2) comprising a rotating cylindrical cathode (3) and an anode (4). Said anode (4) is, in the case shown in Figure 1, a radial type, but it may alternatively be a flat anode. The rotating cylindrical cathode (3) and the anode (4) are connected to a negative or a positive one, respectively. Stretched Pole (5) · - 4 - Strip steel passes a rotating cylindrical cathode that has cathode function in an electrochemical cell. The electrolyte is supplied to the intermediate container (6) through a gap between the rotating cylindrical cathode (3) and the anode (4) that flows between the electrodes. This entire file is located in the tank (7). There is a reaction (I) on the rotating cylindrical cathode (3), whereby a layer containing zinc of the electrolyte containing chloride anions is deposited on the surface of the belt-shell. The zinc depleted electrolyte used is collected at the bottom of the tank (7) and drawn through a pipe (8) to the circulation tank (9). From this tank, the electrolyte is pumped by means of a pump (10) to the pipes (11) and (12) to the regenerator (13) where zinc (14) dissolves and enriches it, more particularly zinc cations. , according to reaction (III). The zinc cation electrolyte thus enriched goes into the electrochemical cell line (15), the circulation tank (9) and the line (10) and (16). In the apparatus shown schematically in FIG. 1, gaseous hydrogen is formed as a result of reaction (III) and is collected in the regenerator (13), optionally washed and conveyed via line (17) to the anode (4) so specifically this anode site (4) was averted from cathode (J) for use in reaction (V). The anode (4) used in this apparatus is a gas diffusion hydrogen yes as already mentioned.

Figure 2 shows a scheme of the principle of gas diffusion of the hydrogen anode. The anode (4) has a hydrophobic portion (18) where it conducts a hydrogen-containing gas to the anode at a point which is removed from the cathode. This part is roughly porous. In a special device, this portion consists of an active carbon (19) dispersed in Teflon (20) and is used as a hydrophobic portion with a Carbon Felt (21) (Torag Piper) layer to aid the electrode carrier and assist conductivity. Further, the anode (4) has a hydrophilic fine-pored portion (22) with an electrolyte catalyst. In a specific embodiment, the hydrophilic portion of activated charcoal (23) is platinum-catalysed (24), 70- 120 K on the Teflon matrix and its thickness. (23). The action of the catalyst (24) produces H + ions on this surface. Hydrogen-containing gas can be hydrogen or a mixture of hydrogen with one or more gases, or a compound such as natural gas, etc. A gas based on hydrogen is preferred.

The invention will now be explained in more detail by way of Example. Example of the Invention Example In this Example, a zinc-containing strip of steel is electrolytically coated using apparatus shown in Figure 1 and Figure 2.

The steel strip had a width of 1,200 mm and the strip was moving at a speed of 100 m / min · Zinc coating was applied to this strip of steel at a rate of 70 g / m · In this production, an electrolyte containing zinc and chloride ions was used and had a value of The pH of about 2 řeba The consumption of H 2 hydrogen at the anode is about 32 kg per hour. This anode consists of porous graphics and platinum is the catalyst. Zinc is supplied to the electrolyte from the regenerator. Hydrogen gas evolving in the regenerator (after scrubbing) is supplied to the anode.

The electrolyte contains these substances at the following concentrations:

ZnCl 2 135 g / l

NaCl 230 g / l A1C13.6H2O 22.5 g / l giving the following ion concentrations:

Zn2 + 60 g / l

Cl “250 g / l 2

The current density is 200 A / dm, the anode / cathode distance is 2 mm and the voltage drop across the cell is 6 V.

The reaction (IV) at the anode is completely suppressed.

Similar methods are used for electrolytic plating at a current density in the range of about 100 A / dm to about 200 A / dm and in a voltage range of about 3 V to 10 V. The distance between the anode and cathode may be about 8 mm up to 12 mm. Typical conditions in this electroplating process - 6 - are: the width of the steel strip from about 1,000 mm to 1,600 mm, the speed at which the steel strip moves from about 70 m / min p to 200 m / min and the applied zinc layer about 30 to 100 g / m 2. The production capacity of such a zinc coating process on the production line can be about 300,000 tons of zinc-plated steel per year, determined by a current flow rectifier of 1,000 kA.

Industrial Applicability A preferred method of electrolytic plating of a zinc-containing sheet. This method is particularly suitable for large production capacities and for the continuous method of moving metal strip steel.

Claims (8)

A method for electrolytically plating lp-steel (i) with a metal layer which at least partially contains zinc in an electrochemical cell with an insoluble anode (4), wherein the steel strip acts as a cathode, characterized in that that the electrolyte in the cell contains chloride ions and hydrogen in the form of hydrogen gas or a gas that contains hydrogen, which is supplied to the anode (4), whereby the reaction proceeds: 2 H + + 2 e "(V) wherein said reaction (V) predominates at the anode above the reaction: 2 Cl" Cl 2f + 2 e "(IV) 2 · The process of claim 1, wherein the reaction rate (V) at the anode is at least three times the rate of reaction (IV) · Method according to claim 2, characterized in that the reaction (IV) at the anode is completely suppressed · 4. A process according to any one of claims 1 to 3, wherein said reaction (V) is an anode of the presence of a catalyst which acts on the reaction. Process according to one of Claims 1 to 4, characterized in that said anode is porous, carrying a catalyst and is equipped for supplying hydrogen or a gas containing hydrogen on a front face which is a reversal of said cathode, so that the gas is in contact with the electrolyte in the pores of said anode, and at the gas, electrolyte and anode interface, said catalyst (V) is subjected to said reaction. Method according to one of Claims 1 to 5, characterized in that the voltage drop between the anode and the cathode is greater than 10 V. - 8 Method according to any one of claims 1 to 6, wherein the supply of zinc cations to the electrolyte of the dissolution of the zinc in the hydrogen-producing regenerator is further incorporated, wherein hydrogen is supplied to the anode for the reaction (V). Method according to one of Claims 1 to 7, characterized in that said metal layer is selected from the group consisting of zinc, zinc-nickel and zinc-iron.