Method of electrolytically coating a steel strip with a metal layer at least partly of zinc

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 ↑ + 2e⁻ (IV).

Description: The invention relates to a method of electrolytically coating a steel strip with a layer at least partly consisting of zinc in an electrochemical cell provided with a non-soluble anode, wherein the layer containing zinc is deposited onto the steel strip which is connected as the cathode. The strip is usually moving continuously. A strip-coating method as described above is in use. In the known method, an electrolyte with a sulphate environment is universally used. A conventional dimension stable anode (DSA) is used as non-soluble anode, for example consisting of titanium with a catalytic coating. Zinc is dissolved in a regenerator, to replenish the electrolyte. Reactions occurring in the known method are: .at the cathode: Zn²⁺ + 2e⁻ → Zn (I) .at the anode: 2H₂O → 4H⁺ + 4e⁻ + O₂ ↑ (II) .in the regenerator: Zn + 2H⁺ → Zn²⁺ + H₂ ↑ (III) However, if an electrolyte containing chloride is used for electrolytically coating a steel strip with a zinc layer in a cell with a non-soluble anode, then instead of reaction (II) the following reaction occurs: .at the anode: 2Cl⁻ → Cl₂ ↑ + 2e⁻ (IV) The chlorine gas formed in reaction (IV) creates such great complications in the apparatus and for efficient processing, that the use of an electrolyte containing chloride ions has to be avoided. Suitable chloride-containing electrolytes are nevertheless available. The intention aims to solve the problem discussed above and to provide an improved method for electrolytically coating a steel strip with a layer containing zinc with the aid of a non-soluble anode while using an electrolyte containing chloride. According to the invention, there is provided a method of electrolytically coating a steel strip with a metal layer at least partly of zinc in an electrochemical cell having an insoluble anode, wherein the strip acts as cathode and the electrolyte in the cell is a chloride electrolyte and hydrogen in the form of a hydrogen gas or gas containing hydrogen is supplied to the anode, the anode being such that the anode reaction: H₂ → 2H⁺ + 2e⁻ (V) takes place, said reaction (V) predominating at the anode over the reaction: 2Cl⁻ → Cl₂ ↑ + 2e⁻ (IV) thereby reducing the voltage drop in the cell due to the higher conductivity of the chloride electrolyte. It has been found that, because of reaction (V) resulting from the supply of hydrogen to the anode, reaction (IV) does not occur or is minor. Consequently chlorine gas no longer forms or forms only slightly so that it is possible to make use of an electrolyte containing chloride ions. The hydrogen ions formed become incorporated into the electrolyte by reaction (V) and may be used in a regenerator for supplementing the zinc in the electrolyte, for example according to reaction (III). In a preferred embodiment of the invention, hydrogen gas formed during reaction (III) in a regenerator is collected and then supplied to the anode for the reaction (V). Preferably the rate of reaction(V) at the anode is at least three times the rate of reaction (IV). Most preferably, the reaction (IV) at the anode is wholly suppressed. As an anode suitable for causing reaction (V), it is preferable to use an anode of the hydrogen gas diffusion anode type, in which the anode is a porous anode carrying a catalyst and has means for feeding the hydrogen gas or gas containing hydrogen to a face of the anode directed away from the cathode, so that the gas contacts the electrolyte in pores of the anode and at the boundary of the gas, the electrolyte and the anode the reaction (V) takes place under the influence of the catalyst. It is mentioned that in Dutch patent application NL-A-8801511, it has been proposed to use a so-called gas diffusion anode in an electrode deposition process. This prior art document is concerned with suppression of the reaction (II) occurring at the non-soluble anode during tinplating in order to improve the limited service life of the non-soluble anode which was shortened as a result of corrosion by the oxygen formed. However, the present invention is concerned with suppression of reaction (IV) at the anode. Preferably the voltage drop between the anode and the cathode is equal to or less than 10 V. The advantage obtained with the invention consists primarily in that the electrolyte containing chloride ions can typically have conductivity approximately three times higher than the electrolyte with sulphate ions. For this reason when a chloride electrolyte is used, at one and the same current density, a voltage drop occurs which is approximately 15 V less than when a sulphate electrolyte is used. In a zinc-coating line with a 300,000 ton per annum production this means a saving in electricity of approximately 75 GWh per annum, representing a saving of approximately FL 7,500,000.-- (Netherlands currency) each year at the present electricity price. This takes into account the costs of the hydrogen gas required. The invention is described above for coating a steel strip with a layer of zinc. However, the invention may equally be used for coating a steel strip with a layer having zinc as the main constituent, in which a part of the zinc is replaced by another metal in order to improve the properties of the layer. Examples of such alloys are zinc-nickel with 10-15% nickel and zinc-iron with 10-20% iron. In the case of zinc-iron the invention offers the additional advantage that there is no risk of Fe(OH)₃ precipitation, in view of the fact that, if a chloride ion electrolyte with a chloride environment is used, no Fe³⁺ ions form, whereas they do form in a sulphate ion electrolyte. The invention will now be illustrated by reference to the drawings and by a non-limitative Example. In the drawings:- Fig. 1 shows an apparatus for electrolytically coating a steel strip with a layer containing zinc by a method of the invention; and Fig. 2 shows a detail of a gas diffusion anode. Fig. 1 shows a steel strip 1 coated with a layer containing zinc being produced in an electrochemical cell 2, comprising a rotating cathode roller 3 and an anode 4. The anode 4 shown in Fig. 1 is a radial type anode but it may alternatively be flat. The roller 3 and the anode 4 are connected to the negative and positive poles respectively of voltage supply 5. This permits steel strip 1 passing around the roller to function as the cathode in the electrochemical cell. Electrolyte is supplied at 6 at the gap between the cathode roller 3 and anode 4 and flows along this gap. The whole assembly is placed in a tank 7. On the cathode roller 3 reaction (I) causes a layer containing zinc to precipitate from the electrolyte, which contains chloride anions on to the steel strip. The used and zinc-ion impoverished electrolyte is collected at the bottom of the tank and taken by means of a pipe 8 to a circulation tank 9. From there the electrolyte is conveyed by a pump 10 through pipes 11 and 12 to a regenerator 13 where zinc 14 dissolves and enriches the electrolyte with zinc ions according to the reaction (III). The electrolyte thus enriched is taken to the electrochemical cell by means of a pipe 15, the circulation tank 9 and the pipes 11 and 16. In the apparatus shown in Fig. 1 the hydrogen gas formed and collected as a result of reaction (III) in the regenerator 13, optionally following scrubbing, is conveyed by means of a pipe 17 to the anode 4 and specifically to that side of anode 4 facing away from the cathode 3 for use in the reaction (V). The anode 4 used in the apparatus is a hydrogen gas diffusion anode which is described below. Fig. 2 shows the principle of a hydrogen gas diffusion anode. The anode 4 has a hydrophobic part 18 where the gas containing hydrogen is taken to the anode on the side of the anode facing away from the cathode. This part has coarse pores. In a specific embodiment the hydrophobic part consists of active carbon 19 held in a Teflon matrix 20 and the hydrophobic part is provided with a layer of Carbon Felt 21 (Torag paper) to help support the electrode and to help conductivity. Further, the anode 4 has a hydrophilic part 22 with fine pores and a catalyst on the electrolyte side. In a specific embodiment the hydrophilic part consists of active carbon 23 charged with platinum 24 as catalyst, in a Teflon matrix and is 70 to 120 µm thick. The reaction (V) takes place in the fine pores at the three-way boundary surface of hydrogen gas, electrolyte and the active carbon 23. Under the influence of the catalyst 24 H⁺ ions form at this boundary surface. The gas containing hydrogen may be hydrogen or a mixture of hydrogen with one or more other gases or a compound of hydrogen such as natural gas, for example. However, preference is given to a gas that consists essentially of hydrogen. EXAMPLE In this example a steel strip is coated electrolytically with a layer containing zinc using the apparatus as shown in Figs. 1 and 2. The strip width is 1200 mm and the strip velocity of 100 m/min. A zinc coating weight of 70 g/m² is applied. Use is made of an electrolyte containing zinc and chloride ions at a pH of about 2. The H₂ consumption at the anode is about 32 kg per hour. The anode consists of porous graphite and the catalyst on it is Pt. Zn is supplied into the electrolyte in the regenerator. The hydrogen gas released in the regenerator is supplied (after a scrubbing process) to the anode. The concentrations in the electrolyte are: ZnCl₂ 135 g/l NaCl 230 g/l AlCl₃.6H₂O 22.5 g/l providing the ionic concentrations Zn²⁺ 60 g/l Cl⁻ 250 g/l The current density is 200 a/dm², the anode/cathode spacing is 2 mm and the voltage drop across the cell is 6V. The reaction (IV) at the anode is wholly suppressed. Similar processes were successfully performed at the current density ranges from 100 to 200 A/dm² and the voltage range from 3 to 10 V. The anode/cathode spacing may be as much as 8 to 12 mm. Typical conditions for such a process are a strip width of 1,000 to 1,600 mm, a strip velocity of 70 to 200 m/min and a coating thickness of 30 to 100 g/m² of zinc. On this basis, a zinc coating production line may have a capacity of approximately 300,000 tons of zinc-coated steel strip per annum, which is determined by the current rectifier capacity of 1,000 kA.

Claims: Method of electrolytically coating a steel strip (1) with a metal layer at least partly of zinc in an electrochemical cell having an insoluble anode (4), wherein the strip acts as cathode, characterized in that the electrolyte in the cell is a chloride electrolyte and contains hydrogen in the form of a hydrogen gas or gas containing hydrogen is supplied to the anode (4), the anode (4) being such that the anode reaction: H₂ → 2H⁺ + 2e⁻ (V) takes place, said reaction (V) predominating at the anode over the reaction: 2Cl⁻ → Cl₂ ↑ + 2e⁻ (IV) thereby reducing the voltage drop in the cell due to the higher conductivity of the chloride electrolyte. Method according to claim 1 wherein the rate of reaction (V) at the anode is at least three times the rate of reaction (IV). Method according to claim 2 wherein the reaction (IV) at the anode is wholly suppressed. Method according to any one of claims 1 to 3 wherein said reaction (V) takes place at the anode under the influence of a catalyst. Method according to any one of claims 1 to 4 wherein said anode is a porous anode carrying a catalyst and has means for feeding said hydrogen gas or gas containing hydrogen to a face of said anode directed away from said cathode, so that the gas contacts the electrolyte in pores of said anode and at the boundary of the gas, the electrolyte and the anode said reaction (V) taking place under the influence of said catalyst. Method according to any one of claims 1 to 5 wherein the voltage drop between anode and cathode is not more than 10V. Method according to any one of claims 1 to 6 further including, supplying zinc ions to the electrolyte by dissolving zinc therein in a regenerator with production of hydrogen, which hydrogen is supplied to the anode for reaction (V). Method according to any one of claims 1 to 7 wherein said metal layer is selected from zinc, zinc-nickel and zinc-iron.

Inventors: MOOIJ JOOP NICOLAAS, VAN HAASTRECHT GIJSBERTUS CORN, MOOIJ, JOOP NICOLAAS, VAN HAASTRECHT, GIJSBERTUS CORNELIS
Applicants: HOOGOVENS GROEP BV, HOOGOVENS GROEP B.V.
Application year: 1992
Main classification: C25D3
IPC codes: C25D3, C25D21, C25D7