EP0493479B1

EP0493479B1 - Improvements in or relating to the electrodeposition of zinc or zinc alloy coatings

Info

Publication number: EP0493479B1
Application number: EP90914540A
Authority: EP
Inventors: Colin Roscoe
Original assignee: EA Technology Ltd
Current assignee: EA Technology Ltd
Priority date: 1989-09-22
Filing date: 1990-09-18
Publication date: 1994-04-27
Anticipated expiration: 2010-09-18
Also published as: WO1991004359A1; GB8921439D0; DE69008537D1; EP0493479A1

Abstract

A method for the electrodeposition onto a substrate of a coating of zinc and/or a zinc alloy which method comprises immersing in an electrolyte which contains ferrous ions and zinc ions at a pH of below 1.0, a non-consumable anode which is resistant to corrosion by ferric ions and the substrate which acts as the cathode, passing an electric current therebetween, the reaction at the anode comprising the conversion of ferrous ions to ferric ions, and concurrently with or after the electrodeposition passing the electrolyte to one or more regenerating tanks comprising metallic iron and/or metallic zinc as feedstock, the rate of dissolution of the feedstock being dependent upon the concentration of ferric ions in solution.

Description

The present invention relates to the electrodeposition of zinc or zinc alloy coatings.
It is known to electrogalvanise metals, such as steel strip or wire, using either a consumable zinc anode together with a variety of electrolytes, or using a non-consumable lead anode in conjunction with a sulphate based electrolyte.
In general terms, non-consumable anodes lend themselves to more flexible plant design and modes of operation. More specifically, their use simplifies the task of electrodespositing alloys having a narrow range of compositions. Conventionally in the electrodeposition of zinc alloy coatings the use of a non-consumable anode is associated with oxygen evolution as the anodic reaction.
We have now developed a method for the electrodeposition of zinc or zinc alloy coatings in which a non-consumable anode is used, but which does not involve the production of gaseous oxygen as the anodic reaction.
Accordingly, the present invention provides a method for the electrodeposition onto a substrate of a coating of zinc and/or a zinc alloy which method comprises immersing in an electrolyte which contains ferrous ions and zinc ions at a pH of below 1.0, a non-consumable anode which is resistant to corrosion by ferric ions and the substrate which acts as the cathode, passing an electric current therebetween, the reaction at the anode comprising the conversion of ferrous ions to ferric ions, and concurrently with or after the electrodeposition passing the electrolyte to one or more regenerating tanks comprising metallic iron and/or metallic zinc as feedstock, the rate of dissolution of the feedstock being dependent upon the concentration of ferric ions in solution.
The anode which is used in the method of the invention may be an anode of the type currently used in the chlorine industry. These generally consist of titanium having a coating of ruthenia/titania thereon. Other anodes suitable for use in the present invention are those incorporating platinum and iridium.
In the method of the present invention the cathodic reaction comprises the deposition of a zinc or zinc alloy coating, whilst the anodic reaction comprises the conversion of ferrous ions to ferric ions. The rate of production of ferric ions at the anode will be the same as the combined rate of deposition of zinc and any other metal at the cathode.
The electrolyte which contains ferrous ions and zinc ions has a pH of below 1.0 and preferably has a pH in the range of from 0.0 to 0.7. The acidity of the electrolyte solution prevents the hydrolysis of ferric ions to form ferric hydroxide. If it is desired to electrodeposit zinc as an alloy of zinc with one or more other metals then the electrolyte must contain cations of the other metal or metals. Metals which can be electrodeposited together with zinc include nickel, cobalt, chromium and iron which is, of course, already present in the electrolyte solution. The anion or anions for the electrolyte solution include chloride, sulphate, tetrafluoroborate, or mixtures thereof. Chloride is particularly preferred and is the anion of choice.
The concentration of the ferrous ions and zinc ions in the electrolyte may vary within wide limits. Preferably the concentration of ferrous ion will be in the range of from 0.5 to 3.0M, more preferably in the range of from 0.5 to 2.0M. The concentration of the zinc ions is preferably in the range of from 0.1 to 2.5M. The zinc ion and ferrous ion concentrations are generally linked so that when the ferrous ion concentration lies towards the lower end of the permitted range the zinc ion concentration will tend to lie towards the lower end of its range excepting that a very high zinc level is required in the alloy.
Zinc rich deposits containing typically less than 10% by weight of iron may be deposited from electrolytes having a ferrous ion concentration in the range of from 0.5 to 1.5M and a zinc concentration in the range of from 0.4 to 1.5M. If higher ferrous ion concentrations are used then an increase must be made, within the limits specified above, of the zinc ion concentration.
The method of the invention also encompasses the possibility of electrodepositing iron rich zinc-iron coatings. These iron rich coatings may be electrodeposited from electrolytes having compositions based upon the ferrous ion and zinc ion concentration ranges stipulated above. For example, at a ferrous ion concentration of 0.9M and a zinc ion concentration of 0.15M a coating containing 70% by weight zinc and 30% by weight of iron can be electrodeposited, whereas when the ferrous ion concentration is raised to 1.8M, the zinc ion concentration in the electrolyte has to be raised to 2.1M to produce the same coating. A zinc ion concentrate of 0.15M would under these circumstances yield an alloy containing from 85 to 90% of iron.
The temperature at which the electrodeposition of the invention is carried out is generally in the range of from ambient to 60°C but higher temperatures up to 80°C can be employed for the deposition of iron rich alloys. The temperature of the electrolyte is generally closely controlled to within ±1°C of a chosen value within the above stated range.
The current density is generally maintained within the range of 0.1A to 1.5Acm⁻², and is preferably maintained to within ±2% of a value lying within this range.
The cathode onto which the zinc or zinc alloy coating is deposited may be any article which it is deposited to coat. For example, the cathode may be steel in the form of strip, sheet, wire or sections.
The electrodeposited coating produced accordingly to the invention may have a thickness varying within wide limits for example in the range of from 0.5µ to 125µ. Coatings having a thickness in the range of from 1 to 10µ are particularly preferred for sheet material and coiled strip. Some specialised applications for steel wire demand the heavy coating weights.
The electrodeposition according to the method of the present invention is carried out in a single cell or in a plurality of cells. The rate of flow of the electrolyte through the cell or cells is generally controlled in order to prevent the build-up of ferric ions in the cell and therefore keeping the rate of chemical milling of the deposit by the ferric ions to an acceptable level. The spent electrolyte is then passed through one or more conditioning or regeneration tanks which contain metallic iron and/or metallic zinc, and optionally another metal as feedstock. The feedstock may be a low grade scrap material comprising the requisite assortment of metals, e.g. iron, zinc, etc. By way of example galvanised steel provides a low cost feedstock. The stripped steel being sold at premium rates for re-processing via a pyrometallurgical route. Furthermore, spent alkaline cells may also be used as a feedstock after a suitable pretreatment to remove the managanese dioxide and neutralise the alkali content. The feedstock may be fed into a common tank or may be fed into separate regeneration tanks. The feedstock may also contain another metal if the intention is to deposit a zinc alloy coating. Thus, if alloys of zinc with nickel, cobalt or chromium are desired the feedstock has to contain nickel, cobalt or chromium so as to provide the appropriate cations in solution. The rate of dissolution of the feedstock is dependent upon the concentration of ferric ions in solution, the ferric ions having been generated at the anode during the electrodeposition reaction. The ferric ions in solution react with the constituents of the feedstocks in a controlled way so as precisely to maintain the composition of the electrolyte. It is therefore possible to divert all or a part of the spent electrolyte to a particular tank and likewise to draw a particular predetermined volume of the feed electrolyte from a given tank, thereby maintaining the electrolyte compositions within a very close tolerance. The overall concentration of the electrolyte is virtually automatically controlled by the anodic reaction. It is, however, necessary to replenish the anions in the electrolyte solution to replace those lost from the electrodeposition by drag out and by spillages.
The present invention will be further described with reference to the following Examples.

Example 1

An electrolyte having a pH of 0.3 and concentration of 0.9M ferrous ions, 5 gram per litre of ferric ions and 45 grams per litre of zinc was maintained at 50°C and flowed through an electrolytic cell. The electrolytic cell comprises a ruthenised titania anode and a strip of steel as the cathode.
At a current density of 0.25Acm⁻¹, a coating comprising 96% by weight of zinc and 4% by weight of iron was deposited to a coating thickness of 2.5µ after a period of time of 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 2

The procedure of Example 1 was repeated except that the concentration of zinc ions in the electrolyte was only 10 grams per litre. A coating comprising 70% by weight of zinc and 30% by weight of iron was deposited to a thickness of 8µ in 90 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 3

The procedure of Example 1 was repeated except that the concentration of ferrous ions was 1.4M and the concentration of zinc ions was 45 grams per litre. A coating comprising 80% by weight of zinc and 20% by weight of iron was deposited to a thickness of 2.5µ in 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 4

The procedure of Example 1 was repeated except that the concentration of ferrous ions was 1.4M and the concentration of zinc ions was 25 grams per litre. A coating comprising 70% by weight of zinc and 30% by weight of iron was deposited to a thickness of 2.5µ in 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 5

The procedure of Example 1 was repeated except that the concentration of ferrous ions in the electrolyte was 1.8M and the concentration of zinc ions 120 grams per litre. A coating comprising 65% by weight of zinc and 35% by weight of iron was deposited to a thickness of 8µ in 90 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 6

The procedure of Example 5 was repeated except that the concentration of zinc ions was adjusted to 90 grams per litre. A coating comprising 50% by weight of zinc and 50% by weight of iron was deposited to a thickness of 2µ in 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 7

The procedure of Example 1 was repeated except that the concentration of ferrous ions was 2.5M and the concentration of zinc ions was 90 grams per litre. A coating comprising 27% by weight of zinc and 73% by weight of iron was deposited to a thickness of 2.0µ in 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Example 8

The procedure of Example 1 was repeated except that the concentration of ferrous ions was 2.9M and the concentration of zinc ions was 60 grams per litre and the electrolyte temperature was raised to 75°C. A coating comprising 15% by weight of zinc and 85% by weight of iron was deposited to a thickness of 2.0µ in 30 seconds. The ferric ions generated at the anode being converted back to ferrous ions by contact with a metallic mixture or iron and zinc held in an electrolyte tank, thereby maintaining the desired electrolyte composition.

Claims

A method for the electrodeposition onto a substrate of a coating of zinc and/or a zinc alloy which method comprises immersing in an electrolyte which contains ferrous ions and zinc ions at a pH of below 1.0, a non-consumable anode which is resistant to corrosion by ferric ions and the substrate which acts as the cathode, passing an electric current therebetween, the reaction at the anode comprising the conversion of ferrous ions to ferric ions, and concurrently with or after the electrodeposition passing the electrolyte to one or more regenerating tanks comprising metallic iron and/or metallic zinc as feedstock, the rate of dissolution of the feedstock being dependent upon the concentration of ferric ions in solution.
A method as claimed in claim 1 wherein the anode is titanium having a coating of ruthenia/titania thereon.
A method as claimed in claim 1 or claim 2 wherein the electrolyte is maintained at a pH in the range of from 0.0 to 0.7.
A method as claimed in any one of the preceding claims wherein the concentration of ferrous ions in the electrolyte is in the range of from 0.5 to 3.0M.
A method as claimed in claim 4 wherein the concentration of ferrous ions in the electrolyte is in the range of from 0.5 to 2.0M.
A method as claimed in any one of the preceding claims wherein the concentration of zinc ions in the electrolyte is in the range of from 0.1 to 2.5M.
A method as claimed in any one of the preceding claims wherein the electrolyte additionally contains cations of one or more of nickel, cobalt and/or chromium and the feedstock additionally contains the appropriate metal to provide the said cations.
A method as claimed in any one of the preceding claims wherein the electrodeposition is effected at a temperature in the range of from ambient to 80°C.
A method as claimed in any one of the preceding claims wherein the electrodeposition is effected at a current density in the range of 0.1A to 0.5Acm⁻².
A method as claimed in any one of the preceding claims wherein the substrate is in the form of a strip, sheet, wire or section.
A method as claimed in any one of the preceding claims wherein the anion or anions for the electrolyte solution comprises chloride, sulphate, tetrafluoroborate, or a mixture thereof.
A method as claimed in any one of the preceding claims wherein the electrodeposited coating has a thickness in the range of from 0.5 to 125µ, preferably 1 to 10µ.
A method as claimed in any one of the preceding claims wherein the feedstock is a low grade scrap material comprising the requisite assortment of metals.
A method as claimed in any one of the preceding claims wherein the anions in the electrolyte are also replenished.