Method for the electrolytic deposition of chromium and chromium alloys

Abstract

The invention relates to a method for depositing chromium and/or chromium alloys on metals, and particularly on sheet steel, wherein an alloy layer is electrolytically deposited on the metal from a solution containing chromium ions and/or chromium ions and further metal ions such as Zn, Cd, In, Pb, Bi, Mo, Cu, Fe, Ni, Co, Mn, Al, Sb, Ag, Sn, Mg, wherein chromium hydroxide precipitated from a chromium (III) solution is used for regenerating the electrolyte solution using chromium (III) ions.

Description

FIELD OF THE INVENTION

The invention relates to a method for the deposition of chromium and chromium alloys on metals, in particular on steel sheets.

BACKGROUND OF THE INVENTION

EP 0 285 931 A1 has disclosed a corrosion-resistant coated steel sheet and a method for manufacturing it. In the method according to this prior art, a zinc/chrome-based alloy, which contains a quantity of more than 5 wt. % but no more than 40 wt. % chromium and whose remainder is composed of zinc, is deposited on at least one side of the steel sheet. In the method, zinc ions and trivalent chromium ions or a mixture of trivalent chromium ions with a mixture of at least one metal from the iron family and zinc are deposited from an acidic electrolyte solution. This solution should contain approximately 10 g/l-150 g/l of zinc ions, 10 g/l-100 g/l of chromium ions, and 10 g/l-100 g/l of ferrous metal ions. The content of zinc and chromium ions here should be from 0.2 mol/l-3 mol/l. The anions are either sulfate or chloride ions, with the addition of a complexing agent and an antioxidant. In order to deposit a zinc/chromium alloy layer that contains more than 5 wt. % of chromium, it should be necessary to maintain the zinc content and the chromium ion content in the coating solution at a requisite high level. If trivalent chromium ions in the form of chromium sulfate or chromium chloride are introduced into the coating solution, this increases the sulfate or chloride content, which interferes with the coating process. Trivalent chromium ions cannot be introduced in the form of chromium oxide or metallic chromium here because these substances are not soluble, even at very low pH values. Trivalent chromium ions could be introduced into the coating solution in the form of chromium hydroxide or chromium carbonate. Both compounds, however, are unstable and transform into insoluble chromium oxide as they age.

In the method according to EP 0 285 931 A1, the trivalent chromium ions are introduced by mixing metallic zinc and an aqueous solution of hexavalent chromium with the acidic coating solution, which contains zinc ions and trivalent chromium ions. In this case, metallic zinc dissolves and the hexavalent chromium ions are transformed into trivalent ones.

This method has the disadvantage of requiring the use of hexavalent chromium solutions. But the use of hexavalent chromium has been sensibly restricted for environmental reasons.

JP 08 246 198 A has disclosed adding chromium ions to a corresponding coating solution by reducing Cr (VI) with hydrogen peroxide. This, too, requires the use of hexavalent chromium, which is unacceptable. Moreover, the use of hydrogen peroxide results in a powerful reaction that is too dangerous for use on an industrial scale.

JP 80 91 842 A has therefore disclosed using organic reagents to reduce Cr (VI) ions to Cr (III) ions, but this is also unacceptable due to the presence of Cr (VI) ions in the process. In addition, the electrolyte is mixed with undesirable organic reagents.

JP 08 049 099 A has disclosed electrolytically dissolving metallic chromium in a process that requires destruction of the passive film through mechanical contact with zinc, magnesium, or aluminum. The disadvantage here is that this is particularly expensive. In summary, the prior art either uses Cr (VI) or carries out an enrichment with other elements that likewise have a negative impact on the electrolytic coating.

DE 10 2006 035 871 B3 has disclosed a method for depositing chromium layers in the form of a hard chromium plating, an electroplating bath, hard chromium plated surfaces, and their use. In this case, an ammonium chromium sulfate, chromium chloride, and chromium sulfate are added to the electroplating bath. The reference mentions that a metered addition and removal of sulfuric acid is required; chromium must be removed from the electrolyte during the chromium plating and subsequently dissolved in the form of a sulfate or chloride. The enrichment with sulfate in this case must be prevented through complicated separation of the anode chamber and cathode chamber with continuous removal of acid (pH value regulation). The resulting constant rise in sulfate concentration leads to the complete replacement of the bath after a certain time.

DE 195 23 307 A1 has disclosed a chromium plating method using trivalent chromium in which a trivalent chromium-containing chromium plating bath and an electrode are used. The trivalent chromium should be selected from the group comprised of chromium (III) sulfate, chromium (III) chloride, chromium (III) oxalate, chromium (III) carbonate, and chromium (III) hydroxide. It should, however, be noted that chromium (III) hydroxide is not commonly used and is not even available on the market because it is not stable. In addition, DE 195 23 307 A1 mentions that trivalent chromium chloride is added to chromium; also, the testing of the pH value with sodium hydroxide does not hinder the enrichment of chlorides in electrolytes.

DE 35 30 223 C2 relates to an acidic bath for galvanic depositing of alloys of chromium with at least one of the metals iron, nickel, or cobalt; chromium (III) salts are also mentioned here, but not chromium hydroxide. This reference also does not disclose a solution to the problem of sulfate enrichment.

DE 26 57 012 C2 relates to a galvanic chromium bath; the chromium plating in this prior art is carried out as a piece-by-piece chromium plating, with the baths being replenished with the required reagents until they are no longer to be used and are discarded completely. This is not possible with continuous processes, where the concentration of anions and cations must remain constant.

DE 24 57 582 C3 relates to an aqueous, acidic galvanic chromium bath based on chromium (III), containing trivalent chromium, bromide ions, ammonia ions, and acid-containing anions of complexing substances. For example, the chromium can be added in the form of chromium (III) chloride or chromium (III) sulfate, using chromium tanning liquor, e.g. in the form of a three-percent alkaline chromium sulfate solution obtained by reducing sodium dichromate with sulfur dioxide as a suitable chromium source. Other suitable salts include chromium formate or acetate.

The object of the invention is to disclose a method for electrolytically depositing chromium and chromium alloys on metals, which is not problematic in terms of environmental protection and workplace safety and reliably permits continuous depositing of chromium and chromium alloys with no negative impact on quality.

SUMMARY OF THE INVENTION

The method according to the invention is described by way of example below in the context of a deposition of a zinc/chromium alloy on a steel sheet:

As explained above, for the deposition of zinc/chromium, it is necessary to continuously replenish the electrolyte with zinc and chromium during production in order to assure uniform production results. In this case, due to the acidic pH value, zinc can easily be dissolved metallically in the electrolyte. Metallic chromium does not dissolve in the acidic electrolyte; Cr (III) ions are required for the deposition, though. There is, however, no suitable Cr (III) compound that dissolves well in the electrolyte and does not introduce foreign anions as in the prior art. Chromium oxide does not dissolve well and is not suitable. Chromium sulfate does in fact dissolve, but enriches the electrolyte with sulfate; in the case of alkaline chromium sulfate, a sodium enrichment also occurs. Chromium (VI) compounds can no longer be used for reasons of workplace safety and environmental protection. Reducing CrO₃ with hydrogen peroxide, which is also known, is too dangerous.

According to the invention, chromium hydroxide (Cr(OH)₃) freshly precipitated from Cr (III) ions is used to replenish the electrolyte solution. Cr(OH)₃ is not stable and after a certain amount of time, transforms into insoluble chromium oxide. In order to be able to control this reaction, with the method according to the invention, zinc oxide and possibly a liquor (ammonia, caustic soda solution, and the like) is/are added to a solution containing alkaline chromium sulfate or another water-soluble chromium (III) compound such as chromium potassium sulfate, ammonium chromium sulfate, chromium chloride, chromium acetate, chromium nitrate, and the like to produce a cake composed of zinc oxide and chromium hydroxide. The zinc oxide provides the increase in pH value required for the precipitation, as well as a good filterability of the filter cake.

In case of a deposition of chromium and chromium alloys without zinc, the addition of zinc oxide is omitted and the pH value is raised using other suitable substances.

This precipitate of zinc oxide and chromium hydroxide is separated out from the precipitation solution directly after the precipitation and is dissolved in the electrolyte thus raising the zinc and chromium concentration, but avoiding the formation of insoluble chromium oxide. In a simple and easy-to-control way, this introduces chromium (III) ions from the original chromium (III) solution into the electrolyte without also introducing an undesirable anion.

In this case, it is advantageous that the chromium (III) compound used does not have to be of high purity since the precipitation process purifies the product. Alkaline chromium sulfate is an easy-to-obtain mass-produced product. The sodium that it contains in addition to the sulfate likewise does not negatively impact this process since it remains in the filtrate and is completely separated out by the precipitation.

A typical alkaline chromium sulfate with an alkalinity of 33% has the following typical composition in terms of oxides:

24%-26% Cr₂O₃,

25%-27% SO₃,

22%-25% Na₂SO₄

22%-25% H₂O.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained by way of example in conjunction with a diagram; the sole FIGURE is a very schematic flowchart of the process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first reaction vessel 1, for example a catch basin, water, zinc oxide, and alkaline chromium sulfate are mixed; the initial solution is taken to represent 100% portions of zinc, chromium, sodium, and sulfate ions. This mixture is appropriately stirred and the precipitation reaction occurs. The supernatant solution and lime sludge are fed via a pump 2 to a filter system. The sludge can also be separated from the filtrate by means of sedimentation, centrifugation, or another solid/fluid separation method. The lime sludge and the supernatant solution are separated in the filter system, the filtrate—i.e. the supernatant solution—is separated out and of the initial quantity of 100% each, still contains 24% zinc, the 0% chromium, 100% sodium, and 65% SO₄. The resulting filter cake, after a single washing, contains 76% zinc, 100% chromium, 0% sodium, and 25% SO₄ as compared to the initial solution. The sulfate quantity can be further reduced through additional washing of the cake. The resulting filter cake is added to the electrolyte vessel 4 or more precisely, to the electrolyte, and is dissolved therein, if need be with the aid of mechanical devices.

The dissolution of the filter cake in the electrolyte can occur in vessels that do in fact communicate with the electrolyte vessels in which the actual deposition is carried out, but are spaced apart from them and situated in such a way relative to the supply line diameter that the dissolution of the filter cake does not interfere with the deposition. It is then possible to provide pumps between such a dissolving vessel (not shown) and the actual electrolytic bath so that a continuous exchange takes place, for example in the form of a flow circuit. The required replenishment of Cr (III) ions during the deposition is determined and the filter cake is produced in a correspondingly discontinuous way, but if possible, is furnished in such a way that after a first filter cake is dissolved, the next respective filter cake is ready to be dissolved, but does not yet show any signs of aging due to its storage time.

The washed filter cake is prevented from drying. This makes it possible to also bridge interruptions of the production process without aging of the precipitated chromium hydroxide.

An advantage of the invention is the fact that with an acceptably low equipment cost and the use of commercially available, easily obtainable chemicals, a continuous replenishment of electrolyte solutions for the deposition of chromium and chromium alloys is achieved, which is not problematic in terms of environmental protection and workplace safety.

Claims

1. A method for depositing chromium and/or chromium alloys on metals and particularly on steel sheets, comprising:

electrolytically depositing an alloy layer on the metal from a solution containing chromium ions alone and/or chromium ions and other metal ions selected from the group consisting of Zn, Cd, In, Pb, Bi, Mo, Cu, Fe, Ni, Co, Mn, Al, Sb, Ag, Sn, and Mg, and using chromium hydroxide precipitated from a chromium (III) solution to replenish the electrolyte solution with chromium (III) ions.

2. The method as recited in claim 1, wherein the time between the precipitation of the chromium hydroxide from the chromium (III) solution and the introduction of the precipitated chromium hydroxide into the electrolytic solution is calculated so that the chromium hydroxide does not transform into a poorly soluble product.

3. The method as recited in claim 1, comprising using an aqueous chromium (III) compound selected from the group consisting of alkaline chromium sulfate, chromium potassium sulfate, ammonium chromium sulfate, chromium chloride, chromium fluoride, chromium acetate, and chromium nitrate to produce the chromium (III) solution.

4. The method as recited in claim 1, comprising using alkaline chromium sulfate as the soluble chromium (III) compound having the following composition in terms of oxides:

20%-45% Cr2O3,

20%-45% SO3,

15%-40% Na2SO4

10%-60% H2O.

5. The method as recited in claim 1, comprising mixing in a first reaction vessel, water and the soluble chromium (III) compound, comprising alkaline chromium sulfate, with each other and then through the addition of an alkaline substance, raising the pH value in the solution and precipitating out chromium (III) hydroxide.

6. The method as recited in claim 1, comprising mixing in a first reaction vessel, water, zinc oxide, and the soluble chromium (III) compound, comprising alkaline chromium sulfate, with one another and then through the addition of zinc oxide or other alkaline substances, raising the pH value in the solution and precipitating out chromium (III) hydroxide.

7. The method as recited in claim 1, comprising filtering out the chromium hydroxide, together with excess ZnO that acts as a filtration agent, from sludge and then washing the chromium hydroxide.

8. The method as recited in claim 7, comprising reducing a remaining sulfate content through single or multiple washing of the sludge; filtering the sludge and adding a filter cake composed of zinc oxide and chromium hydroxide to the electrolyte solution for the deposition of zinc chromium on metallic surfaces.

9. A use of chromium hydroxide, which is precipitated from alkaline chromium sulfate by raising the pH value, for the replenishment of electrolytic solutions with chromium (III) ions.