EP1570115A2

EP1570115A2 - Method for the electrolytic deposition of magnesium on galvanised sheet metal

Info

Publication number: EP1570115A2
Application number: EP03767590A
Authority: EP
Inventors: Horst MITTELSTÄDT; Stefan WIENSTRÖER; Cetin Nazikkol; Bernd Schuhmacher; Christian Schwerdt
Original assignee: ThyssenKrupp Stahl AG; ThyssenKrupp Steel AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2002-12-10
Filing date: 2003-11-21
Publication date: 2005-09-07
Anticipated expiration: 2023-11-21
Also published as: AU2003292060A8; WO2004053203A3; DE10257737B3; ATE323186T1; PL375788A1; WO2004053203A2; ES2261976T3; EP1570115B1; DE50302990D1; AU2003292060A1

Abstract

Process for the electrolytic magnesium deposition on a substrate (1) made from sheet metal with a zinc or zinc alloy coating comprises carrying out the electrodeposition in a solvent (3a) having a lower acidity than water and heat treating the coated substrate to form a magnesium-zinc alloy phase in the zinc layer.

Description

Process for electrolytic magnesium deposition on galvanized sheet

The invention relates to a method for electrolytic magnesium deposition on a substrate made of sheet metal with a zinc or zinc alloy coating, in particular steel sheet.

In the automotive industry there is a great need for materials with high corrosion resistance and at the same time good processing properties. The galvanizing of body panels made of steel (hot-dip process or electrolytic coating) for the purpose of corrosion protection has largely become established in recent decades. The steel sheets galvanized using the hot-dip process or by means of electrolytic deposition are distinguished by good adhesion of the zinc layer to the steel sheet and good workability.

As an alternative to the galvanizing process, significantly improved corrosion properties can be achieved by applying a magnesium layer on the uncoated steel sheet. When a magnesium-coated steel sheet is stored in air, the magnesium layer is immediately oxidized, which means that the sheet surface is passivated. Accordingly, the steel underneath is no longer attacked. Disadvantageous Magnesium-coated sheets, however, are their increased surface roughness compared to galvanized steel sheets due to the formation of the oxide layer.

Various processes for magnesium deposition on galvanized steel sheet are known from the prior art.

JP 62109966 A describes a steel sheet, on the surface of which a zinc layer is first applied and then a magnesium layer, each by vapor deposition in a vacuum chamber. Due to the formation of an oxide layer on the magnesium surface, the sheet has very good corrosion properties. However, the vacuum process is problematic because it requires a very high level of equipment and process engineering. Furthermore, vacuum-coated sheets have a non-optimal adhesive strength of the magnesium layer.

DE 195 27 515 Cl specifies a method for producing corrosion-protected steel sheet. A layer of one or more metals other than zinc or a zinc-free alloy is applied to a galvanized steel sheet by means of vacuum coating. The sheet metal coated in this way is then subjected to a heat treatment without exposure to an oxidizing atmosphere. As a result, a diffusion layer is formed on the surface of the double-coated steel sheet from the metal or alloy applied by vacuum coating and the zinc underneath. so Coated sheets are characterized by a good surface quality and high corrosion protection. In addition, due to its small thickness, the diffusion layer is sufficiently ductile relative to the thickness of the zinc coating to ensure that the steel sheet can continue to be formed well. The central disadvantage of this method is the high level of equipment and process technology associated with vacuum coating.

DE 100 39 375 AI also describes a method for producing corrosion-protected steel sheet. This is similar to the process described in DE 195 27 515 Cl in that the magnesium is deposited from the vacuum phase on a galvanized or alloy-galvanized steel sheet to be subsequently heat-treated. In this case, however, the heat treatment is controlled so that there is a zinc-magnesium phase formation over the entire thickness of the zinc coating. This leads to a coating with the positive properties described above with a further improved corrosion protection. However, the disadvantages of vacuum coating mentioned above still exist.

The direct deposition of Zn-Mg or Zn-Mg-Al alloy coatings on the surface of a steel sheet as a complete layer in the hot-dip process, described for example in EP 0 905 270 A2 and US 3,505,043, also involves considerable technical difficulties. The weld pool, especially compliance with a constant Mg content, can only be managed with great technical effort due to the high formation tendency of Mg slag formation and the inevitable combustion. In addition, the surface quality of the coatings is only low, so that the possible areas of use of these products are severely restricted. Furthermore, the microstructure of the coatings obtained is unfavorable, since the overall proportion of intermetallic phases in the coating is generally too high and the magnesium-containing phases are unfavorably distributed, since a targeted phase formation by means of heat treatment does not take place. This has a negative impact on both the corrosion resistance and the forming behavior of the coatings.

The electrolytic deposition of magnesium in an aqueous electrolyte, which is conceivable as an alternative deposition method, is opposed by the strongly negative normal potential of magnesium (-2.363 V). In an electrolytic cell with an aqueous electrolyte, instead of the deposition of elemental magnesium, almost exclusively the reduction of protons to hydrogen gas takes place at the cathode.

In EP 1 036 862 AI the electrolytic deposition of a Zn-Mg alloy layer on a metal sheet, consisting of iron, an iron alloy or of copper, aluminum or titanium or their alloys, is described in an aqueous acidic electrolyte, the one nonionic or cationic surfactant is added. The electrolytically deposited alloy layer draws are characterized by good formability and corrosion resistance according to the information in this document. The latter is further increased by the incorporation of carbon from the organic surfactant. A disadvantage of this process, however, is its low current efficiency, since the charge transport in the electrolyte is largely carried out via protons and the formation of gaseous hydrogen in the course of the magnesium deposition cannot be prevented. This must be compensated for either by increasing the current density or the dwell time of the sheet to be coated in the electrolytic cell, which in both cases leads to a reduction in process efficiency.

The electrolytic deposition of magnesium in an aprotic (proton-free) solvent is disclosed in the dissertation "Galvanic deposition of aluminum and magnesium alloys" (TH Leuna-Merseburg, 1985). The deposition takes place in an electrolyte based on tetrahydrofuran (THF) on substrates made of nickel, copper, platinum and a low-alloy steel This document only demonstrates the basic feasibility of the process on a laboratory scale, without, however, optimizing it with regard to possible industrial applications.

US Pat. No. 3,520,780 also describes electrolytic magnesium deposition in THF as an aprotic solvent. However, this is not a coating process, but rather a process for electroforming, ie for a controlled process Production of bodies and components from electrodeposited magnesium.

The object of the invention is to provide a process for the electrolytic magnesium coating of sheet metal with a zinc or zinc alloy coating, in particular steel sheet, which is characterized by low specific costs and wherein a sheet coated in accordance with the process should have a high surface quality and formability with simultaneously improved corrosion properties.

The object is achieved by a method of the type mentioned in that

- The electrolytic deposition takes place in a solvent with a lower acidity than water, preferably in an essentially aprotic solvent, and

- The coated substrate is then subjected to a heat treatment to form a Mg-Zn alloy phase in the zinc layer.

By choosing a less acidic solvent (pK _A > 16 with K _A : acid dissociation constant) as water (pK _A = 14) the concentration of protons in the solvent is significantly reduced, so that mainly the reduction of the protons to elemental hydrogen at the Cathode takes place, but the deposition of elemental magnesium. The concentration of protons in the solvent can be further reduced by adding suitable bases to the solvent. In addition, by Addition of other suitable organic components to partially suppress the deposition of hydrogen gas at the cathode. Solvents that are designated as aprotic because of their extremely low acidity, because they contain approximately no free protons, appear to be particularly suitable.

The use of an aprotic solvent, preferably tetrahydrofuran and / or diethyl ether, enables a deposition process which is known to be efficient with respect to the current yield and which, in contrast to the methods of the prior art which are based on magnesium deposition in aqueous solution, with a comparatively low current density and / or Deposition time can take place. The particularly high cost-effectiveness of the method according to the invention in relation to the area-specific coating costs results from the simple implementation in terms of plant technology, in particular from the point of view of possible implementation in a continuous process.

In principle, all magnesium salts are suitable for transporting the magnesium ions from the solvent to the cathode as a result of the applied voltage, which dissolve either completely or only partially ionically in the abovementioned solvents. Examples of such magnesium salts include the magnesium halides, magnesium grignard compounds, magnesium alcoholates or magnesium carboxylates. The electrolytically deposited magnesium layers have a significantly higher density and better adhesion than is the case with the usual vacuum coatings. This enables problem-free further processing in the subsequent heat treatment. In a continuous conveyor belt process, for example, problematic growth of the magnesium on the rolls, often the cause of surface defects on the finished product, can be avoided.

Due to the heat treatment of the sheet following the electrolytic magnesium coating, a Mg-Zn alloy layer is generated due to diffusion. This effectively prevents rapid oxidation of the electrolytically deposited magnesium layer, which would result in a rough surface. Before and during the heat treatment, the magnesium layer is preferably not exposed to an oxidizing atmosphere. The heat treatment can take place in a wide temperature range from 250 to 420 ° C, which results in different layer structures. The preferred layer structure results from the intended application. If the process is to be based on pure solid diffusion, treatment is preferably carried out at a temperature of 300 ° C. If the modification is to be controlled via the formation of a eutectic, a temperature of 380 ° C. is preferred. The treatment time for a zinc coating of 7.5 μm customary in the automobile is a maximum of 60 s, preferably 6 s in the case of solid-state diffusion or 2 s in the case of the formation of a eutectic. In addition, it can be seen that the formation of the alloy phase improves the corrosion properties to such an extent that sheet metal with a reduced thickness of the zinc coating can be used for the process according to the invention. This, in turn, results in an improved formability of the sheet with sufficient or even improved corrosion resistance. Accordingly, the specific advantages of a zinc and a magnesium coating are optimally combined in the method according to the invention. A sheet coated according to the invention thus combines the properties of conventionally galvanized sheets with the extreme corrosion resistance of one

Magnesium layer. At the same time, savings in zinc result in further cost advantages.

According to a first embodiment of the invention, the deposition of metal on the zinc or

Zinc alloy coating can be expanded by simultaneously electrolytically depositing zinc in metallic form in addition to magnesium.

To achieve the above-mentioned properties, it is sufficient if the magnesium layer is deposited in a mass ratio of 0.1 to 10 mass%, preferably 1 to 2 mass%, to the zinc layer present on the galvanized sheet surface, which is one with regard to the material used enables economical execution of the method according to the invention. For a uniform coating result, it is necessary that the concentration of the magnesium ions or the magnesium-containing molecular ions in the electrolyte is kept essentially constant. In terms of process engineering, this can be done in two ways. On the one hand, it is possible to introduce the magnesium ions into the aprotic solvent by means of a magnesium anode that gradually dissolves. On the other hand, a constant ion concentration can also be achieved in that the magnesium ions are introduced into the aprotic solvent by the essentially continuous addition of a magnesium-containing substance using an inert anode.

The use of, for example, electrolytically galvanized sheet metal as the substrate material offers the advantage that the galvanizing and the magnesium deposition and heat treatment of the coated sheet according to the invention can be carried out directly in succession in one system, since the electrolytic cells used for the galvanizing on the one hand and for the subsequent magnesium coating on the other hand essentially can be constructed identically. This advantage can be used in particular if the galvanized sheet is coated as a continuous material in a continuous process. In this case, the band-shaped material first passes through several cells for applying the zinc layer and then through a last cell for magnesium deposition. The expansion of a conventional coil coating system for the execution of the The inventive method is therefore possible with very little effort.

The invention is explained in more detail below with reference to a drawing which shows an exemplary embodiment and shows a schematic representation of a plant for magnesium deposition on a galvanized steel strip.

According to the drawing, a substrate in the form of a steel strip 1 is first passed in a transport direction T over a roller guide 1 a through three successively arranged, identically constructed electrolysis cells 2 and thereby provided with a zinc layer with a total thickness of, for example, approximately 7.5 μm. The steel strip 1 thus functions as a cathode in the electrolysis process. The electrolytic cells 2 are each filled with an aqueous electrolyte 2a, in each of which two anodes 2b consisting of elemental zinc are immersed, which continuously release zinc ions into the electrolyte 2a during the coating process. The steel strip 1 then passes through a last electrolysis cell 3 which is filled with an electrolyte 3a based on an aprotic solvent, for example a mixture of tetrahydrofuran and diethyl ether. Two anodes 3b made of elemental magnesium are immersed in the electrolytes 3a in a manner comparable to the electrolytic cells 2, which anodes in turn continuously release magnesium ions into the electrolytes 3a in the course of the coating. In this electrolysis cell 3, the galvanized steel strip 1 is provided with a magnesium layer with a thickness of, for example, approx. 0.5 μm. After leaving the last electrolysis cell 3, the strip 1 is fed directly to a heating device 4, which is arranged in a housing 5 filled with inert gas 5a, such as argon or nitrogen.

A belt running concept with further roller contacts between the electrolytic cell 3 and the heating device 4 is easily possible here, since the electrolytically deposited Mg layers have good adhesion to the substrate, so that there is no growth of magnesium on the rollers due to abrasion. In the housing 5 provided with the heating device 4, the strip section passing through is heat-treated at a temperature of, for example, 300 ° C. and a treatment time of e.g. 6s, so that an Mg-Zn alloy layer is formed on the surface of the coated steel strip 1 by diffusion. It goes without saying that the length of the route on which the heat treatment is carried out and the transport speed of the belt 1 must be coordinated with one another in order to maintain the desired treatment time.

After leaving the housing 5, the coated and heat-treated steel strip 1, which now has a shiny metallic, highly corrosion-resistant surface, can be subjected to further processing steps or wound up into a coil.

Claims

PATENT TO SPEECH

1. A method for electrolytic magnesium deposition on a substrate made of sheet metal with a zinc or zinc alloy coating (1), in particular steel sheet, wherein

- The electrolytic deposition takes place in a solvent with a lower acidity than water and

- The coated substrate (1) is then subjected to a heat treatment to form a Mg-Zn alloy phase in the zinc layer.

2. The method according to claim 1, which also means that the solvent used in the electrodeposition is an essentially aprotic solvent.

3. The method according to claim 1 or 2, characterized in that a Mg layer is deposited in a mass ratio of 0.1 to 10 mass%, preferably 1 to 2 mass%, to the present zinc layer on the galvanized sheet surface. Method according to one of claims 1 to 3, characterized in that the magnesium ions are introduced into the solvent (3a) by means of a magnesium anode (3b).

Method according to one of Claims 1 to 4, that the magnesium ions are introduced into the solvent (3a) by adding a magnesium-containing substance.

Method according to one of Claims 1 to 5, that the sheet (1) is coated with a zinc or zinc alloy coating as a continuous material in a continuous process.

Method according to one of claims 1 to 6, that the subsequent heat treatment is carried out at a temperature of 250 to 359 ° C, preferably 300 ° C, with a treatment time <60s, preferably 6s.

Method according to one of claims 1 to 7, characterized in that the subsequent heat treatment is carried out at a temperature of 359 to 420 ° C, preferably 380 ° C, with a treatment time <60s, preferably 2s. Method according to one of claims 1 to 8, characterized in that ß is also deposited in metallic form on the zinc or zinc alloy coating during electrolytic magnesium deposition and a Mg-Zn alloy phase is formed during the heat treatment.