EP1570115B1 - Method for the electrolytic deposition of magnesium on galvanised sheet metal - Google Patents

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

The invention relates to a method for the electrolytic magnesium deposition on a substrate made of sheet metal with 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 steel body panels (hot dip or electrolytic coating) for the purpose of corrosion protection has largely prevailed in recent decades. The galvanized steel plates in the hot dip process or by means of electrolytic deposition are characterized by good adhesion of the zinc layer to the steel sheet and good processability.

Significantly improved corrosion properties can be achieved by applying a magnesium layer to the uncoated steel sheet, as an alternative to the galvanizing process. Thus, storage of a magnesium-coated steel sheet in air causes immediate oxidation of the magnesium layer, thereby passivating the sheet surface. Accordingly, the underlying steel is not further attacked. Disadvantageous However, magnesium-coated sheets is their surface roughness increased compared to galvanized steel sheets due to the formation of the oxide layer.

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

In JP 62109966 A a steel sheet is described, on the surface of which first a zinc layer and then a magnesium layer is applied in each case 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 considered to be problematic because it requires a very high equipment and process engineering effort. Furthermore, vacuum-coated sheets have a non-optimal adhesion of the magnesium layer.

In DE 195 27 515 C1 a process for the production of corrosion-protected steel sheet is specified. On a galvanized steel sheet, a layer of one or more metals except zinc or a zinc-free alloy is applied by vacuum coating. Subsequently, the thus coated sheet is subjected to a heat treatment without exposure to oxidizing atmosphere. As a result, a diffusion layer of the vacuum-deposited metal or alloy and the underlying zinc is formed on the surface of the double-coated steel sheet. so Coated sheets are characterized by good surface quality and high corrosion protection. In addition, the diffusion layer is due to their small thickness relative to the thickness of the zinc coating sufficiently ductile, to ensure a further good formability of the steel sheet. The main disadvantage of this process is the high apparatus and process complexity associated with the vacuum coating.

DE 100 39 375 A1 likewise describes a process for producing corrosion-protected steel sheet steel. This is similar to the method described in DE 195 27 515 C1 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 zinc-magnesium phase formation occurs over the entire thickness of the zinc coating. This leads to a coating with the above-described positive properties with a further improved corrosion protection. However, the mentioned disadvantages of vacuum coating are still given.

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 molten bath, in particular the observance of a constant Mg content, is due to the high oxidation tendency due to Mg slag formation and unavoidable burning manageable only with great technical effort. In addition, the surface quality of the coatings is low, so that the possible applications of these products are severely limited. Furthermore, the microstructure of the coatings obtained is unfavorable, since the proportion of intermetallic phases in the coating as a rule is generally too high and the magnesium-containing phases are unfavorably distributed, since a targeted phase formation does not take place by means of heat treatment. This has a negative effect on both the corrosion resistance and on the forming behavior of the coatings.

The proposed as an alternative deposition method electrolytic deposition of magnesium in an aqueous electrolyte is contrary to the strong negative normal potential of magnesium (-2,363 V). In an electrolytic cell with an aqueous electrolyte, the reduction of protons to hydrogen gas takes place at the cathode instead of the deposition of elemental magnesium almost exclusively.

Nevertheless, EP 1 036 862 A1 describes the electrolytic deposition of a Zn-Mg alloy layer on a metal sheet consisting of iron, an iron alloy or copper, aluminum or titanium or their alloys, in an aqueous-acidic electrolyte nonionic or cationic surfactant is added. The electrodeposited alloy layer records according to the information in this document by good formability and corrosion resistance. The latter is increased by the incorporation of carbon from the organic surfactant. A disadvantage of this method, however, is its low current efficiency, since the charge transport in the electrolyte takes place to a considerable extent via protons and thus the formation of gaseous hydrogen in the course of magnesium deposition can not be prevented. This must be compensated either by increasing the current density or the residence time of the sheet to be coated in the electrolysis cell, which leads in both cases to a reduction of the 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 of nickel, copper, platinum and a low-alloyed steel. In this document, only the principle feasibility of the method is demonstrated on a laboratory scale, but without 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 a process for electroforming, ie the controlled Production of bodies and components made of electrolytically deposited magnesium.

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

• die elektrolytische Abscheidung in einem Lösungsmittel mit geringerer Acidität als Wasser, bevorzugt in einem im wesentlichen aprotischen Lösungsmittel, erfolgt und
• das beschichtete Substrat anschließend einer Wärmebehandlung zur Ausbildung einer Mg-Zn-Legierungsphase in der Zinkschicht unterzogen wird.

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

• the electrolytic deposition in a solvent having a lower acidity than water, preferably in an essentially aprotic solvent, takes place 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) than water (pK _A = 14), the concentration of protons in the solvent is significantly lowered, so that no longer the reduction of protons to elemental hydrogen at the Cathode takes place, but the deposition of elemental magnesium. By adding suitable bases to the solvent, the concentration of protons in the solvent can be further reduced. In addition, through Addition of other suitable organic components, the deposition of hydrogen gas at the cathode are partially suppressed. Solvents which are said to be aprotic due to 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 in terms of current efficiency and which, unlike the prior art processes based on magnesium deposition in aqueous solution, at comparatively low current density and / or Deposition time can be made. The particularly high cost-effectiveness of the method according to the invention with respect to the surface-specific coating costs results from the facility-technically simple feasibility, in particular under the aspect of a possible embodiment in a continuous process.

To transport the magnesium ions from the solvent to the cathode as a result of the applied voltage are in principle all magnesium salts that dissolve in the above solvents either completely or only partially ionic. Examples of such magnesium salts include 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 allows easy further processing in the subsequent heat treatment. Thus, e.g. In a continuous conveyor system problematic growth of magnesium on the rollers, often cause surface defects on the finished product, avoided.

Due to the heat treatment of the sheet subsequent to the electrolytic magnesium coating, a Mg-Zn alloy layer is produced by diffusion. This effectively prevents rapid oxidation of the electrodeposited magnesium layer, which would result in a rough surface. Before and during the heat treatment, the magnesium layer is preferably exposed to no oxidizing atmosphere. The heat treatment can take place in a wide temperature range of 250 to 420 ° C, resulting in different layer structures. The preferred layer structure results from the respective intended application. If the process is based on a pure solid state diffusion, it is preferably treated at a temperature of 300 ° C. If the modification is to be controlled by the formation of a eutectic, a temperature of 380 ° C. is preferred. The treatment time amounts to a maximum of 60s, preferably 6s in the case of solid-state diffusion or 2s in the case of the formation of a eutectic, for a customarily customary zinc coating of 7.5 μm.

In addition, it has been found that the corrosion properties are improved to such an extent by the formation of the alloy phase that sheets of 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 while still sufficient or even improved corrosion resistance. Accordingly, the specific advantages of a zinc coating and a magnesium coating are optimally combined in the method according to the invention. Thus, a sheet coated according to the invention combines the properties of conventionally galvanized sheets with the extreme corrosion resistance of a magnesium layer. At the same time, further cost savings are achieved by saving on zinc.

According to a first embodiment of the invention, the deposition of metal on the zinc or zinc alloy coating can be extended by simultaneously zinc being deposited in metallic form in addition to the magnesium and electrolytically.

For attaining the above-mentioned properties, it is sufficient if the magnesium layer is deposited on the galvanized sheet surface in a mass ratio of 0.1 to 10 mass%, preferably 1 to 2 mass%, to the present zinc layer, which is one in terms of material usage economic execution of the method allows.

For a uniform coating result, it is necessary that the concentration of magnesium ions or magnesium-containing molecular ions in the electrolyte be kept substantially constant. This can be done procedurally in two ways. On the one hand, it is possible to introduce the magnesium ions by means of a gradually dissolving magnesium anode in the aprotic solvent. On the other hand, a constant ion concentration can also be achieved by introducing the magnesium ions into the aprotic solvent by the substantially continuous addition of a magnesium-containing substance using an inert anode.

The use of, for example, electrolytically galvanized sheet metal as a 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 immediately after one another in a plant, since the electrolytic cells used for galvanizing on the one hand and for the subsequent magnesium coating on the other hand substantially can be constructed identically. This advantage can be used in particular when the galvanized sheet is coated as a continuous material in a continuous process. The band-shaped material, for example, in this case initially passes through a plurality of cells for application of the zinc layer and then a last cell for magnesium deposition. The extension of a conventional coil coating plant for the execution of the inventive method is therefore possible with very little effort.

In the following the invention will be explained in more detail with reference to a drawing illustrating an embodiment, which shows a plant for magnesium deposition on a galvanized steel strip in a schematic representation.

According to the drawing, a substrate in the form of a steel strip 1 in a transport direction T via a roller guide la initially passed through three successively arranged, identically constructed electrolytic cells 2 and thereby provided with a zinc layer of a total thickness, for example, about 7.5 .mu.m . The steel strip 1 thus acts as a cathode in the electrolysis process. The electrolytic cells 2 are each filled with an aqueous electrolyte 2 a, in each of which two anodes 2 a consisting of elemental zinc are immersed, which continuously release zinc ions into the electrolyte 2 a during the coating process. Thereafter, the steel strip 1 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 of elemental magnesium are immersed in the electrolytes 3a in a manner comparable to the electrolytic cells 2, which in turn continuously emit magnesium ions into the electrolyte 3a in the course of the coating. In this electrolysis cell 3, the galvanized steel strip 1 is provided with a magnesium layer having a thickness of, for example, about 0.5 .mu.m .

After leaving the last electrolysis cell 3, the strip 1 is fed directly to a heating device 4, which is placed in an inert gas 5a, e.g. Argon or nitrogen, filled housing 5 is arranged.

A strip running concept with further roller contacts between the electrolysis cell 3 and the heating device 4 is readily possible here, since the electrolytically deposited Mg layers have good adhesion to the substrate, so that magnesium does not grow on the rollers due to abrasion. In the housing 5 provided with the heater 4, a heat treatment of each passing band portion is carried out at a temperature of, for example, 300 ° C and a treatment time of e.g. 6s, so that a Mg-Zn alloy layer is formed on the surface of the coated steel strip 1 by diffusion. It is understood that for the maintenance of the desired treatment time, the length of the distance on which the heat treatment takes place, and the transport speed of the belt 1 must be coordinated.

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

Claims (9)

1. Method for the electrolytic deposition of magnesium on a substrate consisting of sheet metal with a zinc or zinc alloy coating (1), in particular sheet steel, wherein the electrolytic deposition takes place in a solvent having a lower acidity than water and the coated substrate (1) is subsequently subjected to a heat treatment for forming a magnesium/zinc alloy phase in the zinc layer.
2. Method according to Claim 1, characterised in that the solvent used in the electrolytic deposition is a substantially aprotic solvent.
3. Method according to either Claim 1 or Claim 2, characterised in that a magnesium layer is deposited on the zinc-plated sheet metal surface in a mass ratio from 0.1 to 10% mass, preferably 1 to 2% mass, with respect to the present zinc layer.
4. Method according to any one of Claims 1 to 3, characterised in that the magnesium ions are introduced into the solvent (3a) using a magnesium anode (3b).
5. Method according to any one of Claims 1 to 4, characterised in that the magnesium ions are introduced into the solvent (3a) by adding a substance containing magnesium.
6. Method according to any one of Claims 1 to 5, characterised in that the sheet metal (1) is coated with zinc or zinc alloy coating as a continuous material in an ongoing process.
7. Method according to any one of Claims 1 to 6, characterised in that the subsequent heat treatment is carried out at a temperature from 250 to 359°C, preferably at 300°C, for a treatment time of less than 60 seconds, preferably for 6 seconds.
8. Method according to any one of Claims 1 to 6, characterised in that the subsequent heat treatment is carried out at a temperature from 359 to 420°C, preferably at 380°C, for a treatment time of less than 60 seconds, preferably for 2 seconds.
9. Method according to any one of Claims 1 to 8, characterised in that, during the electrolytic deposition of magnesium, zinc is also deposited in metallic form on the zinc or zinc alloy coating and, during the heat treatment, a magnesium/zinc alloy phase is formed.

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