EP3103897A1 - Method for the electrochemical deposition of thin inorganic layers - Google Patents

Abstract

The invention relates to a process for the electrochemical deposition of a thin inorganic coating on at least one surface of a metallic, electrically conductive substrate which is moved during the deposition process by an electrolyte containing components that form the coating as a result of the deposition process. In order to accelerate such a coating process and to be able to deposit particularly thin layers on a metallic substrate in an environment-harmless and at the same time economical manner, the invention proposes that the surface of the substrate to be coated with the inorganic coating is cathodically polarized during passage through the electrolyte.

Description

The invention relates to a method for the electrochemically assisted deposition of particularly thin inorganic layers on a metallic, electrically conductive substrate, which is held in an electrolyte during the deposition process.

In particular, the invention relates to a method for accelerated deposition of so-called "conversion layers" on flat steel products. As "flat steel products" from rolled steel sheets or strips as well as blanks, blanks and the like obtained from it are called.

Flat steel products, but also other corrosion-susceptible metallic substrates, are usually provided with a multilayer coating system to improve their corrosion resistance and to optimize their optical appearance. For this purpose, the respective substrate is first provided with a metallic protective layer, in particular an alloy layer based on zinc, magnesium or aluminum. This metallic protective layer sacrifices itself in a corrosive attack and protects in this way the underlying corrosion-sensitive metal. The conversion layer, which is typically an inorganic phosphate layer, is usually wet-chemically applied to the metallic protective layer or, if omitted, directly onto the uncoated metal substrate. On the one hand, this conversion layer protects the metallic protective layer or the metal substrate against corrosive attacks. On the other hand, the conversion layer improves the bonding of a subsequently applied organic coating to the protective layer or the metallic substrate. The outer organic coating, which is typically a paint or a laminated polymer layer, also serves as protection against contact with water and associated oxidation, and additionally has a decorative function.

The lifetime of coating composite systems of the type described above is determined in principle by the interfacial chemistry of the different phase boundaries between the individual layers. The composite "metallic substrate / metallic protective layer" is here, as far as its durability considered, usually considered as uncritical due to the chemical relationship of the interconnected substances.

In the event that the outer coating comes into direct contact with the metallic substrate or a metallic protective layer applied thereto, the phase boundary between the metallic substrate or the metallic protective layer applied thereto and the outer organic coating proves to be significantly more complex. A fundamental problem here is that the outer coating is usually semipermeable for corrosive media, especially oxygen and water. As a result, at least locally, an electron transfer reaction may occur at the interface between the metal of the substrate or the metallic protective layer and the polymer of the outer coating. The result is a corrosion process in which the metallic substrate or the metallic protective layer optionally present thereon is anodically oxidized. At the same time, oxygen is cathodically reduced to hydroxyl ions, which can lead to a substrate-near alkalization of the composite interface and to a delamination of the outer coating.

In order to inhibit this process as much as possible, a conversion layer is built up on the metal substrate to be protected or the metallic protective layer applied thereon, on which then lies the outer polymer layer.

Technically established processes for the production of suitable conversion layers are based on the use of phosphates and chromates. In addition to the high energy input that such processes typically require, these have the distinct disadvantage of using heavy metals (nickel, manganese, chromium). Due to the carcinogenic effects of hexavalent chromium, known as Cr (VI) compounds, such metal treatment systems are already banned in many countries and regions.

A possible alternative to the classical systems are Zr-based conversion layers, in which an acidic pickling attack of the metal surface to be treated achieves a local alkalization of the interface, so that a thin ZrO.sub.2 layer precipitates. The principal mechanisms for the deposition of these Zrbasierenden conversion layers are described in the literature ( P. Puomi, HM Fagerholm, JB Rosenholm, K. Jyrkäs, Comparison of different commercial pretreatment methods for hot-dip galvanized and galfan coated steel, Surface and Coatings Technology 115 (1999) 70-78 ; P. Puomi, HM Fagerholm, JB Rosenholm, R. Sipilä, Optimization of commercial zirconic acid based pretreatment on hot-dip galvanized and galfan coated steel, Surface and Coatings Technology 115 (1999) 79-86 ; T. Lostak, A. Maljusch, B. Klink, S. Krebs, M. Kimpel, J. Flock, S. Schulz, W. Schuhmann, Zr-Al-Mg Zr-Al-Mg alloy coated steel sheets: insights into the formation mechanism, Electrochimica Acta 137 (2014) 65-74 ).

In addition, the layer deposition could be significantly accelerated by means of Cu 2+ ions ( T. Lostak, S. Krebs, A. Maljusch, T. Gothe, M. Giza, M. Kimpel, J. Flock, S. Schulz, Formation and characterization of Fe3 + / Cu2 + -modified zirconium oxide conversion layers on zinc alloy coated steel sheets, Electrochimica Acta 112 (2013) 14-23 ).

However, this means an additive of the conversion baths with an undesirable noble metal. In addition, practical tests show that layers applied in this way often do not meet the requirements placed on their function as an electron barrier fulfill.

Another challenge in the production of conversion layers on a metallic substrate or a protective layer applied thereto is the reliable integration of the necessary steps in the processes of a large-scale manufacturing process. This requires that the conversion layers are formed very quickly, to which conventional conversion baths still react too slowly.

Fast-moving coating processes require pretreatment systems whose deposition kinetics are sufficiently high to produce a closed conversion layer at short contact times with the conversion bath. By means of various chemical additives, the film formation kinetics can be significantly increased. Often these additives are toxic or ecologically questionable for other reasons. Therefore, alternative processes are sought, with which layers can be accelerated on the respective substrate.

From the EP 1 694 879 B1 ( US Pat. No. 7,959,982 B2 A method of continuously coating a moving substrate is known in which an ultrafine film between 10 nm and 100 nm thick is deposited on the substrate. The film is thereby formed from a solution which contains oxidized nanoparticles under controlled pH conditions, wherein the substrate is more than 120 ° C. during the coating and the total duration of the deposition is less than 5 seconds, preferably less than one second. There is at least one chemical additive in the solution introduced, which has an antagonistic effect with respect to the formation of the coating. By counteracting the process of the deposition process chemical additive, it should be possible to control this process so that an undesirable rapid growth of the layer is suppressed and thus the desired minimized layer thickness is achieved. The aim of this procedure is a method which makes it possible to build the thin film serving as adhesion promoter between the respective substrate and a lacquer layer applied in a following process step on the substrate so fast that the method can be included in a continuous overall process the substrate is suitably provided and subsequently painted. In particular, in the case of a steel substrate, the provision of the substrate may include a metallic corrosive attack protective layer coating included in the continuous process of the overall process.

In another method for coating a flat product having a core layer of a metal material, in particular of a steel material, and a corrosion protective metallic protective coating optionally formed on the core layer, the flat product is cleaned with an alkaline detergent, then rinsed with demineralized water and then dried. On the dried substrate, a conversion solution is applied which contains in aqueous solution a Zr or Ti compound which dissociates into zirconium or titanium fluoro complexes. In this way, a conversion layer is generated, according to a first Alternatively, the conversion solution as an adhesion promoter additionally an organosilane is added, which contains an epoxy group and is water-soluble, whereas according to a second alternative, first the conversion solution is applied to the flat product, then the flat product rinsed with demineralized water or service water and then an aqueous solution of a suitable adhesion promoter serving organosilane containing an epoxy group is applied. Finally, the resulting flat product is dried. In this way, flat products can be produced with a 20-200 nm, in particular 20-50 nm, thick coating system containing a conversion layer with inorganic constituents, which improves the adhesion of a later to be applied, at least one polymer outer layer to the core layer. The method is the subject of the German patent application 10 2013 113 731.8 of 9 December 2013 and the International Patent Application filed claiming priority PCT / EP2014 / 076987 , The contents of said German and International Patent Application are incorporated by reference into the disclosure of the present application.

Against the background of the above-described prior art, the object has arisen to name a high-speed coating process, which is particularly thin layers, in particular layers of the above-mentioned German patent application 10 2013 113 731.8 and International Patent Application PCT / EP2014 / 076987 mentioned type, on a metallic substrate in a respect to the impact on the Separate environmentally safe and at the same time economic way.

The invention has achieved this object by a method with the steps specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

Accordingly, in a method of electrochemically depositing a thin inorganic coating on at least one surface of a metallic, electrically conductive substrate held during the deposition process in an electrolyte containing constituents which form the coating as a result of the deposition processes, the invention proposes the surface of the substrate to be coated with the inorganic coating is cathodically polarized during the residence in the electrolyte.

The invention is based on the idea of polarizing the substrate to be coated, which is moved in an electrolyte, such that the desired thin inorganic conversion layer forms on the electrically conductive substrate surface. According to the invention, the application of the thin coating is therefore supported electrochemically in such a way that the desired thin layer forms within a short time due to the cathodic polarization provided according to the invention during the deposition process.

In this case, the procedure according to the invention can be implemented cost-effectively on conventional electrolytic coating systems in such a way that it can also be integrated on a large industrial scale into a continuous overall process for the production of metal sheets or strips, which are typically the case with the metallic substrates coated according to the invention.

The latter applies in particular when the substrate to be coated according to the invention is a flat steel product. The flat steel product serving as the substrate to be coated according to the invention may be provided in a manner known per se with a metallic coating, such as, for example, a Zn coating consisting of zinc or a zinc-containing alloy.

• Potentiostatische Verfahren, bei dem die elektrisch leitfähige Substratoberfläche auf ein zeitlich konstantes, elektrisches Potential polarisiert wird. Ein solches geeignetes Potential bzw. das daraus resultierende Elektrodenpotential ist negativer als das Ruhepotenzial der Oberfläche des jeweiligen Substrats in dem jeweiligen Elektrolyten, das um mehr als 100 mV in kathodischer Richtung verschoben ist.
• Galvanostatische Verfahren, bei dem an die elektrisch leitfähige Substratoberfläche eine zeitlich konstante Stromdichte angelegt wird. Dabei ist das zu beschichtende Substrat als Minuspol zu schalten.

For the polarization according to the invention suitable electrochemically assisting processes are, for example:

• Potentiostatic method in which the electrically conductive substrate surface is polarized to a temporally constant, electrical potential. Such a suitable potential or the resulting electrode potential is more negative than the quiescent potential of the surface of the respective substrate in the respective electrolyte, which is shifted by more than 100 mV in the cathodic direction.
• Galvanostatische method, in which to the electrically conductive substrate surface a temporally constant Current density is applied. In this case, the substrate to be coated is to be switched as a negative pole.

The bases of the mentioned electrochemical processes are, for example, in Koryta, J., Dvorak, J., Bohackova, V. "Textbook of Electrochemistry", Springer Verlag, ISBN 978-3-7091-8418-9, 1975, XVI , described.

In the potentiostatic process, a time-constant potential is applied to the metal surface to be coated, and in the case of galvanostatic processes, a time-constant current. The metal surface is the negative pole, ie cathodically polarized.

The height of the potential or the current density is individually adapted to the liquid electrolyte used. In this case, the electrolyte resistance, competing electrode reactions or side reactions, the temperature of the electrolyte, the speed of the moving substrate and the electrical conductivity of the substrate surface are taken into account.

Suitable electrolytes for the purposes according to the invention are, for example, electrolytes from the group of aqueous and acidic electrolyte solutions.

In the case of the application of a potentiostatic method suitable electrical potentials are at least 100 mV more negative than the resting or corrosion potential of the substrate surface in the electrolyte used (cathodic polarization), or a voltage of the order 1 V - 30 V voltage between the substrate (Cathode) and the circuit closing inert counter electrode (eg dimensionally stable anode).

In the case of the application of a galvanostatic method suitable current densities are in the order of 0.1 mA / cm ² - 500 mA / cm ² between the substrate (cathode) and the circuit closing inert counter electrode (eg dimensionally stable anode).

With regard to the operational implementation, it may be advantageous to use the galvanostatic procedure, since in this case it is possible to work with a so-called two-electrode arrangement, whereas a potentiostatic polarization requires a third electrode, a reference electrode with a constant referenceable potential.

Both electrochemical process variants mentioned here are applicable in a modification as pulse or jump method. Suitable pulse methods are known from the specialist literature, s. for example WO 2010/046392 A2 ,

In principle, when carrying out the method according to the invention, it is conceivable to keep the substrate to be coated in the respective electrolyte for a sufficient period of time or to perform the coating so that no relative movement takes place between the electrolyte and the substrate. However, it proves to be particularly practical and advantageous with regard to the effectiveness of the method when the substrate is moved through the electrolyte in the passage.

The speeds at which the electrically conductive substrate is moved through the electrolyte may be in the range of 0.5-15 m / s. In addition to the speed with which the electrically conductive substrate is moved through the electrolyte, and the choice of the electrolyte, a defined layer thickness of the conversion layer can be applied accurately and reproducibly via potential level or current density.

If necessary, the substrate can be electrochemically pre-cleaned in a step preceding the steps carried out according to the invention.

As applied in the prior art, the thin coating applied according to the invention can assume the dual function of "corrosion protection / adhesion promoter" as the conversion layer. However, it is equally possible, by suitable choice of each deposited components, the inorganic coating formed according to the invention so that in each case only one or the other function is in the foreground, so that the coating essentially only to improve the adhesion or only to improve the corrosion protection contributes.

The invention is particularly suitable for depositing a ZrO ₂ -based thin film. In this case, the electrolyte used is an aqueous solution containing a Zr or Ti compound which dissociates into zirconium or titanium fluoro complexes.

Especially the ZrO ₂ -Abscheidung can by the inventive Procedure be accelerated so that within a short application time, a coating with sufficient electronic barrier effect can be generated.

The reaction mechanism of the ZrO ₂ deposition is based on an alkalization of the metallic surface to be coated, which precipitates out of the conversion solution ZrO ₂ . The following reactions take place at the interface metal substrate | Conversion solution from:

(1) Zn ²⁺ + ZrF ₆ ^2- → ZnF ₆ ^4- + Zr ⁴⁺

(2) 2 H ⁺ + 2 e ^- → H ₂

(3) O ₂ + 2 H ₂ O + 4 e ^- → 4 OH ^-

(4) ZrF ₆ ^2- + 5H ₂ O → ZrO ₂ .H ₂ O + 6HF + 2 OH ^-

According to the invention, the alkalization is accelerated by cathodic polarization of the surface to be coated (Eq. (2) and (3)). As a result, the ZrO ₂ precipitation is forced. The polarization can either take place moderately on the one hand, ie with relatively weak cathodic polarization, in which essentially the oxygen conversion takes place in the sense of an oxygen-consuming cathode (equation (3)).

On the other hand, taking into account the respective pH value of the electrolyte, the polarization can be carried out in such a way that mainly a hydrogen evolution (Eq. (2)) starts. In this case, a suitable process window must be chosen, since a too weak polarization is not sufficiently accelerated alkalization and too strong a polarization counterproductive for the layer formation, due to the evolution of hydrogen, which leads to a Swirling or convection of the electrolyte leads and also occupied the surface to be coated with gas bubbles. Therefore, according to the invention, the process window is set in the manner already explained above.

Typical thicknesses of the inorganic layers deposited on the respective metallic substrate according to the invention are in the range from 20 to 200 nm.

Fig. 1

ein Diagramm, in dem die durch Anwendung eines erfindungsgemäßen Verfahrens und die durch eine konventionelle Verfahrensweise erzielten Schichtdicken gegenübergestellt sind;

Fig. 2

eine Gegenüberstellung der Zyklovoltammogramme einer durch Feuerverzinken beschichteten und anschließend dressierten Stahlflachproduktprobe nach 60 s potenzialfreier und potenzialunterstützter ZrO₂-Abscheidung (-1,8 V (vs. SHE) für 60 s);

Fig. 3

eine Gegenüberstellung der Zyklovoltammogramme einer durch Feuerverzinken beschichteten und anschließend dressierten Stahlflachproduktprobe nach 60 s potenzialfreier und potenzialunterstützter ZrO₂-Abscheidung (-1,4 mA/cm² für 5 s und anschließend -0,3 mA/cm² für 55 s) ;

Fig. 4

eine zur Durchführung des erfindungsgemäßen Verfahrens geeignete Vorrichtung.

The invention will be explained in more detail with reference to a drawing illustrating an exemplary embodiment. Each show schematically:

Fig. 1

a diagram in which the achieved by applying a method according to the invention and achieved by a conventional procedure layer thicknesses are compared;

Fig. 2

a comparison of the cyclic voltammograms of a hot dip galvanized and subsequently dressed steel flat product sample after 60 s of potential-free and potential-assisted ZrO ₂ deposition (-1.8 V (vs. SHE) for 60 s);

Fig. 3

a comparison of the cyclic voltammograms of a steel flat product sample coated by hot dip galvanizing and subsequently dressed after 60 s of potential-free and potential-assisted ZrO ₂ deposition (-1.4 mA / cm ² for 5 seconds and then -0.3 mA / cm ² for 55 seconds);

Fig. 4

a device suitable for carrying out the method according to the invention.

To demonstrate the invention, sheet metal samples have been provided from a typical automotive deep-drawing steel, which is suitable by its property profile particularly for the manufacture of components for the outer skin of automobiles. A deep-drawing steel of this type is known under the name HC180B (material number 1.0395).

The sheet metal samples have been divided from both sides galvanized and tempered steel strip to protect against corrosion in a hot dip galvanizing process. The zinc layer thickness was on average 10 μm in the samples. Such zinc coatings are also known in the art under the abbreviation "Z100". Alternatively, the sheet samples may also be provided with a zinc-magnesium coating.

The sheet samples were pre-degreased with an alkaline detergent and neutralized by rinsing with water. Subsequently, the cleaned surface has been dried in the heated air stream.

• H₂ZrF₆-Gehalt von 0,003 mol/l bis 0,1 mol/l der Konversionslösung, insbesondere 0,01 mol/l;
• pH-Bereich: 4 - 5,5, insbesondere bevorzugt 4, beispielsweise eingestellt mittels 10 Gew.-% Ammoniumbicarbonat-Lösung;
• Elektrische Leitfähigkeit 2000 +/- 500 µS/m, die erforderlichenfalls beispielsweise durch Zugabe eines geeigneten Leitsalzes, wie beispielsweise Natriumsulfat, eingestellt werden kann.
• Optionale Konvektion des Elektrolyten während der Schichtabscheidung.
• Optionale Begasung des Elektrolyten mit Luft oder Sauerstoff.

Subsequently, a predominantly aqueous formulation was applied to the sheet samples, which is characterized as follows:

• H ₂ ZrF ₆ content of 0.003 mol / l to 0.1 mol / l of the conversion solution, in particular 0.01 mol / l;
• pH range: 4-5.5, particularly preferably 4, for example, adjusted by means of 10% by weight ammonium bicarbonate solution;
• Electrical conductivity 2000 +/- 500 μS / m, which can be adjusted if necessary, for example, by adding a suitable conducting salt, such as sodium sulfate.
• Optional convection of the electrolyte during the layer deposition.
• Optional gassing of the electrolyte with air or oxygen.

• Beispiel 1 (Potentiostatisches Verfahren)
  • Potenzial max. -2,4 V vs. SHE, insbesondere -1,8 ± 0,1 V vs. SHE;
  • Dauer der potentiostatischen Polarisation bis 900 s, insbesondere 15 s;
  • Optional kann das vorgereinigte Substrat zwecks einer zusätzlichen anodischen Reinigung für 2 - 5 sec um 0,4 ± 0,1 V positiver relativ zum Ruhepotential polarisiert werden.
• Beispiel 2 (Galvanostatisches Verfahren)
  • Stromdichte max. -2,7 mA/cm², insbesondere -0,3 ± 0,1 mA/cm²;
  • Dauer der galvanostatischen Polarisation bis 900 s, insbesondere 15 s;
  • Optional kann das vorgereinigte Substrat zwecks einer zusätzlichen anodischen Reinigung für 2 - 5 sec anodisch polarisiert werden.
• Beispiel 3 (Puls- oder Potenzialsprungverfahren bzw. Stromsprungverfahren)

The barrier properties of the applied conversion layers were characterized by cyclic voltammetry and compared with corresponding electrolessly deposited ZrO ₂ layers ( Figures 2 and 3 ). The application time was 60 s in each case. The comparison of the cyclic voltammograms shows that the barrier properties are higher in the case of the potential- or current-assisted variant than in the case of potential-free or current-free deposition. In addition, complementary surface-analytical measurements (XPS) showed that the potential-aided variant has a significantly thicker ZrO ₂ layer ( Fig. 1 ).

• Example 1 (Potentiostatic method)
  • Potential max. -2.4 V vs. SHE, in particular -1.8 ± 0.1 V vs. SHE;
  • Duration of potentiostatic polarization up to 900 s, in particular 15 s;
  • Optionally, the pre-cleaned substrate may be polarized for 0.4- 0.1 V more positive relative to the quiescent potential for 2-5 sec for additional anodic cleaning.
• Example 2 (galvanostatic process)
  • Current density max. -2.7 mA / cm ² , especially -0.3 ± 0.1 mA / cm ² ;
  • Duration of galvanostatic polarization up to 900 s, in particular 15 s;
  • Optionally, the pre-cleaned substrate may be anodically polarized for 2 - 5 seconds for additional anodic cleaning.
• Example 3 (pulse or potential jump method or current jump method)

• Zunächst Potenzial max. -3,5 V vs. SHE, insbesondere -2,2 ± 0,1 V vs. SHE, für 5 s, anschließend Potenzialsprung auf
• Potenzial max. -1,9 V vs. SHE, insbesondere -1,3 ± 0,1 V vs. SHE, für 55 s;

Potential jump:

• First of all, potential max. -3.5 V vs. SHE, in particular -2.2 ± 0.1 V vs. SHE, for 5 s, then potential jump on
• Potential max. -1.9 V vs. SHE, in particular -1.3 ± 0.1 V vs. SHE, for 55 s;

• Stromdichte max. -2,7 mA/cm², insbesondere -1,4 ± 0,1 mA/cm², für 5 s,
  anschließend Stromsprung auf
• Stromdichte max. -0,7 mA/cm², insbesondere -0,3 ± 0,1 mA/cm², für 55 s.
• Optional kann das vorgereinigte Substrat zwecks einer zusätzlichen anodischen Reinigung für 2 - 5 sec anodisch polarisiert werden.

Current jump:

• Current density max. -2.7 mA / cm ² , in particular -1.4 ± 0.1 mA / cm ² , for 5 s,
  then jump to power
• Current density max. -0.7 mA / cm ² , in particular -0.3 ± 0.1 mA / cm ² , for 55 s.
• Optionally, the pre-cleaned substrate may be anodically polarized for 2 - 5 seconds for additional anodic cleaning.

The experiments described above have been carried out in the laboratory (beaker experiments) with practical parameters and conditions.

For the deposition of the deposited in Examples 1 - 3 ZrO ₂ coating can in practice each a device 1 of in Fig. 4 been shown schematically type used. Devices constructed according to this principle have hitherto been used, for example, for electrolytic belt cleaning.

The apparatus 1 comprises a plurality of so-called horizontal cells 2, 3, 4, each traversed in the conveying direction F, each having a basin 5, which is filled with the above-described aqueous solution as electrolyte, and through which the sheet sample P to be coated is oriented in a horizontal orientation is encouraged. The electrolyte is conveyed by a pump 6 in circulation and applied via nozzles 7 in a conventional manner so on the top and bottom of the sheet sample P, that the exiting electrolyte flow flows over a large area on the relevant side of the sheet sample P.

At the entrance and exit of the respective basin 5, a pair of rollers is arranged in each case. Each of the roller pairs comprises a current roller 8, 9 arranged above the conveying path of the sheet metal sample P and a pressure roller 10, 11 arranged below the conveying path, which presses the sheet metal sample P against the associated current roller 8, 9.

The power rollers 8,9 are connected to the negative pole (-) of a rectifier / power supply unit 12 whose positive pole (+) is connected via bus bars 13 with anodes 14, which are above and below the conveying path between the power rollers / pressure roller pairs 8,10; 9.11 extend.

Alternatively to the device according to Fig. 4 The polarization according to the invention can also be carried out in an electrically sufficiently dense spray curtain or in a suitably designed roller coating process, in which case a downstream exhaust zone or directly connected drying in a heating section with heated air or IR drying support can be provided, so that the drying at 90 ° C +/- 10 ° C within 1 - 10 seconds.

The conditions set forth above in a device of in Fig. 4 The type of deposition shown within a treatment duration of 60 s results in a Zr conversion layer whose coating weight is 20 and 200 mg / m ² . In addition forms a Si-pad with a coating weight of 5 - 500 mg / m ² .

In order to ensure anticorrosive protection during transport to the processor of the sheet samples P, the sheet samples P may be coated with a non-corrosive protective oil or with a formability-promoting agent having a typical coating weight of typically up to 1.2 g / m ² . The conversion layer according to the invention also allows overlay weights significantly below this limit value, so that the protective oil or the formability-improving agent can, if necessary, be removed particularly easily for further processing from the respective sample or the component formed therefrom.

In large-scale production, instead of the sheet metal samples P, usually steel strips are processed which pass through the device 1 in a continuous pass. The resulting flat steel product is typically wound into a coil and made storable.

In the production of body components, the sheets or strips obtained from the above-described manner according to the invention are converted into the respective component with which the vehicle body is then formed. Before the assembly of the vehicle body, the outer coating is applied, whose adhesion to the steel substrate is ensured by the conversion layer applied in accordance with the invention.

In a further experiment, a sample P provided in the manner already explained above was also pre-degreased with an alkaline detergent and neutralized by water rinse. The cleaned surface was then dried in the heated air stream.

The achievable Oberflächenzustandskonvertierende assignment is by application of a predominantly aqueous formulation in the dipping process (with 15 sec direct dipping time) with subsequent exhaust time of up to 30 sec at room temperature and optionally subsequent forced drying in, for example, heated to 140 ° C convection oven dried. A temperature increase of the first drying step up to 90 ° C compensates for increased belt speeds.

The applied conversion solution contained as a predominantly aqueous formulation Zr resulting from the H ₂ ZrF ₆ content of 0.003 mol / l to 0.1 mol / l of the conversion solution or in combination with 1 to 1.5 wt .-% organosilane resulting from the combined Epoxy-silicones containing epoxy groups.

The electrolyte system has been adjusted to a pH range of 4-5.5, in particular 4-4.2. This pH range has been stabilized by the addition of ammonium bicarbonate or another buffer system known in the art.

In the example, an electrical conductivity of the application electrolyte of 2000 +/- 500 mS / m, which is adjusted by a supporting electrolyte, and an electrode distance of 30 mm +/- 10 mm has been used.

The pre-cleaned sample was additionally polarized by 400 +/- 100 mV more positive relative to the rest potential for 2..5 s.

1. A) Es erfolgt eine Polarisation bis auf 2400 mV negativer relativ zum Ruhepotential (bevorzugt 1500 mV) für mindestens 8 sec, insbesondere 15 sec.
2. B) Alternativ kann eine gepulste Polarisation durchgeführt werden. Das 500 mV negativer relativ zum Ruhepotential polarisierte Substrat wird wie folgt mit einem gepulsten Potential beaufschlagt:
   • Pulshöhe: bis 2400 mV negativer relativ zum Ruhepotential
   • Pulsdauer: 0,1 msec
   • Zeitabstand zwischen zwei Pulsen: 1 msec
3. C) Die Zr-Belegung kann durch gezielte Kombination von Puls- und Konstantpolarisationsphasen eingestellt werden.

The following variations of the layer production are possible:

1. A) Polarization takes place up to 2400 mV negative relative to the resting potential (preferably 1500 mV) for at least 8 seconds, in particular 15 seconds.
2. B) Alternatively, a pulsed polarization can be performed. The 500 mV negative relative to the rest potential polarized substrate is applied to a pulsed potential as follows:
   • Pulse height: up to 2400 mV negative relative to the resting potential
   • Pulse duration: 0.1 msec
   • Time interval between two pulses: 1 msec
3. C) The Zr assignment can be adjusted by a specific combination of pulse and constant polarization phases.

The method according to the invention is in particular for the application of layers of the German patent application already mentioned in the introduction, incorporated by reference into the disclosure of the present application 10 2013 113 731.8 dated December 9, 2013 and International Patent Application PCT / EP2014 / 076987 suitable.

REFERENCE NUMBERS

1

Device for separating

2, 3, 4

Horizontal cells

5

pool

6

pump

7

jet

8, 9

current roles

10, 11

pressure rollers

12

Rectifier / Stromversorgungseineit

13

busbars

14

anodes

F

conveying direction

P

sheet sample

Claims (14)

A method of electrochemically depositing a thin inorganic coating on at least one surface of a metallic, electrically conductive substrate held during the deposition process in an electrolyte containing components that form the coating as a result of the deposition process, characterized in that the inorganic Coating surface of the substrate is cathodically polarized during the stay in the electrolyte. A method according to claim 1, characterized in that the metallic substrate is a flat steel product. A method according to claim 2, characterized in that the flat steel product is coated with a metallic protective layer. Method according to one of the preceding claims, characterized in that an electrolyte from the group of aqueous and acidic electrolyte solutions is used as the electrolyte. A method according to claim 4, characterized in that the electrolyte is an aqueous solution containing a Zr or Ti compound which dissociates into zirconium or titanium fluoro complexes. Method according to one of the preceding claims, characterized in that the cathodic polarization is carried out by using a potentiostatic method. A method according to claim 6, characterized in that a time-constant potential is applied to the metal surface to be coated, which corresponds to the resting potential of a substrate in the respective electrolyte and is shifted by more than 100 mV in the cathodic direction. Method according to one of claims 1 to 5,
characterized in that the cathodic polarization is effected by using a galvanostatic method. A method according to claim 8, characterized in that current densities of magnitude 0.1 mA / cm ^{2 are} set. Method according to one of the preceding claims,
characterized in that the polarization is used in a pulse or jump method. Method according to one of the preceding claims,
characterized in that the substrate is passed in passage through the electrolyte. A method according to claim 11, characterized in that the relative speed at which the electrically conductive substrate is moved through the electrolyte is 0.5-15 m / s. Method according to one of the preceding claims, characterized in that the thickness of the inorganic coating obtained on the substrate by the deposition is 20-200 nm. Method according to one of the preceding claims, characterized in that the substrate is electrochemically prepurified in a step preceding a deposition process.

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