EP2391742A1

EP2391742A1 - Zinc diffusion coating method

Info

Publication number: EP2391742A1
Application number: EP10702601A
Authority: EP
Inventors: Frank Natrup
Original assignee: Graf Wolfram; Bodycote Waermebehandlung GmbH
Current assignee: Graf Wolfram; Bodycote Waermebehandlung GmbH
Priority date: 2009-01-27
Filing date: 2010-01-27
Publication date: 2011-12-07
Also published as: SG173143A1; WO2010086151A8; US20120006450A1; WO2010086151A1; DE102009006190A1

Abstract

A method for diffusion coating a surface of a metal substrate with zinc comprises the steps of: a₁) applying a suspension, comprising a liquid and zinc and/or a zinc alloy as a diffusion source or zinc donor, onto the surface of the substrate to be coated, or a₂) applying a liquid onto the surface of the substrate to be coated and subsequently applying zinc and/or a zinc alloy as a diffusion source or zinc donor onto the liquid; and b) drying the substrate obtained in step a₁) or a₂); c) carrying out a heat treatment of the substrate obtained in step b) at a temperature between 200 and 500°C, but below the melting temperature of the diffusion source in a shielding gas atmosphere; and d) removing the mixture applied in step a₁) or a₂).

Description

Zinc diffusion coating process

The present invention relates to a method for diffusion coating a surface of a metallic substrate with zinc, wherein the substrate to be coated is heat-treated together with zinc as a diffusion source at a temperature between 200 and 500 ^° C, but below the melting temperature of the diffusion source.

Components made of corrosion-susceptible material, such as iron and steel, have been galvanized for a long time, i. provided on its surface with a comparatively thin layer of zinc to increase the corrosion resistance of the components. Examples of such components are connecting and fastening elements, such as screws and bolts, body parts for motor vehicles, crash barriers, railings, external stairs and the like. As galvanizing, for example, the hot-dip galvanizing, electroplating and Sherardizing are known.

In hot-dip galvanizing, the substrate to be coated, after a corresponding pretreatment, which conventionally comprises the steps degreasing, pickling, fluxing and drying, is immersed in a molten zinc, which usually has a temperature between 440 to 460 ^° C., and for a sufficient time in this melt leave before the thus coated substrate removed from the melt, cooled and optionally post-treated. A disadvantage of hot-dip galvanizing is found in the attempt to coat coated high-strength steel parts, because under the influence of the relatively high process temperature, these can lose crucial strength and thus become unusable. P2010 / 000489

In electrogalvanizing, the application of the zinc layer to the galvanizing material takes place by electrochemical deposition from a zinc electrolyte. However, this method is only conditionally applicable to complicated shaped substrates. Furthermore, tempered high-strength steel parts must not be treated with this method in order to exclude the risk of hydrogen embrittlement.

Another known galvanizing is the Sherardisierverfah- ren, in which the galvanizing zinc powder is usually heat treated at a temperature between 320 and 420 ⁰ C in admixture with an inert material or filler, such as sand or ceramic, for example alumina. Usually, the method is carried out in a heatable rotating drum, which is also referred to as a retort, in which the galvanizing material is embedded in the mixture of zinc powder and filler, before the drum is sealed airtight after being infested and heated to the required temperature. The filler used in the sherardization process has several functions. On the one hand, this ensures uniform heating, gentle cleaning of the components and a homogeneous distribution of the zinc powder. In addition, this prevents in the case of bulk material collapsing and thus damage to the components. The sherardization process is a diffusion coating process in which zinc from the vapor phase, which is formed by sublimation due to the comparatively high vapor pressure of zinc at the temperature used for the heat treatment, diffuses into the surface layer of the substrate to be galvanized. Such processes are described for example in DE-PS 134 594, in DE-PS 273 654 and EV Proskurkin & NS Gorbunov, "Galvanizing, sherardizing and other zinc diffusion coatings", Technicopy Limited, England, 1972, pages 1-68. 89

With the Sherardisierverfahren even with the use of complicated shaped substrates very uniform, firmly adhering to the substrate Zinkbe laminations are produced with a layer thickness of up to 100 microns, which have excellent corrosion resistance. However, the known Sherardisierverfahren have some disadvantages.

A disadvantage of the known Sherardisierverfahren is the comparatively high zinc consumption, which is caused by the combustion of zinc in the initially present in the reaction space air atmosphere with an oxygen concentration of 21 vol .-%, because the burned zinc is no longer available for coating.

In addition, there are disadvantages associated with the use of the filler, both for complex components, which are usually fixed in racks in the drum, but also for the process itself. Because of the weight of the filler, undesirable deformations of the batt can occur , Furthermore, due to the heat capacity of the filler for the process significantly more heating energy is needed than would be dispensed with the filler. Furthermore, the filler requires additional process steps and handling devices. Thus, the filler must be added to the drum prior to sherardization and subsequently separated from the coating material and cleaned for reuse. The fine dust load due to filler abrasion and zinc ash is of concern for health reasons and therefore requires special precautions for occupational safety, such as extraction systems or respiratory masks. If the filler is dispensed with in order to avoid these disadvantages, it is found in practice that the process no longer leads to film formation under normal zinc dosing and thus fails, or that the zinc consumption related to the galvanized surface is considerably increased.

Furthermore, the use of loose zinc dust during the heat treatment is complicated, because this requires a sufficient distribution of the zinc dust in the retort, which is usually achieved by a retort moved during the heat treatment, in particular because of the handling of Dust from the environmental and work-technical point of view is questionable.

Finally, the retorts commonly used in the sherardizing process are also disadvantageous. Usually, for the above reason, retorts are used for this purpose, which are rotated or moved in an oven so as to achieve a uniform distribution of the zinc dust and the filler, and to make the number of ovens lower than that of the Retorts can be kept. Because the retorts rotate within the heating chamber of the furnace, the seals are also within the combustion chamber. Since these are not completely tight, air and gaseous oxygen-containing combustion products can enter the retort during the heat treatment, which impairs the process reliability and, in particular, increases the zinc consumption for the above reasons.

The object of the present invention is therefore to provide a method for diffusion coating a surface of a substrate with zinc, which can be used to obtain very uniform zinc coatings with excellent corrosion resistance, even when complicatedly formed substrates are used a rotating or otherwise moving retort must be performed, which, based on the surface of the to zinciferous substrate, low zinc consumption and can be dispensed with the use of filler and the use of loose zinc dust during the heat treatment.

According to the invention, this object is achieved by providing a method according to claim 1 and in particular a method for diffusion-coating a surface of a metallic substrate with zinc, which comprises the following steps:

ai) applying a suspension, which contains a liquid, preferably an adhesion-promoting liquid, and zinc and / or a zinc alloy as a diffusion source or zinc donor, to the surface of the substrate to be coated, or & 2) applying a liquid, preferably an adhesion promoter, to the to be coated surface of the substrate and then

Application of zinc and / or a zinc alloy as a diffusion source or zinc donor to the liquid,

and

b) drying the substrate obtained in process step a ^ or a2), c) carrying out a heat treatment of the substrate obtained in process step b) at a temperature between 200 and 500 ^° C, but preferably below the melting temperature of the diffusion source under a protective gas atmosphere forming a zinc-containing diffusion layer on the substrate, and d) removing the mixture applied in step a ^ or aa). Within the scope of the present invention it has surprisingly been found that in a zinc diffusion based process for coating a surface of a metallic substrate during the heat treatment on the use of filler, on the use of loose zinc powder and on a movement of the retort in which the heat treatment can be omitted, if in the process before the heat treatment by the process step ai) or process step & 2) in combination with the drying according to process step b) zinc is applied firmly adhering to the substrate surface to be coated and then the substrate with the adhering thereto zinc is subjected to a heat treatment under a protective gas atmosphere. In contrast to the classical Sherardisierverfahren, in which loose zinc powder or zinc dust is used as the diffusion source during the heat treatment, in the inventive method as a diffusion source on the surface to be coated of the substrate firmly adhering zinc or zinc alloy is used. For this reason, both the use of loose zinc dust as well as the use of filler can be dispensed with in the inventive method. This achieves, inter alia, that the method according to the invention has only a low energy requirement and that deformations of the substrate to be coated are reliably prevented. In addition, for this reason, the loading density of the substrate to be coated in the retort, in which the heat treatment takes place, can be considerably increased because no volume of the retort for filler and loose zinc dust is lost.

Finally, this also allows an at least substantially dust-free and thus environmentally friendly process management.

In addition, in the method according to the invention, by eliminating the need for the retort in which the heat treatment is carried out, 2oi move, so therefore can be dispensed with the use of a movable or rotatable retort, the reaction chambers used in the process according to the invention, unlike the Sherardisierverfahren usually used in ovens positioned movable retorts can be made much simpler, which reduces investment costs. In particular, in the process according to the invention, the substrates to be coated in the reaction space do not have to be stored on a rack, in contrast to the movable retorts conventionally used in sherardization processes. In addition, it is possible in the method according to the invention due to the omission of movable retorts, which must be rotatably arranged in the combustion chamber of a furnace, to use conventional inert gas or vacuum furnace technology. Due to the gas-tight design, air penetration during the heat treatment can reliably be prevented. This is achieved in combination with the heat treatment under a protective gas atmosphere that takes place during the heat treatment no burning of zinc by reaction with oxygen, so that the zinc consumption, based on the surface of the substrate to be galvanized, is very low, even if the use of a filler is dispensed with. In addition, the zinc consumption is further reduced in the inventive method, because it is possible due to the application of the zinc as a firmly adhering layer on the substrate surface to be coated before performing the heat treatment, only as much zinc on the substrate surface, as for the later Coating is required. For this reason, the zinc used in the process according to the invention is at least almost completely consumed. For this reason too, the process costs in the process according to the invention are significantly lower than in the processes known from the prior art. For the purposes of the present patent application, a protective gas atmosphere is understood as meaning a gas or gas mixture which is less than 10%, preferably less than 1%, more preferably less than 100 ppm, even more preferably less than 10 ppm, very preferably less than 1 ppm and most preferably contains at most 0, 1 ppm oxygen.

According to a first preferred embodiment of the present invention, the application of the diffusion source zinc according to the process step ai), ie it is applied to the surface to be coated of the substrate, a suspension containing a liquid and zinc and / or a zinc alloy. The liquid used is preferably water, and more preferably distilled water. Since liquids containing water or water react or react with zinc with hydrogen formation at elevated temperatures, a cooled suspension is preferably used in process step a 1, more preferably one at a temperature of less than 20 ^° C., and most preferably cooled to a temperature of less than 10 ⁰ C suspension. As an alternative to water, it is also possible to use liquid hydrocarbons, alcohols or ketones or other liquids or liquid mixtures as liquid, provided that they are volatile, if necessary react to a limited extent with zinc and preferably dissolve salts. Preferred examples of these are trichlorethylene, dichloromethane, benzene, xylene, toluene or C 1 -C 10 alcohols, in particular methanol, ethanol, propanol and butanol. Due to the low viscosity of the suspension, a low-cost process with water makes it possible to apply the suspension uniformly to difficult-to-reach areas of the substrate surface, even in complicatedly shaped substrates to be coated, so that these areas are also affected by the subsequent heat treatment Substrate surface are uniformly coated with zinc. In this first embodiment of the present invention, good adhesion of the zinc or of the zinc alloy to the substrate surface to be coated is achieved by the drying of the suspension applied to the substrate surface in process step b). To improve the adhesion of the zinc or of the zinc alloy on the substrate surface to be coated after drying, the suspension used in process step a ^ may be added to at least one further compound, which is selected from the group, in addition to the liquid and the zinc or zinc alloy which consists of dispersants, binders, wetting agents and any mixtures of two or more of the aforementioned compounds. While the dispersant improves the homogeneity of the suspension, and thus uniform application of the diffusion source to the substrate surface, and the wetting agent facilitates wetting of the substrate surface, the binder acts as an adhesion enhancing adhesive.

According to a second preferred embodiment of the present invention, the application of the diffusion source zinc is carried out according to method step a2), ie first a liquid, preferably an adhesion promoter, is applied to the surface of the substrate to be coated before subsequently applying to the liquid or liquid Adhesive zinc and / or a zinc alloy is applied. As the liquid or adhesion promoter, a solvent is optionally used in admixture with at least one further compound selected from the group consisting of dispersants, binders, wetting agents and any mixtures of two or more of the aforementioned compounds. The solvent used is preferably water, and more preferably distilled water. alternative to Water can also be used as solvent liquid hydrocarbons, alcohols or ketones or other liquids or liquid mixtures, provided that they are volatile, if necessary, react with limited zinc and preferably salts can solve. Preferred examples of these are trichlorethylene, dichloromethane, benzene, xylene, toluene or C ₁ -C ₁ 0 alcohols, in particular methanol, ethanol, propanol and butanol.

In this second embodiment of the present invention, too, good adhesion of the zinc or zinc alloy to the substrate surface to be coated is achieved by drying the liquid applied to the substrate surface or the adhesive applied to the substrate surface in process step b) Zinc reached. In order to improve the adhesion of the zinc or of the zinc alloy on the substrate surface to be coated after drying, at least one compound selected from the group consisting of dispersant, binder, and the liquid / adhesion promoter used in process step a2) may be added. Wetting agent and any mixtures of two or more of the aforementioned compounds.

Regardless of whether the diffusion source is applied according to method step a 1 or alternatively according to method step a 2), it is preferred that the suspension applied in method step a 1 or the liquid / adhesion promoter applied in method step a 2) at least one binder, wherein the binder is preferably selected from the group consisting of halogen salts, sulfates, sulfites, phosphates, silicates, boron compounds, water glass and any mixtures of two or more of the aforementioned compounds. Particularly good results are achieved, in particular, when the binder is selected from zinc chloride, sodium chloride, potassium chloride, ammonium chloride, zinc sulfate and any mixtures of two or more of the aforementioned compounds. These compounds have the advantage that they are thermally stable at the temperature at which the heat treatment is carried leads, ie between 200 and 500 ⁰ C. In addition, they are readily soluble in water, which is why they can easily be removed again from the coated substrate after the heat treatment. Furthermore, the abovementioned salts also act as fluxes and activators. Finally, the salts corrosively attack the substrate and zinc dust, thereby removing any oxides present on the substrate surface.

As wetting agent, the suspension or the liquid / adhesion promoter applied in process step a) may preferably contain at least one compound which is selected from the group consisting of surfactants and alcohols. Examples of suitable alcohols are methanol, ethanol, propanol, butanol or pentanol.

Preferably, the applied suspension or applied in the step a2), liquid prior to application to the substrate to a temperature of less than 20 ° C and cooled in step ai) preferably to a temperature of less than 10 ⁰ C.

An exemplary suspension suitable for carrying out the first embodiment of the present invention may be, for example, water saturated with sodium chloride, zinc, zinc chloride (for example, in an amount of 1% by weight) and alcohol (preferably ethanol or methanol). An example of a suitable adhesion promoter for carrying out the second embodiment of the present invention is, for example, a solution consisting of water, sodium chloride, zinc chloride and alcohol (preferably ethanol or methanol).

For reasons of good suspensibility, zinc in the form of zinc powder or zinc dust having an average particle size of not more than 100 μm, preferably not more than 20 μm and particularly preferably not more than 10 μm, is used in method step ai) or alternatively in method step a2). Especially with zinc powder or

Zinc dust with a mean particle size between 3 and 6 μm and with a maximum particle size of 70 μm will get good results.

Furthermore, it is preferred that the zinc used has a zinc content between 90 and 100 wt .-% and particularly preferably a zinc content between 99 and 100 wt .-%.

With regard to the way in which the suspension in process step ai) or the liquid or the adhesion promoter in process step a2) is applied to the substrate, the present invention is not limited. This can be done for example by spraying, dipping, brushing, rolling or brushing.

By the drying carried out in process step b), the diffusion medium, ie the zinc or the zinc alloy, is adhered to the substrate surface. Preferably, the drying in the process step b) is carried out at a temperature between 40 and 250 ⁰ C. In order to assist the drying process, the substrate may additionally lent before coating, ie before step c) and preferably before Step b), preferably to a temperature between 40 and 250 ⁰ C are heated.

The heat treatment in process step c) can in principle be carried out at any temperature and duration known for conventional sherardization processes. Preferably, the heat treatment is carried out at a temperature between 300 and 420 ⁰ C and particularly preferably at a temperature between 340 and 410 ⁰ C. However, the heat treatment should preferably be carried out at a temperature lower than the melting temperature of the diffusion source, ie, the zinc or the zinc alloy applied in the step ai) and / or Si2), otherwise there is a fear of sintering of the substrate surface ,

The duration of the heat treatment may be, for example, between 10 minutes and 10 hours.

As stated above, the heat treatment according to the invention is carried out under a protective gas atmosphere, under protective gas atmosphere here is a gas or a gas mixture is called which contains less than 10% oxygen. Particularly good results are obtained when the protective gas atmosphere during the heat treatment in process step c) less than 1%, more preferably less than 100 ppm, even more preferably less than 10 ppm, most preferably less than 1 ppm and most preferably not more than 0 Contains 1 ppm oxygen.

Examples of corresponding protective gases are noble gases, nitrogen, methane, C ₁ -C 4 -alkanes, C ₁ -C 4 -alkenes, C ₁ -C 4 -alkynes, silanes, hydrogen and ammonia. Basically, the present invention is not limited in the pressure at which the heat treatment is performed. For example, the heat treatment can be carried out at atmospheric pressure or a slight overpressure of up to 1.5 bar, preferably between 1.02 and 1.2 bar. This ensures that in the event of accidental leakage in the retort during the heat treatment, no air enters the retort.

However, it is equally possible to carry out the heat treatment at a reduced pressure, for example at a pressure of between 10 ² and 0.99 bar, and preferably between 1 and 10 mbar.

As stated above, the process according to the invention is preferably carried out without filler. However, it is also possible, although less preferred, to add a little filler to the retort. In this less preferred case, however, it is advantageous if the filler content in the retort during the heat treatment, based on the volume of the reaction space, is less than 20%, more preferably less than 10% and most preferably less than 1% ,

To carry out the heat treatment, the retort with the substrate to be coated and the diffusion source adhered thereto is first heated to the heat treatment temperature before the temperature is maintained for a time sufficient for the heat treatment and the retort is finally cooled to room temperature. For the purposes of the present patent application, heat treatment is understood to be the period of time during which the temperature required for heat treatment is maintained, ie excludes during the heating and cooling phase. According to the invention, the heat treatment is carried out under a protective gas atmosphere, whereas during the heating rate and / or the cooling phase no Intergasatmo- sphere must be present, although this is less preferred.

According to a further preferred embodiment of the present invention, it is provided that the oxygen content in the atmosphere contained in the reaction space in which the heat treatment is carried out already before the start of the heat treatment, i. before the temperature at which the heat treatment is carried out, ie already before or at the latest during the heating phase, to less than 10%, preferably less than 1%, more preferably less than 100 ppm, even more preferably less than 10 ppm, more preferably less than 1 ppm and most preferably not more than 0.1 ppm. This reliably prevents zinc from being burnt when the reaction chamber is heated up by oxygen.

Furthermore, according to a further preferred embodiment of the present invention, it is likewise preferred in the reaction space in which the process step c) is carried out to reduce the oxygen content in the protective gas atmosphere contained in the reaction space to less than 10%, preferably to less than 1%. , more preferably less than 100 ppm, even more preferably less than 10 ppm, most preferably less than 1 ppm, and most preferably not more than 0.1 ppm oxygen to be adjusted or maintained until the temperature in the reaction space after completion of the heat treatment is cooled to less than 200 ⁰ C / has been. As a result, the formation of a sinter cake can be reliably prevented. Further, it is preferable that the mixture or the diffusion source applied after the heat treatment is removed from the coated substrate by, for example, washing, ultrasonic treatment or brushing of the coated substrate in the process step a or the process step.

In addition, the coated substrate can be passivated after the heat treatment, preferably after removal of the mixture applied in process step ai) or in process step a2).

In a further development of the concept of the invention, it is proposed to clean the substrate prior to application according to method step ai) or method step a2) by preferably mechanical surface treatment. This can be done for example with a blasting agent, by pickling in alkaline or acidic solutions and / or by treatment with a flux.

The method according to the invention is particularly suitable for coating substrates which consist of a zinc-settable metal, preferably iron and its alloys, such as steel and cast iron, copper and its alloys and / or aluminum and its alloys.

Claims

claims:

A method of diffusion coating a surface of a metallic substrate with zinc comprising the steps of:

ai) applying a suspension comprising a liquid and

Zinc and / or a zinc alloy as a diffusion source or zinc dispenser, on the surface to be coated of the

Substrate, or

Α2) applying a liquid to the surface of the substrate to be coated and then applying zinc and / or a zinc alloy as diffusion source or zinc donor to the liquid,

and

b) drying the substrate obtained in step ai) or a2), c) carrying out a heat treatment of the substrate obtained in step b) at a temperature between 200 and 500 ^° C. but below the melting temperature of the diffusion source under a protective gas atmosphere and d) Removing the mixture applied in step ai) or a2).

2. The method according to claim 1, characterized in that the suspension applied in step ai) in addition to the liquid and the zinc and / or the zinc alloy at least one white contains a compound which is selected from the group consisting of dispersants, binders, wetting agents and any mixtures of two or more of the aforementioned compounds.

A process according to claim 1, characterized in that the liquid applied in step a.2) contains a solvent and optionally at least one further compound selected from the group consisting of dispersant, binder, wetting agent and any mixtures of two or more of the aforementioned compounds.

4. The method according to at least one of claims 1 to 3, characterized in that the liquid of the applied in the step ai) suspension or the solvent of the liquid applied in step a2) contains or is a compound which consists of water , Alcohols, liquid hydrocarbons, ketones and any mixtures of two or more of the aforementioned compounds.

5. The method according to claim 4, characterized in that the liquid of the applied in step ai) suspension o- the solvent of the applied in step a2) liquid trichlorethylene, dichloromethane, benzene, xylene, toluene or a Ci-Cio-alcohol , preferably methanol, ethanol, propanol or butanol, contains or consists thereof.

6. The method according to at least one of claims 2 to 5, characterized in that the suspension applied in step a ^ or the liquid applied in step a2) contains at least one binder which consists of halogen salts, sulphates, sulphites, Phosphates, silicates, boron compounds, water glass and any mixtures of two or more of the aforementioned compounds.

7. The method according to claim 6, characterized in that in the step ι) applied suspension or the applied in the step a2) liquid contains at least one binder which from the zinc chloride, sodium chloride, potassium chloride, ammonium chloride, zinc sulfate and any Mixtures of two or more of the aforementioned compounds existing group is selected.

8. The method according to at least one of claims 2 to 7, characterized in that the suspension applied in step ai) or the liquid applied in step a2) contains at least one wetting agent which consists of surfactants and alcohols, preferably methanol, ethanol , Propanol, butanol or pentanol, existing group is selected.

9. The method according to at least one of the preceding claims, characterized in that the applied in step a ^ suspension or the applied in the step BL2) liquid before application to the Substrate is cooled to a temperature of less than 20 ⁰ C and preferably to a temperature of less than 10 ⁰ C.

10. The method according to at least one of the preceding claims, characterized in that in step ai) or in step a2) zinc in the form of zinc powder or zinc dust having an average particle size of at most 100 .mu.m, preferably of at most 20 .mu.m and more preferably of maximum 10 microns is used.

11. The method according to claim 10, characterized in that in step a ^ or in step a2) zinc in the form of zinc powder or zinc dust with an average particle size between 3 and 6 microns and a maximum particle size of 70 microns is used.

12. The method according to at least one of the preceding claims, characterized in that the suspension in step a ^ or the liquid in the step a2) is applied to the substrate by means of spraying, dipping, brushing, rolling or brushing.

13. The method according to at least one of the preceding claims, characterized in that the drying in step b) is carried out at a temperature between 40 and 250 ⁰ C.

14. The method according to at least one of the preceding claims, characterized in that the substrate is heated to a temperature between 40 and 25O ⁰ C prior to coating.

15. The method according to at least one of the preceding claims, characterized in that the heat treatment in the step c) for a time between 10 minutes and 10 hours at a temperature between 300 and 420 ⁰ C and preferably at a temperature between 340 and 410 ° C. is carried out.

16. The method according to at least one of the preceding claims, characterized in that the heat treatment in step c) is carried out under a protective gas atmosphere which is less than 10%, preferably less than 1%, more preferably less than 100 ppm, even further preferably less than 10 ppm, most preferably less than 1 ppm and most preferably not more than 0.1 ppm oxygen.

17. The method according to at least one of the preceding claims, characterized in that the protective gas is selected from the group consisting of noble gases, nitrogen, methane, C ₁ -C ₄ -AIkBHCn, Ci-Cv alkenes, Silanes, hydrogen, ammonia and any combination of two or more of the aforementioned compounds.

18. The method according to at least one of the preceding claims, characterized in that the heat treatment is carried out at a pressure between 1 and 1.5 bar and preferably between 1.02 and 1.2 bar.

19. The method according to at least one of claims 1 to 17, characterized in that the heat treatment at a pressure between 10 ² and 0.99 bar and preferably between 1 and 10 mbar is performed.

20. The method according to at least one of the preceding claims, characterized in that in the reaction space during the heat treatment, based on the volume of the reaction space, less than 20% filler, preferably less than 10% filler, more preferably less than 1% filler and completely particularly preferably no filler is present.

21. The method according to at least one of the preceding claims, characterized in that before the start of the heat treatment in the reaction space in which the step c) is carried out, the oxygen content in the atmosphere contained in the reaction space to less than 10%, preferably less than 1%, more preferably less than

100 ppm, more preferably less than 10 ppm, most preferably less than 1 ppm, and most preferably not more than 0.1 ppm, and then the heat treatment is started and carried out in the reaction space in the atmosphere thus prepared.

22. The method according to at least one of the preceding claims, characterized in that in the reaction space in which the step c) is carried out, the oxygen content in the protective space contained in the reaction space gas atmosphere so long to less than 10%, preferably to less than 1%, more preferably less than 100 ppm, even more preferably less than 10 ppm, most preferably less than 1 ppm and most preferably not more than 0.1 ppm Oxygen is adjusted or held until the temperature in the reaction chamber after the completion of the heat treatment has cooled to less than 200 ⁰ C.

23. A method according to at least one of the preceding claims, characterized in that the coated substrate is cleaned in step d) by washing, ultrasonic treatment or brushing in order to remove the mixture applied in step ai) or a2).

24. The method according to at least one of the preceding claims, characterized in that the coated substrate is passivated after the heat treatment.

25. The method according to at least one of the preceding claims, characterized in that the substrate prior to the application step according to step ai) or a2) by mechanical surface treatment with a blasting agent, by pickling in alkaline or acidic solutions and / or by treatment with a flux purified becomes.

26. The method according to at least one of the preceding claims, characterized in that the substrate consists of a zinc-alloyable metal, preferably of iron and its alloys, such as steel and cast iron, copper and its alloys and / or aluminum and its alloys ,