WO2010112481A1 - Method for producing sol-gel anti-corrosion coatings for solar absorbers - Google Patents

Abstract

The invention relates to a method for producing sol-gel anti-corrosion coatings for solar absorbers, the surfaces of which are composed of thin layers of cermet materials, which are deposited on a metallic substrate. The method comprises mixing at least one mercaptosilane dissolved in an organic solvent with water and an acid catalyst and with an optionally present further silane, bringing a substrate into contact with the resulting sol mixture over a period of time that is sufficient for allowing a film of at least monomolecular thickness developing on the substrate to be formed on the substrate, removing the sol mixture from the substrate, and drying and finally heat-treating the substrate.

Description

 ALANOD Aluminium-Veredlung GmbH & Co. KG, Egerstr. 12, D-58256 Ennepetal

"Process for the preparation of sol-gel corrosion protection coatings for solar absorbers"

The invention relates to a process for the preparation of sol-gel corrosion protection coatings for solar absorbers. Such solar absorber surfaces are made of thin layers of cermet materials deposited on a metallic substrate.

Material destruction by environmental influences, in particular in the case of metallic or metallized devices or devices, causes high costs, which can be avoided by the use of suitable protective coatings that can be applied by various techniques. This also applies to solar panels. For example, the growing demand for replacing fossil fuel for building heating, which accounts for approximately 50 percent of household energy consumption, has led to the development of façade solar collector systems. If such absorbers are used without additional glazing, the effect of the weather to which they are exposed must be taken into account in a different way. Thus, while the numerous commercial cermet coatings, z. Dark color, a good compromise in terms of color and spectral selectivity, but it must, before one such coating can be used, its corrosion resistance can be improved.

Known investigations on corrosion stability - such as: S. Brunold; U. Free; Carlsson; K. Moller; M. Kohl, Solar Energy Materials and Solar CeIIs 2000, 61, (3), 239-253 - are naturally concerned with the importance of the long-term stability of solar absorber coatings up to the efficiency of the conversion of solar radiation to heat. Such problems are not easy to solve in that, in addition to a detailed understanding and the evaluation of possibilities for preventing corrosion, they also require knowledge for the production of barrier layers, which can prevent the corrosive degradation of coatings without changing their optical properties.

One known way to remedy insufficient long-term stability of absorbers is the production of absorber surfaces, which consist of numerous layers, which u. a. improve the corrosion stability, as z. As described in EP 1 867 934 A1. However, such a multilayer structure can only be produced in many successive coating steps, which considerably increases the price of such an absorber. Although satisfactory corrosion inhibition can be achieved by the application of thin coatings (below 1000 nm) by means of chemical vapor deposition (CVD), physical vapor deposition (PVD) or sputtering and plasma chemical vapor deposition (PECVD) Deposition costly vacuum deposition techniques, as described by way of example in the ES 2 061 399 A1.

In contrast, a sol-gel treatment can be accomplished with relatively little equipment and with mild processing and environmental conditions. Here, silanes having the general formula (R-Si (OR ¹ ) ₃ wherein R is an aminoalkyl, acryloxypropyl, glycidoxypropyl, isocyanatopropyl, alkylaryl, or other similar radical, and OR ^{1 is} ethoxy, methoxy , Acetoxy or another As well as a bis-end-capped (OR'-polymer-OR ') silane are the most commonly used precursors for the production of thin films for corrosion protection. These coatings act as a corrosion barrier by keeping corrosive media away from the metal to be protected where otherwise cathodic or anodic corrosion reactions are diffusion controlled. Since the anticorrosive effect is dependent on the layer thickness, comparatively thick coatings must be applied in order to achieve adequate protection, as described in US Pat. Nos. 3,935,349, 4,754,012, 5,814,137, 6,261,638 and 7,141 306, WO 2007/085339 A1 and in: M. Fir; Orel; AS Vuk; A. Vilcnik; R. Jese; V. Francetic, Langmuir 2007, 23, (10), 5505-5514.

As described in WO 02/072283 A1 and US Pat. No. 5,750,197, mercaptosilanes have also been used mainly for the production of protective films on aluminum alloys, but they have not been recognized as effective corrosion inhibitors for spectrally selective surfaces.

In the past, even ultrathin uniform sol-gel coatings have been made that are invisible to the human eye, but where the anticorrosive performance of aging has decreased significantly. Such a process, known for example from US Pat. No. 5,175,027, also suffers from a very short possible processing time from the production of the sol to its application. For solar reflectors, which are used for the bundling of sunlight in high-temperature collectors, relatively thick sol-gel coatings were applied, but there was no requirement for a low thermal emissivity, as this z. B. in WO 2007/059883 A1 is collected. For the application of protective layers on a solar absorber, such a method proves to be insufficient, since both the layer thickness, as well as a still insufficient corrosion stability too great an influence on the thermal emissivity and the solar absorbance show. The object of the present invention is the development of a method of the type mentioned for the production of ultra-thin, ie from 1 to 500 nm thick coatings, which on the one hand ensure adequate corrosion protection, but on the other hand, do not degrade the optical properties of solar selective surfaces. In particular, this means that the solar absorptivity α _s should not decrease and the thermal emittance ετ should remain below 0.20.

The method according to the invention comprises mixing at least one mercaptosilane dissolved in an organic solvent with water and an acid catalyst, applying the resulting sol mixture to a substrate for an exposure time sufficient to form an at least monomolecular film on the substrate, the separation of sol and film-coated substrate and a subsequent drying and a final heat treatment. Optionally, the initial mixing of the mercaptosilane can also be carried out with at least one further silane.

In accordance with the invention, mercaptosilane-based sol-gel corrosion protection films, preferably having a thickness of between 0.5 and 500 nm, can be produced on various metallic substrates, including metallic films provided with optically effective layer systems on their surface selective cermet layers, such as chromoxynitride, titanium oxynitride or titanium-aluminum oxynitride, used in solar heat applications. Most notable is the application of these coatings to solar heat absorber surfaces. The coatings according to the invention advantageously ensure very good corrosion protection and do not impair the thermal emissivity of these surfaces, which can increase by at most a few percentage points, with the solar absorption capacity even increasing. Further advantageous design features and advantages of the invention are contained in the following description and in the dependent claims, wherein the method according to the invention is explained in more detail with reference to preferred embodiments illustrated in the accompanying drawings. Showing:

1 photos of cermet-coated samples exposed in salt spray chambers, differently anticorrosive treated,

2 in comparison, potentiodynamic curves of a coated according to the invention and an uncoated cermet,

3 shows a comparison, hemispherical reflection spectra of a coated according to the invention and an uncoated cermet,

Fig. 4 Photos of exposed in Salzsprühkammern, different anti-corrosive treated, aluminum samples.

According to the invention, the production of nanodynic anti-corrosion films - in particular with a thickness in the range from 0.5 to 500 nm - can be carried out on solar, spectrally selective surfaces, as are typical of the solar absorbers commercially available under the names Sunselect, TiNOX and Eta plus, which made of sheet metal, such as aluminum, copper, iron or their alloys, and with a, for example, chromium and titanium oxynitride existing cermet absorber layer and optionally further, z. B. superficially arranged antireflection and / or protective layers of tin or silicon oxide are coated.

1 shows the increased anticorrosive effect of the coating according to the invention on commercially available Sunselect material. This material is used as an absorber coating for solar panels and differs from other selective coatings for the solar industry by its extremely low emissivity and blue coloration. Sunselect is an absorber material in which a multilayer optical system is located on a copper carrier, the uppermost layer of which is an antireflection coating made of tin oxide SnO 2. An immediately underlying layer is an absorbing layer consisting of chromium oxynitride CrNO _x . Under this layer, in turn, lies a metallic IR reflection layer. The various layers are sputtered over a large area onto the metal substrate.

Fig. 1 shows photographs of three differently protected 5 cm x 5 cm samples exposed in a chamber to a salt spray test according to ASTM B 117-07a: ("Standard Practice for Operating Salt Spray (Fog) Apparatus").

The left sample (a) had an unprotected cermet surface already showing corrosion after 24 hours, the middle sample (b) had a cermet surface protected by a known non-inventive coating and exhibiting corrosion after 120 hours of corrosion, which was prepared on the basis of polyphenylsilsesquioxane and the right sample (c) had a cermet surface protected with 3-mercaptopropyltrimethoxysilane sol (MPTMS), which showed no signs of corrosion even after 480 hours. It can be seen that protection with MPTMS significantly improves the corrosion stability in a salt spray chamber.

FIG. 2 shows potentiodynamic curves of unprotected (curve a) and MPTMS-protected Sunselect (curve b). The potentiodynamic measurements were carried out with the potentiostat galvanostat device AUTOLAB PGSTAT30, using a three-electrode cell K0235 from Princton Applied Research. The sample to be examined was switched as a working electrode, while a platinum grid was used as the counter electrode and Ag / AgCl as the reference electrode. In a measuring range of -0.5 V to 0.9 V, potential difference polarization curves were recorded at a scanning speed of 0.5 mV / s in a 0.5 molar NaCl electrolyte. As the comparison of the two curves a, b illustrates, the coating causes it to decrease the corrosion current and a shift of the corrosion potential to more negative values, indicating that MPTMS acts as a mixed inhibitor (anodic and cathodic inhibitor) with a more pronounced cathodic efficiency.

FIG. 3 shows the hemispherical reflection spectrum of protected (curve a) and unshielded Sunselect (curve b) with calculated values of the solar absorptivity α _s and the thermal emissivity ε j. The optical properties of the samples were measured by measuring the IR absorption and reflection spectra on samples with a minimum size of 5 cm * 5 cm. Visible (VIS) and near-infrared (NIR) reflectance spectra were determined with the Perkin Elmer Lambda 950 UVA / is / NIR device using an integrating sphere (150 mm module), while the mid IR spectra were determined were determined using a Bruker IFS 66 / S spectrometer equipped with an integrating sphere (OPTOSOL). As standard for the diffuse reflection, a gold plate was used. The values of the solar absorptivity α _s and the thermal emissivity εj were calculated according to a standard method described in M. Kohl, G. Jorgensen, AW Czandera: Performance and Durability Assessment, Optical Materials for Solar Thermal Systems, Elsevier, The Netherlands, 2004 and by MG Hutchins: Spectrally selective matehals for efficient visible, solar and thermal radiation control, described in: M. Santamouris (ed.), Solar Thermal Technologies for Buildings, James & James, London, 2003.

The curve comparison indicates no significant change in selectivity through the coating. The increase in the thermal emissivity of a sample protected according to the invention was less than 0.35, typically less than 0.10. An increase of less than 0.05 could be achieved if one were to pay even more attention to a corresponding preparation of the coating according to the invention. The preservation of selectivity on the one hand, and the increase in corrosion stability, on the other hand, are the most important effects of the present invention.

The inventive method for increasing the corrosion protection is also suitable for bare metallic substrates such as copper, aluminum, iron and their alloys. This is illustrated by Fig. 4, where photos of AI 2024-T3 samples of size 5 cm * 5 cm are shown, which are also in salt spray chambers a salt spray test after the o. Standard ASTM B 117-07a were subjected. The exposure of the samples took place over different periods. The upper row shows samples not adequately protected by a layer based on isobutyltrimethoxysilane according to the invention, while the photos in the lower row represent samples which were protected according to the invention with a sol containing MPTMS according to Embodiment 3 described below. It can be seen that the samples protected according to the invention show excellent corrosion stability in the salt spray chamber, corrosion only beginning after 13 days, whereas in comparison the samples with the coating containing no MPTMS were completely corroded after one day.

The protection of a substrate can be achieved according to the invention in particular by the application of a mixture of the following constituents:

i) 30% by mass to 98% by mass, preferably more than 60%

Percent by mass, of an organic solvent, preferably an aliphatic alcohol, in particular ethanol, or else 3-butoxy-2-propanol or propylene glycol monobutyl ether;

ii) 0.01% by mass to 70% by mass, preferably 0.02% by mass to 60% by mass, of at least one mercaptosilane of the general formula:

wherein R ¹ , R ^{2 are} independently selected from alkyl (1 - 14 C), cycloalkyl or aryl groups, wherein each n is an integer from 0 to 20, m is an integer from 0 to 2 and r is an integer from 1 to 3, where: m + r = 3;

iii) an acid catalyst (any compound whose aqueous solution has a pH between 0-6.9) selected from the group consisting of HCl, HBr, HI, HNO ₃ , H ₂ SO ₄ , H ₃ PO ₄ , ants -, acetic, propionic, butyric, salicylic, trifluoroacetic, trichloroacetic, trifluoromethanesulfonic and methanesulfonic acid, so that the final concentration is 1 .mu.mol to 1 mmol;

iv) 0.001% by mass to 50% by mass, preferably 0.01% by mass to 45% by mass, of water;

v) 0% by mass to 50% by mass; preferably 0% by mass to 40% by mass of another, singly end-capped, hydrolyzable silane having the general formula:

worin - jedoch nicht darauf beschränkt - R¹, R² unabhängig voneinander ausgewählt sind aus Alkyl- (1 - 10 C-Atome), Cycloalkyl- (1 - 10 C- Atome) oder Arylgruppen, wobei jeweils m eine ganze Zahl von 0 bis 2, n eine ganze Zahl von 0 bis 3 und r eine ganze Zahl von 1 bis 4 ist, wobei gilt: m + n + r = 4, und worin - jedoch nicht darauf beschränkt - R³ unabhängig von R¹ und R² ausgewählt ist aus Alkyl-, Cycloalkyl-, substituierten Alkyl-, substituierten Cycloalkyl-, Arylgruppen, substituierten Arylgruppen, fluorierten Alkylgruppen, fluorierten Arylgruppen oder fluorierten Cyclo- alkylgruppen,

wherein R ¹ , R ^{2 are} independently selected from alkyl (1-10 C-atoms), cycloalkyl- (1-10 C-atoms) or aryl groups, wherein each m is an integer from 0 to 2, n is an integer from 0 to 3 and r is an integer from 1 to 4, where: m + n + r = 4, and wherein, but not limited to, R ^{3 is} independently selected from R ¹ and R ² is selected from alkyl, cycloalkyl, substituted alkyl, substituted cycloalkyl, aryl groups, substituted aryl groups, fluorinated alkyl groups, fluorinated aryl groups or fluorinated cycloalkyl groups,

and / or another double-end-capped hydrolyzable silane having the general formula:

wherein R ¹ , R ^{2 are} independently selected from alkyl (1-10 C-atoms), cycloalkyl- (1-10 C-atoms) or aryl groups, wherein each m is an integer from 0 to 2 and r is a whole Number from 1 to 3, where: m + r = 4, and wherein, but not limited to, R ^{4 is} independently selected from R ¹ and R ² from differently terminated or substituted alkyl chains, polydialkylsiloxane, polyether chains or perfluoroalkylated chains.

The mixture is prepared in such a way that only monomeric and oligomeric species are formed by the SoI gel method; Preferably, the sol consists only of fully hydrolyzed monomers and dimers of very low Amount of fully hydrolyzed linear thmeres. The regulation of the hydrolysis / condensation ratio is achieved with an appropriate amount of added water. Therefore, it is advantageous to use more than one equivalent of water for each hydrolyzable alkoxy group. The mixing time for preparing the sol-mixture can be in the range of one minute to seven days.

In particular, the substrate is immersed in the sol mixture so long that it can soak up in the event that it has a porous surface, the immersion time being sufficient for the sol to be one, preferably virtually self-forming, at least monomolecular Gel layer formed on the surface of the substrate. Commercial solar absorber surfaces are porous, so this step is of great importance. If the SoI can penetrate deeply into this porous structure, better corrosion protection is possible. The formation of a layer that preferably bonds itself together so that the condensation reaction proceeds and the penetration of the sol can take place in particular within a period in the range of one second to five hours, wherein the immersion time is preferably more than five seconds and less than four hours should be.

The removal of the sol can be done in a number of ways, but it should be understood that the rate at which the removal of the sol occurs significantly affects the thickness and anti-corrosive effect of the later protective layer. Too thick a film can degrade the solar spectral selectivity of the sample. Therefore, the speed of the sample removal from the sol should be between 0.1 cm / s and 100 cm / s, whereby in a discontinuous process the sample can be pulled out of the sol, especially on a laboratory scale by hand.

The sample is then after removal of the sol or from the sol over a period in the range between one second and two days, preferably in Range between one second and 36 hours, air dried to remove the solvents.

At the end, the sample is subjected to a heat treatment, also referred to as baking, to complete the condensation and achieve complete crosslinking. In this step, the air-dried gel forms a dense uniform water impermeable film. The heat treatment can be carried out at various temperatures, whereby the burn-in time can be reduced with increasing temperature. The heat treatment of the samples is carried out in the temperature range from 70 ⁰ C to 300 ⁰ C for a time in the range of one second to 5 hours, preferably in the range of 85 ⁰ C to 250 ⁰ C over a period of time in the range of one second to four hours.

The coating of samples may be accomplished using different techniques for applying the sol to the substrate. If the substrate is sheet-shaped, it can be coated by means of the above-described - also referred to as a batch process - dip coating process.

However, it is of particular advantage if the substrate is in the form of a tape wound in roll form - a so-called coil - because then a continuous coating process can be used.

For this purpose, the substrate strip is preferably guided from a coil consisting of uncoated substrate by means of a transport cylinder arrangement through a container which contains the sol mixture for producing the coating, wherein the intended immersion time, which may correspond to that of the batch process, is maintained , Thereafter, the tape is pulled out of the bath, dried by blowing with air and then passed through an oven where it is heated to complete the condensation. Finally, it is cooled to room temperature and rolled up into a coil of coated substrate. To achieve an optimal coating, is a moving speed of the tape taking into account the above conditions, in particular the residence times and temperatures in each section of the coating process, to the respective section lengths of the entire machine assembly, in particular the bath container and the furnace set.

The method according to the invention is not restricted to immersion methods with regard to the manner of application of the sol, but other application techniques known for the realization of sol-gel methods, such as spraying, roller application or application by means of a slot nozzle, may also be provided ,

To demonstrate various ways of producing the coating, the following three examples are given.

Example 1: Thin MPTMS coating on absorber material

4 g of 3-mercaptopropyltrimethoxysilane (MPTMS) were dissolved in 25 g of ethanol. The solution was acidified with 0.1 ml of 0.1 molar hydrochloric acid and stirred for four hours. During this time, only hydrolyzed monomers and dimers formed. During the hydrolysis, the sol was additionally diluted with 75 ml of ethanol. The SoI was applied to a commercially available Sunselect material. In a reaction time of 240 s it was achieved that the sample was able to absorb SoI, preferably to soak with the sol, with the monomers and dimers penetrating into the porous structure of the material. Thereafter, the samples at a rate of 10 cm / s from the SoI were extracted for 60 minutes and then air-dried at 140 subjected to ⁰ C for 60 minutes heat treatment. The Sunselect protected in this way showed excellent anti-corrosion properties. It was stable for more than 20 days in the salt spray chamber test described above, and no significant deterioration in optical properties was observed. The increase in solar absorptivity was in the range of 0.00-0.05, and the increase in the thermal emissivity was 0.00-0.10, preferably less than 0.05, which makes this coating superior to other similar coatings. Nevertheless, Sunselect was resistant to fingerprints.

Example 2: Hydrophobic M PTMS M isch coating on absorber material

3 g of 3-mercaptopropyltrimethoxysilane (MPTMS) were mixed with 1 g of 1H, 1H, 2H, 2H-perfluorooctylthethoxysilane and dissolved in 25 g of ethanol. The solution was acidified with 0.1 ml of 0.1 molar hydrochloric acid and stirred for four hours. During this time, only hydrolyzed monomers and dimers formed. During the hydrolysis, the sol was additionally diluted with 75 ml of ethanol. The SoI was applied to a commercially available Sunselect material. In a reaction time of 240 s it was achieved that the sample was able to absorb SoI, preferably to soak with the sol, with the monomers and dimers penetrating into the porous structure of the material. Thereafter, the samples at a rate of 10 cm / s from the SoI were extracted for 60 minutes, air dried and subjected at 140 ⁰ C for 60 minutes heat treatment. The sunscreen thus protected showed excellent anticorrosive properties (stability for more than 20 days in the salt spray chamber) and no significant deterioration in optical properties was observed. The increase in solar absorbance was in the range of 0.00-0.05, and the increase in thermal emissivity was 0.00-0.10, preferably less than 0.05, making this coating superior to other similar coatings. Nevertheless, Sunselect was resistant to fingerprints. Contact angle determined with water of this coating were significantly higher in comparison with the coating according to Example 1, namely at 100 °, instead of 75 °. The contact angles were determined by means of a Krüss type G-1 contact angle measuring device using a static drop measurement technique with 20 μl drops of MiIIi-Q water with result formation from three measured values. Example 3: Hydrophobic MPTMS Mixed Coating on Aluminum Alloy

3 g of 3-mercaptopropyltrimethoxysilane (MPTMS) were mixed with 1 g of trimethoxysilylpropylureido-terminated polydimethylsiloxane 1000 (PDMSU 1000) and dissolved in 25 g of ethanol. The solution was acidified with 0.1 ml of 0.1 molar hydrochloric acid and stirred for four hours. During this time, only hydrolyzed monomers and dimers formed. During the hydrolysis, the sol was additionally diluted with 75 ml of ethanol. The sol was applied to samples of AI 2204-T3 alloy. In a contact time of 240 s, a film formation on the sample was achieved. Thereafter, the samples at a rate of 10 cm / s from the SoI were extracted for 60 minutes and then air-dried at 140 subjected to ⁰ C for 60 minutes heat treatment. The aluminum alloy thus protected showed excellent anticorrosive properties, ie it was stable in the salt spray chamber for more than 13 days. The contact angles for water were 100 ° compared to 75 ° for the coating according to Example 1.

As already apparent from the foregoing, the present invention is not limited to the illustrated embodiments, but includes all the same means and measures in the context of the invention. Furthermore, the invention is not limited to the feature combination defined in claim 1, but may be defined by any other combination of certain features of all the individual features disclosed overall. This means that in principle virtually every individual feature of the independent claim can be omitted or replaced by at least one individual feature disclosed elsewhere in the application. In this respect, the claims are to be understood merely as a first formulation attempt for an invention.

Claims

claims A process for producing a sol-gel corrosion protection coating for solar absorbers with a nanodurant anti-corrosion film comprising applying a sol mixture prepared from at least one mercaptosilane, solvent, acid catalyst and water to a substrate, exposing said sol mixture to said substrate over an exposure time sufficient to form from the sol mixture an at least monomolecular layer on the surface of the substrate, a separation of the coated substrate from the sol mixture and a subsequent drying and a final heat treatment of the coated substrate. 2. The method according to claim 1, wherein the mercaptosilane is a compound having the following chemical structure:

wherein R ¹ , R ^{2 are} independently selected from alkyl (1 - 14 C), aryl groups, wherein each n is an integer from 0 to 20, m is an integer from 0 to 2 and r is an integer from 1 to 3, where: m + r = 3. 3. The method according to any one of claims 1 to 2, wherein in the sol mixture, a final concentration of the mercaptosilane in the range 0.01 mass percent to 50 percent by mass is set. 4. The method according to any one of claims 1 to 3, wherein the sol mixture in addition to the mercaptosilane at least one further hydrolyzable silane is mixed. 5. A method according to claim 4, wherein in the sol mixture, a final concentration of the further hydrolyzable silane added is set in the range of 0% by mass to 70% by mass. 6. The method according to claim 4 or 5, wherein as a further hydrolyzable silane, a compound having the following chemical structure is used:

wherein R ¹ , R ^{2 are} independently selected from alkyl (1-10 C-atoms), cycloalkyl (1-10 C-atoms) or aryl groups, wherein each m is an integer from 0 to 2, n is an integer from 0 to 3 and r is an integer from 1 to 4, where: m + n + r = 4, and wherein R ^{3 is} independently of R ¹ and R ² selected from alkyl, cycloalkyl, substituted alkyl, substituted cycloalkyl, aryl groups, substituted aryl groups, fluorinated alkyl groups, fluorinated aryl groups or fluorinated cycloalkyl groups. 7. The method according to any one of claims 4 to 6, wherein is used as a further hydrolyzable, in particular doubly endcapped, silane, a compound having the following chemical structure:

wherein R ¹ , R ^{2 are} independently selected from alkyl (1 - 10 C atoms), cycloalkyl (1 - 10 C atoms) or aryl groups, wherein each m is an integer from 0 to 2 and r is a whole Number from 1 to 3, where: m + r = 4, and wherein R ^{4 is} independently selected from R ¹ and R ² selected from differently terminated or substituted alkyl chains, polydialkylsiloxane, polyether chains or perfluoroalkylated chains. 8. The method according to any one of claims 1 to 7, wherein the solvent is an aliphatic alcohol, in particular ethanol, or 3-butoxy-2-propanol or propylene glycol monobutyl ether. 9. The method according to any one of claims 1 to 8, wherein the acidic catalyst is selected from the group HCl, HBr, Hl, HNO ₃ , H ₂ SO ₄ , H ₃ PO ₄ , formic, acetic, propionic, butyric , Salicylic, trifluoroacetic, trichloroacetic, trifluoromethanesulfonic and methanesulfonic acid, and the concentration of added catalyst is 1 μmol to 1 mmol throughout the composition. A process according to any one of claims 1 to 9, wherein the amount of water used is between 0.001% by mass to 80% by mass of the sol mixture. A process according to any one of claims 1 to 10, wherein a mixing time for preparing the sol mixture is in the range of 1 minute to 7 days. 12. The method according to any one of claims 1 to 11, wherein the substrate comprises a spectrally selective solar cermet, preferably a Chromoxynitrid-, Titanoxyni- trid- or titanium-aluminum oxynitride absorber coating, which on a metallic support such as copper, aluminum , Iron or their alloys, is deposited. 13. The method according to any one of claims 1 to 12, wherein the substrate has a bare metallic surface, which consists in particular of copper, aluminum, iron or their alloys. 14. The method according to any one of claims 1 to 13, wherein the contact time of the sol mixture on the substrate in the range of 0.1 s to 5 h. 15. A method according to any of claims 1 to 14, wherein the separation of the coated substrate from the SoI mixture occurs at a rate in the range of 0.1 cm / s to 100 cm / s. 16. The method according to any one of claims 1 to 15, wherein the drying of the coated substrate is carried out at room temperature, wherein a drying time in the range of 1 s to 36 h. 17. The method according to any one of claims 1 to 16, wherein the heat treatment in the temperature range from 70 ⁰ C to 300 ⁰ C, in particular over a time in the range of 1 s to 5 h, preferably in the range of 85 ⁰ C to 250 ⁰ C, especially over a period of time in the range of 1 second to 4 hours. 18. The method according to any one of claims 1 to 17, wherein a thickness of the protective film of less than 1000 nm, preferably in the range of 5 nm to 500 nm is set. 19. The method according to any one of claims 1 to 18, wherein during the coating for the thermal emission coefficient of the coated substrate sets a value which is not greater than 0.35 to the thermal emission coefficient of the substrate. 20. The method according to any one of claims 1 to 19, wherein the application of the sol mixture to the substrate by immersing the substrate in the SoI mixture, spraying the substrate, roller application of the sol mixture or application by means of a slot nozzle. 21. The method according to any one of claims 1 to 20, wherein the coating is carried out batchwise or continuously, in particular from coil to coil.