EP3305941B1 - Method for producing an adhering sol-gel-layer on a metal surface - Google Patents

Description

Background of the invention

The present invention relates to a method of adhering a sol-gel layer to a metal surface.

Sol-gel coatings are non-metallic inorganic or hybrid polymeric materials from colloidal dispersions, the so-called sols. The starting materials are also referred to as precursors. From them arise in solution in first basic reactions finest particles. Special processing of the brine can produce powders, fibers, layers or aerogels. The hydrolysis of precursor molecules and the condensation between resulting reactive species are the major basic reactions of the sol-gel process. The processes involved and the properties of the precursor molecules have a decisive influence on the resulting material properties.

The adhesion of sol-gel coatings to metal surfaces will vary widely depending on the coated metal, depending on the metal to be coated and the pretreatment.

Extensive investigations have shown that good adhesion to metal surfaces is only achieved if the metal surface to be coated has a homogeneous layer of metal oxides. On metallic surfaces without an oxide layer, no adherent sol-gel coatings can be achieved. Based on this experience, it is preferred to coat metals which already form oxide layers with the oxygen from the environment, such as aluminum, stainless steels and titanium.

State of the art

As a rule, sol-gel coatings are applied to stainless steels, aluminum and titanium after pretreatment by degreasing on the already existing, naturally formed oxide layers. This usually gives good results with stainless steels and titanium, with the exception of mechanically polished surfaces. For aluminum is usually for pretreatment
the coating additionally applied a thin, about 1 to 3 microns thick oxide layer by electrochemical oxidation (anodizing).

An adhesive coating is not possible on carbon steels and Korten steel, because the naturally formed oxide layers do not allow sufficient bonding of the coating and are not themselves resistant to corrosion. This leads to poor adhesion of sol-gel layers and to the detachment of the layers by rusting.

On mechanically polished surfaces on stainless steel and aluminum is generally not sufficient adhesive strength to achieve. Obviously, the high mechanical stress and deformation of the material in conjunction with the action of polishing agents to form oxide layers with poor conditions for a high-quality, homogeneous and firmly adhering sol-gel coating.

The aim of the invention is to provide a method for adherent application of a sol-gel layer on a metal surface. This is achieved by the claimed method.

From the EP 1 816 219 A1 is a method for heat treatment of steel strips by means of direct flame heating known.

The EP 1 975 128 A1 discloses a process for producing a titanium oxide coating on various substrates.

The EP 0 967 297 A1 relates to a process for producing articles provided with a coating of a silica film.

Brief description of the invention

The present invention relates to a method for adhering a sol-gel layer to a metal surface, comprising: forming an oxide layer on a metal surface by treating the metal surface at a temperature of 600 ° C to 1500 ° C in an atmosphere having an oxygen content of 22 -100 vol .-% or by means of oxygen plasma, applying a liquid sol on the metal surface, wherein the sol in a solvent comprising silanes of the formula Si (OR ') _4-n R " _n comprises n = 0.1 or 2, wherein each OR 'independently represents a hydroxy, alkoxy and / or cycloalkoxy radical and each R ", when present, independently represents an alkyl and / or cycloalkyl radical, reacting the sol to a gel by evaporation of the solvent, and curing the sol gel.

A metal surface may be a smooth surface of a metal. A metal surface may also be a pretreated, such as a polished or a roughened surface of a metal. A metal surface may also be a surface of a metal already present with a passivation layer, such as an oxide layer. The oxide layer provided on this metal surface by the method according to the invention differs from any passivation layer already present in the form of an oxide layer on the metal surface.

The oxygen content in the atmosphere is 22-100 vol.%, Preferably 30-90 vol.%, More preferably 40-80 vol.%.

The treatment of the metal surfaces is preferably carried out in a period of 1 to 20 seconds.

It is particularly preferred that the atmosphere contains no sulfur compounds. It is also preferred that the method not include an additional step of removing organic residues from the at least one metal surface.

The layer thickness of the oxide layer is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm.

The particular results of the process according to the invention can probably be explained by the fact that in the case of the short-term treatment of the metal surface at temperatures above 1000 ° C., the formation of chemically very stable oxides of iron and aluminum. Thus, alumina α-Al ₂ O _{3 can form} above 1000 ° C and thus above the melting temperature of the aluminum. At temperatures above 1000 ° C also α-Fe ₂ O _{3 can} form, which is also very stable chemically. Both oxide forms are hardly attacked by hydrolysis or by acids. The very short treatment time of the method according to the invention also avoids a critical heating of the deeper (underlying) metal surfaces.

A method of adhering a sol-gel layer to a metal surface comprises forming oxide layers on metal surfaces as described above. The application of the sol-gel layer usually follows directly, without further process steps being provided. Furthermore, the method comprises, on the correspondingly treated metal surface, applying a liquid sol on the at least one metal surface, preferably by dipping, flooding, spraying or brushing, and allowing the sol to react to a gel, preferably by evaporation of the solvent. Optionally, an additional

Step of curing the gel, preferably at 160 ° C to 300 ° C for a period of 10 to 45 minutes.

The sol comprises silanes dissolved in a solvent of the formula Si (OR ') _4-n R " _n where n = 0, 1 or 2 wherein each OR' independently represents a hydroxy, alkoxy and / or cycloalkoxy radical and each R ", when present, independently represents an alkyl and / or cycloalkyl radical.

The sol further preferably comprises one or more elements selected from the group consisting of Al, Ti, Zr, Mg, Ca and Zn.

The layer thickness of the oxide layer is preferably 100 nm to 2 μm, more preferably 200 nm to 1.8 μm, more preferably 300 nm to 1.7 μm, even more preferably 500 nm to 1.5 μm, and even more preferably 800 nm to 1 μm.

By means of the method according to the invention, a material comprising a metal having at least one oxidized metal surface and a sol-gel layer can be produced thereon.

The layer thickness of the oxide layer is preferably 100 nm to 2 μm, more preferably 200 nm to 1.8 μm, more preferably 300 nm to 1.7 μm, even more preferably 500 nm to 1.5 μm, and even more preferably 800 nm to 1 μm.

The layer thickness of the sol-gel layer is preferably about 6 μm, more preferably about 0.5-5.0 μm, more preferably 1.0-5.0 μm, or 0.5-3.0 μm, and most preferably 1.0-4.0 μm. Preferably, the sol-gel layer has a uniform thickness with variations of less than 10% of the layer thickness.

The metal is preferably selected from carbon steel, Korten steel, metal with chrome plated surfaces, metal with high gloss polished surfaces, stainless steel and aluminum.

Detailed description of the invention

In the course of these investigations it has been shown that the quality of oxide layers can be different and depending on the conditions under which they originated. In addition, it has been shown that the structure and properties of the oxide layers have a strong influence on the adhesion of subsequently applied sol-gel layers. The chemical stability and corrosion resistance of oxide layers is an important factor influencing the permanent adhesion of sol-gel layers to oxide layers as a primer. Sol-gel layers are usually not completely free of pores. These pores allow corrosive media to penetrate and attack the oxide layer. The degradation of oxide layers with low corrosion resistance can lead to later detachment of the sol gel layers.

It has been shown that the conditions under which oxide layers are formed can lead to noticeable differences in their structure and properties. This has a direct influence on the adhesion of later applied sol-gel layers.

Extensive investigations into the parameters of the formation of oxide layers on metal surfaces and the resulting properties and their influence on the adhesion of sol-gel layers have shown that the temperature and the oxygen content of the atmosphere during the formation of the oxide layers have a significant influence on the adhesion of subsequently applied sol-gel layers. This surprising and new realization led subsequently to the development of the methods underlying the subject invention.

The methods known and used in the prior art for the pretreatment of metal surfaces prior to the sol-gel coating are used at temperatures below 100 ° C and consist of wet-chemical processes.

The step of forming an oxide layer on a metal surface comprises treating the metal surfaces at high temperatures in the range of preferably 800 ° C to 1200 ° C, more preferably 1000 ° C to 1200 ° C, in combination with a rich oxygen supply of over 22 vol .-%, ie an oxygen content higher than that of the earth's atmosphere, preferably from 22 to 100 vol%, more preferably from 30 to 90 vol%. The appropriate temperature is chosen depending on the metal to be coated and its surface quality.

The oxygen content in the atmosphere is therefore 22-100 vol .-%, preferably 30-90 vol .-%.

The treatment is preferably carried out either by treatment with a gas flame (flaming), which has a marked excess of oxygen and thus has a strong oxidizing effect, or by treatment by means of an oxygen plasma. In the case of a gas flame, the other components that are used in the combustion in the flame and make up the atmosphere, fuels such as propane, butane, a propane / butane mixture, hydrogen, methane.

As a rule, the duration of the treatment is 1 to 10 seconds. In this case, only a thin layer of material is heated on the surface, without the underlying metal undergoes significant changes. The treatment of the metal surfaces is preferably carried out in a period of 1 to 10 seconds.

It is particularly preferred that the oxygen-excess atmosphere does not contain sulfur compounds. Suitable gases for flame treatment are gases which have no components of sulfur or sulfur compounds. These can lead to the formation of metal sulfides and impair the coatability.

By contrast, oxide layers from atmospheric plasma are not suitable for pretreatment since the high nitrogen content of around 78% by volume in the natural atmosphere leads to the embroidering of the metal surfaces. Subsequently, no firmly adhering sol-gel layer can be applied to these surfaces.

The oxides formed at high temperature under excess oxygen lead to a significantly improved adhesive strength and homogeneity of sol-gel layers applied thereon. There may be some metals in the episode as well adherent and high-quality sol-gel coatings on which this was previously not possible, such as carbon steels, Korten steel and chrome-plated surfaces as well as on mechanically polished surfaces on stainless steel and aluminum.

Another advantage of the treatment according to the invention is that organic residues on surfaces to be coated, which may adversely affect the adhesion of the coatings, are also reliably eliminated. Therefore, in just one step, both the metal surface can be prepared with an oxide layer, as well as organic residues are removed, so that a simplified process is provided. Therefore, it is preferred that the method not include an additional step of removing organic residues from the at least one metal surface.

The method of the invention preferably comprises applying the liquid sol on the at least one metal surface by immersion, flooding, spraying or brushing, and reacting the sol to a gel by evaporation of the solvent and curing of the gel, preferably at 160 ° C to 300 ° C for a period of 10 to 45 minutes.

The sol comprises silanes dissolved in a solvent of the formula Si (OR ') _4-n R " _n where n = 0, 1 or 2 wherein each OR' independently represents a hydroxy, alkoxy and / or cycloalkoxy radical and each R ", when present, independently represents an alkyl and / or cycloalkyl radical.

The sol further preferably comprises one or more elements selected from the group consisting of Al, Ti, Zr, Mg, Ca and Zn.

Sol-gel layers on surfaces treated according to the invention are distinguished by a significantly improved adhesive strength and homogeneity in comparison to sol-gel layers on conventionally pretreated surfaces.

To determine the adhesion, test methods such as the Rockwell test, the cross cut test, the scratch hardness test, the pull-off test or the bending test are used.

In the Rockwell test (VDI Guideline 3198) a diamond cone with a defined force is pressed into the layer surface. In the vicinity of the hardness impression, the layer is damaged, which can be seen in the microscope as a crack network or as a layer eruptions in the edge region of the impression. The evaluation of the flaking around the impression can either be done according to the VDI Guideline 3198 by division into classes 1-6, or by digital image analysis of the chipped area shares, which gives a more objective and finely subdivided rating.

In the cross-cut test, the adhesive strength is determined by making continuous cuts at right angles to each other down to the substrate so that a grid is formed. In the scratch hardness test used to evaluate the adhesion of organic layer systems, such as e.g. Paints and paints used on flat, smooth sample plates, the sample material is applied with a uniform layer thickness on flat test plates with the same surface finish. After drying, the adhesive strength is determined by passing the sheets against a rounded pin or chisel that is loaded with weights until the paint layer detaches from the substrate. The pull-off test determines the minimum tensile force needed to peel or tear off a single-coat or multi-layer finish perpendicular to the surface. In contrast to the shear stress of the scratch hardness test, this method loads the coating with maximum tensile force. For the test, first a dolly is glued vertically to the coating. After the adhesive has cured, a tester is attached to the anvil and aligned to create a voltage perpendicular to the test area. The force is gradually increased and observed controlled until the coating surface goes off or a certain value is reached.

In the bending test, coated metal sheets with a defined radius are bent through a mandrel by 90 °, depending on the sheet thickness D (x times D). Detected is the starting radius from which the coating peels off in the bending area. The multiplier serves as a parameter. A second parameter is the value by how many degrees the sheet can be bent beyond 90 ° without the coating peeling off.

By means of flame treatment or oxygen plasma, firmly adhering sol-gel layers can also be achieved on carbon steel, Korten steel and chrome-plated surfaces as well as on mechanically polished surfaces on stainless steel and aluminum.

It will be understood by those skilled in the art that a sol-gel layer is first applied in the form of a liquid sol having colloidal particles suspended therein, which subsequently converts to a gel and eventually forms a solid, hard lacquer layer. So if the "application of the sol-gel varnish" or the "hardening of the sol-gel varnish" is mentioned, the expert knows in which state the sol-gel system is located.

Sol-gel coatings usually consist of two reaction components, which are mixed in a fixed ratio shortly before processing. This mixture is last added as a third component, a dilution, usually an alcohol. Dilution sets the concentration of the reaction mixture and the viscosity of the final batch. The sol-gel is preferably a silica sol based on silanes which are dissolved in solvents, wherein the silica sol preferably also contains one or more further sol-forming elements, preferably one or more elements from the group consisting of Al, Ti, Zr, Mg, Ca and Zn, these elements replacing the Si atoms in the colloidal structures. Preferred sol-gel coatings / sol-gel coatings are in EP 2145980 described. Reference is made in particular to the in EP 2145980 described sol-gel coatings and the method for their use.

The starting compounds for the formation of the sols and finally of the sol-gel lacquer are hydrolyzable silanes of the formula SiR ₄ , where the 4 radicals R 2-4 comprise hydrolyzable radicals OR 'and 0-2 comprise non-hydrolyzable radicals R "These starting silanes Thus, they can also be represented as Si (OR ') _4-n R " _n with n = 0,1 or 2. If additional sol-forming elements as just described are used, corresponding compounds should be selected according to the valences of the elements as starting compounds, such as AlR ₃ , etc.

The hydrolyzable radicals OR 'are hydroxy, alkoxy and / or cycloalkoxy radicals. Suitable examples thereof include, for example, hydroxy, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, i-butoxy, t-butoxy, pentoxy, hexoxy, cyclopentyloxy, cyclohexyloxy, in particular Ethoxy, n-propoxy and isopropoxy are preferred. The hydrolyzable radicals OR 'may be identical or different from one another.

The non-hydrolyzable radicals R ", if present, are alkyl and / or cycloalkyl radicals, suitable examples of which include, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, i-butyl, t-butyl, pentyl, hexyl, cyclopentyl, cyclohexyl radicals, with particular preference being given to methyl, ethyl, n-propyl and isopropyl radicals The nonhydrolyzable radicals R "may likewise be identical or different from one another.

The brine starting compounds may consist of a single type of silane, but often they will comprise mixtures of several silanes (and optionally additional sol-forming starting compounds of other elements). It is preferred that at least one of the components of the starting compounds is a silane of the formula Si (OR ') _4-n R _"n with n = 0, ie Si (OR') ₄ . For example, a preferred sol-gel layer may include the starting materials TEOS (tetraethoxyorthosilane) and MTES (methyltriethoxysilane) and / or DMDES (dimethyldiethoxysilane).

In addition, it is of course also possible to use other additives customary in the field of sol-gel systems, for example additional network formers, such as acryloxypropyltrimethoxysilane or methacryloxypropyltrimethoxysilane, which can provide further organic crosslinks, especially if a not inconsiderable proportion of the starting compounds are so-called network-converting compounds of the formula Si (OR ') _4-n R " _n where n = 1 or 2.

In the sol, the starting compounds are partially hydrolyzed to the corresponding hydroxy compounds (such as orthosilicic acid, trihydroxyalkylsilane, etc.), which can be promoted by the addition of a catalyst such as acid. Due to the high tendency for condensation of these hydroxy compounds, these can now condense with elimination of water to form smaller siloxane networks. The sol already contains colloidal particles containing siloxane bonds. Siloxane bonds are bonds of the form ≡Si-O-Si≡, where "≡" symbolizes any three independent bonds with other elements, in particular OH, OR 'and R ", thus forming a three-dimensional crosslinked structure in the colloidal particles Where OR 'and R "have the same meaning as above.

The application of the sol-gel lacquer can be done in any way, such as by immersion, flooding, spraying or brushing. Preferably, however, it is done by spraying, since this allows precise control of the amount applied per unit area.

The amount can be adjusted as needed. For example, a sol-gel layer may have a layer thickness of up to about 6 μm, or about 0.5-5.0 μm, preferably 1.0-5.0 μm, or 0.5-3.0 μm and most preferably 1.0-4.0 microns. Preferably, the sol-gel layer has a uniform thickness with variations of preferably less than 10% of the layer thickness.

The viscosity of the sol-gel varnish can be adjusted by a person skilled in the art. It is known that the sol, with a correspondingly high dilution in its solvent, is sufficiently low-viscosity to penetrate into any pores of a surface which may be present. It is known that the sol, with a correspondingly high dilution in its solvent, is sufficiently low-viscosity to be applied by spraying, spraying, rolling or brushing.

Suitable solvents for the sol are water and especially alcohols such as methanol, ethanol, n-propanol or isopropanol, with ethanol and isopropanol being preferred because of their physical properties and the low toxicity of their vapors.

Then the applied sol is allowed to react to a gel. This reaction converts the liquid sol into a solid gel layer, in which the colloidal particles of the sol crosslink with each other and with not yet hydrolyzed and condensed starting compounds by further hydrolysis and condensation. This can be done, for example, by evaporation of the alcoholic solvent during drying.

After drying the surfaces, the sol-gel coatings can be baked become, thereby forming a glass-ceramic structure that is firmly adhering, resistant to aging and insensitive to environmental influences. The baking of the coating can be carried out by a person skilled in the art according to the usual procedure.

For example, the gel-coated surfaces will undergo thermal curing, e.g. 250 ° C subjected. This occurs at elevated temperatures, with the gel transforming into a colorless, transparent, glassy layer. The silica sol constituents convert into an even more highly crosslinked silica which, depending on the composition of the underlying sol, may contain other constituents such as aluminum oxide, titanium oxide or zirconium oxide. These layers are hard, closed and resistant to many of the chemicals that a surface may come into contact under ordinary circumstances, and to temperatures up to about 500 ° C.

According to a preferred embodiment, the coated surface is exposed in the subsequent curing of the gel temperatures of 160 ° C to 300 ° C, more preferably from 160 ° C to 250 ° C and more preferably from 160 ° C to 220 ° C. This curing should be for a period of at least 10 minutes, preferably 20 to 45 minutes, for example 30 minutes. The curing is preferably carried out at temperatures between 180 ° C and 250 ° C, for example at 200 ° C, but also temperatures below 180 ° C are suitable for this purpose. The gel transforms into a hard, colorless and transparent, vitreous lacquer that tightly seals the surface, has no cracks and gives the surface high hardness and wear resistance.

The processes of gel formation and the curing of the gel can merge into one another, since, for example, gelation by drying and evaporation of the solvent can at least partly also take place at the beginning of the treatment for hardening. Also, such a method in which the processes of gelation and curing of the gel merge into one another, is encompassed by the invention.

However, other conventional curing methods may be used.

The invention also relates to a treated surface Korten steel containing a colorless, transparent, coating on a surface of the Korten steel, the surface treatment being by a process according to the present invention Invention is performed. The Korten steel according to the invention differs structurally from known Korten steels, which is recognizable by the color and the properties, such as stability against corrosion.

As a sol-gel coating / sol-gel lacquer z. POLIANT or POLISEAL from POLIGRAT.

The invention also provides a metal comprising an oxidized metal surface produced by a method as described above.

The layer thickness of the oxide layer is preferably 100 nm to 2 μm, more preferably 200 nm to 1.8 μm, more preferably 300 nm to 1.7 μm, even more preferably 500 nm to 1.5 μm, and even more preferably 800 nm to 1 μm. The invention also provides a material comprising a metal having at least one oxidized metal surface and a sol-gel layer thereon, wherein the material has been produced according to a method described above.

The layer thickness of the oxide layer is preferably 100 nm to 2 μm, more preferably 200 nm to 1.8 μm, more preferably 300 nm to 1.7 μm, even more preferably 500 nm to 1.5 μm, and even more preferably 800 nm to 1 μm.

The layer thickness of the sol-gel layer is preferably about 6 μm, more preferably about 0.5-5.0 μm, more preferably 1.0-5.0 μm, or 0.5-3.0 μm, and most preferably 1.0-4.0 μm. Preferably, the sol-gel layer has a uniform thickness with variations of less than 10% of the layer thickness.

The metal is preferably selected from carbon steel, Korten steel, metal with chrome plated surfaces, metal with high gloss polished surfaces, stainless steel and aluminum.

Examples: Example 1 (comparative example)

A stainless steel sheet of size DIN A5 of 1 mm thickness of material 1.4301 with a mechanically mirror-polished surface was cleaned in a dipping degreasing, rinsed with water, dried in air and spray-coated with a sol-gel layer. Subsequently, the surface was cured for 30 minutes at a temperature of 250 ° C in air.

After cooling, the coating could be rubbed in places with the thumb. In a bending test at 90 ° with a bending radius of 5 times the sheet thickness, the coating completely dissolved in the area of deformation.

Example 2:

A stainless steel sheet according to that in Example 1 was degreased like this and dried. Subsequently, the surface was exposed to an oxygen-rich (blue) gas flame for a period of 3 seconds and, after cooling, spray-coated with a sol-gel layer. Subsequently, the surface was cured for 30 minutes at a temperature of 250 ° C.

After cooling, the coating was firmly adherent and could not be rubbed off with the thumb. In a bending test of 180 ° with a bending radius of 5 times the sheet thickness, the coating showed no separation or cracking in the area of deformation.

Example 3: (Comparative Example)

A sheet of Korten steel of size DIN A5 1 mm thick and with some slight rusting was cleaned in a dip degreasing, rinsed with water and it was applied without further pretreatment by spraying a sol-gel coating. This was then cured for 30 minutes at 250 ° C in air.

After cooling, the layer was wiped off the surface of the surface with a finger. In a bending test by 90 ° with a radius of 5 × D, the coating completely dissolved in the bending area.

Example 4:

A sheet of Korten steel as in the example was cleaned in a Tauchentfettung, rinsed with water and dried. It was then flashed for 5 seconds with an oxygen-rich (blue) gas flame. After cooling The coating was firmly adherent and could not be rubbed with the thumb. In a bending test by 180 ° with a radius of 5 times the sheet thickness, the coating did not peel off and showed no delamination or cracks in the bending area. In a subsequent salt spray test over a period of 400 hours, the surface showed no corrosion.

Example 5: (Comparative Example)

A sheet of Korten steel was oxidized by pretreatment in a solution of 15% hydrogen peroxide, and then a sol-gel coating was applied by spraying and baked at 250 ° C for 30 minutes.

The layer could not be rubbed off with a finger and passed the bending test with a radius of 5 times sheet thickness by 90 °, without peeling off. In a subsequent salt spray test, the surface showed significant corrosion after just 40 hours. The high corrosion resistance of the surface in Example 4 compared to Example 5 shows the influence of the quality of the oxide layer produced by the method according to the invention in Example 4 as the basis of the coating.

Claims (5)

1. A method for applying an adherent sol-gel layer to a metal surface, comprising:

   - producing an oxide layer on a metal surface by treating the metal surface at a temperature of 600°C to 1500°C in an atmosphere having an oxygen content of 22-100 vol-%, or by means of oxygen plasma,

   - applying a liquid sol to the metal surface, wherein the sol contains silanes of formula Si(OR')_4-nR"_n, where n = 0, 1, or 2, dissolved in a solvent, wherein each OR' independently represents a hydroxy, alkoxy, and/or cycloalkoxy functional group, and each R", if present, independently represents an alkyl and/or cycloalkyl functional group,

   - reacting the sol to form a gel by evaporating the solvent and curing the gel.
2. The method for applying an adherent sol-gel layer to a metal surface according to Claim 1, wherein the oxygen content in the atmosphere is 30-90 vol-%.
3. The method for applying an adherent sol-gel layer to a metal surface according to Claim 2, wherein the oxygen content in the atmosphere is 40-80 vol-%.
4. The method for applying an adherent sol-gel layer to a metal surface according to one of Claims 1 to 3, wherein the treatment of the metal surfaces takes place in a time period of 1 to 10 seconds.
5. The method for applying an adherent sol-gel layer to a metal surface according to any of the preceding claims, wherein the metal is selected from carbon steel, Corten steel, metal with chrome-plated surfaces, metal with surfaces that are mechanically polished to a high gloss, stainless steel, and aluminum.