EP1381711B1 - Method for improving metal surfaces to prevent thermal tarnishing - Google Patents

Description

The present invention relates to a method of preventing or at least reducing the yellowing of metallic surfaces (e.g., stainless steel, copper, brass and bronze) exposed to elevated temperatures.

Conventional stainless steels such as grades 1.4301 (chrome nickel steel) and 1.4016 (chrome steel) corrode at temperatures of 200 ° C - 230 ° C in air atmosphere. Oxygen layers form on the surface as a result of the incorporation of oxygen, which leads to discoloration which is often undesirable to the user, for example yellowing (tarnish colors). In this context, particular consideration should be given to those household appliances which by their function have to be exposed to high temperatures (for example, up to 500 ° C) (for example ovens and stoves, in particular pyrolysis ovens, slide-in parts such as gratings or baking trays, covers).

There are methods of increasing corrosion resistance by treating the steel surfaces. Such methods include annealing treatments in an inert atmosphere coupled with pickling operations as described in Japanese Patent Application (Application No. JP 06079990, published on 19.04.94). Furthermore, the corrosion resistance can be increased by electrolytic polishing.

From EP 00 101 186.5 (published as EP-A 1 022 357) it is additionally known that the tarnishing of stainless steels can be suppressed by selective oxidation and pickling processes as a result of the temperatures occurring in the usual household up to 350 ° C , Otherwise, no quality is described so far, which avoids thermally induced discoloration at temperatures of about 230 ° C in continuous use without the application of a protective layer.

Accordingly, another approach to suppress tarnish is to apply protective layers by wet chemical methods. These include, on the one hand, the application of water glass to metallic surfaces and the application of layers by means of the sol-gel process (see, for example, DE-A 197 14 949, Applicant: INM). Such layers act as a diffusion barrier for oxygen. In order to avoid interference colors, they are applied in thicknesses over 1 μm of baked-in thickness (DE-A 197 14 949). Thinner layers e.g. on a sol-gel basis lead to optically disturbing interferences.

Sol-gel processes are used in particular for the application of vitreous layers. The technique of the sol-gel process is well known to those skilled in the art and is described in detail, for example, in Brinker-Scherer, The Physics and Chemistry of Sol-Gel Processing, Sol-Gel Science, Academic Press (1990). Sol-gel processes are hydrolysis-condensation reactions (for example of silanes such as R _n SiX _4-n or a mixture of several such silanes, where R is, for example, hydrogen or an aliphatic or aromatic radical and X is a hydrolyzable radical, such as Alkoxy or phenoxy) in which, after complete removal of the reaction product by dehydration (chemically in the sense of condensation, water from the solvent, if present), with simultaneous branching and crosslinking of this product, structures with, for example, Si-O bonds are formed , The particle size (particle diameter) in the structures is 100 nm or less. Removal of the solvent forms a gel (of increased viscosity and increased degree of crosslinking), which is then dried to airgel and finally (by heating at about 500 ° C) to become (in the case of silanes: glassy) a layer which becomes contains both silicon and oxygen (in a stoichiometric ratio of about 1: 2). These glassy layers based on Si and O will hereinafter be referred to as Si-O layers.

Such a sol-gel process is described for silanes of the general formula R _n SiX _4-n in DE-A 197 14 949. The glass-like layers described therein improve not only the corrosion / tarnish protection but also the possibility of cleaning and, depending on the thickness, also the scratch sensitivity of the substrate. However, they are susceptible to cracking, probably due to shrinkage processes and differences in the expansion coefficients, at layer thicknesses of 2 μm and above. This crack sensitivity is based on the fact that the thus treated layers at temperatures above about 350 ° C due to outgassing the organic components lose their flexibility. In addition, complicated geometries can not be coated in terms of production technology with these thickness tolerances. If the layers are applied with a smaller thickness (below 1000 nm), they are not sensitive to the formation of cracks and can also be applied controllably via dilutions, but show interference colors, which the user regularly views as undesirable.

Due to the tendency to form cracks, thicker sol-gel layers (layer thickness> 2000 nm) are on surfaces of stainless steel, but also on other metals such as copper, brass and bronze, especially in the case of their use in the home (ovens, stoves , etc.), but technically and practically uninteresting, since the cracking leads to loss of function.

To build up their protective effect, the vitreous Si-O layers require temperatures which are above the start-up temperature of the respective metal, for example conventional stainless steels (the start-up temperatures usually approach 200 ± 20 ° C. for steel). By the term "structure of the protective effect" are on the one hand densification processes of the layer, wherein the compacted layer then acts as a diffusion barrier for oxygen, but on the other hand meant chemical reactions at the interface to steel or metal / alloy, which prevent the formation of visually disturbing oxide layers ,

It is crucial, if working in oxygen-containing atmosphere (eg air), that the structure of the protective effect at temperatures or at times takes place (is) below which or before visually visible tarnish colors (could) occur. As noted in the previous paragraph, this is not the case without further resources. For this reason, in the case of sol-gel processes (especially in the case of the use of silanes for the formation of Si-O layers), such auxiliaries are added in the form of alkalis (as network converters). Typical alkali sources are those mentioned in DE-A 197 14 949 (column 3, last paragraph), in particular NaOH, KOH, Mg (OH) ₂ , Ca (OH) ₂ . These network converters are incorporated into the Si-O network and interrupt it, so that the so modified Si-O network more or less closely approaches the water glass depending on the concentration of the used alkali (s). Among other things, the effect of the network converters is to lower the compaction temperatures of the layers. In other words, the build-up of the protective effect and thus the protection against oxygen can be generated at lower temperatures compared to sol-gel processes without the use of network converters. This in turn causes the temporal or reversed in temperature order: the tarnish protection layer can form at times or at temperatures before or below which visible tarnish colors occur.

On the other hand, however, the use of network converters has a significant disadvantage: it usually reduces the chemical resistance of the layers. If, therefore, chemically highly stable (glassy) layers are to be obtained, they must be baked in an oxygen-free atmosphere (for example under nitrogen or possibly also argon as protective gas) without the need for network converters. However, this in turn requires a relatively high cost, which makes a sol-gel process under inert gas atmosphere economically less interesting.

In contrast to the use of silanes in sol-gel processes, sol-gel processes based on suitable Ti, Zr, Al and / or B compounds are not used. This is partly because the build up of the protective effect does not take place at temperatures below the start-up temperature, meaning that the stainless steel / metal / alloy already yellows / starts during the protective treatment.

One object which the inventors have faced in the light of the prior art, therefore, was to provide a method which makes it possible to produce surfaces of stainless steel, but also of other metals or alloys, such as copper, brass and bronze, without the use of To coat network converters and yet prevent the tarnish protection layer forms only at times or at temperatures, after or above which visible tarnish colors have already occurred. After carrying out such a process, the original metallic impression of the surface should be retained, even if the sol-gel process is carried out on the basis of suitable Ti, Zr, Al and / or B compounds.

A second object of the present inventors was to provide a process which would provide good corrosion / tarnish protection of the stainless steel or other metals and alloys even at continuous use temperatures up to 450 ° C, preferably up to 500 ° C and even to 550 ° C, while maintaining the metallic original impression and the possibility of a simple or improved cleaning of the substrate, ie metal or alloy, while ensuring the occurrence of interference colors at low Layer thicknesses preferably prevented, but at least significantly reduced. Due to the low layer thicknesses, the problem of keeping the crack-sensitivity of the coating low is also solved.

Finally, a final task of the inventors was to provide a method which solves both of the aforementioned objects simultaneously in one method.

• gegebenenfalls Schritt (i), der eine Behandlung der metallischen Oberfläche vorsieht, um ihre Anlauftemperatur zu erhöhen und damit die erste der oben genannten drei Aufgaben zu lösen;
• Schritt (ii), der eine mechanische und/oder chemische Aufrauhung der zu beschichtenden metallischen Oberfläche beinhaltet, um die zweite der oben genannten Aufgaben zu lösen; und schließlich
• Schritt (iii), der die Beschichtung der aufgerauhten Oberfläche mittels z.B. eines Sol-Gel-Prozesses umfasst, wobei die Schicht in einer Dicke von weniger als 1000 nm, vorzugsweise 800 um oder weniger, 600 um oder weniger, 500 nm oder weniger, oder 400 nm oder weniger, aufgetragen wird, und das die dritte Aufgabe löst, wenn es Schritt (ii) nachfolgt.

The inventors of the present application have now found that, in order to solve these objects, a method requires the following steps:

• optionally, step (i), which provides a treatment of the metallic surface to increase its start-up temperature and thus to accomplish the first of the above three objects;
• Step (ii) involving mechanical and / or chemical roughening of the metallic surface to be coated in order to achieve the second of the above-mentioned objects; and finally
• Step (iii) comprising coating the roughened surface by, for example, a sol-gel process, the layer being in a thickness of less than 1000 nm, preferably 800 μm or less, 600 μm or less, 500 nm or less, or 400 nm or less, and that solves the third object if it succeeds step (ii).

In this case, the thin, translucent layer of 100 nm to less than 1 .mu.m thickness obtained on the basis of Si, Zr, Ti, B, Al compounds, obtained according to step (iii), is applied on the basis of Si compounds ,

A variant of this method also comprises the optional step (i) and subsequently carrying out step (ii) simultaneously with the coating step (iii), wherein step (ii) represents the introduction of a second phase and the layer is less than 1000 nm thick , preferably 800 nm or less, 600 nm or less, 500 nm or less, or 400 nm or less.

Thus, one aspect of the present invention relates to the methods outlined above. Another aspect of the present invention relates to a component, e.g. a metal sheet of chromium-nickel steel, which has been subjected to such a process.

• Ggf. kann auf Schritt (i) verzichtet werden, ohne dass die Lösung der oben definierten Aufgaben gefährdet würde. Dies kann nämlich dadurch geschehen, dass ein Sonderstahl zur Verwendung ausgewählt wird, der (selbst in Sauerstoff haltiger Atmosphäre) relativ spät anläuft. Beispiele solcher Sonderstähle sind Cronifer 45 bzw. Cronifer 2 von Krupp VDM.

The methods are described in more detail below:

• Possibly. Step (i) can be dispensed with without jeopardizing the solution to the above defined tasks. This can be done by using a special steel for use is selected, which starts (even in oxygen-containing atmosphere) relatively late. Examples of such special steels are Cronifer 45 and Cronifer 2 from Krupp VDM.

It goes without saying for the person skilled in the art that step (i) is not necessary even if the baking takes place in an inert or non-oxidizing atmosphere (then, according to the prior art, no network converter is necessary).
However, step (i) is indispensable in the remaining cases if the task (s) to be resolved are to be provided with surfaces free of tarnish colors and if the aforementioned preconditions are not met ( no use of special steel as described in the penultimate paragraph, no network converters, no working in a non-oxidizing atmosphere).

The metallic surfaces to be treated are preferably those of stainless steels, in particular surfaces of steel grades 1.4301 and 1.4016 (chromium-nickel or chromium steel), which are otherwise untreated at working temperatures of 200.degree and higher in the air atmosphere and, as a result, turn yellow during substep (iii) (in the absence of network transducers).

According to the findings, which the inventors of the present invention have obtained, chemically resistant (because network converter free) sol-gel layers can be applied to substrates, even without tarnish colors forming, if / because the substrates or their surfaces after the above-mentioned step (i) have start-up temperatures significantly above 200 ° C, eg at 250 ° C, preferably at 300 ° C, are. That is, according to a preferred embodiment (α) of the present invention, a first step (i) of the method according to the invention is a treatment of the metallic surface to increase its start-up temperature and thus to achieve the first of the above three objects.

Step (i) of the preferred embodiment (α) may be by any method in which the metal can form a tarnish before it becomes a discoloring oxide layer. Preferably, this step is the process described in EP-A 1 022 357. Preferably, step (i) comprises the steps of heating the metallic surface up to 550 ° C and then pickling the heated surface with mineral acid (as described in EP-A 1 022 357). It is particularly preferred to increase the start-up temperature of the metallic surface to about 300 ° C, so that the start-up temperature is above the temperature at which the protective effect of the example Si-O layer occurs, because after such a step (i) (and In the subsequent step (ii)), step (iii) can be carried out in an oxygen atmosphere without network converters.

Hereinafter, this step will be referred to as a "step to increase the startup temperature" or a "step to increase the startup temperature". This is followed by step (ii), by means of which the metallic surface is roughened, and step (iii), a conventional coating process, e.g. a sol-gel process, one after the other, with the result that the tarnish protection of the treated metal / alloy such as steel, copper, brass or bronze is not lost even at temperatures up to 550 ° C.

According to a preferred embodiment of the present invention, the organic constituents (eg methyl, ethyl, 1-propyl, isopropyl residues, for chemistry in general and the organic residues in particular see below, page 9) of the layers are not completely burned out , Then you get a cleaning-friendly, tarnish-resistant surface lower. Surface energy. For the expert, it requires only little effort to test at which temperature the burnout has to be made according to this preferred embodiment. An exact temperature range, or even value, can not be determined because it depends on numerous parameters known to those skilled in the art (e.g., chemical qualitative and quantitative composition). Burning out regularly takes place at a temperature which is above the (later) application temperatures. That is, if the surface treated metal is to be installed in a hearth where it is to be exposed to a temperature of up to 450 ° C, the burnout at temperatures of 450 ° C or above 450 ° C, preferably at about 470, at about 480, at about 490 or at about 500 ° C, take place.

It has furthermore been found that the interference colors of the layers which occur at low layer thicknesses can be suppressed by mechanical and / or chemical and / or physical roughening of the (noble) steel surface. Physical roughening is defined according to the invention as the (physical) introduction of second phases (such as light-scattering particles or pores). Examples of the different types of roughening are Grinding or blasting, in particular sand or shot peening (mechanical), etching, for example with acids such as phosphoric, sulfuric or hydrochloric acid (chemical) to produce a microstructure in the surface to be treated (in contrast to the etching is that in EP-A 1 022 357 and according to the invention as step (i) to be used pickling a pure cleaning process for removing the oxide layer without itself in the (substrate) surface to be treated to form a microstructure), but also the incorporation of light scattering particles and / or pores (physically).

The pores are preferably produced by air-filled particle interspaces. The expert knows how to install these interparticle spaces (see also the next but one paragraph). Particularly suitable as light-scattering particles are TiO ₂ and ZrO _2, in general all those particles whose refractive index is greater than that of the respective layer. In any case, the geometries of the mechanical, chemical or physical roughening which break the interferences according to the invention are of the order of 2 to 1000 nm, preferably in the range of 15 to 500 nm, in the range of 40 to 300 nm, in the range of 50-250 nm or in a range of 100-200 nm (ranges in each case based on the diameter). Preferred ranges for the chemical and mechanical roughnesses are 50-1000 nm, in particular 200-500 nm. Preferred ranges for the (light-scattering) particles (first form of physical roughening) are 2 to 30 nm, in particular 5 to 25 or 10-20 nm (depending essentially on the type of particles and their refractive index). Preferred areas for the pores (second form of physical roughening) are 2 - 100 nm, in particular 5 - 50 nm.

When using the light-diffusing particles or pores in step (ii) to prevent interference, attention must be paid to a certain ratio between Me (e.g., Si) of the matrix on the one hand and pores on the other. It is essential that the volume fraction of particles / pores in the fired layer is 0.05-20%, more preferably 0.1-15%, most preferably 1-5%.

It is well known to those skilled in the art to introduce pore or light scattering particles as well as mechanical or chemical roughness into the layers. Nevertheless, the procedure for the case of pores and particles is roughly sketched. Particles can be incorporated by adding light-scattering particles during the sol-gel process Finally, due to their refractive index (which is different from that of the matrix, ie the layer) and small size of about 2 - 30 nm (eg 20 nm, indicated as particle diameter) prevent the occurrence of interference colors or at least significantly reduce their intensity , Suitable particles are, for example, Al ₂ O ₃ , TiO ₂ , ZrO ₂ and SiO ₂ .

When it comes to the pores to be installed to avoid the interference colors, there are basically three options for achieving this. First, during the sol-gel process, a blowing agent is added which escapes leaving pores behind at the latest during the process of baking, ie during the conversion of the airgel into the coating. Or one lowers the concentration of the starting substances for the hydrolysis-condensation reactions (for example the silanes) in order to be able to incorporate pores (air) into the matrix. Or one controls the sol-gel process so that it leads without particle addition by incomplete cross-linking / compression to a pore-containing layer.

The coated according to the present invention layers are transparent, so do not change the metallic surface in appearance.

On the chemistry of the sol-gel process according to the present invention

Starting compounds for the hydrolysis and subsequent condensation according to the invention are compounds of the general formula R _n MeX _4-n , where X and R are as described in DE-A 197 14 949 (column 2, lines 18-34, column 3, lines 1-9). where n is 0, 1, 2 or 3 and Me is selected from Si, Al, Zr, B and Ti. In the case of Me = Al or B, it will be understood by those skilled in the art that the above formula because of the triviality of the central atoms A1 and BR _n MeX _3-n , must be. Preference is given to compounds with Me = Si; with R = hydrogen, methyl, ethyl, i-propyl, n-propyl, vinyl, allyl or phenyl, where not all R must be the same; with X = OH, methoxy, ethoxy or phenoxy or Hal (F, Cl, Br, I, preferably Cl and Br), wherein not all X must be the same; and with n = 0, 1 or 2. The organic radicals R and X generally have 1 to 16 carbon atoms, with 1 to 12, in particular 1 to 8 carbon atoms being preferred (for the aryl radicals, of course, only 6 or 10 carbon atoms are preferred). Particular preference is given to radicals having 1 to 4 (alkyl, alkenyl, alkynyl) or 6 (aryl) or 7 to 10 (aralkyl, alkaryl) carbon atoms.

Particularly preferred are compounds with Me = Si; with R = hydrogen, methyl, ethyl, or phenyl, where not all R must be the same; with X = OH, methoxy, ethoxy or phenoxy, wherein not all X must be the same; and with n = 0 or 1.

At least one compound of the general formula R _n MeX4-n must be a compound in which n = 2, 1 or 0 or at least one compound of the general formula R _n MeX _3-n must be a compound in which n = 1 or 0, since otherwise no layer formation is possible (at n = 3 or 2, for example, silane / borane has only one hydrolyzable radical X and consequently can only react with one molecule).

Preferably, two, three or more compounds of the general formula R _n MeX _4-n or R _n MeX _{3-n are used} in combination, wherein the ratio R: Me (corresponding to n) on a molar basis is preferably 0.2 on average to 1.5.

The hydrolysis and condensation reactions (sol-gel processes) are more preferred. Mann carried out in a solvent mixture of water and an organic solvent such as methanol, ethanol, acetone, ethyl acetate, DMSO or dimethyl sulfone. The organic solvent may also be a mixture of two or more solvents. The solvents mentioned and usable according to the invention are all miscible with water, so that the hydrolysis can proceed without phase separation.

The coating (composition) can be applied to the metallic surfaces in various known ways: by dipping, spinning, spraying, flooding or rubbing; the metallic surface in the bath of e.g. Dipping silanes is a preferred process.

The thickness with which the layers are applied according to the invention is in a range from 100 to below 1000 nm, more preferably in a range from 200 to 850 nm, particularly preferably in a range from 300 to 750 nm, most preferably at 350 to 600 nm. But layer thicknesses of 100 to 300 nm, more preferably 100 to 200 nm, are preferred for the purposes of the present invention.

example

After the process described in EP-A 1 022 357 (step (i)) and then shot blasted (step (ii)) chromium steel 1.4016 (no tarnish colors) was treated with a 5% solution of Dynasil GH 02 (The Dynasil solution is according to the manufacturer, the Degussa Hüls, on hydrolyzed and partially condensed silanes) dip-coated in 1-butanol, dried and baked at 550 ° C. The steel did not run after treatment even at a temperature of 500 ° C (10h hold time). No interference colors were observed.

Claims (10)

1. Method of coating metallic surfaces of ovens, cookers and the insert parts as well as coverings thereof, excluding lithographic plates, characterised in that the method comprises either, in this sequence,

   (a) a step (ii), which includes a mechanical and/or chemical roughening of the metallic surface to be coated, and

   (b) a step (iii), which includes coating of the roughened surface, wherein the coating is applied in a thickness of 100 nm to less than 1 micron, or

   (c) the introduction of a second phase as step (ii) simultaneously with the coating step (iii), wherein the coating is applied in a thickness of 100 nm to less than 1 micron, and

   (d) the thin, translucent coating, which is obtained after step (iii), of 100 nm to less than 1 micron thickness on the basis of Si, Zi, Pi, B, Al compounds, in preferred manner is applied on the basis of Si compounds.
2. The method according to claim 1, characterised in that the introduction of the second phase is carried out by inclusion of light-dispersing particles, particularly TiO₂, Al₂O₃, ZiO₂ and/or SiO₂.
3. The method according to claim 2, characterised in that the geometry of the mechanical and chemical roughening lies in an order of magnitude of 50 - 1000 nm or 200 - 500 nm and that the geometry for introduction of the second phase of physical roughening lies in the range of 2 - 100 nm, particularly 5 - 50, 2 - 30, 5 - 25 or 10 - 20 nm.
4. The method according to one of the preceding claims, characterised in that the metallic surface to be coated is a steel surface, in preferred manner a surface containing chromium and/or nickel.
5. The method according to one of the preceding claims, characterised in that the coating is applied in a thickness which lies in a range of 200 to 850 nm, in preferred manner in a range of 300 to 750 nm and in particularly preferred manner in a range of 350 to 600 nm.
6. The method according to one of the preceding claims, characterised in that preceding the steps (iii) and (iii) is a step (i) which provides a treatment of the metallic surface in order to increase the tarnishing temperature of this metallic surface to approximately 300° C so that the tarnishing temperature lies above the temperature at which the protective effect of, for example, the Si-O coating occurs.
7. The method according to claim 6, characterised in that the step (i) comprises the steps of heating the metallic surface up to 550° C and subsequently pickling the heated surface in mineral acid.
8. The method according to claim 6 or 7, characterised in that the coating is carried out in step (iii) in a wet-chemical manner, preferably by means of a sol-gel process.
9. The method according to one of the preceding claims, characterised in that as starting compounds in step (iii), particularly for the sol-gel process, use is made of compounds of the general formula R_nMeX_4-n or R_nMeX_3-n, wherein X is hydrolysable groups or hydroxyl groups, wherein R is hydrogen, alkyl, alkenyl and alkinyl groups with up to 12 carbon atoms and aryl, aralkyl and alkaryl groups with 6 to 10 carbon atoms and n is 0, 1 or 2, with the proviso that at least one compound with n = 1 or 2 is used and wherein Me is selected from Si, Al, Zr, B and Ti.
10. Components treated by means of the method according to one of the preceding claims.