Underglaze composition, underglaze layer, product containing the same and production method thereof

Abstract

The present invention relates to an underglaze composition for producing an adhesion promoter layer between steel and at least one top glaze, the at least one top glaze being for producing a glaze-based coating highly resistant to mechanical, thermal and chemical actions, wherein the underglaze composition comprises boric oxide (B2O3) and one or more alkali metal oxides, in particular Li2O, Na2O and / or K2O, in the weight ratios according to the following table, and SiO2 as a first main component, the weight percentage ratio of which ranges from 35% by weight to 70% by weight, preferably from 40% by weight to 65% by weight, and Fe2O3 as a second main component, the weight percentage ratio of which ranges from 5% by weight to 28% by weight, preferably from 7% by weight to 23% by weight and particularly preferably from 8% by weight to 15% by weight; and also relates to an underglaze layer produced from such an underglaze composition; a highly corrosion-resistant article having such an underglaze layer; a method for producing such an underglaze layer and also relates to a method for using such an underglaze composition to produce a highly corrosion-resistant article and the use of such an underglaze composition for producing a highly corrosion-resistant article.

Description

[0001] The present invention relates to: a ground coat enamel composition according to the preamble of patent claim 1, a ground coat layer produced from such a ground coat composition according to the preamble of patent claim 5, an article having a high corrosion resistance to mechanical, thermal and chemical effects and having such a ground coat layer according to the preamble of patent claim 13, a method for producing such a ground coat layer according to the preamble of patent claim 15, a method for producing a highly corrosion-resistant article according to the preamble of patent claim 16 and the use of the ground coat composition according to the preamble of patent claim 17 for producing a highly corrosion-resistant article.

[0002] Underglaze compositions have been known for some time and are indispensable for the production of highly corrosion-resistant articles, which typically have a highly corrosion-resistant surface formed by a cover coat enamel. Underglaze compositions are used here to produce an underglaze layer, which forms a kind of adhesion promoter layer between the steel of the substrate of the highly corrosion-resistant article to be produced and the cover coat enamel layer which provides the article with its high corrosion resistance. The cover coat enamel layer firstly has an extremely smooth and, moreover, mechanically extremely stable and chemically inert surface. The combination of the steel of the substrate, the underglaze layer and the cover coat enamel layer forms a steel-enamel composite material.

[0003] Today, in the chemical and pharmaceutical industries, such steel-enamel composites are firmly established in process management with highly aggressive media or in sterile high-purity applications. For example, when a specific product purity is involved, when coating formation is to be avoided or when the required hygienic process steps require sterilization, for example, the extremely smooth, stable and chemically inert surface of the chemical enamel (also referred to as steel-enamel composite as described above) provides optimal conditions.

[0004] Glaze is a glassy, ​​solidified silicate melt which is fused to a metal carrier material. Extremely high requirements are placed on the carrier material of the substrate (usually a steel plate) in terms of the surface quality of the metal plate used and in terms of its chemical composition. For example, boilerplate is currently used in particular as a carrier material of the substrate. For reasons of good adhesion of the glaze to the boilerplate or steel plate, the maximum permissible carbon content in the metal plate does not exceed 0.16% by weight according to current standards. The reason is that the glaze must undergo a chemical reaction with the steel so that the glaze layer can be chemically bonded to the steel. This bonding of the glaze layer to the steel occurs against the background of a chemical reaction in the course of which the silicate melt is bonded to the steel, but also in the course of which, as a side reaction, carbon oxide gases are also formed from the carbon present in the steel and the oxygen originating from the silicate melt, which gases remain dissolved in the glaze in the form of bubbles and have a lasting adverse effect on the properties of the glaze applied to the steel.

[0005] Since good adhesion of the applied glaze to steel is essential, as mentioned above, the typical practice is to first apply a base glaze layer to the steel. In order to improve the adhesion of this base glaze layer to steel, so-called adhesion oxides have been used in the base glaze composition in the past, which are mixed with the base glaze composition in the past. These are conventionally nickel oxide, cobalt oxide and / or manganese oxide. Since nickel oxide is a toxic substance, and in order to avoid the toxic properties of nickel oxide, attempts have been made to find substitute oxides in the past. Therefore, recent developments in this regard preferably use rare earth oxides and also oxides of molybdenum and tungsten as adhesion oxides, so as to improve the chemical reaction of the base glaze with the steel surface and optimize the adhesion of the base glaze with the steel surface. In addition, if possible, the cobalt oxide that acts as an adhesion oxide should also be replaced, because in addition to the health hazards caused by cobalt oxide, the production or mining of cobalt oxide also occurs under social and environmental critical conditions. Incidentally, cobalt is also indispensable for electric vehicles at present, which means that cobalt is not only expensive, but also this raw material has signs of shortage.

[0006] In all enamelling methods known to date from the prior art and commercial use, during the actual enamelling process, i.e. during the production of the base enamel layer on the steel substrate, a chemical redox reaction takes place at temperatures between 800° C. and 960° C. between the glaze melt or glass melt (which is more liquid at these temperatures) and the underlying steel substrate. Due to their chemically defined properties of being more noble than iron, the metal ions of the above-mentioned oxides of cobalt (Co), nickel (Ni), manganese (Mn), molybdenum (Mo), tungsten (W) and / or rare earth metals are reduced to their metallic state at these elevated temperatures and form an alloy with the iron (Fe) of the steel surface. At the same time, conversely, a conversion of metallic iron (Fe) to Fe 2+ and Fe 3+ In addition, the carbon used or present in the steel is also oxidized to carbon monoxide (CO), but mainly to carbon dioxide (CO 2 ).

[0007] Especially the latter, i.e. the oxidation of carbon present in the steel by oxygen originating in particular from the adhering oxides of the ground enamel composition, which inevitably occurs during the enameling process, is very disadvantageous, since carbon dioxide in particular leads to the formation of gas bubbles and very large-volume bubble structures within the ground enamel and in particular along the steel-enamel interface layer, e.g. Figure 1 This is illustrated by the cross-sectional view of a steel sheet coated with a base glaze layer and a plurality of top glaze layers shown in FIG. Both the bubble formation itself and the bubbles distributed within the base glaze disrupt the mechanical homogeneity and therefore also the mechanical stability of the finished glaze layer after glazing. Therefore, a practically reasonable and also particularly feasible possibility to avoid such bubble formation is to limit the carbon content of the metal sheet to be used in advance.

[0008] A further disadvantage of the presence of bound oxide ions in the base enamel composition is that the reduction of the metal of the bound oxide ions to its metallic state and its subsequent alloying with the iron of the steel substrate proceeds on the one hand as an exothermic process and leads to uncontrollable alloying of the steel surface, but on the other hand is necessary in conventional base enamel compositions of the past for chemical bonding of the enamel layer to the steel.

[0009] Another difficulty with conventional base enamel compositions of the past for obtaining a uniform and well-adhering coating on the steel surface of the article is that when applying these past base enamel compositions, it is very difficult, if not impossible, to obtain a completely uniform layer application of the base enamel composition slip on the steel surface of the article due to the complex geometry of the container and especially of turbines and agitators. However, in order for the base enamel to react completely uniformly and uniformly on the steel surface, it is necessary to coat the steel surface of the article with the base enamel composition as uniformly and uniformly as possible. For example, in most cases, the different geometries within the components and the highly fluctuating steel thicknesses with tolerances of up to 200% lead to the fact that the first application of the base enamel composition slip on the steel surface of the article is defective and only results in an inadequate, defective base enamel layer, and therefore a second application of the base enamel composition slip on the first base enamel layer is necessary. In turn, the disadvantage resulting from this is that the base enamel does not adhere completely uniformly to the steel surface of the article, in particular also due to the second base enamel composition application on the defective first base enamel layer, which in turn leads to an uneven adhesion reaction between the enamel layer and the steel.

[0010] Another disadvantage of the aforementioned alloying of the oxide-adhering metal with the steel surface of the article to be coated is that this alloying of the steel surface usually occurs inhomogeneously, and then the current flow in the interface layer at the steel surface locally leads to the formation of electrochemical elements, which further amplifies the inequality of the alloying of the steel surface. This "overreaction" to form a "stainless steel surface" on the steel surface reduces the adhesion of the base enamel layer to the steel surface, with the result that, in the worst case, the enamel layer may spontaneously fall off locally.

[0011] During the complete glazing process for producing highly corrosion-resistant articles, first, as mentioned above and as required, one to two base glaze layers are applied to the metal support material. The purpose of the base glaze layer is to produce adhesion between the chemically resistant top glaze layer and the support material, i.e. the steel of the substrate. The base glaze has a relatively low chemical resistance compared to the top glaze and should therefore usually be applied only as a thin adhesion promoter layer. However, as mentioned above, if the first base glaze layer is not uniform enough and therefore one or more further base glaze layers are required, it is necessary to apply a second and possibly a third base glaze layer. In the prior art to date, the layer thickness of the base glaze that can be obtained by repeated spraying and firing of articles coated with the base glaze composition typically varies between 0.2 and 0.9 mm, wherein the layer thickness of the entire base glaze is often thicker and lies in the range from 0.3 to 0.6 mm.

[0012] However, the problem with such a large total layer thickness of the base glaze layer is that for commercial glazing, the total layer thickness of all glaze layer thicknesses, that is to say the total layer thickness of the base glaze and the top glaze, is specified in the DIN / ISO standards. The total layer thickness of the base glaze and the top glaze together permitted according to these standards is in the range between 1 mm and 2.2 mm, with a permissible tolerance of 0.2 mm above or below.

[0013] However, since only the top glaze layer has the good glazing properties required for the desired corrosion resistance, this layer should be as thick as possible and, in contrast, the base glaze layer should be as thin as possible. In combination with the frequently necessary repeated base glaze coating, this in turn has the result that only a few tenths of a millimeter remain for the layer thickness of the top glaze layer required for chemical and also mechanical corrosion resistance, with the result that the glaze coating of the article in accordance with DIN / ISO Standard 28721-1 is smaller than desired, which in turn has a negative impact on the service life of the article and often requires premature reconditioning of the glaze coating of the steel substrate.

[0014] Starting from these problems known in the prior art, the object of the present invention is to provide a base enamel composition which makes it possible to provide a base enamel layer for producing highly corrosion-resistant products while avoiding and / or reducing the above-mentioned problems, as well as a method for producing such a base enamel layer and additionally a method for producing highly corrosion-resistant products using such a base enamel composition, and furthermore the use of such a base enamel composition for producing highly corrosion-resistant products.

[0015] This object is achieved by the following items: the base enamel composition as described in patent claim 1, the base enamel layer as described in patent claim 5 produced from the base enamel composition, the highly corrosion-resistant product having the base enamel layer as described in patent claim 13, and also the method for producing the base enamel layer as described in patent claim 15, the method for producing highly corrosion-resistant products using the base enamel composition as described in patent claim 16, and the use of the base enamel composition for producing highly corrosion-resistant products as described in patent claim 17.

[0016] In particular, the object of the present invention is achieved by a base enamel composition for producing an adhesion promoter layer between steel and at least one layer of enamel for producing an enamel-based coating that is highly resistant to mechanical, thermal and chemical effects, wherein the base enamel composition comprises boron oxide (B 2 O 3 ) and one or more alkali metal oxides, especially lithium oxide (Li 2 O), sodium oxide (Na 2 O) and / or potassium oxide (K 2 O)

[0017]

[0018] and SiO as the first main component 2 , the weight percentage ratio of which is in the range of from 35% by weight to 70% by weight, preferably in the range of from 40% by weight to 65% by weight, and Fe as the second main component 2 O 3 , whose weight percentage proportion is in the range from 5% by weight to 28% by weight, preferably in the range from 7% by weight to 23% by weight and particularly preferably in the range from 8% by weight to 15% by weight.

[0019] The essential point of the invention is that due to the presence of iron (III) oxide in the underglaze composition, during application to the steel surface of the substrate, a neutralization reaction of iron (III) and iron (0) to iron (II) takes place at elevated temperatures together with metallic iron from the steel surface of the substrate, which is required for the production of the underglaze layer. The iron (II) then further reacts with the silicon dioxide also present in the underglaze composition according to the invention to give iron silicates. Since this reaction of iron (III) oxide with elemental metallic iron takes place directly at the interface between steel and glaze (i.e. underglaze), a very good and direct bonding of the iron silicates to the steel surface is produced. Since this reaction takes place at elevated temperatures during the glazing process over the entire surface of the steel substrate coated with the underglaze composition according to the invention, a continuous iron silicate layer appears over the entire surface of the steel substrate, by means of which the surface of the steel substrate is protected from external influences, so that in particular the ingress of oxygen originating from the past underglaze composition into the carbon present in the steel and therefore also the formation of carbon oxides (i.e. carbon monoxide and carbon dioxide) is effectively suppressed. The main advantage of the base enamel composition according to the invention is therefore that, when coating the surface of a steel substrate with the base enamel composition according to the invention, there is no longer any fear of bubble formation in the base enamel layer, which according to the prior art in principle continues with each heating of the article and therefore also with each heating of the base enamel layer and the steel, which leads to a considerable improvement in the chemical and mechanical corrosion resistance of the article coated with the base enamel composition according to the invention.

[0020] According to one embodiment of the present invention, the base enamel composition of the present invention, in addition to the two main components of silicon dioxide and iron (III) oxide, also has the aforementioned boron oxide (B 2 O 3 ) and one or more alkali metal oxides, especially lithium oxide (Li 2 O), sodium oxide (Na 2 O) and / or potassium oxide (K 2 In addition to aluminum oxide (AlO), if desired, it also contains aluminum oxide (AlO) in the weight proportions according to the following table: 2 O 3 ) and one or more alkaline earth metal oxides, especially calcium oxide:

[0021]

[0022] In addition, the base enamel composition may further comprise at least one substance, in particular zinc oxide (ZnO), titanium dioxide (TiO 2 ) and / or calcium fluoride (CaF 2 The latter substances can advantageously be used to control the rheology of the melt of the base enamel composition, wherein the weight proportions of the substances given in the following table have proven to be advantageous:

[0023]

[0024]

[0025] In this case, the actual amounts or weight proportions of the abovementioned substances in the underglaze composition according to the invention can be selected, depending on the desired overglaze composition and on the geometry of the steel substrate, within the ranges specified in the two tables above, wherein the weight proportions of silicon dioxide, iron (III) oxide, boron oxide, the sum of the alkali metal oxides, the sum of aluminum oxide and alkaline earth metal oxides and the substances for adjusting the rheology of the melt of the underglaze composition in each case add up to 100 weight percent. In this case, the weight figures are in each case based on the dry weight of the underglaze composition according to the invention and not on the weight of the underglaze composition slip in the form of which the underglaze composition is applied to the respective surface of the steel substrate.

[0026] Thus, according to the invention, the undercoat composition is advantageously substantially free of oxides of the elements nickel, cobalt and manganese (which according to the prior art are often referred to as "adhesion oxides"), and is also in particular substantially free of rare earth elements, and particularly preferably substantially free of the elements cobalt, nickel, manganese, tungsten, vanadium, niobium, molybdenum, chromium, antimony, arsenic, bismuth, zinc, tin, lead and thallium.

[0027] In an extremely advantageous manner, the base enamel composition according to the invention therefore contains neither toxic heavy metals nor other substances or elements which are undesirable or problematic in terms of health or the environment.

[0028] Another advantageous and highly desirable effect of the undercoat composition according to the invention further lies in its readily available and inexpensive components, which are readily available, do not require environmentally harmful mining and are also completely unproblematic with regard to raw material shortages, which are already evident in the case of several oxide-adhering metals used hitherto.

[0029] Furthermore, the object of the invention is also achieved by a base enamel layer applied to a steel sheet surface, which base enamel layer has been produced from a base enamel coating according to the above statement.

[0030] The ground enamel layer according to the invention comprises iron silicate at the steel-ground enamel contact zone, which is formed during the ground enamelling process and at the temperatures required for such a process in the range from 890° C. to 950° C. by reaction of the metallic iron of the steel substrate with iron(III) oxide added to the ground enamel composition in the presence of silicon dioxide. This steel-ground enamel contact zone extends from the steel surface in the direction of the ground enamel, wherein the iron silicate in the cooled state (i.e. in the form of the finished ground enamel layer) adheres extremely firmly to the surface of the steel substrate and forms thereon a solid coating extending over the entire surface and in this way protects the ground enamel-coated surface from further external influences.

[0031] A particular advantage of the ground enamel layer according to the invention is that it can have a layer thickness in the range from not less than 0.05 mm to not more than 0.8 mm, but preferably in the range from 0.1 mm to 0.4 mm and particularly preferably in the range from 0.1 mm to 0.3 mm.

[0032] Since the base enamel layer according to the invention can have such a low layer thickness of much less than half a millimeter, there is still considerable room for applying one or more top enamel layers in order to produce highly corrosion-resistant coatings in accordance with DIN / ISO standards compared to the prior art. This is especially the case considering that according to the invention it is not necessary to apply more than one base enamel layer to the steel substrate.

[0033] An important advantage of the present invention further lies in the fact that the iron silicate according to the present invention is crystalline, in particular substantially (ie predominantly) in the form of fayalite crystals Fe 2 SiO 4 In the form of fayalite crystals. These fayalite crystals have a very high melting point of over 1000° C. and therefore withstand repeated high temperatures in further downstream firing processes. The iron silicate is formed on the steel surface of the steel substrate in the form of fayalite crystals, instead of a continuous solid, crystalline and extremely resistant layer, which for its part has a layer thickness of less than 80 μm, preferably less than 50 μm, for example in the range from 15 μm to 50 μm. It should further be pointed out at this point that the iron silicate according to the invention does not necessarily have to be in the form of fayalite crystals only, but in the presence of other metals, such as, for example, magnesium or calcium, can also be in the form of mixed silicates, for example in the form of olivine (Mg, Fe) 2 SiO 4 or calcium iron pyroxene (CaFe)(Si 2 O 6 ) if such metals are present in the base enamel composition. However, the essence of the present invention is that the adhesion of the base enamel layer to the steel surface of the substrate is achieved in any case by utilizing the Fe-O-Si bonding structure present in the iron silicate.

[0034] As mentioned above, the iron silicate forms, in particular over the entire surface, a crystalline layer at the steel-bottom enamel contact zone, which is suitable for forming a barrier layer between the steel surface of the substrate and, for example, a glassy or amorphous phase of the bottom enamel layer directly adjacent to the fayalite crystalline layer, and in particular and particularly advantageously between the steel surface of the substrate and at least one top enamel layer of a highly corrosion-resistant article produced using the bottom enamel composition according to the invention. Due to this barrier layer property of the crystalline layer, reactions of components of the steel substrate with components of the enamel layer or layers are effectively prevented, wherein the layer thickness of the crystalline layer is in the range from 10 μm to 65 μm, preferably in the range from 15 μm to 50 μm and particularly preferably does not exceed 50 μm, and effective and good protection is formed against reactions as are carried out in the previous bottom enamel coatings known from the prior art.

[0035] To this end, the base enamel layer and also especially the crystal layer according to the present invention is essentially free of bubbles and in particular also essentially free of carbon monoxide and / or carbon dioxide, which significantly improves and increases both the chemical and in particular also the mechanical stability of the base enamel layer produced using such a base enamel composition according to the present invention, and therefore also significantly improves and increases the chemical and mechanical stability of highly corrosion-resistant products produced using such a base enamel composition according to the present invention compared to previous highly corrosion-resistant products.

[0036] Since the crystalline layers of the base enamel layer according to the invention provide such a good mutual barrier effect both with respect to the entry of material into the steel surface of the substrate and the exit of material from the steel of the substrate, according to the invention a steel substrate can be used whose steel sheet, especially in the steel-base enamel contact area, has a carbon content in the range from 0% by weight to 0.5% by weight, preferably in the range from 0.01% by weight to 0.45% by weight and particularly preferably in the range from 0.08% by weight to 0.3% by weight.

[0037] Thus, in an extremely advantageous manner, steels having a very high carbon content can be used compared to what was required in the past in the production of highly corrosion-resistant articles. The undercoat composition according to the invention thus also enables a more cost-effective production of highly corrosion-resistant articles, since according to the invention it is not necessary to resort to very low-carbon and usually expensive steels, and conventional steel grades can be used instead.

[0038] Another important aspect of the present invention is that the base enamel layer produced using the base enamel composition according to the present invention has a self-repairing mechanism. Therefore, the base enamel composition according to the present invention combines two properties that are extremely useful and important for the production of highly corrosion-resistant products. The first of these two properties is the ability to form iron silicate crystals with the metallic iron in the steel matrix, which form a barrier layer on the surface of the steel matrix as a firmly adhered and full-surface layer resistant to high temperatures. The second characteristic of the base enamel composition according to the present invention further consists in the formation of a bonding layer, i.e., providing an adhesion layer at which optimal bonding with the top enamel layer can be achieved.

[0039] If, in a rather theoretical case, for example due to the mechanical action of forces, the crystal layer firmly adhering to the surface of the steel layer is damaged and, for example, holes or thinning points are caused, which is theoretically conceivable, the above-mentioned self-healing mechanism automatically takes effect, because when the crystal layer is damaged, in the case of heating, the instantaneous and automatic reformation of the fayalite crystals takes place at the damaged location, because at this location the metallic iron (0) reacts again with the iron (III) oxide present in the underglaze composition according to the invention to produce iron (II) and then immediately further reacts with the silicon dioxide likewise present in the underglaze composition according to the invention to produce iron silicate. Since the layer thickness of the iron silicate crystal layer is initially still small, this reaction takes place as long as the iron silicate crystal layer permits, and likewise ends automatically when the layer thickness of the iron silicate crystal layer reaches a maximum layer thickness of approximately 65 μm to 80 μm.

[0040] The first growth of the iron silicate crystal layer on the surface of the steel substrate ends in the same way.

[0041] Furthermore, the object of the invention is achieved by a highly corrosion-resistant article with respect to mechanical, thermal and chemical effects, which has a base enamel layer applied to a steel sheet and in the form described above, and at least one top enamel layer applied to the base enamel layer.

[0042] According to the invention, the total layer thickness of the base enamel layer and at least one top enamel layer of the highly corrosion-resistant article produced using the base enamel composition according to the invention is in the range from 0.5 mm to 3 mm, preferably in the range from 0.8 mm to 2.6 mm and particularly preferably not more than 2.4 mm. In this way, since extremely thin base enamel layers can be achieved according to the invention, highly corrosion-resistant articles with enhanced high corrosion resistance compared to conventional highly corrosion-resistant articles with the same enamel layer thickness can advantageously be produced, since the base enamel layer according to the invention (which only needs to be present as one layer) allows or enables the application of more top enamel layers than before and nevertheless still meets DIN / ISO standard 28721-1.

[0043] In addition, the object of the present invention is also achieved in particular by a method for producing a base enamel layer having the above-mentioned characteristics, the method comprising the following steps:

[0044] i. Provide steel plates;

[0045] ii. optionally removing rust on the surface, especially loose rust;

[0046] iii. applying the base enamel composition according to the above statement;

[0047] iv. firing the base enamel composition at a temperature in the range of from 890°C to 950°C, preferably in the range of from 900°C to 940°C and particularly preferably in the range of from 920°C to 930°C, for a time period in the range of from 20 min to 80 min, preferably in the range of from 25 min to 70 min and particularly preferably in the range of from 28 min to 60 min.

[0048] In this respect, it should be pointed out at this point that in principle the base enamel layer according to the invention can be applied to new steel substrates using the base enamel composition according to the invention, but such application of the base enamel layer is also possible at any time to used steel substrates, for example in order to reuse the steel substrate after damage or wear. In the latter case, all that is required according to the invention is to remove the previously defective enamel layer from the steel substrate, for example by blasting, and to loosen the components. After this, a re-coating with the base enamel composition according to the invention can be carried out, with all the associated advantages.

[0049] In addition, the object of the present invention is further achieved in particular by a method for producing highly corrosion-resistant products, in particular newly produced or reconditioned highly corrosion-resistant products, the method comprising the following steps:

[0050] a) providing a new or used highly corrosion-resistant product made of steel sheet, which in particular has a damaged bottom glaze layer and / or top glaze layer;

[0051] b) cleaning the surface of the article to be coated, in particular mechanically, for example by blasting with at least one abrasive substance, in order to substantially remove any loosely adhered matter, such as rust, for example, and / or one or more previous coatings, in particular defective coatings;

[0052] c) once the base enamel layer has been produced on the cleaned steel sheet to be coated according to or analogously to the above statements regarding the method for producing a base enamel layer;

[0053] d) applying a slip of a top glaze composition to subsequently form a top glaze layer on the base glaze layer;

[0054] e) drying the glaze composition slip;

[0055] f) heating the article having the ground glaze layer and the overglaze composition, more precisely the dried overglaze composition slip, to a firing temperature in the range of from 780° C. to 870° C., preferably in the range of from 800° C. to 860° C. and particularly preferably in the range of from 800° C. to 840° C.;

[0056] g) maintaining the firing temperature for a period of time in the range of from 6 min to 125 min, preferably in the range of from 6.75 min to 100 min, and particularly preferably in the range of from 7.5 min to 90 min, to produce the glaze layer;

[0057] h) cooling the article in a controlled manner;

[0058] i) If necessary, repeat the application of the overglaze composition slip, similar to the previous five steps d) to h), to subsequently form additional overglaze layers on the previous overglaze layer.

[0059] The method according to the invention for producing highly corrosion-resistant articles thus exhibits numerous advantages, firstly based on the fact that even for geometrically difficult articles, a single coating with the underglaze composition is sufficient, since a crystalline layer that acts as a barrier is formed at all points of the article, as long as this crystalline layer has not yet reached a thickness that would terminate the reaction of the metallic iron from the steel substrate with the iron (III) oxide and the silicon dioxide from the underglaze composition. Since the thickness of the crystalline layer measured on the geometry of a typical steel substrate is very thin, i.e., generally less than 50 μm, it is not necessary according to the invention to apply the underglaze composition according to the invention with a uniform layer thickness at all locations on the article to be coated, since, in particular at the high temperatures required for coating, the reactive components in any case migrate sufficiently to thin points and / or defects, where the layer thickness of the crystalline layer may not have grown sufficiently thick. Such thin points and / or defects are therefore virtually automatically repaired and / or supplemented by means of the underglaze composition according to the invention until a sufficient layer thickness of the crystalline layer is reached. According to the present invention, since both iron (III) oxide and silicon dioxide are present in excess in the undercoat composition according to the present invention in addition to iron (0) originating from the steel surface of the steel substrate, there is always sufficient iron (0), iron (III) oxide and silicon dioxide in any case to enable full-surface and dense formation of a crystal layer of fayalite crystals. This fact also contributes to the extremely favorable self-repair mechanism of the fayalite crystal layer according to the present invention.

[0060] Another advantage of the method according to the invention for producing highly corrosion-resistant articles is furthermore that the barrier layer of the steel fayalite crystals protecting the steel substrate is very thin and therefore also enables a very thin base glaze layer, which means that more top glaze layers can be applied to the base glaze layer than was previously possible. This firstly enables the highly corrosion-resistant articles produced by the method according to the invention to achieve a significantly higher corrosion resistance and also a higher mechanical stability.

[0061] Furthermore, the objects of the present invention are also achieved in particular by the use of the undercoat composition according to the above statements for producing highly corrosion-resistant articles as described above.

[0062] The core of the present invention and its advantages can be summarized as follows.

[0063] The crucial core of the present invention is to provide a completely new method for base enamel adhesion.

[0064] Therefore, in order to overcome the difficulties known from the art for producing base enamel layers on the one hand and for producing highly corrosion-resistant articles on the other hand and also to at least reduce the amount of past adhering oxides, in particular to zero, a new adhesion mechanism is provided.

[0065] The method according to the invention completely avoids the use of all the metal oxides described so far to form the above-mentioned alloys between the adhesion oxide and the steel, which has been necessary so far to form a chemically stable adhesion of the enamel on the steel.

[0066] The novel adhesion mechanism according to the present invention uses Fe 2 O 3 As a binding substance, it is used to create a chemical bond between the base enamel layer and the steel.

[0067] Therefore, when Fe is added to a glaze that does not contain adhering oxides 2 O 3 When the Fe 2 O 3 With metallic iron (Fe 0 ) has a redox reaction. The Fe from the glaze layer 3+ Converted to Fe 2+ , and at the same time the iron Fe from the steel surface 0 Oxidized to Fe 2+ .Fe 2+ Local supersaturation of SiO 2 reacts and forms iron silicate. Since the liquid glass melt is now Fe 2+Supersaturation, iron silicate crystals crystallize along the interface layer with the steel - and only there. In order to make this supersaturation possible, the invention uses iron (III) oxide in a weight percentage range of from 5 weight percent to 28 weight percent, so that a sufficient amount of Fe is always present in the base enamel composition according to the invention. 2 O 3 Such Fe 2 O 3 The content is ideal so that the glaze melt reacts with the steel to form a crystalline layer as early as during the first firing process, i.e. during the first and only firing process for forming the ground enamel. The duration of the firing process depends on the thickness of the steel sheet and is, according to the invention, a time period in the range from 20 minutes to 80 minutes, the time required for firing the ground enamel layer increasing with increasing layer thickness of the steel sheet. In this respect, the time period of 20 minutes to 80 minutes is indicated in relation to how long the temperature required for firing the ground enamel layer is maintained after the firing temperature has been reached.

[0068] During this first firing process, due to the presence of Fe in the glaze melt, 2 O 3 and SiO 2 A continuous high-melting-point iron silicate crystal layer is formed along the glaze melt-steel interface, which is basically in the form of fayalite (i.e., Fe 2 SiO 4 ). The crystals that form have a melting point of more than 1000° C.; they thus form a continuous solid and crystalline layer, which does not decompose again even in the subsequent firing process. The crystal layer thus effectively prevents further reactions of the glaze melt with the steel. Depending on the thickness of the applied ground glaze layer, the crystal layer particularly preferably has a layer thickness of from 15 μm to 50 μm. When a continuous crystal layer is formed along the steel-glaze interface, the crystal growth itself also stops automatically. Therefore, even during long additional firing processes, no further growth of the crystal layer along the interface layer occurs.

[0069] Since the crystal layer is still very thin, it requires only a relatively small amount of glaze to form it. When using the base glaze composition according to the invention, it is possible to apply a top glaze even if the base glaze composition according to the invention is normally applied insufficiently and too thinly to the steel surface, which application in the past would have resulted in an insufficient formation of the base glaze-adhering layer and would therefore have required a second primer application process or even resulted in the glaze layer chipping or flaking off. Even in the case that the base glaze composition itself does not provide a sufficient amount of silicon dioxide, this does not result in an insufficient or unusable base glaze layer according to the invention, since in this case the subsequently applied top glaze will provide the required amount of SiO 2In order to enable the crystallization and formation of iron silicate crystals to be achieved. As already mentioned above, this effect is also essential for the extremely advantageous self-repairing mechanism of the underglaze layer according to the invention.

[0070] Fe 2 O 3 To Fe 2+ The reduction reaction and metal Fe 0 To Fe 2+ Oxidation and SiO 2 The further reaction and crystallization of iron silicate is an exothermic process that promotes chemical adhesion. An extremely stable and strong bond is formed via Fe-O-Si-.

[0071] Incidentally, since there is no difference in the electronegativity of the metal of the hitherto used adhesion oxide and the steel substrate of the base body, there is also no possibility of uncontrolled further reactions in the sense of alloy formation and / or redox reactions along the steel-enamel interface layer and / or adhesion layer. The crystal formation reaction stops automatically when the crystal layer is completely formed. The driving force behind the adhesion reaction is the formation of the crystal layer. The ground enamel is therefore significantly more resistant to prolonged firing temperatures and firing times than the ground enamels known to date according to the prior art (which function using adhesion oxides).

[0072] Another important advantage of the base enamel composition according to the present invention is that it reduces the CO 2 and CO bubbles are generated because the solidified iron silicate crystals prevent further reaction on the steel surface.

[0073] Therefore, the advantages of the present invention are as follows:

[0074] The hitherto conventional and problematic adhering oxides of cobalt oxide, manganese oxide, nickel oxide can be dispensed with to form a chemically stable adhesion of the enamel on the steel.

[0075] • Rare earth oxides can be dispensed with to form a chemically stable adhesion of the enamel on steel.

[0076] Other heavy metal oxides, especially toxic heavy metal oxides such as molybdenum (Mo), vanadium (V) and / or tungsten (W) can be dispensed with to form a chemically stable adhesion of the enamel on the steel.

[0077] The minimum layer thickness of the base glaze layer can be reduced to less than 0.1 mm.

[0078] · The second base coat can be dispensed with.

[0079] The thickness of the base enamel layer required for adhesion can be reduced to approximately 0.1 mm to 0.3 mm.

[0080] The base glaze layer has an inherent self-healing function, especially even if the base glaze is not applied sufficiently.

[0081] During the underglazing process, the crystalline layer has formed an oxidation protection for the steel.

[0082] When a sufficient layer thickness is reached, the growth of the crystal slows down drastically and automatically.

[0083] Under normal conditions, the thickness of the crystal layer along the steel surface does not exceed 50 μm.

[0084] When a steel sheet having a carbon content higher than 0.14% by weight is used, the annealing process can be omitted.

[0085] • Steel sheets with a relatively high carbon content of up to 0.25% by weight, possibly even up to 0.5% by weight, can be used directly.

[0086] The base enamel according to the invention does not contain any adhering oxides, any rare earth metals or any toxic heavy metals, in particular the following elements Co, Ni, Mn, W, V, Nb, Mo, Cr, Sb, As, Bi, Pb, Tl.

[0087] The adhesion reaction of the base enamel layer to the steel surface is carried out via a crystallization process through Fe-O-Si-bonds.

[0088] No alloying with more noble partners or metals (Co, Ni, Mn, W, V, Nb, Mo, Cr, Sb, As, Bi, Pb, Tl) along the steel interface; according to the present invention, such alloying, which was necessary according to the prior art in the past to produce adhesion, is not required.

[0089] Further embodiments of the invention emerge from the dependent claims.

[0090] The invention will be described below with reference to exemplary embodiments which are described in more detail based on the accompanying drawings. In the drawings:

[0091] Figure 1 shows a cross-sectional view of a conventional highly corrosion resistant article according to the prior art; and

[0092] Figure 2 A cross-sectional view of a highly corrosion resistant article produced in accordance with the present invention is shown.

[0093] In the following description, the same reference numerals are used for identical and identically acting parts.

[0094] Figure 1A cross-sectional view of a conventional highly corrosion resistant article 10 is shown. The article 10 consists of a steel sheet 20 to which a base enamel layer 30 is applied. The base enamel layer 30 adjoins the steel sheet 20 along a steel-base enamel contact area 60, wherein a layer of iron oxide dissolved in the base enamel is formed along the contact area 60, which adjoins the glassy base enamel layer 30 filled with a large number of bubbles 50. Arranged above the base enamel layer 30 are a plurality of top enamel layers 40, which are also rich in bubbles.

[0095] Figure 2 A cross-sectional view of a highly corrosion-resistant article 10 produced according to the invention using an underglaze composition according to the invention is shown. Thus, the article 10 produced according to the invention comprises a steel layer in the form of a steel plate 20, onto which an underglaze layer 30 is applied. For its part, the underglaze layer 30 has a crystalline layer 35 along the steel-underglaze contact zone 60, which covers the steel plate 20 over the entire surface and protects it from the overlying underglaze layer 30 and also from the top glaze layer 40 located still further above. The crystalline layer 35 consists of fayalite crystals and is bubble-free. The thickness of the crystalline layer 35 is substantially 50 μm. From Figure 2 It can be easily seen in the figure that any bubbles that exist are only in the area of ​​the bottom glaze layer 30 adjacent to the top glaze layer 40, and the bottom glaze layer 30 is bubble-free elsewhere. Further bubble formation does not occur; on the contrary, the area of ​​the bottom glaze layer adjacent to the crystal layer 35 is also bubble-free.

[0096] Exemplary formulations of glass compositions according to the present invention are given in the following table.

[0097]

[0098]

[0099] At this point it should be pointed out that all the parts described above, taken individually and in any combination, and in particular the details shown in the drawings, are considered essential to the present invention. Modifications thereof are familiar to those skilled in the art.

[0100] List of Reference Numerals

[0101] 10 Highly corrosion-resistant products (detailed drawing)

[0102] 20 Steel Plate

[0103] 30 Base glaze layer

[0104] 35 Crystal Layer

[0105] 40 glaze layer

[0106] 50 bubbles

[0107] 60 Steel-enamel contact area

Claims

1. A base enamel layer (30) applied to a steel sheet (20), produced from a base enamel composition for producing an adhesion promoter layer between steel and at least one layer of enamel, the at least one layer of enamel being used to produce an enamel-based coating having a high corrosion resistance to mechanical, thermal and chemical effects, in, The base enamel composition comprises boron oxide (B 2 O 3 ) and one or more alkali metal oxides, the one or more alkali metal oxides being Li 2 O、Na 2 O and / or K 2 Oh, and SiO as the first main component 2 , the weight percentage ratio of which is in the range of from 35% by weight to 70% by weight, and Fe as the second main component 2 O 3 , the weight percentage ratio of which is in the range of from 5% by weight to 28% by weight; wherein the steel-base enamel contact area comprises iron silicate; The invention is characterized in that the iron silicate forms a full-surface crystal layer (35) at the steel-bottom enamel contact area.

2. The base glaze layer according to claim 1, It is characterized in that The base enamel composition further comprises Al in a weight ratio according to the following table: 2 O 3 and one or more alkaline earth metal oxides: 。 3. The base glaze layer according to claim 2, It is characterized in that The base enamel composition further comprises at least one substance in a weight ratio according to the following table, the substance being ZnO, TiO 2 and / or CaF 2 , for controlling the rheology of the melt of the base enamel composition: 。 4. The base enamel layer according to any one of the preceding claims 1 and 2, It is characterized in that The undercoat composition is free of adhering oxides, namely oxides of the elements nickel, cobalt and manganese.

5. The base glaze layer according to claim 1, It is characterized in that The ground enamel layer (30) has a layer thickness in the range from 0.05 mm to 0.8 mm.

6. The base glaze layer as claimed in claim 1, It is characterized in that The iron silicate is crystalline.

7. The base glaze layer according to claim 6, It is characterized in that The layer thickness of the crystalline layer (35) is in the range from 10 µm to 65 µm.

8. The base glaze layer as claimed in claim 1, It is characterized in that The base glaze layer (30) is bubble-free and does not contain CO and / or CO 2 .

9. The base glaze layer as claimed in claim 1, It is characterized in that The steel plate (20), at the steel-base enamel contact area, has a carbon content ranging from 0% by weight to 0.5% by weight.

10. An article (10) having a high degree of corrosion resistance to mechanical, thermal and chemical effects, comprising a base enamel layer (30) as claimed in any one of claims 1 to 9 applied to a steel plate (20) and at least one top enamel layer (40) applied to the base enamel layer (30).

11. The highly corrosion-resistant article (10) according to claim 10, It is characterized in that The total layer thickness of the base glaze layer (30) and the at least one top glaze layer (40) is in the range from 0.5 mm to 3 mm.

12. A method for producing an underglaze layer (30) as claimed in any one of claims 1 to 9, It is characterized in that Follow these steps: i. Providing a steel plate (20); ii. Remove rust on the surface; iii. applying the base enamel composition; iv. firing the ground enamel composition at a temperature ranging from 890°C to 950°C for a time period ranging from 20 min to 80 min.

13. A method for the new production or restoration of a used highly corrosion-resistant article (10) according to any one of the preceding claims 10 and 11, It is characterized in that Follow these steps: a) providing a new or used highly corrosion-resistant product (10) made of a steel plate (20) having a damaged bottom glaze layer (30) and / or a top glaze layer (40); b) cleaning the surface of the article to be coated, mechanically by blasting with at least one abrasive substance, in order to remove any loosely adhered matter, and / or one or more previous coatings; c) once the base enamel layer (30) has been produced on the cleaned steel sheet (20) to be coated according to claim 12; d) applying a slip of a top glaze composition to subsequently form a top glaze layer (40) on the base glaze layer (30); e) drying the glaze composition slip; f) heating the article having the base glaze layer (30) and the top glaze composition to a firing temperature in the range of from 780°C to 870°C; g) maintaining the firing temperature for a period of time ranging from 6 min to 125 min to produce the glaze layer (40); h) cooling the article in a controlled manner; i) Repeated application of the overglaze composition slip to subsequently form additional overglaze layers on the previous overglaze layer (40).

14. Use of the above-mentioned base enamel composition for producing a highly corrosion-resistant article (10) as claimed in any one of claims 10 to 11.

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