Base enamel composition, base enamel coating, article having such a base enamel coating, and method for producing the same

Abstract

The present invention relates to a base enamel composition for producing an adhesion-promoting coating between steel and at least one top enamel, in order to produce an enamel-based coating that is highly resistant to corrosion against mechanical, thermal and chemical influences, said base enamel composition comprising boron oxide (BO) and alkali metal oxides, in particular LiO, NaO and / or KO, in weight fractions according to the following table, and also SiO, as a first main component, in a weight percentage fraction ranging from 35% to 70% by weight, preferably ranging from 40% to 65% by weight, and 5% by weight, as a second main component. The present invention relates to a base enamel composition comprising FeO in a weight percent fraction in the range of 100 to 28% by weight, preferably in the range of 7% to 23% by weight, and more preferably in the range of 8% to 15% by weight, as well as to a base enamel coating made from such a base enamel composition, a highly corrosion-resistant article having such a base enamel coating, a method for making such a base enamel coating, and also to a method for making a highly corrosion-resistant article using such a base enamel composition, and to the use of such a base enamel composition for making a highly corrosion-resistant article.

Description

[Technical Field]

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

[0002] Base coating enamel compositions have been known for some time and are typically essential for the production of highly corrosion-resistant articles having a highly corrosion-resistant surface formed from the overcoating enamel. Here, the base coating enamel composition serves to produce a base coating enamel layer that forms a kind of adhesion-promoting layer between the substrate steel of the highly corrosion-resistant article being produced and the overcoating enamel layer, providing the article with high corrosion resistance. The overcoating enamel layer has a very smooth, mechanically very stable, and chemically inert surface. The combination of the substrate steel, the base coating enamel layer, and the overcoating enamel layer forms a steel-enamel composite.

[0003] This type of steel-enamel composite material is now firmly established in the chemical and pharmaceutical industries, in the control of processes using highly corrosive media and in aseptic high-purity applications. For example, if specific product purity is an issue, if the formation of coatings needs to be avoided, or if sterilization is required in required hygienic process steps, the very smooth, stable, and chemically inert surface of chemical enamel (see also the steel-enamel composite material mentioned above) provides optimal conditions.

[0004] Enamel is a glassy, ​​solidified silicate melt fused to a metal carrier material. Extremely high requirements are placed on the substrate carrier material, usually steel sheet, regarding the surface quality and chemical composition of the sheet metal used. For example, boiler plate is currently used as the substrate carrier material. Due to the need for good adhesion of enamel to boiler plate or steel sheet, the maximum permissible carbon content of sheet metal according to current standards is 0.16% by weight or less. This is because, in order for the enamel layer to chemically bond to the steel, the enamel must undergo a chemical reaction with the steel. This bonding between the enamel layer and the steel occurs during the chemical reaction that bonds the silicate melt to the steel. However, during this process, as a side reaction, carbon dioxide gases are formed from the carbon present in the steel and the oxygen from the silicate melt. These gases remain dissolved in the enamel as bubbles, permanently affecting the properties of the enamel applied to the steel.

[0005] Because good adhesion of the enamel applied to the steel is essential, typical practice is to first apply a base coat enamel layer to the steel, as described above. To improve the adhesion of this base coat enamel layer to the steel, known as adhesion oxides, have been mixed with conventional base coat enamel compositions. These have traditionally been nickel oxide, cobalt oxide, and / or manganese oxide. Because nickel oxide is a toxic material, attempts have been made in the past to find alternative oxides to avoid its toxicity. Therefore, more recent developments have prioritized the use of rare earth oxides and / or molybdenum and tungsten oxides as adhesion oxides to improve the chemical reaction between the base coat enamel and the steel surface and optimize adhesion of the base coat enamel to the steel surface. In addition to the health hazards associated with cobalt oxide, its production or mining also occurs under challenging social and environmental conditions. Therefore, cobalt oxide, which functions as an adhesion oxide, should also be replaced, if possible. Cobalt also happens to be essential for electromobility at the moment, which means that not only is it expensive, but there are already signs that this raw material is running short.

[0006] In all enameling methods known from the prior art and commercially used to date, during the actual enameling process, i.e., during the production of the base coating enamel layer on the steel substrate at temperatures between 800°C and 960°C, there is a chemical redox reaction between the enamel melt or glass melt, which is more liquid at these temperatures, and the underlying steel substrate. Due to their chemically defined, inert nature compared to iron, the above-mentioned metal ions of cobalt (Co), nickel (Ni), manganese (Mn), molybdenum (Mo), tungsten (W) and / or rare earth metals are reduced to their metallic state at these high temperatures and form alloys with the iron (Fe) on the steel surface. At the same time, conversely, the Fe of metallic iron (Fe) is reduced. 2+ and Fe 3+In addition, carbon used or present in the steel is also oxidized to carbon monoxide (CO), but primarily to carbon dioxide (CO2).

[0007] The latter, specifically the oxidation of carbon present in the steel by oxygen, resulting from the adhesion oxidation of the base coat enamel composition that inevitably occurs during the enameling process, is highly detrimental because carbon dioxide, in particular, can lead to the formation of bubbles and a large volume of bubbles in the base coat enamel, especially along the steel-enamel interface, as shown in the cross-section of a steel sheet coated with a base coat enamel layer and multiple overcoat enamel layers in Figure 1. Both the bubble formation itself and the distribution of bubbles in the base coat enamel disrupt the mechanical uniformity and therefore the mechanical stability of the finished enamel layer after enameling. Therefore, a practically reasonable and particularly feasible way to avoid such bubble formation is to limit the carbon content of the sheet metal used in advance.

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

[0009] Another difficulty with conventional basecoat enamel compositions for achieving a uniform, well-adhered coating on the steel surface of an article is that, when applying these conventional basecoat enamel compositions, the complex geometries of containers, particularly turbines and stirrers, make it extremely difficult, if not impossible, to apply a completely uniform layer of the basecoat enamel composition slip to the steel surface of the article. However, for a completely consistent and uniform adhesion reaction of the basecoat enamel to the steel surface, it is necessary for the steel surface of the article to be coated as consistently and uniformly as possible with the basecoat enamel composition. For example, in most cases, due to the highly variable steel thicknesses, with tolerances of up to 200% within different shapes and components, it has become the standard practice to first apply the basecoat enamel composition slip to the steel surface of the article, which only leads to a defective and insufficient basecoat enamel layer. Therefore, it is necessary to apply the basecoat enamel composition slip a second time on top of the first basecoat enamel layer. In particular, applying a second basecoat enamel composition to a defective first layer of basecoat enamel results in an uneven adhesion reaction between the enamel layer and the steel, and therefore a drawback resulting is that the basecoat enamel does not adhere completely uniformly to the steel surface of the article.

[0010] A further drawback of the aforementioned alloying of the metal of the adhered oxide with the steel surface of the article to be coated is that such alloying of the steel surface generally occurs unevenly, which then leads to the local formation of electrochemical elements within the interfacial layer at the steel surface due to the current, which further amplifies the uneven alloying of the steel surface. Such "over-reactions" forming a "stainless steel surface" across the steel surface can reduce the established adhesion of the underlying coating enamel layer on the steel surface and, in the worst case, can result in local spontaneous peeling of the enamel layer.

[0011] During the complete enameling process to produce highly corrosion-resistant articles, one or two basecoat enamel layers are first applied to the metal carrier material, as necessary, as described above. The purpose of the basecoat enamel layer is to create adhesion between the chemically resistant overcoat enamel layer and the carrier material, i.e., the steel substrate. Basecoat enamel has relatively low chemical resistance compared to overcoat enamel, and therefore generally only needs to be applied as a thin, adhesion-promoting layer. However, as described above, if the first basecoat enamel layer is not sufficiently uniform and therefore one or more additional basecoat enamel layers are required, a second, and possibly a third, basecoat enamel layer must be applied. In the prior art, the basecoat enamel layer thickness achieved by repeated spraying and firing of articles coated with the basecoat enamel composition typically ranges between 0.2 mm and 0.9 mm, with the overall basecoat enamel layer thickness often being thicker, in the range of 0.3 to 0.6 mm.

[0012] However, the problem with such a large total thickness of the base coat enamel layer is that all enamel layer thicknesses, i.e., the total thickness of both the base coat enamel and the overcoat enamel, are specified in DIN / ISO standards for commercial enameling. According to these standards, the total thickness of the base coat enamel plus the overcoat enamel allowed ranges from 1 mm to 2.2 mm, with a tolerance of 0.2 mm above and below.

[0013] However, since only the overcoat enamel layer has the good enamel properties necessary for the desired corrosion resistance, this layer must be as thick as possible, whereas the undercoat enamel layer must be as thin as possible. In combination with the often-necessary repeated undercoat enamel coatings, this results in the overcoat enamel layer thickness required for chemical and / or mechanical corrosion resistance remaining at only a few tenths of a millimeter, resulting in an enamel coating of the article that is smaller than necessary in accordance with DIN / ISO standard 28721-1, which in turn has a negative effect on the service life of the article and often requires an early readjustment of the enamel coating of the steel substrate. Summary of the Invention [Problem to be solved by the invention]

[0014] Proceeding from these problems known from the prior art, the object of the present invention is to provide a base coating enamel composition that makes it possible to provide a base coating enamel layer for producing highly corrosion-resistant articles, and also a method for producing such a base coating enamel layer, and in addition a method for producing highly corrosion-resistant articles using such a base coating enamel composition, and also the use of such a base coating enamel composition for producing highly corrosion-resistant articles, while avoiding and / or reducing the above-mentioned problems. [Means for solving the problem]

[0015] This object is achieved by a base coating enamel composition according to claim 1, by a base coating enamel layer made from such a base coating enamel composition according to claim 5, by a highly corrosion-resistant article having such a base coating enamel layer according to claim 13, and also by a method for making such a base coating enamel layer according to claim 15, by a method for making a highly corrosion-resistant article using such a base coating enamel composition according to claim 16, and by the use of such a base coating enamel composition for making a highly corrosion-resistant article according to claim 17.

[0016] In particular, the object of the present invention is achieved by a base coat enamel composition for creating an adhesion-promoting layer between steel and at least one overcoat enamel in order to create an enamel-based coating with high corrosion resistance to mechanical, thermal and chemical influences, the base coat enamel composition comprising boron oxide (BO) and alkali metal oxides, in particular lithium oxide (LiO), sodium oxide (NaO) and / or potassium oxide (KO), in weight proportions according to the following table:

[0017] [Table 1]

[0018] The first main component contains SiO2 in a weight percentage ratio in the range of 35% to 70% by weight, preferably 40% to 65% by weight, and the second main component contains Fe2O3 in a weight percentage ratio in the range of 5% to 28% by weight, preferably 7% to 23% by weight, particularly preferably 8% to 15% by weight.

[0019] The essence of the present invention is that, as a result of the presence of iron(III) oxide in the base coat enamel composition, during application to the steel surface of the substrate together with metallic iron from the steel surface of the substrate at the high temperatures required to produce the base coat enamel layer, the iron(III) and iron(O) are converted to iron(II). The iron(II) then further reacts with silicon dioxide, also present in the base coat enamel composition according to the present invention, to give iron silicate. This reaction between iron(III) oxide and elemental metallic iron occurs directly at the interface between the steel and the enamel, i.e., the base coat enamel, resulting in a very good direct bond between the iron silicate and the steel surface. This reaction occurs during the enameling process at high temperatures over the entire surface of the steel substrate coated with the base coating enamel composition according to the invention, resulting in a continuous iron silicate layer over the entire surface of the steel substrate, which protects the surface of the steel substrate from external influences, and as a result, in particular, oxygen from conventional base coating enamel compositions migrates into the carbon present in the steel, thereby effectively suppressing the formation of carbon oxides, i.e., carbon monoxide and carbon dioxide. Therefore, an essential advantage of the base coating enamel composition according to the invention is that the formation of bubbles in the base coating enamel layer, which, according to the prior art, in principle continued every time the article, and therefore the base coating enamel layer and the steel, were heated, is no longer a concern when coating the surface of a steel substrate with the base coating enamel composition according to the invention, thereby significantly improving the chemical and mechanical corrosion resistance of the article coated with the base coating enamel composition according to the invention.

[0020] According to one embodiment of the present invention, the base coat enamel composition according to the invention comprises, in addition to the two main components silicon dioxide and iron(III) oxide, and also the aforementioned boron oxide (BO) and alkali metal oxides, in particular lithium oxide (LiO), sodium oxide (NaO) and / or potassium oxide (KO), optionally also aluminum oxide (AlO) and alkaline earth metal oxides, in particular calcium oxide, in weight proportions according to the following table:

[0021] [Table 2]

[0022] Furthermore, the base coat enamel composition may further comprise at least one material, in particular zinc oxide (ZnO), titanium dioxide (TiO2) and / or calcium fluoride (CaF2). The latter material may be advantageously used to control the rheology of the melt of the base coat enamel composition, the weight ratios of materials shown in the table below having proven advantageous:

[0023] [Table 3]

[0024] In this case, the actual amounts or weight ratios of the aforementioned materials in the base coat enamel composition according to the invention can be selected within the limits specified in the two tables above depending on the desired overcoat enamel composition and on the silicon dioxide, iron(III) oxide, boron oxide, the sum of the alkali metal oxides, aluminum oxide, and the sum of the alkaline earth oxides, and the materials for adjusting the rheology of the melt of the base coat enamel composition, which in each case total 100 weight percent. In this case, the weight figures are in each case based on the dry weight of the base coat enamel composition according to the invention, and not on the weight of the base coat enamel composition slip in the form in which the base coat enamel composition is applied to the respective surface of the steel substrate.

[0025] Therefore, according to the invention, the basecoat enamel composition is advantageously essentially free of oxides of the elements nickel, cobalt and manganese, which have hitherto been called "adhesion oxides" according to the prior art, and in particular is essentially free of rare earth elements, particularly preferably the elements cobalt, nickel, manganese, tungsten, vanadium, niobium, molybdenum, chromium, antimony, arsenic, bismuth, zinc, tin, lead and thallium.

[0026] Thus, in a very advantageous manner, the base coat enamel composition according to the invention does not contain toxic heavy metals or other materials or elements that are undesirable or problematic in health or environmentally related respects.

[0027] A further advantageous and highly desirable effect of the base coat enamel composition according to the invention is its readily available and inexpensive components, which are readily available, do not require environmentally harmful mining and do not present any problems with regard to raw material shortages, which is already evident in the case of some of the adhesive oxide metals used so far.

[0028] Furthermore, the object of the present invention is also achieved by a basecoat enamel layer applied to the surface of a steel sheet, the former being made from a basecoat enamel coating according to the above description.

[0029] The basecoat enamel layer according to the invention in the steel-basecoat enamel contact area comprises iron silicate, which is formed during the basecoat enameling process at temperatures required for such process in the range of 890°C to 950°C from the reaction of iron(III) oxide added to the basecoat enamel composition with metallic iron of the steel substrate in the presence of silicon dioxide. This steel-basecoat enamel contact area here extends from the steel surface towards the basecoat enamel, and the iron silicate, in the cooled state, i.e. in the form of the finished basecoat enamel layer, adheres very firmly to the surface of the steel substrate and forms a solid coating thereon that extends over the entire surface, thus protecting the surface coated with the basecoat enamel from further external influences.

[0030] A particular advantage of the base coat enamel layer according to the invention is that it can have a layer thickness in the range from 0.05 mm to 0.8 mm, but preferably in the range from 0.1 mm to 0.4 mm, particularly preferably in the range from 0.1 mm to 0.3 mm.

[0031] The base coat enamel layer according to the invention can have a layer thickness much less than 0.5 millimeters, which leaves considerable room for applying one or more overcoat enamel layers in comparison with the prior art to produce a highly corrosion-resistant coating in accordance with DIN / ISO standards, especially considering that according to the invention there is no need to apply more than one base coat enamel layer to the steel substrate.

[0032] A further important advantage of the present invention is that the iron silicate according to the present invention is crystalline, and in particular, essentially, i.e., primarily, in the form of fayalite crystals, Fe2SiO4. These fayalite crystals have a very high melting point, exceeding 1000°C, and therefore can withstand even the repeated intense heat of the downstream firing process. The iron silicate forms, to some extent, as part of fayalite crystals on the steel surface of the steel substrate, in a continuous, solid, crystalline, and highly resistant layer having a layer thickness of less than 80 μm, preferably less than 50 μm, for example, in the range of 15 μm to 50 μm. It should be further pointed out at this point that the iron silicate according to the present invention does not necessarily have to be exclusively in the form of fayalite crystals, but can instead be in the form of a mixed silicate, for example, in the form of olivine (Mg,Fe)2SiO4 or hedenbergite (CaFe)(SiO6), in the presence of other metals, such as magnesium or calcium, if such metals are present in the base coating enamel composition. However, the essence of the present invention is that in all cases adhesion of the undercoat enamel layer to the steel surface of the substrate is achieved by taking advantage of the Fe-O-Si bonding structure present in iron silicates.

[0033] As mentioned above, the iron silicate in the steel-base coating enamel contact area forms, particularly over the entire surface, a crystalline layer suitable for forming a barrier layer between the steel surface of the substrate and the glassy or amorphous phase of the base coating enamel layer, for example, directly adjacent to the fayalite crystalline layer, particularly advantageously between the steel surface of the substrate and at least one overlying base coating enamel layer of a highly corrosion-resistant article produced using the base coating enamel composition according to the invention. Due to this barrier property of the crystalline layer, reactions between the components of the steel substrate and the components of the enamel layer are effectively prevented, and the crystalline layer, with a thickness in the range of 10 μm to 65 μm, preferably in the range of 15 μm to 50 μm, particularly preferably 50 μm or less, provides effective and good protection against reactions such as those occurring in conventional base coating enamel coatings known in the prior art.

[0034] For this reason, the base coating enamel layer according to the present invention, in particular the crystalline layer, is also essentially bubble-free, in particular also essentially free of carbon monoxide and / or carbon dioxide, which significantly improves and increases both the chemical stability and in particular the mechanical stability of the base coating enamel layer produced using such a base coating enamel composition according to the present invention, and consequently also improves and increases the chemical and mechanical stability of the highly corrosion-resistant articles produced using such a base coating enamel composition according to the present invention compared to conventional highly corrosion-resistant articles.

[0035] Since the crystalline layer of the base coating enamel layer according to the invention provides such a good mutual barrier effect both with respect to the transfer of materials to the steel surface of the substrate and with respect to the egress of materials from the steel of the substrate, it is possible according to the invention to use a steel substrate, the steel sheet of which has a carbon content in the range of 0% to 0.5% by weight, preferably in the range of 0.01% to 0.45% by weight, particularly preferably in the range of 0.08% to 0.3% by weight, particularly in the steel-base coating enamel contact area.

[0036] It is therefore possible to use steels with very high carbon contents compared to conventional requirements in the production of highly corrosion resistant articles in a very advantageous manner. Thus, the base coating enamel composition according to the invention also allows for a more cost-effective production of highly corrosion resistant articles, since it is not necessary to resort to very low carbon, often expensive, steels, but instead conventional steel grades can be used.

[0037] Another important aspect of the present invention is that the base coat enamel layer produced using the base coat enamel composition of the present invention has a self-repairing mechanism. Therefore, the base coat enamel composition of the present invention combines two properties that are extremely useful and important for producing highly corrosion-resistant articles. The first of these two properties is the ability to form iron silicate crystals with the metallic iron in the steel substrate, which form a barrier layer on the surface of the steel substrate as a full-surface layer that is firmly adhered at high temperatures. The second property of the base coat enamel composition of the present invention is the ability to form a bonding layer, i.e., an adhesive layer that can achieve optimal bonding to the overcoat enamel layer.

[0038] In the rather theoretical scenario of damage to the crystalline layer firmly adhering to the surface of the steel layer, for example, resulting in a theoretically conceivable hole or thinned spot due to the mechanical action of forces, the aforementioned self-repair mechanism becomes automatically activated, since when the crystalline layer is damaged, upon heating, instantaneous and automatic reformation of fayalite crystals occurs at the damaged site, where metallic iron(II) again reacts with iron(III) oxide present in the base coat enamel composition according to the invention to form iron(II), which then immediately reacts further with silicon dioxide also present in the base coat enamel composition according to the invention to form iron silicate. This reaction occurs as long as possible due to the initial thinness of the iron silicate crystalline layer, and likewise ends automatically when the thickness of the iron silicate crystalline layer reaches a maximum thickness of approximately 65 μm to 80 μm.

[0039] The primary growth of the iron silicate crystal layer on the surface of the steel substrate is also terminated in the same manner.

[0040] The object of the present invention is also achieved by an article having high corrosion resistance to mechanical, thermal and chemical influences, comprising a base coating enamel layer applied to a steel sheet in the form described above, and at least one overcoating enamel layer applied to the base coating enamel layer.

[0041] According to the present invention, the total thickness of the base coating enamel layer and at least one overcoat enamel layer of the highly corrosion-resistant article produced using the base coating enamel composition according to the present invention is in the range of 0.5 mm to 3 mm, preferably in the range of 0.8 mm to 2.6 mm, and particularly preferably 2.4 mm or less. Thus, due to the very thin base coating enamel layer achievable according to the present invention, it is possible to produce highly corrosion-resistant articles with increased corrosion resistance compared to conventional highly corrosion-resistant articles with the same enamel layer thickness, since the base coating enamel layer according to the present invention only needs to be present as a single layer, and more overcoat enamel layers can or can be applied than before, while still meeting DIN / ISO standard 28721-1.

[0042] In addition, the object of the present invention is also achieved in particular by a method for producing a basecoat enamel layer having the above-mentioned properties, comprising the following steps: i. providing a steel sheet; ii. optionally, superficially removing rust, especially loose rust; iii. applying a base coat enamel composition as described above; iv. Firing the base coating enamel composition at a temperature in the range of 890°C to 950°C, preferably in the range of 900°C to 940°C, particularly preferably in the range of 920°C to 930°C, for a period in the range of 20 minutes to 80 minutes, preferably in the range of 25 minutes to 70 minutes, particularly preferably in the range of 28 minutes to 60 minutes.

[0043] In this regard, it should be pointed out at this point that the base coat enamel layer according to the invention can, in principle, be applied to new steel substrates at any time using the base coat enamel composition according to the invention, as well as to used steel substrates, for example, 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 initial defective enamel layer and free components from the steel substrate, for example, by blasting. Following this, a new coating can be applied using the base coat enamel composition according to the invention, and all the associated advantages can be utilized.

[0044] In addition, the object of the present invention is also further achieved by a method for producing a highly corrosion resistant article, in particular a method for newly producing or reconditioning a used highly corrosion resistant article, which method comprises carrying out the following steps: a) providing a new article, in particular made from steel sheet, or a used highly corrosion-resistant article having a damaged base coating enamel layer and / or overcoating enamel layer, b) cleaning the surface of the article to be coated, in particular mechanically, for example by blasting with at least one abrasive material, in order to substantially remove any loose deposits, such as for example rust and / or one or more previous, particularly defective, coatings; c) preparing a base coat enamel layer once on the cleaned steel sheet to be coated according to or similar to the above description of the method for preparing the base coat enamel layer; d) applying a coating enamel composition slip to subsequently form a coating enamel layer on the base coating enamel layer; e) drying the coated enamel composition slip; f) heating the article comprising the base coating enamel layer and the overcoating enamel composition, or rather the dried overcoating enamel composition slip, to a firing temperature in the range of 780°C to 870°C, preferably 800°C to 860°C, particularly preferably 800°C to 840°C, g) maintaining the baking temperature for a period of 6 to 125 minutes, preferably 6.75 to 100 minutes, particularly preferably 7.5 to 90 minutes, to produce a coated enamel layer; h) cooling the article in a controlled manner; i) if necessary, repeatedly applying the coating enamel composition slip to subsequently form further coating enamel layers on the coating enamel layer, similar to the five aforementioned steps d) to h).

[0045] Thus, the method for producing highly corrosion-resistant articles according to the present invention offers numerous advantages. First, a single coating with the base coat enamel composition is sufficient even for geometrically difficult-to-coat articles, because the crystalline layer acting as a barrier is formed at all points on the article, provided it does not reach a thickness sufficient to terminate the reaction between the metallic iron from the steel substrate and the iron(III) oxide and silicon dioxide from the base coat enamel composition. Because 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, according to the present invention, it is not necessary to apply the base coat enamel composition according to the present invention in a uniform layer thickness at all locations on the article to be coated, because, especially at the high temperatures required for coating, there is sufficient migration of reactive components to thin spots and / or defects where the crystalline layer may not have grown thick enough. Therefore, such thin spots and / or defects are virtually automatically repaired and / or filled by the base coat enamel composition according to the present invention until a sufficient crystalline layer thickness is reached. According to the present invention, in addition to the iron (0) coming from the steel surface of the steel substrate, both iron (III) oxide and silicon dioxide are present in excess in the base coat enamel composition according to the present invention, so that there is always sufficient iron (0), iron (III) oxide and silicon dioxide to allow the formation of a full and dense crystalline layer of fayalite crystals, which also contributes to the highly advantageous self-repair mechanism of the fayalite crystalline layer, according to the present invention.

[0046] A further advantage of the method of the present invention for producing highly corrosion-resistant articles is that the barrier layer of fayalite crystals protecting the steel of the steel substrate is very thin, thus allowing for a very thin undercoat enamel, which means that it is now possible to apply a larger overcoat enamel layer on top of the undercoat enamel layer, which firstly allows for significantly higher corrosion resistance and also for greater mechanical stability of the highly corrosion-resistant articles produced by the method of the present invention.

[0047] Furthermore, the objects of the present invention are also achieved in particular by the use of a basecoat enamel composition according to the above description for producing the above-mentioned highly corrosion-resistant article.

[0048] The essence of the present invention and its advantages can be summarized as follows:

[0049] The essential core of the present invention is that a completely new approach to basecoat enamel adhesion is provided.

[0050] Thus, in order to overcome the difficulties known from the prior art for producing an underlying coating enamel layer on the one hand and for producing highly corrosion-resistant articles on the other hand, and to reduce the amount of conventional adhesion oxides at least, in particular to zero, a new adhesion mechanism is provided.

[0051] The approach according to the invention completely avoids the use of all metal oxides previously described for the formation of an alloy between the adhesive oxide and the steel, which was previously necessary for the formation of a chemically stable bond of the enamel on the steel.

[0052] The new adhesion mechanism according to the present invention uses Fe2O3 as a bonding material to create a chemical bond between the underlying coating enamel layer and the steel.

[0053] Therefore, when Fe2O3 is added to enamel without adhesive oxide, the Fe2O3 in the enamel layer reacts with the steel sheet at the enamel-steel interface, and the metallic iron (Fe 0 ) oxidation-reduction reaction occurs between the Fe in the enamel layer. 3+ Fe 2+ At the same time, iron on the steel surface is converted into 0 is Fe 2+ Along the interface layer, Fe 2+ The former immediately reacts further with SiO2 to form iron silicates.2+ Because the steel is supersaturated with Fe, iron silicate crystals crystallize along the interface with the steel—and only there. To enable this supersaturation, a weight percentage of iron(III) oxide ranging from 5% to 28% by weight is used according to the present invention, ensuring that a sufficient amount of FeO is always present in the base coat enamel composition according to the present invention. Ideally, this FeO content ensures that the enamel melt reacts with the steel to form a crystalline layer as quickly as possible during the first firing step, i.e., the first and only firing step for forming the base coat enamel. The duration of the firing step depends on the thickness of the steel sheet and, according to the present invention, is in the range of 20 to 80 minutes. The time required for firing the base coat enamel layer increases with the thickness of the steel sheet. In this regard, it should be noted that the 20 to 80 minute period relates to the time required to maintain the temperature required for firing the base coat enamel layer after the firing temperature has been reached.

[0054] During this first firing step, the Fe2O3 and SiO2 present in the enamel melt form a continuous layer of high-melting-point iron silicate crystals, essentially in the form of fayalite, i.e., Fe2SiO4, along the enamel melt-steel interface. The resulting crystals have a melting point above 1000°C and thus form a continuous, solid crystalline layer that is not broken down during subsequent firing processes. This crystalline layer therefore effectively blocks further reaction between the enamel melt and the steel. Depending on the applied thickness of the basecoat enamel, the crystalline layer preferably has a thickness of 15 to 50 μm. By the time a continuous crystalline layer forms along the steel-enamel interface, crystal growth itself automatically stops. Therefore, even during prolonged further firing steps, no further growth of the crystalline layer along the interface occurs.

[0055] Because the crystalline layer remains very thin, the amount of enamel required to form it is significantly reduced. With conventional base coat enamels, even insufficient and excessively thin applications of the base coat enamel composition of the present invention to the steel surface would lead to insufficient formation of a base coat enamel-adhesion layer, thus requiring a second base coat application step, or resulting in the enamel layer peeling or delaminating. Even if the base coat enamel composition itself does not provide a sufficient amount of silicon dioxide, this does not result in an insufficient or unusable base coat enamel layer of the present invention, since the subsequently applied top coat enamel will provide the necessary amount of SiO2 to enable crystallization and the formation of iron silicate crystals. As already mentioned above, this effect is also essential for the highly advantageous self-repair mechanism of the base coat enamel layer of the present invention.

[0056] Fe in Fe2O3 2+ Reduction reaction to Fe and metallic Fe 0 Fe 2+ The oxidation reaction to Fe-O-Si, and further reaction with SiO2 to form iron silicate, is an exothermic process that promotes chemical bonding. An extremely stable and strong bond is formed via Fe-O-Si.

[0057] Due to the coincidence that there is no difference in the electronegativity between the metal of the adhesive oxide used so far and the steel substrate of the base material, no uncontrollable further reactions in the sense of alloy formation and / or redox reactions can occur along the steel-enamel interface layer and / or adhesive layer. Once the crystalline layer is completely formed, the crystal formation reaction stops automatically. The driving force behind the adhesive reaction is the formation of the crystalline layer. As a result, the base coat enamel is significantly more resistant to long firing temperatures and firing times than base coat enamels known so far according to the prior art that work with adhesive oxides.

[0058] A further essential advantage of the base coat enamel composition according to the invention is that the formation of CO and CO bubbles in the base coat enamel is reduced, since the solidified iron silicate crystals prevent further reaction of the steel surface.

[0059] Therefore, the advantages of the present invention are apparent as follows: To form a chemically stable bond of the enamel to the steel, it is possible to avoid the use of adhesive oxides, cobalt oxide, manganese oxide and nickel oxide, which have traditionally been problematic. · It is possible to avoid the use of rare earth oxides to form a chemically stable bond of the enamel to the steel. To form a chemically stable bond of the enamel to the steel, it is possible to avoid the use of other, particularly toxic, heavy metal oxides such as molybdenum (Mo), vanadium (V), and / or tungsten (W). The minimum thickness of the base coating enamel layer can be reduced to less than 0.1 mm. It is possible to dispense with the application of a second basecoat enamel layer. -The thickness of the base coating enamel layer required for adhesion can be reduced to approximately 0.1mm to 0.3mm. The base coat enamel layer has an inherent self-repairing property, especially when the base coat enamel is poorly applied. The crystalline layer is already present during the base coating enamelling process, forming an oxidation barrier on the steel. Once a sufficient thickness is reached, the crystal growth slows down rapidly and automatically. The thickness of the crystalline layer along the steel surface does not exceed 50 μm under normal conditions. When using steel sheets with a carbon content of more than 0.14% by weight, it is possible to avoid the annealing process. The direct use of steel sheets with a relatively high carbon content of 0.25% by weight, and in some cases even up to 0.5% by weight, is possible. The base coating enamel according to the invention does not contain any adhesive oxides, any rare earth metals or any toxic heavy metals, in particular it does not contain any of the following elements: Co, Ni, Mn, W, V, Nb, Mo, Cr, Sb, As, Bi, Pb, Tl. The adhesion reaction of the base coating enamel layer to the steel surface proceeds via a crystallization process of Fe-O-Si bonds. No alloying with more inert partners or metals (Co, Ni, Mn, W, V, Nb, Mo, Cr, Sb, As, Bi, Pb, Tl) occurs along the steel interface, and such alloying, which was previously required in the prior art to produce adhesion, is not required in the present invention.

[0060] Further embodiments of the invention emerge from the dependent claims.

[0061] The invention will be explained below with reference to exemplary embodiments which will be elucidated in more detail on the basis of the drawings. [Brief explanation of the drawings]

[0062] [Figure 1] 1 shows a cross section through a conventional highly corrosion resistant article according to the prior art. [Figure 2] 1 shows a cross section through a highly corrosion resistant article made in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0063] In the following description, the same reference numerals are used for identical and functionally identical parts.

[0064] FIG. 1 shows a cross-section through a conventional highly corrosion-resistant article 10. Article 10 comprises a steel sheet 20 having applied thereto a base coating enamel layer 30. Base coating enamel layer 30 is adjacent to steel sheet 20 along a steel-base coating enamel contact area 60, along which a layer of iron oxide dissolved in the base coating enamel is formed, adjacent to base coating enamel layer 30 in a glassy state containing many gas bubbles 50. Disposed on base coating enamel layer 30 are multiple overcoat coating enamel layers 40, which are also many gas bubbles.

[0065] FIG. 2 shows a cross-section through a highly corrosion-resistant article 10 made in accordance with the present invention using a base coating enamel composition according to the present invention. Thus, the article 10 made in accordance with the present invention includes a steel layer in the form of a steel sheet 20 onto which a base coating enamel layer 30 has been applied. The base coating enamel layer 30 includes, as part thereof, a crystalline layer 35 that completely covers the steel sheet 20 along the steel-base coating enamel contact area 60 and shields it from influences from the overlying base coating enamel layer 30 and the overlying overcoat enamel layer 40. The crystalline layer 35 is composed of fayalite crystals and is bubble-free. The thickness of the crystalline layer 35 is substantially 50 μm. As can be readily seen from FIG. 2, any bubbles present are present only in the region of the base coating enamel layer 30 adjacent to the overcoat enamel layer 40; the base coating enamel layer 30 is otherwise bubble-free. No further bubble formation occurs; instead, the area of ​​the underlying coating enamel layer adjacent to the crystalline layer 35 is also bubble-free.

[0066] Exemplary formulations for glass compositions according to the present invention are shown in the table below.

[0067] [Table 4]

[0068] It should be pointed out at this point that all the above-mentioned parts, both alone and in any combination, and in particular the details illustrated in the drawings, are claimed as essential to the invention, modifications of which are known to those skilled in the art.

[0069] List of Reference Numbers 10 Highly corrosion resistant articles (details) 20 steel sheets 30 Base coating enamel layer 35 Crystal Layer 40 Coating enamel layer 50 bubbles 60 Steel-Based Coating Enamel Contact Area

[0070] A base coating enamel composition, a base coating enamel layer made from such a base coating enamel composition, a highly corrosion-resistant article having such a base coating enamel layer, a method for making such a base coating enamel layer, a method for making a highly corrosion-resistant article using such a base coating enamel composition, and the use of such a base coating enamel composition for making a highly corrosion-resistant article.

Claims

1. A base coating enamel composition for producing an adhesion-promoting layer between steel and at least one overcoating enamel to produce an enamel-based coating that is highly resistant to corrosion against mechanical, thermal and chemical influences, said base coating enamel composition containing boron oxide (B ２ O ３ ) and alkali metal oxides, especially Li ２ O, Na ２ O and / or K ２ O in weight proportions according to the following table: Table 1 And as the first main component, SiO in a weight percent ratio in the range of 35 wt% to 70 wt%, preferably in the range of 40 wt% to 65 wt%. ２ and as a second main component, Fe in a weight percentage ratio in the range of 5 wt.% to 28 wt.%, preferably in the range of 7 wt.% to 23 wt.%, particularly preferably in the range of 8 wt.% to 15 wt.%. ２ O ３ A base coating enamel composition comprising:

2. The base coating enamel composition is Al ２ O ３ and alkaline earth metal oxides, in particular calcium oxide, in weight proportions according to the following table: Table 2

3. The base coat enamel composition contains at least one material, in particular ZnO, TiO, in weight proportions according to the following table, to control the rheology of the melt of the base coat enamel composition: ２ and / or CaF ２ The base coating enamel composition of claim 2, further comprising: Table 3

4. 3. The basecoat enamel composition according to claim 1 or 2, characterized in that it is essentially free of adhesion oxides, i.e. oxides of the elements nickel, cobalt and manganese, and in particular is essentially free of rare earth elements, particularly preferably is essentially free of the elements cobalt, nickel, manganese, tungsten, vanadium, niobium, molybdenum, chromium, antimony, arsenic, bismuth, zinc, tin, lead and thallium.

5. A base coating enamel layer (30) applied to a steel sheet (20), characterized in that the base coating enamel layer (30) is made from the base coating enamel coating according to any one of claims 1 to 4.

6. 6. The base coating enamel layer according to claim 5, wherein the steel-base coating enamel contact area comprises iron silicate.

7. 7. The base coating enamel layer according to claim 5 or 6, characterized in that the base coating enamel layer (30) has a layer thickness in the range of 0.05 mm to 0.8 mm, preferably in the range of 0.1 mm to 0.4 mm, particularly preferably in the range of 0.1 mm to 0.3 mm.

8. The iron silicate is crystalline, in particular essentially composed of fayalite crystals, Fe ２ SiO ４ 8. The base coating enamel layer according to claim 6 or 7, characterized in that it is in the form of:

9. 9. A base coating enamel layer according to any one of claims 6 to 8, characterized in that the iron silicate in the steel-base coating enamel contact area forms a crystalline layer (35), in particular on the entire surface.

10. 10. The base coating enamel layer according to claim 9, characterized in that the thickness of the crystalline layer (35) is in the range of 10 μm to 65 μm, preferably in the range of 15 μm to 50 μm, particularly preferably 50 μm or less.

11. The undercoating enamel layer (30), in particular the crystalline layer (35), is essentially free of bubbles, in particular CO and / or CO ２ The base coating enamel layer according to any one of claims 5 to 10, characterized in that it does not contain

12. 12. The base coating enamel layer according to any one of claims 5 to 11, characterized in that the steel sheet (20), in particular in the steel-base coating enamel contact area, has a carbon content in the range of 0 to 0.5% by weight, preferably in the range of 0.01 to 0.45% by weight, particularly preferably in the range of 0.08 to 0.3% by weight.

13. An article (10) having high corrosion resistance to mechanical, thermal, and chemical influences, comprising a base coating enamel layer (30) according to any one of claims 5 to 12 applied to a steel sheet (20) and at least one overcoating enamel layer (40) applied to the base coating enamel layer (30).

14. 14. A highly corrosion-resistant article according to claim 13, characterized in that the total layer thickness of the base coating enamel layer (30) and the at least one overcoating enamel layer (40) is in the range of 0.5 mm to 3 mm, preferably in the range of 0.8 mm to 2.6 mm, particularly preferably 2.4 mm or less.

15. The following steps: i. providing a steel sheet (20); ii. Optionally, superficially removing rust, especially loose rust; iii. Applying the base coating enamel composition according to any one of claims 1 to 4; iv. firing the base coat enamel composition at a temperature in the range of 890°C to 950°C, preferably in the range of 900°C to 940°C, particularly preferably in the range of 920°C to 930°C, for a period in the range of 20 minutes to 80 minutes, preferably in the range of 25 minutes to 70 minutes, particularly preferably in the range of 28 minutes to 60 minutes; A method for producing an undercoating enamel layer (30) according to any one of claims 5 to 12, characterized in that

16. 15. A method for producing a highly corrosion-resistant article (10), in particular a method for newly producing or reconditioning a used highly corrosion-resistant article (10), according to claim 13 or 14, comprising the following steps: a) providing a new article made from a steel sheet (20) or a used highly corrosion-resistant article (10) having a particularly damaged base coating enamel layer (30) and / or overcoating enamel layer (40); b) cleaning the surface of the article to be coated, in particular mechanically, for example by blasting with at least one abrasive material, in order to substantially remove any loose adherent matter, such as for example rust and / or one or more previous, in particular defective, coatings; c) once producing a base coating enamel layer (30) on the cleaned steel sheet (20) to be coated according to claim 15 or similar thereto, d) applying a coating enamel composition slip to subsequently form a coating enamel layer (40) on said base coating enamel layer (30); e) drying the coated enamel composition slip; f) heating the article comprising the base coating enamel layer (30) and the overcoating enamel composition, or rather the dried overcoating enamel composition slip, to a baking temperature in the range of 780°C to 870°C, preferably 800°C to 860°C, particularly preferably 800°C to 840°C, g) maintaining the baking temperature for a period ranging from 6 minutes to 125 minutes, preferably from 6.75 minutes to 100 minutes, particularly preferably from 7.5 minutes to 90 minutes, to produce the coated enamel layer (40); h) cooling the article in a controlled manner; i) if necessary, repeatedly applying the coating enamel composition slip to the coating enamel layer (40) to subsequently form further coating enamel layers (40); A method characterized by:

17. Use of the base coating enamel composition according to any one of claims 1 to 4 for making a highly corrosion resistant article (10) according to claim 16.

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