DE2837012A1 - PROCESS FOR PARTIAL ENAMELLING OF STAINLESS STEEL ARTICLES - Google Patents

Description

VON KREISLER

PATENT LAWYERS Dr.-Ing. by Kreisler + 1973

Dr.-Ing. K. Schönwald, Cologne Dr.-Ing. Th. Meyer, Cologne Dr.-Ing. K. W. Eishold, Bad Soden Dr. J. F. Fues, Cologne Dipl.-Chem. Alek von Kreisler, Cologne Dipl.-Chem. Carola Keller, Cologne Dipl.-Ing. G. Selting, Cologne

5 COLOGNE 1 21 Aug

DEICHMANNHAUS AT THE MAIN RAILWAY STATION

1978 feet / he

Apparatenfabriek Atag B.V. te Ulft

and Friedr. Blasberg GmbH & Co KG, Solingen

Process for the partial enamelling of articles made of stainless steel

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10 41

909811/0754

Telephone: (0221) 234541-4 Telex: 8882307 dopa d Telegram: Dompalent Cologne

The invention relates to a method for the partial enamelling of objects made of stainless steel, the metal surfaces with a decorative appearance that are not covered with enamel in addition to firmly enamelled areas exhibit.

US Pat. No. 2,869,265, for example, deals with the difficulties that arise here. It applies here to overcome a variety of problems stemming from various causes. There is a first problem in ensuring the desired high adhesive strength of the enamel on the base material made of stainless steel. A conventional pickling process is not sufficient for the desired adhesion of the enamel to stainless steel, so that, for example in said US-PS a mechanical pretreatment for roughening the metal surface, for example by sandblasting or rough grinding has been proposed. The liability to be realized with it The enamel on the stainless steel base is improved, but such pretreatment is only limited can be used and is particularly suitable for the production of only partially enamelled objects stainless steel major drawbacks. Depending on the pattern of the desired decorative enamel application, namely, it can be difficult to locate the stainless steel raw material excluded from mechanical pretreatment, which ultimately appear as bare metal areas in the Finished product should be available. If in special cases - for example in those shown in the said US-PS Peeling - such a separation into parts to be roughened and parts not to be roughened is also possible, it is immediately understandable that, for example, when applying a decorative enamel pattern on a flat surface considerable difficulty for only partial

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Roughening of the stainless steel surface can occur. As a result instead, there is an overall sufficiently roughened surface on which the enamel pattern is then applied is applied and baked, so that there is then the need to roughen the now exposed Leveling metal areas and polishing them bright, for example. This is done according to the state of the art mechanical grinding and polishing suggested. It stands to reason that such a procedure is not only is material and labor intensive, but that this subsequent mechanical polishing work on the decorative enameled Stainless steel will only be possible in special cases, without the desired surface finish of the enamelled Endangering surface areas.

To improve the adhesive strength of enamel layers on iron and steel substrates, including rustproof ones Steel are numerous suggestions for applying intermediate adhesive layers been made. Primarily closed or non-closed metal intermediate layers are used here proposed, which are often to be deposited galvanically. This is what DE-PS 688 058 wants improve the adhesive strength of enamel coatings on unalloyed carbon steels or alloyed steels, that first of all an adhesive layer of iron of the highest purity, e.g. electrolyte iron, by plating or by direct electrolysis. An adhesive strength of the enamel layer, which corresponds to today's requirements cannot be set in the way described in this publication will.

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It is also known, for example, stainless steels Chromium steel or chromium-nickel steel or other alloyed steels with a ferritic and austenitic structure To pretreat activation with preferably strongly acidic nickel-containing solutions in such a way that the adhesive strength is reduced Signs of passivity on the stainless steel surface are canceled. This pre-treatment can be both chemically as well as electrolytically. At the same time, metal is deposited at the active sites, that the stainless steel surface before new passivation protects. The metal is usually not deposited in a continuous layer. Rather, it arises a primer made of nickel, cobalt or copper, for example, which randomly distributes the applied metals may contain on the surface of the stainless steel. Even the smallest amounts of these metals have an adhesion-improving effect, so that it can be assumed that even a few atomic layers of the applied metal have an adhesion-improving effect. Usually are metallic intermediate adhesive layers of this type ■ at most 1 to 2, u strong «Baths of this type are used in practice for the gold-plating and silver-plating of stainless steel cutlery as well as, for example, in the processing of machine parts, aircraft components and possibly also used for partial coating.

The relevant literature refers, for example, to W. Pfanhauser "Handbuch für Galvanotechnik" 1941, Volume I _r page 542. Such intermediate adhesive layers, in particular nickel or cobalt, are also used in practice for the full-surface enamelling of low-carbon steels or steel alloys - for example during manufacture used by household appliances such as stoves, washing machines or dishwashers and freezers. However, it has been shown that such intermediate adhesive layers

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Nickel, cobalt or copper are also insufficient for the subsequent enamelling of stainless steels Adhesion strength leads. This applies in particular to highly stressed, partially enamelled objects made of stainless steel as the base material, in which the enamelled surface areas have a considerable mechanical Must withstand stress. Such cases are given, for example, with household appliances, for example in the case of rinsing tubs which are conventionally made from stainless steel, but which are now partially should be provided with an enamel coating. Corresponding areas of application are, for example, "hotplates or other everyday objects.

For the production of such only partially enamelled items made of stainless steel, which are also adherent Enamelled surface areas, in particular metal surfaces not covered by enamel, with a decorative appearance should, another problem arises from a completely different source Enamel compounds required for high chemical and mechanical strength require comparatively high baking temperatures, usually in the range above 750 ° C, preferably above 800 ° C, e.g. in the range between 820 and 850 ° C. For manufacturing reasons, such objects must also be partially enamelled the entire piece of metal with its surface areas covered by enamel compound and not covered by enamel compound exposed to the required high temperatures. However, these temperatures not only lead to the desired Melt flow and thus for drawing the enamel onto the stainless steel, in an undesirable manner are simultaneously the areas not covered by the enamel mass are heavily scaled. For the decorative effect of this enamel-free surfaces reactions with particularly serious consequences can be caused by metallic deposits, those from previous work stages, for example from the application of intermediate adhesive layers. To the

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to give non-enamelled surface areas of the object the desired decorative bare appearance or around them Places for a further decorative refinement, for example for a subsequent metal deposition, such as gold plating or silver plating, to prepare, need the layer of scale and any residues of metallic Intermediate layers are removed. This cleaning should be as efficient as possible in terms of material and labor and should be immediate lead to a stainless steel surface that is as leveled as possible, so that the subsequent work required for polishing and / or smoothing can be kept as low as possible.

It can thus be seen that the object of the invention the manufacture of items made of stainless steel, the next firmly enamelled surface areas Metal surfaces not covered with enamel with decorative Appearance have a multitude of problems thrown on ·. The invention has the task of to develop a process that solves the problems described in the most energy and material-saving form coped with and partially enamelled items based on stainless steel with the desired mechanical Strength values and high quality of the decorative metal r connects surfaces.

The invention accordingly relates to a method of the type mentioned at the outset, which is characterized is that, at least the surface areas to be enamelled of stainless steel after degreasing with an intermediate adhesive layer of nickel, cobalt or Copper and then with an electroplated iron layer and that after the enamel has been fired in, the non-enamelled areas are

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can be cleaned and, if desired, refurbished decoratively.

It has surprisingly been shown that in the combination of thin adhesive intermediate layers of nickel, cobalt or copper and galvanically applied iron adhesive layers a technically simple and safe solution to the problem lies in the required high adhesive strengths of the enamel on the stainless steel base. For the decorative required according to the invention Enamelling with enameled and non-enameled surface areas lying next to one another on the same Subject, however, have found additional particular difficulties in preferred embodiments of the invention can be achieved in the manner described below. A first problem becomes created precisely by the galvanically applied intermediate iron layer, which promotes adhesive strength. One wears this usually only see a thin adhesive layer of iron on the entire surface of the only partially enamelled Stainless steel part, then there are usually undesirable effects in the enamel fire. As already described, During enamel firing, the areas not covered by the enamel scale from the areas that are electrodeposited Oxides form in these areas.

Probably due to the vastly different expansion coefficients of the stainless steel and the scaling iron adhesive layer occur in the non-enamelled surface area Tensions between the stainless steel base material and the applied scaling iron adhesive layer, so that This can flake off in flake form and is whirled up by air movements that cannot be prevented in the kiln. These iron or iron oxide particles fall onto the glassy, softened surface of the hot enamel and stick to it and / or form undesirable spotty discoloration there.

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The preferred embodiment looks accordingly the inventive method provides that the email-free Surface areas are largely free of the adhesive iron layer, at least during the enamel firing. That can take place in that - e.g. by suitable covering of the areas not to be enamelled during the galvanic Iron application - these areas are kept iron-free from the outset or that in the case of a full-area Apply the iron adhesive layer to the iron from the surface areas not to be enamelled before baking of the email is removed again in a suitable manner.

Now, however, it has continued to be shown - and here lies an important preferred element for the invention - that conveniently in the presence of limited amounts the iron adhesive layer in the enamel-free surface areas is not completely dispensed with. This is it around the following elements according to the invention: It is advisable to use the adhesive iron layer at least before the fire the applied enamel mass laterally over the areas covered with this mass with a narrow edge into the enamel-free Surfaces protrude. For the reasons outlined above, however, we are only consciously using one narrow protruding edge of the iron adhesive layer worked. As a rule, the width of this protruding edge is of the adhesive iron layer at least about 0.5, expediently at least about 1 mm. Usually this is outstanding Edge of the iron adhesive layer not wider than about 10 mm, preferably not wider than 5 mm. An outstanding one Edge of the iron adhesive layer in the range of about 1 to 3 mm - or, depending on the thickness of the enamel layer, only 1 to 2 mm be particularly expedient. The meaning of this element of the teaching according to the invention will become apparent from the following evident.

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With the decorative partial enamelling according to the invention of rustproof stainless steel surfaces, enamelled and non-enamelled areas adjoin each other. At the practical use of the finished goods, for example in sinks, stovetops or the like, are now completely especially the edge zones of the enamelled surface areas are exposed to heavy loads. It must be according to the invention so it must be ensured that absolutely reliable adhesion values of the enamel mass on the stainless steel

IQ was present in particular in the edge area of the surface area covered with enamel compound. In order to prevent oxidative undermining of these particularly endangered edge areas during scaling of the metal layers during the firing of the enamel, it is therefore preferred according to the invention to prefer the adhesive iron layer a little in the area not actually covered by the enamel compound. Furthermore, the following phenomenon must also be taken into account: During the baking process, the enamel compound flows slightly outwards, in particular due to the flattening of the edge areas, and thus spreads slightly. Surprisingly, it has been found that with the inventively preferred measure of the narrow protruding edge of the iron adhesive layer, it is possible to ensure the required high-quality adhesive strength precisely in the edge zones of the enamelled surface areas without undesirable side effects - for example undesired discoloration in the outer edge zone - occurring.

In the context of the action according to the invention, it can be expedient be, the enamel-free surface areas also from the adhesive intermediate layer made of nickel, cobalt or copper largely to be kept free. Appropriately, both for this and for the subsequent area is limited Application of the iron adhesive layer worked with means known per se from covering technology. So they can't metallizing areas of the stainless steel surface

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Suitable painting, by masking with adhesive tape or adhesive film or by tightly fitting pre-formed masks to be covered.

The successive stages of the process according to the invention are in the following in sequence individual described, additionally preferred according to the invention in connection with the respective work stage Features are particularly emphasized.

In a first step, the partie'll Stainless steel object to be enamelled degreased and cleaned by methods known per se. Particularly The following sequence of process steps can be suitable:

a) Degreasing the boil with an alkaline cleaner 70 - 90 ° C, treatment time approx. 5 minutes

b) Anodically degreasing in an alkaline electrolyte at 40-60 ° C, 4-8 A / dm ² , voltage 6-10 V, treatment time approx. 2 minutes

c) cold rinsing, approx. 30 seconds

d) Pickling in hydrochloric acid 1: 1, room temperature, 0.5-1 minute

_r .-- fe) Direct further treatment in the wet state in an - -, "electrolytic activation bath. In the case of activation with an adhesion promoting nickel -

J5 deposition can be a NicKeiciiloridbad under the following Process conditions are used: -r_

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Composition: 50 g / l nickel chloride each

100 ml / 1 hydrochloric acid

Process conditions at: working temperature room temperature

Current density 5-10 A / dm ²

Voltage 4 - 10 V.

pH value less than 1

Duration 1-2 minutes.

In a subsequent process step, the electrolytic deposition of the adhesive iron layer takes place. The mean layer thickness of the deposited The iron layer is expediently in the range from about 5 to 20 μm. The iron layer serves as a carrier for the subsequent enamelling. This iron layer is preferably in a particularly pure form deposited, i.e. it is free from foreign metals and organic compounds, in particular also free from carbon. These requirements for the iron layer can make the use of special electrolytes desirable. Boron fluoride electrolytes or also additive-free iron sulfate electrolytes are particularly suitable for this purpose. Is preferred According to the invention, a boron fluoride electrolyte used, the is characterized by its particular stability. Iron electrolytes of this type are known in principle. It for example, reference is made to Dettner-Elze "Handbuch der Galvanotechnik", Volume II, 1966, pages 444 ff., especially pages 449 and 450.

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As an example of one within the scope of the invention Process particularly suitable electrolytes and the working conditions preferably to be observed with this electrolyte the following information is provided:

Composition: 840 g / l iron (II) fluoroborate solution

35 g / l sodium chloride 10 g / l boric acid

0.1-0.5 g / l fluorinated anionic wetting agent

Process parameters: current density 10-20 A / dm

Voltage "'8 - 15 V, temperature 65 - 75 ° C Bath movement Electrolyte movement Treatment time 5-15 minutes.

These numerical data are to be understood as mean values and are given in each individual case Adapted to circumstances.

In a preferred embodiment of the invention the enamel is not applied directly to the iron layer deposited in this way, but rather another one and, if necessary, multi-stage modification of the iron layer is provided.

In a first preferred embodiment, the electrodeposited high-purity iron layer is initially one Subject to pickling. The pickling can be done chemically and / or electrolytically. In detail, for example be worked as follows:

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The workpiece resulting from the iron electroplating is preferably rinsed cold and then placed in the subsequent chemical and / or electrolytic stage Pickle given to roughen the previously formed iron layer. A suitable pickling liquid is, for example aqueous sulfuric acid. If pickling is carried out with a 10% aqueous sulfuric acid, the following process parameters can be used here Find use;

a) Chemically
10

b) Electrolytic

After the staining is finished, the material parts are rinsed again.

Working temperature 75 ° C. iron Min. Treatment time 5 Min. A / dm ² Removal 20 - 40 g / m ² V ch working temperature 40 g / m ² Treatment time - 60 ° C Current density 10 1 tension 5 - 20th Removal 20th - 10 - 40

In the preferred embodiment of the invention In the process, after the iron layer has been pickled, a further intermediate adhesive layer is preferably made Nickel or cobalt and only then the enamel layer is applied, or else it can be applied directly to the pickled Iron layer can be applied in a known manner, a first base enamel layer, which is then later with a Cover enamel is covered. These two alternatives for further treatment can also be combined with one another.

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Is applied to the pickled iron layer again first If a metallic intermediate adhesive layer is applied, nickel or cobalt baths in particular can be used here and electroless or electrolytic operation can be selected. For the currentless Working with nickel, for example, the following bath is particularly suitable under the following process parameters:

Composition of the solution: 10 g / l nickel sulfate pH 2.5 adjusted with Schwe

fic acid

Process parameters: temperature 70 - 75 ° C

Dive time 5 - 8 minutes

2 Deposited amount of nickel 0.5 - 1.5 g / m.

Instead of this exchange nickel-plating bath, electrolytic nickel or cobalt baths or also reduction nickel baths can be used. and cobalt baths are used.

In addition to or instead of these metallic intermediate adhesive layers a base enamel can be applied to the pickled iron layer in a manner known per se, the Adhesive oxides, especially nickel oxide and / or cobalt oxide, have been added.

The parts that have been pretreated in this way will eventually become more familiar Way provided with the cover enamel mass and burned in. The stoving temperatures are usually above 800 ° C. The previously described strong scaling of the non-enamelled stainless steel surface areas occurs here.

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In order to create the desired decorative, non-enamelled surface areas, descaling is the first step the non-enamelled surfaces required. The object of the invention is this descaling and cleaning To carry out labor and material saving and at the same time ensure that the descaling itself provides a steel surface that is as leveled as possible.

Does the arrangement of the decoratively designed non-enamelled surface areas allow in principle, the descaling and refurbishing of these areas can be done mechanically, e.g. by grinding and, if desired, subsequent polishing, take place.

In a further particularly preferred embodiment the invention, however, is the detachment of the scaled coating and any residual stocks of metallic Adhesive intermediate layers made in the non-enamelled areas by chemical and / or galvanic means. A combination of chemical substances is particularly suitable and galvanic measures, which are described in detail below.

The areas are descaled and cleaned using a multi-stage pickling process, which is expedient a combination of electrolytic and chemical pickling is carried out. The electrolytic Pickling anodic and / or cathodic alternatively with alternating current and / or with direct current with superimposed alternating current be performed. A stain that works in three stages has proven to be particularly suitable, that an electrolytic anodic pickling is carried out in a first step, which is followed in a subsequent A chemical pickling step follows, whereupon electrolytically alternating anodic-cathodic is being worked on.

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Essentially sulfuric acid is suitable as the pickling liquid and mixtures of phosphoric acid. A particularly suitable composition can, for example a 10 to 15% aqueous mixture of sulfuric acid and phosphoric acid (e.g. 10% by weight sulfuric acid 4% by weight phosphoric acid). When working with such a pickling solution, the following process parameters then apply, for example:

This solution is first electrolytically anodically pickled for 2 minutes at 60 ° C, 10 to 20 A / dm ² , 5-15 V. Then chemically pickling in the same solution is carried out for 5 minutes at 60 ° C and in the third stage for 10 minutes also electrolytically pickled in the solution of the same composition at 60 ° C, ^{10-20 A / dm 2} , 5-15 V, alternating anodic-cathodic, whereby the polarity of the goods can change every 10-20 seconds.

After this stain, the parts are rinsed and the rest of the time subjected to decorative treatment. For this purpose, according to the invention, for example, an electrolytic Polishing, such as the anodic polishing known per se, can be used. Well-known baths of this type can be used Based on chromic acid and / or sulfuric acid-phosphoric acid or based on perchloric acid and acetic anhydride be constructed. For the relevant literature, reference is made to Dettner-Elze "Handbuch der Galvanotechnik", Volume 1, Part II, Carl Hanser-Verlag, Munich 1964, pages 876, 912 and 913 as well as 925 ff. And H. Benninghoff "Electrolytic polishing of metals in industry", Eugen G. Leuze Verlag 1953.

For example, the following composition can be used in an anodic polishing bath:

Sulfuric acid, specific weight = 1.84 35 - 50 kg Phosphoric acid, 85% 40 - 60 kg

Alkylamines 10 g / l

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Process parameters: temperature 15th 60 ° C Current density 10 - 2OA / dm ² tension - 12 volts Movement of goods 10 - Treatment time 20 minutes

In addition to or instead of such an electropolishing, the non-enameled surface areas can also use further metallic layers are provided and / or colored. For example, it is possible to go through usual electroplating treatment gold, silver, chrome, black chrome, copper, brass, bronze and other metal layers to be deposited. It is also possible to deposit aluminum with subsequent coloring of the Aluminum or aluminum oxide surface. The three-stage pickling process selected according to the invention often makes it possible with all these finishing processes excellent effects directly on the pickled stainless steel surface as To achieve the basis for a subsequent electroplating and / or coloring, without the need for the previously described Electropolishing needs.

The subject matter of the invention also includes those produced in part by the process described enamelled items made of stainless steel, which in addition to the firmly enamelled surfaces with Enamel uncovered metal surfaces with a decorative appearance exhibit.

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Claims (11)

Claims 1) Process for partial enamelling of objects Made of stainless steel, the metal surfaces not covered with enamel in addition to firmly enamelled surfaces have a decorative appearance and at least in the areas to be enamelled a pretreatment to increase the adhesive strength of the enamel, characterized in that at least the to enamelling surface areas after degreasing with an intermediate adhesive layer made of nickel, cobalt or Copper and then provided with an electroplated iron layer and that after the burn-in of the Enamels clean the non-enamelled areas and, if desired, decorate them will· 2) Method according to claim 1, characterized in that the email-free surface areas at least during the The enamel fire are largely free of the adhesive layer of iron. 3 ) Method according to claims 1 and 2, characterized in that the adhesive iron layer protrudes laterally at least before the enamel firing over the surfaces covered with enamel with a narrow edge. 4) Method according to claims 1 to 3, characterized in that the protruding narrow edge of the adhesive iron layer before the enamel firing at least about 1 mm, but preferably no more than about 5 mm and in particular about 1 to 3 mm, is wide. 5) Process according to claims 1 to 4, characterized in that that the enamel-free surface areas also from the adhesive intermediate layer of nickel, cobalt or copper are largely kept free. 909811/0754 6) Process according to claims 1 to 5, characterized in that after the galvanic application of the intermediate adhesive layer from preferably nickel on the stainless steel the iron adhesive layer in an average layer thickness of 5 to 20 .mu.m, preferably from a boron fluoride electrolyte, is electroplated. 7) Process according to claims 1 to 6, characterized in that the adhesive iron layer is pickled before the enamel application is, it being possible to work chemically and / or electrolytically. 8) Process according to claims 1 to 7, characterized in that after the pickling of the iron layer for the subsequent Enamel application of a further intermediate adhesive layer, preferably made of nickel and / or a base enamel layer containing adhesive oxides is applied to the iron layer. 9) Process according to claims 1 to 8, characterized in that after the enamel has been fired, the non-enameled Surface areas cleaned by pickling and then preferably by electrolytic polishing and / or galvanic Metal deposition can be worked up decoratively. 10) Process according to claims 1 to 9, characterized in that in the pickling stage of the enamelled object also the proportions of the applied adhesive layers that may still be present in the non-enamelled surface areas to expose the surface of the stainless steel, preferably with a substantially pickling solution consisting of sulfuric acid and phosphoric acid is used. 09S1 1/0754 11) Method according to claim 10, characterized in that the exposure of the non-enamelled surface areas in several stages by means of a combination of electrolytic and chemical pickling takes place, preferably first electrolytically anodically, then chemically and finally electrolytically anodic-cathodic alternation is carried out. 9811/0754