DE3534009A1 - Process for preparing surfaces of steel suitable for enamelling - Google Patents

Abstract

The process is carried out for preparing for direct-white enamelling of steel surfaces and comprises degreasing, prepickling, intensive pickling, nickel-plating and passivation of the steel surface, these treatments being carried out in the order indicated. To improve the adhesive strength of a white-enamel layer directly applied to the steel surfaces, in particular in the case of two-layer application of the enamel, the steel surfaces are additionally treated with an oxidising prepickling solution immediately before the intensive pickling. <IMAGE>

Description

Process for preparing surfaces that can be enamelled

Stahls The invention relates to a method for preparing surfaces enamellable steel for direct white enamelling (DWE) through successive Degreasing, pre-pickling, intensive pickling, nickel-plating and passivation of the steel surfaces.

Such an elaborate surface preparation is essential when with a DWE, optimal quality of the enamelled surface is to be achieved. The heterogeneous interface reaction between the three phases of steel-enamel penetration atmosphere can only run evenly over the entire surface area if the Surface in terms of its chemical composition and its metallurgical composition Structure homogeneous and largely free of disruptive substances such as scale, corrosion products, Oil and grease residues, rolling wear, drawing aids and carbon residues is.

In the introduction of DE-AS 22 40 767, a known method reports in which to roughen the sheet metal surface by chemical means Pickling a number of pickling acids, including nitric acid, are listed. It deals is essentially a phosphoric acid pickling treatment to remove Scale, roughening of the surface and formation of a thinner iron phosphate layer on the surface.

The effect of nitric acid is commonly seen as roughening the steel surface described. It is also referred to as bright stain, with the minor stain can be easily removed during subsequent rinsing with a jet of water and a shiny, shiny metallic - roughened - surface is created. Accordingly the HNO3 stain is used exclusively as a follow-up stain after the actual intensive stain used. As in this bath sequence the decisive one for replacement nickel plating Pickling agent is missing, can only be achieved by means of electrolytic nickel plating or chemical re- induction nickel plating the necessary Ni layer for the DWE must be applied. The means of such The surface structure achieved with the bright stain is also guaranteed with the latter two Nickel plating process does not give entirely satisfactory enamel adhesion.

GB-PS 763 379 also relates to a pretreatment process to improve the enamel adhesion and to protect against corrosion during storage to the actual enamelling process. The steel sheet is first immersed in a sodium orthosilicate cleaning solution immersed and electrolytically degreased. After rinsing with running water it will be the sheet in sulfuric acid or in a mixture of equal parts ant and Nitric acid in water or in a mixture of two of the acids mentioned immersed in water, whereby the stain can be electrolytically supported. Then there is another wash with running water.

For the method according to GB-PS 763 379 is characteristic of the following the application of an extremely thin coating of Ni, Mo or Co as well as one removable coating made of Zn, Sn or Cd.

AT-PS 288.107 deals with additives to nitrate-containing, sulfuric acid pickling solutions for the abrasive pickling of steel and iron parts with the goal of a constant pickling speed and thus constant pickling removal to reach. One of the bath additives with the mentioned effect is sulfamic acid cited.

According to JP-OS 58-110682, finally, when pickling stainless Steel with HN03-H2F2 mixed acid, if the effect of the added as NOx acceptor Sulphamic acid decreases, HNO3 with 2: 0.5 wt.% H202 instead of HN03 in addition added to the mixed acid.

The last-mentioned known methods thus give way to a great extent away from a preparatory process to DWE with the sequence of Treatment steps; they also have other goals.

The process parameters and the bath compositions for a typical Preparatory treatment as it is currently used are in the table below compiled. The main treatment steps are in each case with the rinses Groups combined.

As a rule, detergents are used for degreasing and cleaning, namely Mixtures of alkaline salts (builders), such as silicates, phosphates or alkali hydroxides or carbonates and surfactants are used. While silicates have a particularly good dirt-carrying capacity have, the phosphates are distinguished by their good dispersing effect dust-like coverings such as rolling and drawing abrasion. The surfactants are for "fat solubility" as a result of lowering the surface tension and desorption of the fat molecules from responsible for the steel surface. The builders enhance the effect of the surfactants considerable, so that there is a synergistic effect between builder and surfactant speaks.

In the case of pre-pickling, which is usually carried out with dilute sulfuric acid The surfaces are derusted and metal soaps split. The resulting cleavage products can be used in the subsequent degreasing process, which part the pre-pickling is to be removed from the surfaces.

The intensive staining and the subsequent nickel plating are decisive for the adhesion of the enamel. The pickling solutions used for intensive pickling are usually sulfuric acid, phosphoric acid or sulfuric acid with small additions of nitric acid - u.zw. up to a maximum of 1 mass% - included. Designation of the composition of the temperature of the duration of the treatment Treatment step solution solution step [° C] [min] Coarse degreasing 2% detergent 90 10 solution Hot rinsing water 50 5 Fine degreasing 2% detergent 90 10 solution Hot rinsing water 50 5 Cold rinse water room temperature 5 Pre-pickling 10% H2SO4 60-80 1 Cold rinse water room temperature 5 Subsequent degreasing 1 - 2% detergent medium solution 90 10 Hot rinsing water 50 5 Cold rinse water room temperature 5 Intensive pickling 10% by mass H2SO4 50-60 2-6 0.1 mass% HNO3 acidic rinse aqueous solution, pH 2 - 3 room temperature 5 Continuation of the table Designation of the composition of the temperature of the duration of the treatment Treatment step solution solution step [° C] [min] Nickel plating a) Exchange of 57 g NiSO4 # 7H2O / 1 nickel plating pH value 2.0 - 2.2 80 4-8 b) Reduction ratio 28.5 g NiSO4 # 7H2O / 1 nickel plating 3.5 g NaH2PO2 / 1 30 4-8 8.0 g CH3COONa / 1 pH 5.0-5.2 Acid rinsing aqueous solution, room temperature 5 pH 2 to 3 Cold rinsing water room temperature 5 Neutralization and 1% soda or Passivation of borax solution (DWE-Naß- 90 5 assignment) or purely organic Washing-up liquid (PUESTA- Assignment) With intensive pickling, a certain amount of material removal - the so-called pickling removal - must be achieved in order to achieve a clean, metallic bright steel surface with high reactivity during enamel firing. In addition, the prerequisites for the trouble-free process of the replacement nickel plating, which is usually carried out, must be created. The steel components and iron remaining on the steel surface as so-called pickling paste react with the nickel bath according to an ion exchange process, with nickel being deposited on the steel surface and iron dissolving. The more noble elements present in the pickling paste, such as C, Cu and Cr, have an electrochemical effect on this exchange. The amount of pickling paste and thus also the pickling removal itself control the nickel plating that can be achieved.

Alternatively, reduction nickel plating can also be carried out, wherein nickel is obtained from a nickel salt solution by reacting the salt with a reducing agent like sodium hypophosphite, for example according to the equation NiSO4 + NaH2P02 + H20 § Ni + NaH2Po3 + H2SO4 is deposited on the steel surface.

It is known that in order to achieve good enamel adhesion, a minimum amount of nickel on the steel surface is required.

The last treatment step is passivation and / or neutralization made to prevent corrosion on the active steel surface. the Salts used in low concentrations in the passivation solutions, such as Borates, nitrites, carbonates and hydroxides remain on the metal surface after drying. All inorganic salts - especially those with an alkaline reaction - cause even down to the lowest concentrations of 0.1 mg / dm2 and even below at Application of the enamelling compound in powder form with strong adhesive strength up to Formation of sheet silver. In contrast, wet enamel slip shows a good absorption capacity for all impurities and there is no impairment of liability. With electrostatic powder coating, there is a residue-free rinsing treatment or the use of an organic, compatible surfactant in the final rinsing bath is a prerequisite to achieve good email quality.

The enamelling is done directly in a single operation with titanium white enamel carried out. The enamelling compound can - as already mentioned above - wet as enamel paste or using the method of electrostatic powder application (PUESTA) be applied. In the case of wet application, for example, a by wet grinding one Mixture of frit, clay, bentonite, sodium aluminate and water obtained slip used, with, for example, recrystallization-clouded boron-titanium-white enamel as the frit is used.

The surface preparation should be the influence of the many, for factors responsible for the adhesion of the enamel, such as steel composition, Enamel frit resp.

-powder quality, firing conditions and enamel thickness, as far as possible be balanced.

Despite optimal preparation, it happens again and again that the adhesion of the enamel on the finished enamelled part is insufficient. this occurs with the types of steel with carbon content that are commonly used for DWE today below 0.01%, u.zw. both with vacuum decarburized steels (Vac steels) and with so-called open coil "(in the open collar) annealed steels (OC- Steels) equally on. Liability problems occur particularly often when from certain Reasons very thick layers of enamel must be applied or a second Enamelling as part of repairs (rework).

The object of the invention is to provide a preparatory process to create, with which an extensive improvement of the previously achievable enamel adhesion on steel surfaces with direct white enamelling, especially with two-layer Email order, is achieved.

In a method, the task at hand becomes that defined at the beginning Art according to the invention solved in that the steel surfaces immediately before Intensive pickling additionally treated with an oxidizing pickling solution. The invention is based on the knowledge that in addition to the nickel plating and its distribution on of the steel surface, the microstructure of the steel surfaces is also crucial Significance for the optimal reaction of the enamelling compound with the base material Enamel penetration and thus for the adhesion of the enamel.

The oxidizing pickling solution creates a larger surface generated as a result of superficial attack on the grain boundaries of the material structure. In doing so, the surface is enlarged, as it were, into the inside of the material. According to the invention The structural improvement achieved is from scanning electron microscope images directly visible on the steel surfaces.

This adheres directly to steel surfaces prepared according to the invention The white enamel applied is far better in those cases than with conventional ones Preparation treatment - especially if through rework a second layer of enamel is applied - sufficient adhesion is not achieved will. Thus, the scrap production of the enamelling plant can be reduced and in addition, an increase in the quality of the enamelled goods can be achieved because the latter a higher resistance to impact and bending stress exhibit.

The ones that accumulate as a pickling product in the oxidizing pre-pickling solution Iron ions have no negative influence on the pre-pickling result up to high concentrations the end.

Likewise, the removal of pickling has little effect on oxidizing pre-pickling within the scope of the usual spreads, the one used to control the nickel plating when replacing nickel plating essential pickling removal during intensive pickling.

An aqueous solution with a content of 1 to is advantageous 20% by mass, preferably 4 to 8% by mass of nitric acid as an oxidizing pickling solution used.

During oxidizing pre-pickling with such a solution, nitrous is formed Gases, so that the pickling solution in this case also expediently acceptors for this Gases are added.

The treatment is preferably carried out with an amidosulfonic acid-containing one Pre-pickling solution carried out.

The acceptors, such as sulfamic acid, react with the nitrous gases and bind them, so that the environmental pollution from these toxic and corrosive gases is drastically reduced. However, the acceptors do not have any adverse effects on the course of the oxidizing process according to the invention.

Treatment with the oxidizing pickling solution becomes beneficial at a temperature of a maximum of 700 ° C., preferably between 20 and 300 ° C., in particular at surrounding ambient temperature, 1 to 3 min, preferably 2 to 19 min long, carried out.

The improvement in enamel adhesion achieved according to the invention is made clear by the diagrams.

From Fig. 1, the improvement in the adhesion of the enamel support is for both sides single-layer and double-layer enamelled by wet enamel application, 1.5 mm thick sheets of vacuum-decarburized steel of DWE quality (in the tests For example, a deep-drawing steel with approx.

0.006% C, about 0.30% Mn, 0.016 to 0.018% P, 0.010 to 0.012% S; approx. 0.02% Cr, 0.02% Ni and 0.02% Cu (impurities) used) the same but conventionally prepared sheets can be seen. The nickel plating was made by replacement nickel plating.

The liability index was determined in each case with a ball drop test arrangement determined, where I stands for the best and V for the worst enamel adhesion.

In this test arrangement, a falling body with a hemispherical Tip with a diameter of 25 mm vertically into a tip placed on the enamel surface dropped cylindrical die with an inner diameter of 26 mm.

The mass of the falling body and its height of fall are chosen so that a fall energy of 10 J results (abbreviation 26 / 25-10). The judgment the impact impression is subjectively based on the above-mentioned 5-part scale, an intermediate value is also possible in each case.

For the assessment of the adhesive strength, not only is essential surface covered with enamel, but rather the one with Enamel residues toothed surface area.

The liability number he V means sheet silver, IV-V stands accordingly for a matt appearance of the surface without enamel residues. In Fig. 1 it can be seen that for example, achieved with one without oxidizing pre-pickling according to the invention Liability index from IV even improved liability by two liability indicators - So on liability number II - with the additional pre-pickling treatment according to the invention can be reached. The improvements that can be achieved are even clearer - namely Up to three units of the adhesion index - with DWE powder application, like them can be seen in FIG. 2.

The one produced by the preparatory process according to the invention - Recognizable in the scanning electron microscope images mentioned earlier - The sponge-like surface of the steel is ideal for both a very good and inhomogeneous - Nickel distribution as well as for a surface enlargement and one in particular reactive surface responsible, so that the adhesion reaction during enamel firing can run optimally. The intergranular attack in the depth of the material enables a good interlocking of the steel with the enamel.

FIGS. 3 and 4 are the adhesion index as a function of the reduction nickel plating The nickel plating produced can be found in the enamelling compound for wet application. the The properties of the sheet metal and the layer of enamel are each the same as in Connection with FIGS. 1 and 2 explained.

The diagram shown in Fig. 3 was based on conventionally prepared for DWE, enamelled steel surfaces determined, whereas in Fig. 4 values are entered are obtained with surfaces prepared according to the invention.

A comparison of FIG. 3 with FIG. 4 clearly shows the improvement Recognize the effect of the additional oxidizing pre-pickling according to the invention. Of the The area of the nickel plating to achieve optimal adhesion is after the additional Pre-pickling treatment widened considerably.

In addition, this is particularly important for two-layer enamel application The adhesion of the enamel is also absolutely better.

Claims (5)

Claims: 1. Process for preparing surfaces that can be enamelled Steel for direct white enamelling (DWE) by successive degreasing, Pre-pickling, intensive pickling, nickel-plating and passivation of the steel surfaces, thereby characterized in that the steel surfaces are added immediately before the intensive pickling treated with an oxidizing pickling solution. 2. The method according to claim 1, characterized in that an aqueous Solution with a content of 1 to 20% by mass, preferably 4 to 8% by mass of nitric acid is used as an oxidizing pre-pickling solution. 3. The method according to at least one of claims 1 and 2, characterized characterized in that the treatment at a temperature of a maximum of 700C, preferably between 20 and 30 ° C. for 1 to 30 minutes, preferably 2 to 10 minutes will. 4. The method according to one or both of claims 2 and 3, characterized marked that the treatment with iner pre-pickling solution, which acceptors for resulting nitrous gases is made. 5. The method according to claim 4, characterized in that the treatment is carried out with a pickling solution containing amidosulfonic acid.