EP0224065B1 - Process for obtaining chromate layers - Google Patents

Abstract

A method of treating a zinc plated steel surface provides improved corrosion resistance and painting properties. The surface is rendered cathodic in the presence of an aqueous treatment liquor containing 5-70 g/l of hexavalent chromium, 0.01 to 5 g/l of trivalent chromium, 5-100 g/l of silica and/or silicate and 0.05-10 g/l of nitrate in which the ratio of Cr3+/Cr6+ is within the range 1/50-1/3.

Description

The invention relates to a process for the production of chromate layers on surfaces of zinc or zinc alloys electrochemically by means of chromium ⁶⁺ containing aqueous Chromatierungsbäder.

It is known to treat surfaces made of zinc or zinc alloys with solutions which contain chromium ⁶⁺ , chromium 3+ and silica (JP-OS 52-17340, JP-OS 52-17341 from 1977). In these cases, the chromate layers are formed chemically by reacting the metal surface with the chromating solution. Although these methods have their advantages, problems arise in particular if chromate layers with an increased layer weight are desired in order to improve the corrosion resistance. However, layers of this type are distinguished by unsatisfactory adhesion to the treated metal. The adhesion of a subsequently applied coating and the uniformity of the chromate layer in its outer appearance also deteriorate.

A difficulty in setting the desired layer weight is also that either the concentration of the treatment solution varies or z. B. the shape or pressure of the application rolls must be changed. These difficulties increase as relatively rapid changes in layer weight are required.

It is also known to produce chromate layers on surfaces of zinc and zinc alloys electrochemically, for example with chromating baths containing chromium ⁶⁺ and sulfuric acid or chromium ⁶⁺ and heavy metal ions (JP-PS 47-44417 (1972); JP-PS 48-43019 (1973)). Such chromate layers, however, have insufficient corrosion resistance, but generally have good properties with regard to subsequent painting. In addition, electrochemical processes also have certain disadvantages with regard to the process control. If zinc-coated steel sheet is treated continuously, zinc ions which have been dissolved out accumulate in the chromating bath and the pH value increases. These zinc ions and the chromium ³⁺ ions formed by reduction during the electrochemical treatment precipitate out, so that the nature of the treatment solution does not remain stable. Among other things, the appearance of the coating formed also fluctuates and the corrosion resistance of the treated workpieces decreases.

The object of the invention is to eliminate the shortcomings of the known processes for producing chromate layers, in particular the abovementioned shortcomings, and to provide a process which leads to chromate layers with an excellent appearance and very good properties with regard to corrosion resistance and adhesion promotion for a subsequently applied organic coating .

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the chromate layers are produced with the aid of a chromating bath which contains 5 to 70 g / ^{l of} chromium ⁶⁺ ions, 0.01 to 5.0 g / I chromium ³⁺ ions, 5 to 100 g / I silicic acid and / or silicate and 0.05 to 10 g / I nitrate ions, the weight ratio of chromium ³⁺ : chromium ⁶⁺ in the range of 1:50 to 1 : 3 lies.

The chromium ⁶⁺ ions can be introduced into the bath individually or as a mixture via chromic anhydride, ammonium and alkali dichromate. The concentration of the chromium ⁶⁺ ions is essential in that in the case of a concentration below 5 g / l with continuous treatment of the workpiece to be machined, the speed of the coating formation drops and it becomes difficult at the same time to achieve uniform coating formation. If the concentration is more than 70 g / l, no improvement in the chromate layer formed can be determined. In addition, such high concentrations lead to a strong and undesirably high zinc dissolution. Finally, considerable quantities of treatment bath are discharged from the treated workpiece, which is ultimately uneconomical.

Can be added to the ion chromating in the form of nitrate or carbonate compounds ^- the Chrom3 +. It is also possible that Chrom3 + ^- ions by the reduction of chromium ^{+ 6 -} ion with organic substances, such as alcohol, starch, or tannic acid to introduce. Due to the Chrom3 + content, an increased deposition of the chromate layer is achieved with the same amount of electricity. The perfect ratio of chromium ³⁺ : chromium ⁶⁺ is also decisive for the aforementioned effect. If the Chrom3 +: Chrom ⁶⁺ ratio is below 1:50, the described effect weakens. With a Chrom3 +: Chrom ⁶⁺ ratio above 1: 3, the ability of the chromate layer to absorb organic coatings decreases.

The addition of silica and / or silicate pursues the goal of converting the chromating bath into a dispersion in which the dispersed particles have a size of approximately 1 to 100 μm. At a concentration below 5 g / l, the corrosion resistance and paint adhesion decrease. With concentrations above 100 g / 1, no further improvement can be achieved. In addition, there is a risk that silicic acid and / or silicates separate out and that too much of the chromating bath is removed from the workpiece to be treated.

The nitrate ions can be added to the chromating bath in the form of nitric acid, ammonium and alkali nitrate individually or as a mixture. The concentration range is important insofar as a perfect layer formation is no longer guaranteed below 0.05 g / l (cf. Comparative Example 3 in Table 3). In particular, corrosion resistance and paint adhesion decrease. At concentrations above 10 g / l there is an additional improvement in the characteristics stik of the chromate layer formed can no longer be determined. In addition, higher nitrate levels lead to increased zinc dissolution and a reduction in layer weight.

With regard to the individual components of the chromating bath, particularly favorable results with regard to layer quality and process control are achieved if, according to a preferred embodiment of the invention, the chromate layer is produced with the aid of a chromating bath which contains 10 to 50 g / l chromium ^{6 +} ions, 0.05 to 5 g / I chromium 3 + ions, 1 0 to 50 g / 1 silica and / or silicate and 0.1 to 3 g / I nitrate ions.

The pH of the chromate bath is not critical. However, it is particularly expedient to generate the chromate layers with the aid of a chromating bath, the pH of which is in the range from 1 to 6. At pH values below 1, the characteristics of the chromate layer do not change, but there is a risk that the amount of zinc released from the metal surface will increase and the weight of the chromate layer formed will decrease. If the pH is higher than 6, the character of the chromate layer formed is also retained, however, a separation of silica and / or silicate can hardly be avoided.

To adjust the pH of the chromating bath you can e.g. Choose ammonium hydroxide, alkali hydroxide or an alkali carbonate.

The temperature of the treatment solution is expediently between room temperature and 70 ° C. Although a higher temperature does not change the character of the chromate layer formed, it should not exceed the limit of 70 ° C. for economic reasons.

Within the method according to the invention, the workpiece is switched as a cathode. For this it is necessary that the surface is cleaned beforehand. However, the cleaning is not required to be complete.

According to a preferred embodiment of the invention, the electrochemical treatment is carried out at a cathodic current density of 3 to 80 A / dm ² . With a current density below 3 A / dm ² , a perfect layer formation is hardly guaranteed. Corrosion resistance and paintability are also becoming increasingly poor. A current density above 80 A / dm ² brings no additional benefit.

The duration of the electrochemical treatment is dimensioned such that the amount of chromium within the chromate layer formed is in the desired range. In addition to the duration of the treatment, the amount of chromium is also determined by the concentration of the individual components of the chromating bath, the pH value, the temperature and the current density. If the latter parameters are specified, the electrolysis time required to generate certain chromate layer weights can be determined after a few tests. Of course, you can also fix the electrolysis time and regulate the layer weight and the chromium content of the layer by changing the current density.

The relationship between the amount of electricity per unit area, expressed as Coulomb / dm ² , and the amount of chromium and silicon deposited in the chromate layer is shown in Fig. 1 using the example of the coating of an electrolytically galvanized steel sheet. The composition of the chromating bath used was

The electrolysis conditions on the basis of which this graph was created are given in Table 7.

Table 7 and Fig. 1 demonstrate that the chromate content in the chromate layer produced can easily be controlled by changing the amount of current according to the product of current density and electrolysis time. In contrast, the content of SiO _{2 in the} chromate layer is largely independent of the electrochemical conditions and remains practically constant. This makes it clear that the problems of purely chemical chromating processes discussed at the beginning can be remedied. In these, the layer weight can be increased to improve the corrosion resistance, but at the same time this also increases the silica content, as a result of which the adhesive properties and paintability of the coating formed decrease. Instead, when using the method according to the invention, chromate layers with excellent corrosion resistance and very good adhesion for a subsequently applied organic coating can be achieved.

Because of the relationships mentioned above and with regard to the production of chromate layers with optimal properties, a preferred embodiment of the invention provides for chromate layers whose chromium content is 10 to 300 mg / m ² , preferably 20 to 150 mg / m ² (calc as Cr), and their silica and / or silicate content is 3 to 30 mg / m ² , preferably 5 to 20 mg / m ² (calculated as Si).

The presence of nitrate ions in the chromating bath not only increases the. Corrosion resistance of the chromate layers formed, but is also responsible for the increase in the stability of the chromate bath and thus for the uniformity of the appearance of the chromate layers produced during continuous treatment. The nitrate ion content largely prevents accumulating zinc and chromium ions from precipitating in the chromating bath.

The surfaces of zinc or zinc alloys treated by the method according to the invention are usually rinsed with water and then dried. If desired, the chromate layers produced by the process can be passivated after drying with the usual aqueous chromate solutions and / or with corrosion-protecting solutions based on organic plastics. The chromate layers produced provide excellent corrosion resistance and are an excellent base for the subsequent application of organic coatings, such as paints.

The method according to the invention, intended for the treatment of surfaces made of zinc or zinc alloys, is suitable for workpieces made of compact zinc or of compact zinc alloys. It is also suitable for workpieces that have a surface coating of zinc or zinc alloy. This overlay can, for example, have been produced from a melt or electrolysis deposition. The method according to the invention is of particular importance for the treatment of galvanized steel strip.

The invention is illustrated by the following examples, for example and in more detail.

Example 1:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, rinsed with water after the treatment and dried. This sheet was compared with that obtained in Comparative Example 1. The result, which is shown in Table 1, shows that the chromate layer from the electrochemical treatment according to this invention had good properties in terms of adhesion, homogeneity, corrosion resistance and paint adhesion compared to the chromate layer obtained by the electroless process.

• Cr⁶⁺: 22,0 g/I (Verwendung von Chromsäureanhydrid)
• Cr³⁺: 4,0 g/l (Reduktion von Cr⁶⁺ durch Stärke)
• Si0₂-Dispersion: 250,0 g/I (20 Gew.-%ig der Fa. Nissan Chemical Industries, Ltd.)
• N0₃: 0,98 g/I (Verwendung von HN0₃)
• Der pH-Wert betrug hier 1,2.

Composition of the chromating bath:

• Cr ⁶⁺ : 22.0 g / I (use of chromic anhydride)
• Cr ³⁺ : 4.0 g / l (reduction of Cr ⁶⁺ through starch)
• Si0 ₂ dispersion: 250.0 g / l (20% by weight from Nissan Chemical Industries, Ltd.)
• N0 ₃ : 0.98 g / I (use of HN0 ₃ )
• The pH here was 1.2.

Conditions of cathodic electrolysis:

Electrolysis time: 3 to 12 s. (The electrolysis time was regulated in order to achieve a certain chromium content in the chromate layer.)

Current density: 10 A / dm ²

Electrolysis temperature: 50 ° C

• Auf gleiches gereinigtes, elektrolytisch verzinktes Stahlblech wurde dieses im Beispiel 1 beschriebene Chromatierungsbad durch Rollen aufgetragen und getrocknet. Der Chromgehalt der Chromatschicht wurde durch entsprechende Dosierung der Auftragsmenge an Chromatierungsbad gesteuert.

Comparative example 9:

• This chromating bath described in Example 1 was applied to the same cleaned, electrolytically galvanized steel sheet by rolling and dried. The chromium content of the chromate layer was controlled by appropriate metering of the amount of chromate bath applied.

Example 2:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, then washed with water and dried. The obtained sample was compared with that of Comparative Example 2, with the result that, as shown in Table 2, the chromate layer from the catholytic treatment according to this invention had better corrosion resistance and paint adhesion than the chromate layers obtained from the catholytic treatment according to Comparative Example 2.

• Cr⁶⁺: 41,6 g/I (Verwendung von Ammondichromat)
• Cr³⁺: 2,4 g/I (Verwendung von basischem Chromcarbonat)
• Si0₂: 20,0 g/I (Verwendung von Na₂0. Si0₂)
• N0₃: 0,98 g/I (Verwendung von HN0₃.)
• Einstellen des pH-Wertes auf 5,0 mittels Ammoniumbicarbonat.

Composition of the chromating bath:

• Cr ⁶⁺ : 41.6 g / I (use of ammond dichromate)
• Cr ³⁺ : 2.4 g / I (use of basic chrome carbonate)
• Si0 ₂ : 20.0 g / l (use of Na ₂ 0. Si0 ₂ )
• N0 ₃ : 0.98 g / I (use of HN0 _3. )
• Adjust the pH to 5.0 using ammonium bicarbonate.

Comparative Example 2:

• Cleaned, electrolytically galvanized steel sheet of the same quality was treated electrolytically under the same conditions as in implementation example 2, but chromating baths were used in which Na _{2 was used} once compared to example ₂ . Si0 ₂ and HN0 ₃ , on the other hand Na ₂ 0-Si0 ₂ or N0 _{3 were} omitted.

Example 3:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, then washed with water and dried. The samples obtained were compared with those of Comparative Example 3, with the result that - as shown in Table 3 - the coatings from the catholytic treatment according to this invention had better corrosion resistance and paint adhesion than the coatings from the catholytic treatment according to Comparative Example 3.

• Cr⁶⁺: 15,2 g/I (Verwendung von Kaliumchromat)
• Cr³⁺: 1,5 g/I (Reduktion von Cr⁶⁺ mit Tanninsäure)
• Si0₂: 10,0 g/I (Pulver, Fabrikat der Fa. Japan Aerosil)
• Die Bäder wurden mit 0,06 bzw. 0,12 bzw. 0,24 g/I N0₃-lonen (eingebracht über NaN0₃) modifiziert und mittels Natriumhydroxid auf pH 5,0 eingestellt.

Composition of the chromating baths:

• Cr ⁶⁺ : 15.2 g / I (use of potassium chromate)
• Cr ³⁺ : 1.5 g / I (reduction of Cr ⁶⁺ with tannic acid)
• Si0 ₂ : 10.0 g / I (powder, manufactured by Japan Aerosil)
• The baths were modified with 0.06, 0.12 and 0.24 g / l N0 ₃ ions (introduced via NaN0 ₃ ) and adjusted to pH 5.0 using sodium hydroxide.

Conditions of cathodic electrolysis:

Electrolysis time: 5 s

Current density: 5 A / dm ²

Electrolysis temperature: 30 ° C

Comparative Example 3:

Sheets of the same quality were treated electrolytically with chromating baths, as indicated in Example 3. However, the N0 ₃ ions were once left out, and their concentration was reduced to 0.03 g / l. The cathodic electrolysis took place under the same conditions as in Example 3. The results are shown in Table 3.

Example 4:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, then washed with water and dried. The samples obtained were compared with those of Comparative Example 4, with the result that, as shown in Table 4, the chromate layers from the catholytic treatment according to this invention had better corrosion resistance and paint adhesion than the Chromate layers of Comparative Example 4.

• Cr⁶⁺: 5,2 g/I (Verwendung von Chromsäureanhydrid)
• Cr³⁺: 0,2 g/I und
• N0₃: 0,48 g/I (Verwendung von HN0₃)
  • mit Zusätzen von 6 bzw. 12 g/I Si0₂, eingebracht als 20 Gew.-%ige Dispersion (Fabrikat Asahi Denka Kogyo KK)
  • mit Ammoniumhydroxid auf pH 3 eingestellt
  • Bedingungen der kathodischen Elektrolyse:
  • Elektrolysedauer: 8 s
  • Stromdichte: 15 A/d_m ²
  • Elektrolysetemperatur: 30° C

Composition of the chromating baths:

• Cr ⁶⁺ : 5.2 g / I (use of chromic anhydride)
• Cr ³⁺ : 0.2 g / I and
• N0 ₃ : 0.48 g / I (use of HN0 ₃ )
  • with additions of 6 or 12 g / l Si0 ₂ , introduced as a 20% by weight dispersion (manufactured by Asahi Denka Kogyo KK)
  • adjusted to pH 3 with ammonium hydroxide
  • Conditions of cathodic electrolysis:
  • Electrolysis time: 8 s
  • Current density: 15 A / d _m ²
  • Electrolysis temperature: 30 ° C

Comparative Example 4:

Cleaned sheets of the quality of Example 4 were treated electrolytically with chromating baths, as indicated in Example 4, under the conditions of Example 4. In one case Si0 _{2 was} omitted, in the other the concentration was reduced to 3 g / l. The results obtained are shown in Table 4.

Example 5:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, then washed with water and dried. The samples obtained were compared with those of Comparative Example 5, with the result that, as shown in Table 5, the chromate layers from the catholytic treatment according to the invention had a higher efficiency with regard to the chromium content in the chromate layer formed and better corrosion resistance and paint adhesion than the chromate layers of Comparative Example 5.

• Cr⁶⁺: 12,0 g/I (Verwendung von Chromsäureanhydrid)
• N0₃: 1,5 g/I (Verwendung von HN0₃) und
• Si0₂: 10,0 g/I (20 Gew.-%ige Dispersion der Fa. Nissan Chemical Industries, Ltd.)

Composition of the chromating baths:

• Cr ⁶⁺ : 12.0 g / I (use of chromic anhydride)
• N0 ₃ : 1.5 g / I (use of HN0 ₃ ) and
• Si0 ₂ : 10.0 g / l (20% by weight dispersion from Nissan Chemical Industries, Ltd.)

The additions of Cr ³⁺ ions (reduction of Cr ⁶⁺ by methanol) were such that the weight ratios of Cr ³⁺ to Cr ^{6+ were} 1:50, 1:10 and 1: 3. The pH was adjusted to 5 with ammonium hydroxide.

Conditions of cathodic electrolysis:

Electrolysis time: 1 s

Current density: 50 A / dm ²

Electrolysis temperature: 30 ° C

Comparative Example 5:

Cleaned sheets according to Example 5 were treated with the same chromating baths under the same conditions of cathodic electrolysis as in Example 5, but the additions of Cr ^{3+ were} such that weight ratios of 1: 100 and 1: 2.5 resulted. The samples are shown in Table 5 as Comparative Examples 5.

Example 6:

Cleaned, electrolytically galvanized steel sheet was treated catholytically under the following conditions, then washed with water and dried. The samples obtained were compared with those of Comparative Example 6 with the result that, as shown in Table 6, the chromate layers according to the invention had better corrosion resistance and paint adhesion than those of Comparative Example 6.

• Cr⁶⁺: 10,4 g/I (Verwendung von Chromsäureanhydrid)
• Cr³⁺: 0,5 g/I (Verwendung von Chromcarbonat)
• SiO₂: 75,0 g/I (20 Gew.-%ige Dispersion der Fa. Nissan Chemical Industries, Ltd.)
• N0₃: 3,0 g/I (Verwendung von HN0₃)
  • mit Natriumcarbonat auf pH 5,0 eingestellt

Composition of the chromating bath:

• Cr ⁶⁺ : 10.4 g / I (use of chromic anhydride)
• Cr ³⁺ : 0.5 g / I (use of chrome carbonate)
• SiO ₂ : 75.0 g / l (20% by weight dispersion from Nissan Chemical Industries, Ltd.)
• N0 ₃ : 3.0 g / I (use of HN0 ₃ )
  • adjusted to pH 5.0 with sodium carbonate

Conditions of cathodic electrolysis:

Electrolysis time: 4 s

Current density: 3 or 6 or 9 A / dm ²

Electrolysis temperature: 50 ° C

Comparative Example 6:

Sheets of the quality of Example 6 were treated with the chromating bath of Example 6 with the current densities 0 and 1.5 A / dm ² . The results are shown in Table 6.

Conditions of cathodic electrolysis:

Electrolysis time: 4 s

Current density: 0 or 1.5 A / dm ²

Electrolysis temperature: 50 ° C

The data listed in Tables 1 to 6 were evaluated according to the following principles:

1. Adhesive properties of the chromate layer:

Cellophane adhesive strips with a width of 50 mm were stuck onto the coating, quickly torn off and the amounts of Cr and Si residues in the coating were shown in percent. The cheapest value is 100%.

2. Homogeneity of the coating:

The coating was checked for discoloration with the naked eye and in the 4 stages

⊙O △ x

rated. ⊙ is optimal.

3. Corrosion resistance:

Salt spray test:

The rust formation according to JIS-Z-2371 was assessed with the naked eye after a certain period of time specified in the tables and in 4 steps

⊙O △ x

rated. ⊙ is optimal.

4. Adhesive properties of the coating layer: (coating: commercially available white alkyd melamine lacquer, application 27 to 30 pm)

(4-1): Erichsen test

The lacquer coating was scratched with a cutter at a distance of 1 mm to the metallic surface, so that a grid with several scratch marks lying at an angle of 90 ° is created. Then the test sheet was slowly dented by pressing in a ball until a bulge of 7 mm in height was formed. The degree of paint removal was then determined by gluing and tearing off an adhesive tape. The extent of the paint layer removed was examined with the naked eye and in 4 steps

⊙O △ x

rated. 0 is optimal.

(4-2): DuPont shock test:

With a DuPont impact tester, impacts were exerted on the paint surface, the extent of the detached layer was viewed with the naked eye and in 4 steps

⊙O △ x

rated. ⊙ is optimal.

Claims (6)

1. Process for forming chromate coatings on surfaces of zinc or zinc alloys in an electrochemical manner by means of chromium⁶⁺ containing aqueous chromating bathes, characterized in that the chromate coatings are formed by means of a chromating bath, containing 5 to 70 g/I chromium⁶⁺ ions, 0.01 to 5.0 g/I chromium³⁺ ions, 5 to 100 g/I silicic acid and/or silicate and 0.05 to 10 g/I nitrate ions, in which the weight ratio of chromium³⁺ to chromium⁶⁺ is in the range of 1:50 to 1:3.

2. Process according to claim 1, characterized in that the chromate coatings are formed by means of a chromating bath, containing 10 to 50 g/I chromium⁶⁺ ions, 0.05 to 5.0 g/I chromium³⁺ ions, 10 to 50 g/I silicic acid and/or silicate and 0.1 to 3.0 g/I nitrate ions.

3. Process according to claim 1 or 2, characterized in that the chromate coatings are formed by means of a chromating bath, of which the pH- value is in the range of 1 to 6 and the temperature in the range of room temperature to 70° C.

4. Process according to claim 1, 2 or 3, characterized in that the weight of the chromate coating is adjusted by an appropriate choice of the current density and of the current quantity.

5. Process according to one or more of the claims 1 to 4, characterized in that the electrochemical treatment is performed at a cathodical current density of 3 to 80 A/dm².

6. Process according to one or more of the claims 1 to 5, characterized in that chromate coatings are formed, which contain chromium in an amount of 10 to 300 mg/m², preferably 20 to 150 mg/m² (calculated as Cr) and silicic acid and/or silicate in an amount of 3 to 30 mg/m², preferably 5 to 20 mg/m² (calculated as Si).

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