EP0224065A1 - 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 made of zinc or zinc alloys electrochemically by means of chromate-containing aqueous chromating baths.

It is known to treat surfaces made of zinc or zinc alloys with solutions containing chromium, chromium 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 external appearance also deteriorate.

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

It is also known to generate chromate layers on surfaces of zinc and zinc alloys electrochemically, for example with chromating baths containing chromium and sulfuric acid or chromium ⁶⁺ and heavy metal ions (JP-PS 47-44417 (1972; JP-PS 48-43019 (1973) However, chromate layers of this type 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 procedure. If zinc-coated steel sheet is treated continuously, zinc ions which have been released accumulate in the chromating bath the pH rises These zinc ions and by reduction during the electrochemical treatment Chrom3 formed. + ^-. ion trap, so that the treatment solution is not stable in nature Among other things, the appearance varies of the coating formed and takes to corrosion responsibility of the treated workpieces.

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 having 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 removed using a Chromatierbades generated, the ions 5 to 70 g / 1 _Ch rom ^6+, 0.01 to 5.0 g / 1 Chrom3 + ^- ions, 5 to 100 g / 1 of silica and / or silicate and 0.05 to 10 g / 1 contains nitrate ions, the weight ratio of chromium ³⁺ : chromium ^{6+ being} in the range from ^1:50 to 1: 3.

The chromium ^{6 + -} can ion-chromic anhydride, ammonium and alkali metal dichromate individually or are incorporated into the mixture in the bath. The concentration of the chromium ⁶⁺ ions is essential in that in the case of a concentration below 5 g / 1 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.

The chromium ions can be added to the chromating bath in the form of nitrate or carbonate compounds. It is also possible that chromium ³⁺ ions by the reduction of chromium ^{+ 6 -} ion with organic substances, such as alcohol, starch, or tannic acid to introduce. Due to the chromium ³⁺ content, an increased deposition of 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 chrome is ³⁺ : Chromium ^{6 + -} ratio below 1: 50, the described effect weakens. With a chromium chromium ^{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 about 1 to 100 _/ um. At a concentration below 5 g / 1, the corrosion resistance and paint adhesion decrease. No further improvement can be achieved with concentrations above 100 g / l. In addition, there is a risk that silicic acid and / or silicates separate out and an excessive amount of chromating bath is discharged 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 / 1, an additional improvement in the characteristics 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 obtained obtained when the chromate by means of a Chromatierungsbades is in accordance with a preferred embodiment of the invention produces, from 10 to 50 g / 1 chromic ^{6 + -} ions, from 0.05 to 5 g / 1 Chrom3 + ^- ions, 10 to 50 g / 1 of silica and / or silicate and 0.1 to 3 g / 1 nitrate ions.

The pH of the chromate bath is not critical. However, it is particularly expedient to generate the chromate layers using a chromating bath, the pH of which is in the range from 1 to 6. The pH of the chromate layer does not change at pH values below 1, 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 advantageously 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 Workpiece switched as 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 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 experiments. 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.

Die Elektrolysebedingungen, aufgrund derer diese Graphik erstellt wurde, sind in Tabelle 7 angegeben.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 one in Fig. 1 using the example of the coating reproduced 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 Si0 _{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 eliminated. 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, chromate layers with excellent corrosion resistance and very good adhesion for a subsequently applied organic coating can be achieved when using the method according to the invention.

Because of the above-mentioned relationships 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 / m2 (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 the nitrate ions in the chromating bath not only increases the corrosion resistance of the chromate layers formed, but is also responsible for increasing the stability of the chromating bath and thus for the uniformity of the appearance of the chromate layers produced during continuous treatment. The content of nitrate ions largely prevents accumulating zinc and chromium ions from precipitating in the chromating bath.

The surfaces of zinc or zinc alloys treated by the process according to the invention are usually rinsed with water and then dried. If desired, the chromate layers produced by the process can be post-treated with the usual aqueous chromate solutions and / or with corrosion-protecting solutions based on organic plastics before drying. The chromate layers produced provide excellent corrosion resistance and are an excellent base for 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 have been produced, for example, 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.

Composition of the chromating bath:

The pH here was 1.2.

• Elektrolysedauer: 3 bis 12 sec (Zum Erzielen eines bestimmten Chromgehaltes in der Chromatschicht wurde die Elektrolysedauer geregelt.) Stromdichte: 10 _A/dm ² Elektrolysetemperatur: 50°C

Conditions of cathodic electrolysis:

• Electrolysis time: 3 to 12 seconds (To achieve a certain chromium content in the chromate layer, the electrolysis time was regulated.) Current density: 10 _{A / dm} ² electrolysis temperature: 50 ° C

Comparative Example 1

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.

Composition of the chromating bath:

Adjust the pH to 5.0 using ammonium bicarbonate.

Fading example 2

Cleaned, electrolytically galvanized steel sheet of the same quality was treated electrolytically under the same conditions as in implementation example 2, wherein however, chromating baths were used in which Na ₂ .Si0 ₂ and HNO ₃ , and on the other hand Na ₂ 0.Si0 ₂ or N0 ₃ , were omitted compared to Example 2.

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.

Composition of the chromating baths:

The baths were modified with 0.06, 0.12 and 0.24 g / 1 N0 ₃ ions (introduced via NaN0 ₃ ) and adjusted to pH 5.0 using sodium hydroxide. Conditions of cathodic electrolysis:

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 / 1. The cathodic electrolysis was carried out under the same conditions as in Example 3. The results are shown in Table 3.

Example 4

mit Ammoniumhydroxid auf pH 3 eingestellt.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. Composition of the chromating baths:

adjusted to pH 3 with ammonium hydroxide.

Conditions of cathodic electrolysis:

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 / 1. 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.

Composition of the chromating baths:

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

Vergleichsbeispiel 5Conditions of cathodic electrolysis:

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 that of Comparative Example 6.

Composition of the chromating bath:

Conditions of cathodic electrolysis:

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:

• 1. Hafteigenschaften der Chromatschicht:
  • Auf den Überzug wurden Cellophanklebstreifen von 50 mm Breite geklebt, rasch abgerissen und die Rückstandsmengen an Cr und Si im Überzug in Prozent ausgewiesen. Der günstigste Wert ist 100 %.
• 2. Homogenität des Überzugs:
  • Der Überzug wurde mit bloßem Auge auf Verfärbungen geprüft und in den 4 Stufen
    • O 0 x
    bewertet. ist optimal.
• 3. Korrosionsbeständigkeit:
  • Salzsprühtest:
    • Es wurde die Rostbildung nach JIS-Z-2371 nach einem bestimmten, in den Tabellen angegebenen Zeitraum mit bloßem Auge beurteilt und in 4 Stufen
      • O Δ x
      bewertet. ist optimal.
• 4. Hafteigenschaften der Anstrichschicht: (Anstrich: handelsüblicher weißer Alkydmelamin-Lack, Auftrag 27 bis 30 _/um)
  • (4-1): Erichsen-Test
    Der Lacküberzug wurde mit einem Schneidegerät im Abstand von l mm bis zum metallischen Untergrund angeritzt, so daß ein Gitter mit mehreren im Winkel von 90° aufeinanderliegenden Ritzspuren entsteht. Dann wurde das Prüfblech durch Eindrücken einer Kugel langsam verbeult, bis sich eine Wölbung von 7 mm Höhe bildet. Die Ermittlung des Grades der Lackentfernung erfolgte anschließend durch Aufkleben und Abreißen eines Klebebandes. Das Ausmaß der entfernten
  Lackschicht wurde mit bloßem Auge untersucht und in 4 Stufen
  • O Δ x
  bewertet. 0 ist optimal.
  • (4-2): DuPont-Stoßprüfung: Mit einem DuPont-Stoßprüfer wurden Stöße auf die Lackfläche ausgeübt, das Ausmaß der abgelösten Schicht mit bloßem Auge betrachtet und in 4 Stufen
    • O Δ x
    bewertet. ist optimal.
    
    
    
    

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 of 50 mm in width were stuck to 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 0 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 Δ x
      rated. is optimal.
• 4. Adhesive properties of the paint layer: (paint: commercially available white alkyd melamine paint, application 27 to 30 _/ um)
  • (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 varnish removal was then determined by sticking and tearing off an adhesive tape. The extent of the removed
  Paint layer was examined with the naked eye and in 4 steps
  • O Δ x
  rated. 0 is optimal.
  • (4-2): DuPont impact test: With a DuPont impact tester, impacts were exerted on the paint surface, the extent of the detached layer was observed with the naked eye and in 4 steps
    • O Δ x
    rated. is optimal.
    
    
    
    

Claims (6)

1. A process for producing chromate layers on surfaces made of zinc or zinc alloys electrochemically by means of aqueous chromating baths containing chromium ⁶⁺ , characterized in that the chromate layers are produced with the aid of a chromating bath which contains 5 to 70 g / 1 chromium ⁶⁺ ions, 0.01 to 5.0 g / 1 Chrom3 + ^- ions, 5 to 100 g / 1 of silica and / or silicate and 0.05 to 10 g / 1 nitrate ions wherein the weight ratio of chromium chromium in the range of 1: 50 to 1: 3 lies. 2. The method according to claim 1, characterized in that the chromate layers are produced with the aid of a chromating bath which contains 10 to 50 g / 1 chromium ^{6 +} ions, 0.05 to 5 g / 1 chromium ³⁺ ions, 10 to 50 Contains g / 1 silica and / or silicate and 0.1 to 3 g / 1 nitrate ions. 3. The method according to claim 1 or 2, characterized in that the chromate layers are produced with the aid of a chromating bath, the pH of which is in the range from 1 to 6 and the temperature is in the range from room temperature to 70 ° C. 4. The method according to claim 1, 2 or 3, characterized in that the weight of the chromate layer is adjusted by a suitable choice of the current density and the amount of current. 5. The method according to one or more of claims 1 to 4, characterized in that one carries out the electrochemical treatment at a cathodic current density of 3 to 80 A / dm ² . 6. The method according to one or more of claims 1 to 5, characterized in that chromate layers are produced whose chromium content is 10 to 300 mg / m ² , preferably 20 to 150 mg / m ² (calculated as Cr) and their content of silica and / or silicate is 3 to 30 mg / m ² , preferably 5 to 20 mg / m ² (calculated as Si).

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