EP0244022A2 - After-treatment process for phosphated metal surfaces - Google Patents

Abstract

In a method for the aftertreatment of phosphated metal surfaces, the treatment with a solution containing chromium VI and chromium III ions is carried out by cathodic electrolysis, the concentration of chromium VI ions being in the range from 0.05 to 10 g / l and the ratio of chromium III / chromium VI is set to less than 1.0. Then it is painted. According to a preferred embodiment of the invention, the treatment is carried out with a solution containing chromium VI and chromium III ions, which additionally contains colloidal silica and / or phosphate ions and / or water-soluble organic polymer and has a pH in the range from 1 , 5 to 5.0 is set. In cathodic electrolysis, a current density of 0.01 to 0.5 A / dm 2 and a treatment time of 2 to 120 sec are expediently set, so that a layer weight of 4 to 100 mg / m 2 (calculated as Cr) results. The subsequent painting is designed particularly favorably as electro-dip painting.

Description

The invention relates to a method for the aftertreatment of phosphate coatings applied to metal surfaces by means of a solution containing chromium VI and chromium III ions and subsequent painting.

It is generally known to post-treat phosphated metal surfaces with rinse solutions based on chromium in order to improve the corrosion resistance after the subsequent painting and the paint adhesion. The phosphated workpieces are brought into contact with the aftertreatment solution either by spraying or immersion. Despite the great advantages that can be achieved with such aftertreatment solutions, it is a disadvantage that the wastewater obtained with the required water rinsing requires thorough processing. Remaining residues of such aftertreatment solutions can also lead to the formation of bubbles in the subsequently applied lacquer if parts of the workpiece which are difficult to access, such as occur, for example, in automobile bodies, are not thoroughly freed from residues of the aftertreatment solution by the water rinsing. Particularly intensive water rinsing can, however, result in the improvement in corrosion resistance achieved with the aftertreatment being reduced by half. This means that the expectations placed on aftertreatment with chromium-based solutions must not be overestimated and that the quality of the phosphate coating previously produced is of additional influence. With car bodies in particular, it is extremely important
that all parts, especially the outer area, such as body floors, fenders, doors and the like, have a high corrosion resistance and excellent paint adhesion.

Because of the aforementioned situation, there has been no shortage of attempts to replace chrome-based rinse solutions with various types of chromium-free solutions in order to simplify the procedure and to achieve results which are always predictable and easy. However, no post-treatment solution of this kind has attained comparable importance to that with solutions containing chromium.

The object of the invention is to provide a process for the aftertreatment of phosphate coatings applied to metal surfaces, which leads to uniformly good results, is as little dependent on the previously produced phosphate coating, improves the corrosion resistance and the paint adhesion and requires little procedural effort.

The problem is solved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the treatment with the solution containing chromium VI and chromium III ions is carried out by cathodic electrolysis, the concentration of chromium VI ions to a value in the range from 0.05 to 10 g / l and the ratio of chromium III / chromium VI to less than 1.0.

In the practical implementation, it is common that First clean the workpieces thoroughly, for example with an alkaline cleaning solution, preferably rinse them with water in several stages, then activate the surface of the workpiece for the subsequent phosphating, e.g. with the help of a colloidal titanium-containing dispersion, then phosphate it and treat it with the method according to the invention.

In the aforementioned process, the nature of the phosphate coating produced is of minor importance. The process according to the invention can be followed by any phosphating process which permits the formation of a dense phosphate coating. It is advisable to subsequently carry out a multi-stage water rinse. Even after the electrolytic treatment with the solution containing chromium VI and chromium III ions, rinsing should be carried out with water and, if possible, also with completely deionized water. If necessary, it is then dried. The final stage of the process is painting.

The method according to the invention, which is designed in particular for the aftertreatment of phosphated automobile bodies, is suitable both for phosphated steel surfaces, e.g. Made of cold-rolled steel, and also suitable for phosphated steel surfaces with zinc or zinc alloy coatings. The workpieces can also partially have aluminum surfaces or surfaces of other phosphatable metals.

The one used in the method according to the invention Solution containing chromium VI / chromium III ions is advantageously prepared by dissolving anhydrous chromic acid and / or its alkali, alkaline earth or ammonium salts. If the concentration falls below 0.05 g / l, the current yield is greatly reduced and the chromium / chromate separation is insufficient. In terms of corrosion resistance and paint adhesion, this leads to results that are far below what can be achieved. At concentrations above 10 g / l, a complex water rinse is required, although blistering is not guaranteed after the subsequent painting. The paint adhesion can also decrease. The preferred range for the concentration of chromium VI ions is in the range of 0.2 to 3.0 g / l.

The presence of chromium III ions stabilizes the properties of the painted workpiece even at very low concentrations. Concentrations corresponding to a ratio of equal to or greater than 1 lead to unstable aftertreatment solutions and the formation of sludge. A ratio of chromium III to chromium VI in the range from 0.1 to 0.5 is particularly favorable.

According to a preferred embodiment of the invention, the treatment is carried out with a solution containing chromium VI and chromium III ions, which additionally contains colloidal silica in a concentration of 0.01 to 5.0 g / l. As a result, the property of the subsequently applied lacquer film is considerably improved. At concentrations below 0.05 g / l, the improvement is practically no longer present; at concentrations above 5 g / l, no further improvement can be achieved. Concentrations in the range from 0.1 to 2.0 g / l are particularly favorable.

A further advantageous embodiment of the invention provides for the treatment to be carried out with a solution which additionally contains phosphate ions in a concentration of 0.01 to 2 g / l. This is associated with an increase in the electrical conductivity of the aftertreatment solution, and the adhesion of the chromate layer is also improved. This in turn improves the "wet adhesion" after painting and the corrosion resistance of the paint film. In this respect, lower phosphate concentrations have practically no effect; larger concentrations are not associated with any additional improvement. Instead, there is a risk of blistering due to insufficient water rinsing. Concentrations in the range from 0.2 to 1.0 g / l are particularly advantageous. The levels of phosphate and silica can have a synergistic effect.

Optionally, fluoride and boron compounds, sulfuric acid or sulfates, if compatible with the solution, can be added to the aftertreatment solutions in addition to the aforementioned inorganic compounds.

An advantageous embodiment of the invention consists in carrying out the treatment with a solution which additionally contains water-soluble organic polymer in a concentration of 0.01 to 5.0 g / l (calculated as dry substance). This configuration results in a particularly strong adhesion of the chromate layer in the pore area of the phosphate layer. The reason is in that the polymer is practically not removed even when rinsed with water, and the adhesion is further increased when drying. At a concentration below 0.0l g / l this effect practically does not yet occur, at concentrations above 5 g / l the appearance of the paint film can be impaired.

The content of water-soluble polymer with the simultaneous presence of silica and / or phosphate has a noticeable additive effect. In this case, it is particularly advantageous to set the polymer concentration to 0.2 to 2.0 g / l.

According to a further advantageous embodiment of the invention, an aftertreatment solution is used which, as the water-soluble organic polymer, contains one based on polyacrylate or polyurethane. However, other water-soluble, in particular cationic, polymers are also suitable, provided that they are stable in the aftertreatment solution containing chromium VI ions. In cathodic electrolysis, they are deposited on the phosphate layer and seal the phosphate layer. A certain caution is required when measuring the concentration of the cationic polymers and the choice of electrolysis conditions, since the deposited polymer has an electrically insulating effect which, if it is designed as an electro-dip coating, can impair the subsequent coating.

It is also particularly expedient to carry out the aftertreatment with a solution whose pH is at one Value is set in the range of 1.5 to 5.0. A pH value below 1.5 can lead to an excessive dissolution of the phosphate layer. At pH values above 5, the aftertreatment solution becomes unstable, ie sludge is formed. Particularly favorable pH values are in the range from 3.5 to 4.5. For pH adjustment, chromic acid, possibly also phosphoric acid, can be used without thereby exceeding the limits of 10 g / l or 2.0 g / l, or alkali metal hydroxide or ammonium hydroxide can be used.

According to a further advantageous embodiment of the invention, the aftertreatment is carried out with a solution containing chromium VI and chromium III ions, the temperature of which is set to a value in the range from ambient temperature to 50 ° C. A temperature of 20 to 40 ° C is particularly favorable.

Further advantageous embodiments of the invention consist in carrying out the cathodic electrolysis at a current density of 0.01 to 0.5 A / dm 2 and in dimensioning the duration of the electrolysis to 2 to 120 seconds. A current density lower than 0.0l A / dm² is practically ineffective and current densities higher than 0.5 A / dm² can lead to gas formation. Best results are achieved in the range of 0.03 to 0.3 A / dm².

The treatment duration is set taking into account the selected current density.

An alternative to adjusting the treatment time and current density is cathodic electrolysis with a current of 0.2 to 30 Coul / dm². The problems mentioned in connection with the current density must be taken into account.

According to a further advantageous embodiment of the invention, the treatment duration and current density or amount of current are selected such that a layer weight of 4 to 100 mg / m² (calculated as Cr) results. A layer weight below 4 mg / m² is practically not associated with an improvement in the corrosion resistance after painting and paint adhesion. Layer weights above 100 mg / m² bring no additional increase in the improvement that has already been achieved. A layer weight in the range of 4 to 30 mg / m² is particularly advantageous.

After the electrolytic treatment, water is rinsed, if possible rinsed with deionized water, and dried if necessary. This is followed by the painting, which according to a further advantageous development of the invention is designed as an electro-dip painting.

The method according to the invention is intended in particular for the aftertreatment of phosphated automobile bodies. By appropriately guiding the bodies in relation to the anode arrangement, it can be achieved that the parts which are exposed to high corrosion stress receive a particularly intensive treatment in the chromium VI / chromium III solution. It should be taken into account that the effect that can be achieved is greatest in the body parts, that directly opposite and are close to the anode (s). In this respect, extensive influence can be exerted by the targeted arrangement of anodes.

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

EXAMPLES

• 1. Alkalische Reinigung bei 50 ± 2°C l80 sec im Tauchen. Gesamtalkalinität des Reinigers l5 ± l Punkte (bestimmt durch Titration einer l0 ml-Badprobe mit 0,l n Schwefelsäure gegen Bromphenolblau).
• 2. Spülung mit Leitungswasser bei Raumtemperatur 20 sec im Spritzen.
• 3. Aktivierung mit einer kolloidales Titan enthaltenden Aktivierungslösung bei Raumtemperatur 20 sec im Spritzen.
• 4. Phosphatierung mit einer Zinkphosphat als Hauptkomponente enthaltenden Phosphatierungslösung:
  
      Freie Säure 0,9 ± 0,l Punkte
  
      Gesamt-Säure l8 ± l Punkte (bestimmt durch Titration einer l0 ml-Badprobe mit 0,l n Natronlauge gegen Bromphenolblau (Freie Säure) bzw. Phenolphthalein (Gesamt-Säure))
  
      Beschleunigerkonzentration l,2 ± 0,2 Punkte (gemessen nach der Saccharometermethode)
  
      Phosphatierbedingungen 53 ± 2°C l20 sec im Tauchen.
• 5. Wasserspülung wie bei 2.
• 6. Nachbehandlung unter den in der Tabelle l genannten Bedingungen.
• 7. Wasserspülung wie bei 2.
• 8. Spülung mit vollentsalztem Wasser bei Raumtemperatur l5 sec im Spritzen
  (spezifischer elektrischer Widerstand des Wassers größer als 5 . l0⁵ Ohm . cm).
• 9. Trocknen bei ll0°C während l20 sec.
• l0. Kathodische Elektrotauchlackierung bei 250 V für die Dauer von l80 sec.
  (Elecron 9200 der Fa. Kansai Paint, Ltd.)
• 11. Wasserspülung wie bei 2.
• 12. Spülung mit vollentsalztem Wasser bei Raumtemperatur 5 sec im Spritzen.
• 13. Einbrennen bei l75°C für die Dauer von 30 min.
• 14. Zwischenlackierung mit einem Lack auf Melaminalkyd-Basis (Amilac N-2 der Fa. Kansai Paint, Ltd.) durch Aufsprühen. Lackdicke 3o µm. Nach l0 bis 20 min Warten Einbrennen bei l40°C während 30 min.
• 15. Decklackierung mit einem Lack auf Melaminalkyd-Basis (Amilackwhite M 3 der Fa. Kansai Paint, Ltd.) durch Aufsprühen. Lackdicke 40 µm. Nach l0 bis 20 min Warten Einbrennen bei l40°C während 30 min.

There were four different sheet qualities, each with the dimensions 70 × 150 × 0.8 mm, namely

cold rolled steel of quality SPCC-SD JIS-G-3l4l (CRS),

electrolytically galvanized steel, overlay 20 g / m² (ground floor),

electrolytically Zn / Ni-plated steel, overlay 20 g / m², Ni content l2% by weight (Electro Zn / Ni) and

Quality JIS-7075-T 6 (AL) aluminum

treated according to the following procedure:

• 1. Alkaline cleaning at 50 ± 2 ° C for 180 seconds while diving. Total alkalinity of the cleaner 15 ± 1 points (determined by titration of a 10 ml bath sample with 0.1 in sulfuric acid against bromophenol blue).
• 2. Rinse with tap water at room temperature for 20 seconds in a spray.
• 3. Activation with an activation solution containing colloidal titanium at room temperature for 20 seconds by spraying.
• 4. Phosphating with a phosphating solution containing zinc phosphate as the main component:
  
  Free acid 0.9 ± 0.1 points
  
  Total acid 18 ± 1 points (determined by titration of a 10 ml bath sample with 0.1 l sodium hydroxide solution against bromophenol blue (free acid) or phenolphthalein (total acid))
  
  Accelerator concentration 1.2 ± 0.2 points (measured according to the saccharometer method)
  
  Phosphating conditions 53 ± 2 ° C 120 sec in diving.
• 5.Washing up as in 2.
• 6. Post-treatment under the conditions listed in Table 1.
• 7.Washing as in 2.
• 8. Rinse with deionized water at room temperature for 15 seconds in a spray
  (specific electrical resistance of water greater than 5.10 l ohm. cm).
• 9. Drying at 120 ° C for 120 seconds.
• l0. Cathodic electrocoating at 250 V for a period of 180 sec.
  (Elecron 9200 from Kansai Paint, Ltd.)
• 11.Washing as in 2.
• 12. Rinse with deionized water at room temperature for 5 seconds in a spray.
• 13. Bake at l75 ° C for 30 minutes.
• 14. Intermediate painting with a melamine alkyd-based paint (Amilac N-2 from Kansai Paint, Ltd.) by spraying. Paint thickness 3o µm. After l0 to 20 min wait bake at l40 ° C for 30 min.
• 15. Top coat with a melamine-based paint (Amilackwhite M 3 from Kansai Paint, Ltd.) by spraying. Lacquer thickness 40 µm. After l0 to 20 min wait bake at l40 ° C for 30 min.

The test panels obtained after the aforementioned complete or partial treatment course were subjected to the following tests:

Salt spray test

Only sample sheets treated up to the stage of electro-dip painting were provided with a cross cut down to the metallic surface and exposed to the spray of a 5% saline solution for 1000 h (JIS-Z-237l). The evaluation was done by Determination of the bubble formation along the cutting line in mm.

In a further test, the bubble formation was determined in mm starting from the edges of the sample sheet.

Wet grip test

For this purpose, treated test panels were immersed in deionized water at 40 ° C. for a period of 10 days until the topcoat. Thereafter, the sheets were provided with a cross-cut extending to the metallic background, so that 100 fields each with an edge length of 2 mm were created. These fields were covered with an adhesive tape, which was then peeled off again. To assess the adhesion, the squares remaining on the metal surface were counted.

Impact test

Sample sheets treated until the topcoat were first "aged" for 24 h at room temperature, then immersed in demineralized water of 40 ± 1 ° C. for a period of 120 h without mutual contact. They were air-dried for 1 hour at room temperature and then fixed at an angle of 45 ° so that the painted surface was facing upwards. The center of the sheet was then subjected to the action of 100 screw nuts (approx. 6 mm; total weight l98 ± 0.5 g), which struck from a height of 4.5 m, guided by a vertical pipe approx. 50 mm in diameter.

The test result was assessed visually and according to

A = little paint removal
B = medium paint removal
C = strong paint removal

graded.

Serve / salt spray test

The test panels subjected to the aforementioned impact test were then subjected to the salt spray test described in more detail above for a period of 72 h. Then they were exposed to the free atmosphere. This alternating stress was carried out four times in total. Finally, the samples were again subjected to the salt spray test for 72 hours.

The test panels were then treated with a metal sponge to remove corrosion products and no longer adhering paint. The evaluation was done visually and took into account the paint removal according to the rating

A = very little
B = little
C = a lot
D = very much.

The test results are summarized in Table 2. It can be seen that the sheets treated by the process according to the invention had excellent results in every respect. In the comparison tests it can be seen that the results of the salt spray tests (Comparative Example 1) or the appearance of the paint film and the wet adhesion (Comparative Example 2) are unsatisfactory. The reason for this lies in particular in the fact that the conditions essential to the invention with regard to the chromium III / chromium VI ratio or the chromium VI concentration were not observed. The aftertreatment according to Comparative Examples 3 and 4, in which the chromate solution was not used electrolytically, showed unsatisfactory to unsatisfactory results, in particular with regard to the salt spray tests, the impact test and the combined impact / salt spray test. Comparative example 5 shows the results for a non-phosphated sample sheet.

Claims (13)

1. A process for the aftertreatment of phosphate coatings applied to metal surfaces by a solution containing chromium VI and chromium III ions and subsequent painting, characterized in that the treatment with the solution containing chromium VI and chromium III ions is carried out by carries out cathodic electrolysis, the concentration of chromium VI ions to a value in the range of 0.05 to 10 g / l and the ratio of chromium III / chromium VI to less than 1.0. 2. The method according to claim l, characterized in that one carries out the treatment with a chromium VI and chromium III ions containing solution containing 0.2 to 3.0 g / l chromium VI ions and a Chromium III / Chromium VI ratio ranges from 0.1 to 0.5. 3. The method according to claim l or 2, characterized in that one carries out the treatment with a solution containing chromium VI and chromium III ions, the additional colloidal silica in a concentration of 0.0l to 5.0 g / l , preferably from 0.1 to 2.0 g / l. 4. The method according to claim 1, 2 or 3, characterized in that one carries out the treatment with a chromium VI and chromium III ions containing solution, which additionally phosphate ions in one Concentration of 0.0l to 2 g / l, preferably 0.2 to 1.0 g / l, contains. 5. The method according to claim 1, 2, 3 or 4, characterized in that one carries out the treatment with a solution containing chromium VI and chromium III ions, which additionally contains water-soluble organic polymer in a concentration of 0.0l to 5 , 0 g / l, preferably from 0.2 to 2.0 g / l (calculated as dry matter). 6. The method according to claim 5, characterized in that one carries out the treatment with a chromium-VI and chromium-III-ion-containing solution, which contains such as a water-soluble organic polymer based on polyacrylate or polyurethane. 7. The method according to one or more of claims 1 to 6, characterized in that one carries out the treatment with a solution containing chromium VI and chromium III ions, the pH of which to a value in the range from 1.5 to 5.0, preferably from 3.5 to 4.5, is set. 8. The method according to one or more of claims l to 7, characterized in that one carries out the treatment with a solution containing chromium VI and chromium III ions, the temperature of which to a value in the range from ambient temperature to 50 ° C, preferably from 20 to 40 ° C. 9. The method according to one or more of claims l to 8, characterized in that one carries out the cathodic electrolysis at a current density of 0.0l to 0.5 A / dm², preferably from 0.03 to 0.3 A / dm² . l0. Method according to one or more of claims 1 to 9, characterized in that the cathodic electrolysis is carried out for a period of 2 to 120 seconds. 11. The method according to one or more of claims l to l0, characterized in that one carries out the cathodic electrolysis with an amount of current of 0.2 to 30 Coul / dm². 12. The method according to one or more of claims l to ll, characterized in that the cathodic electrolysis is carried out in such a way that a layer weight of 4 to 100 mg / m², preferably 4 to 30 mg / m² (calculated as Cr) becomes. 13. The method according to one or more of claims l to l2, characterized in that the subsequent coating is designed as a cathodic electrocoating.