EP0361375A1 - Phosphate-coating process - Google Patents

Abstract

In a process for applying phosphate coatings on metal surfaces with the aid of phosphating solutions based on zinc phosphate, which additionally contain nickel, a phosphating solution is used which contains 0.3 to 5 g / l, preferably 1 to 3 g / l, formic acid (calc. as HCOO) contains. The phosphating solutions preferably contain 0.5 to 2 g / l, in particular 0.7 to 1.5 g / l zinc, 10 to 25 g / l phosphate (calculated as PO4) and additionally 0.5 to 3 g / l nickel , 0.3 to 1.5 g / l manganese and 0.5 to 2.0 g / l simple and / or complex fluoride (calc. As F). The method is used in particular to produce phosphate coatings on steel, galvanized steel and / or alloy-galvanized steel and to prepare metal surfaces for electrocoating, in particular cathodic electrocoating.

Description

The invention relates to a method for applying phosphate coatings on metal surfaces with the aid of phosphating solutions based on zinc phosphate, which additionally contain nickel and formic acid, and its use in dipping to produce phosphate coatings on steel, galvanized steel and / or galvanized steel, in particular as preparation for Metal surfaces for electro dip painting.

It is known to produce phosphate coatings on surfaces of steel, galvanized steel or alloy-galvanized steel with the aid of phosphating solutions which belong to the zinc / nickel-phosphate system. If the surfaces to be treated consist of so-called composite metals such as steel and galvanized steel and / or alloy-galvanized steel, the use of phosphating solutions from the zinc / nickel / manganese-phosphate system has become established. The phosphate coatings produced by these processes increase the corrosion resistance of the treated metal surfaces and the adhesion of a subsequently applied paint, especially if the paint is applied by electrocoating.

The effect of the nickel in the above-mentioned phosphating systems is, in particular, to increase the corrosion resistance after subsequent painting, that of the manganese, and to increase the alkali resistance required for the improvement of the cationic electrocoating. The presence of manganese also improves the so-called wet grip.

In addition to zinc and nickel or zinc, nickel and manganese, the abovementioned phosphating systems contain accelerators, which can in particular be nitrate ions, chlorate ions, nitrite ions and / or nitrobenzenesulfonic acid. The phosphating solutions are generally used at temperatures in the range from 30 to 60 ° C., immersion processes, spraying processes or combined immersion-spraying processes are customary.

On the basis of the knowledge that an increased nickel content in the phosphate coating increases the corrosion resistance, in particular of the phosphated and painted metal surfaces, attempts have been made to increase the nickel content in the layer by increasing the nickel content in the phosphating solution. However, high concentrations of nickel ions in the phosphating solution are associated with high costs. Furthermore, it has been shown that if manganese ions are present at the same time, the nickel content in the phosphate coating cannot be increased to the expected extent. A certain proportion of manganese ions in the phosphating solution is, however, reluctant to do without, since otherwise the alkali resistance and the wet adhesion of the phosphate coating produced decrease.

The object of the invention is to provide a phosphating process for the treatment of metal surfaces which leads to phosphate coatings of high quality, in particular high corrosion resistance, and which is responsible for excellent wet adhesion of the subsequently applied lacquer film, and which can nevertheless be carried out in a simple manner.

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 metal surfaces are brought into contact with a phosphating solution which contains 0.3 to 5 g / l of formic acid (calculated as HCOO).

Surprisingly, it has been found that the above-mentioned formic acid content of the phosphating solution enables phosphate coatings with relatively high nickel contents to be obtained even at comparatively low nickel concentrations.

The formic acid or formate content can be brought about by adding formic acid as such or salts of formic acid, such as alkali metal formate, alkaline earth metal formate, ammonium formate or heavy metal formates to the phosphating solution. Particularly suitable compounds are sodium formate, potassium formate, calcium formate, barium formate, ammonium formate, nickel formate, cobalt formate, iron (III) formate and manganese formate. The concentrations in the range from 0.3 to 5 g / l are important insofar as, when added in accordance with a concentration <0.3 g / l, the effect with regard to the deposition of nickel ions on the metal surface to be treated is inadequate, whereas with additives An increase in the effect of more than 5 g / l is practically no longer achievable. It is particularly advantageous to measure the addition of formic acid or formate in such a way that a concentration of 1.0 to 3.0 g / l results.

A preferred embodiment of the invention provides for the metal surfaces to be brought into contact with a phosphating solution which contains 0.5 to 2 g / l of zinc. At zinc concentrations of less than 0.5 g / l, a uniform phosphate coating is practically no longer achievable, whereas at concentrations of above 2 g / l, the proportion of hopeit (Zn₃ (PO₄) ₂. 4 H₂O) in the phosphate coating rises and thus the latter Suitability as a primer for subsequent electro-dip painting decreases. Particularly favorable results are achieved if the zinc content in the phosphating solution is 0.7 to 1.5 g / l.

According to a further expedient embodiment of the invention, the metal surfaces should be brought into contact with phosphating solution which contain 0.5 to 3.0 g / l of nickel. At a concentration of less than 0.5 g / l, there is a risk that, despite the presence of the reducing formic acid or the reducing formate, the amount of nickel deposited is insufficient, so that the corrosion resistance of the phosphate coating and its adhesion imparted to a subsequently applied lacquer film is insufficient not achieve the optimum. If a concentration of 3 g / l is exceeded, an additional increase in the quality of the phosphate coating produced can no longer be achieved. A concentration of 2 g / l nickel is usually sufficient.

According to a further advantageous embodiment of the invention, the addition of manganese ions, which is responsible for increasing the alkali resistance of the phosphate coating and the wet adhesion of the subsequently applied paint, in particular in the case of partially galvanized or alloy-galvanized steel, should be in the range from 0.3 to 1.5 g / l lie. If the concentration is higher than 1.5 g / l, the properties of the phosphate coating can deteriorate, particularly with regard to the corrosion resistance. As a rule, it is sufficient if the manganese content of the phosphating solution is 1 g / l. At concentrations of the manganese ions below 0.3 g / l, the desired effect can no longer be achieved to the required extent.

Further advantageous embodiments of the invention consist in bringing the metal surfaces into contact with a phosphating solution containing 10 to 25 g / l phosphate (calc. As PO₄) or 0.5 to 2.0 g / l simple and / or complex fluoride (calculated as F) contains. The fluoride ions can be used as hydrofluoric acid or as complex fluorides, for example in the form of fluorosilicate or fluoroborate. The addition of fluorides serves in particular to rapidly form the phosphate coating even at low temperatures, to refine the phosphate crystals and to increase the phosphophyllite content (Zn₂Fe (PO₄) ₂. 4 H₂O) in the phosphate coating when treating steel surfaces. If the fluoride content is less than 0.5 g / l, the effect achieved may be too low, while at concentrations above 2.0 g / l an additional improvement in the effect can no longer be achieved.

The phosphating solution used in the process according to the invention generally contains oxidizing agents which accelerate the formation of the coating. According to a further advantageous development, nitrate, chlorate, nitrite and / or nitrobenzenesulfonate ions are used as accelerators individually or in any combination. Concentrations should be 2 to 15 g / l for nitrate ions, 0.1 to 1 g / l for chlorine ions, 0.01 to 0.2 g / l for nitrite ions and 0.3 to 2.0 g for nitrobenzenesulfonate ions / l. At concentrations of the accelerators below the specified lower limits, the acceleration effect achieved is usually not sufficient, while at concentrations above the upper limit an additional accelerating effect can no longer be achieved or else adverse effects can occur as a result of the entry into the phosphate layer.

The process according to the invention is integrated into the usual process of producing phosphate coatings by first subjecting the metal surfaces to be treated to thorough cleaning, then to activation, for example with colloidal titanium phosphate, and then to the phosphating treatment will. The phosphating treatment is expediently carried out at a temperature in the range from 20 to 55 ° C. for from 30 to 180 seconds.

According to a preferred embodiment of the invention, the method according to the invention is used in diving.

The method can be used with particular advantage for the production of phosphate coatings on steel, galvanized steel and / or alloy-galvanized steel. The galvanizing can be done electrolytically as well as by hot galvanizing. Zinc-nickel and zinc-iron alloys are particularly suitable zinc alloys. The metal surfaces to be treated can be in the form of sheet metal or strip, but also as already deformed workpieces, for example as automobile bodies. The method according to the invention is particularly suitable for the preparation of metal surfaces for the subsequent painting, the electrocoating and in this case the cathodic electrocoating playing a special role.

The advantage of the method according to the invention is that nickel in the phosphating solution is deposited in considerable amounts on the surface of the workpiece to be treated and this nickel acts as a crystallization nucleus for the phosphate formation. The result is that very fine-grained phosphate coatings are formed, which also contain a considerable amount of nickel in the phosphate layer itself. Furthermore, the rate of oxidation of the iron (II) ions leached out during the treatment of iron surfaces is reduced to iron (III) ions, with the result that a considerable proportion of phosphophyllite is incorporated into the phosphate coating.

• (a) bei Behandlung von Stahloberflächen Zn₂Fe(PO₄)₂ . 4 H₂O als Hauptbestandteil und als Nebenbestandteile Zn₂Ni(PO₄)₂ . 4 H₂O, ggf. außerdem Zn₂Mn(PO₄)₂ . 4 H₂O und geringe Mengen Zn₃(PO₄)₂ . 4 H₂O sowie Mikromengen an Ni,
• (b) bei Behandlung von Zinkoberflächen Zn₃(PO₄)₂ . 4 H₂O als Hauptbestandteil und als Nebenbestandteile Zn₂Ni(PO₄)₂ . 4 H₂O, ggf. außerdem Zn₂Mn(PO₄)₂ . 4 H₂O, wenn in der Behandlungslösung Fe²⁺ vorhanden sind: Zn₂Fe(PO₄)₂ . 4 H₂O, sowie Ni

auf.The phosphate coatings produced show

• (a) in the treatment of steel surfaces Zn₂Fe (PO₄) ₂. 4 H₂O as the main component and as secondary components Zn₂Ni (PO₄) ₂. 4 H₂O, possibly also Zn₂Mn (PO₄) ₂. 4 H₂O and small amounts of Zn₃ (PO₄) ₂. 4 H₂O and microns of Ni,
• (b) in the treatment of zinc surfaces Zn₃ (PO₄) ₂. 4 H₂O as the main component and as secondary components Zn₂Ni (PO₄) ₂. 4 H₂O, possibly also Zn₂Mn (PO₄) ₂. 4 H₂O if Fe²⁺ is present in the treatment solution: Zn₂Fe (PO₄) ₂. 4 H₂O, and Ni

on.

The invention is explained in more detail and, for example, using the examples below.

Examples

Three different metal surfaces were treated. This was it
- for steel sheet samples according to SPCC according to JIS-G-3141 (hereinafter referred to as SPC),
- around electrolytically galvanized sheet steel (hereinafter referred to as EC),
- hot-dip galvanized sheet steel (hereinafter referred to as GA).

• a) Entfetten mit einem stark alkalischen Reiniger bei 40 ± 2°C im Tauchen, während 180 sec.
• b) Wasserspülen durch 20 sec langes Spritzen mit Leitungswasser von Raumtemperatur
• c) Konditionieren der Oberfläche durch Behandeln mit einem titanhaltigen Aktivierungsmittel (Preparen ZN) im Tauchen während 30 sec bei Raumtemperatur und einer Konzentration der Aktivierungsmittels von 1 g/l
• d) Phosphatieren durch Tauchen für die Dauer von 120 sec.; die jeweils zur Anwendung gebrachten Phosphatierungslösungen sind in der nachfolgenden Tabelle näher definiert.
• e) Wasserspülen 20 sec, Spritzen mit Leitungswasser von Raumtemperatur
• f) Spülen mit vollentsalztem Wasser im Spritzen für die Dauer von 20 sec
• g) Trocknen bei 110°C für die Dauer von 180 sec.

The respective metal surfaces were treated according to the following procedure

• a) degreasing with a strong alkaline cleaner at 40 ± 2 ° C in immersion, for 180 sec.
• b) Rinse water by spraying with tap water at room temperature for 20 seconds
• c) Conditioning the surface by treatment with a titanium-containing activating agent (Preparen ZN) in immersion for 30 seconds at room temperature and a concentration of the activating agent of 1 g / l
• d) phosphating by dipping for 120 seconds; the phosphating solutions used in each case are defined in more detail in the table below.
• e) water rinsing 20 sec, spraying with tap water at room temperature
• f) Rinsing with deionized water in a syringe for 20 seconds
• g) drying at 110 ° C for 180 sec.

The free acid (F.A.) specified in the table was determined by titration of a 10 ml bath solution with 0.1N NaOH against bromophenol blue as an indicator. The required number of ml 0.1 n-NaOH corresponds to the free acid score.

The total acidity (T.A.) was determined by titrating a 10 ml bath sample with 0.1 n-NaOH against phenolphthalein as an indicator until the color changed from colorless to pink. Again, the consumption of ml 0.1 n-NaOH is equal to the number of points.

Following the phosphating, the individual samples were provided with a cathodic electrocoat material (Elecron 9400, manufacturer Kansai Paint Co. Ltd.) at a bath temperature of 28 ° C. and with the application of a voltage of 250 V for a period of 180 seconds. The thickness of the lacquer layer was 20 µm.

Following the electrodeposition coating, rinsing was carried out first with tap water for 20 seconds and then with deionized water for 5 seconds each at room temperature. Then the paint layer was baked at 175 ° C. for 30 minutes.

After the application of the base coat, a melamine alkyd paint (Amilacque N-2 from Kansai Paint Co. Ltd.) was applied as a filler by spraying. After drying for 10 to 20 minutes, the baking was carried out at 140 ° C. for 30 minutes. The thickness of the lacquer layer produced was 30 µm.

Finally, the top coat was applied by spraying using a melamine alkyd paint (Amilacque WHITE M-3 from Kansai Paint Co. Ltd.). Here too, after drying for 10 to 20 minutes, the stoving was carried out for 30 minutes at 140 ° C. The dry layer thickness was 40 µm.

The coating treatment resulted in a total layer thickness of 90 µm.

• (a) Aussehen des Überzuges:
  o - feiner und gleichmäßiger Phosphatüberzug
  x - schlechte Überzugsausbildung, insbesondere inhomogener Überzug
• (b) Überzugsgewicht:
  • (ba) Von SPC löste man mit einer wäßrigen Lösung von 50 g/l Chromsäureanhydrid den Überzug ab und rechnete das Überzugsgewicht aus den Gewichten vor und nach dem Ablösen aus.
    Einheit: g/m²
  • (bb) beschichtetes Stahlblech (elektrolytisch oder feuerverzinkt):
    Mit einer wäßrigen Lösung, die 20 g/l Ammoniumbichromat und 480 g/l 29-%ige Ammoniak enthielt, wurde der Überzug abgelöst und aus den Gewichten der Proben vor und nach dem Ablösen das Schichtgewicht ermittelt.
    Einheit: g/m²
• (c) Alkalibeständigkeit des Überzuges:
  Das phosphatierte Stahlblech wurde 5 min in 0,1 n-NaOH bei 30°C getaucht. Die Phosphormengen vor und nach dem Tauchen wurden mit einem Röntgenfluoreszenzanalysegeräte gemessen und verglichen. Bewertet wurde die Alkalibeständigkeit jeweils von drei Probestücken nach der Formel

The sheets treated in this way were then subjected to various tests. The quality of the phosphate coating was first assessed.

• (a) Appearance of the coating:
  o - fine and even phosphate coating
  x - poor coating formation, especially inhomogeneous coating
• (b) Coating weight:
  • (ba) The coating was removed from SPC with an aqueous solution of 50 g / l chromic anhydride and the coating weight was calculated from the weights before and after the removal.
    Unit: g / m²
  • (bb) coated steel sheet (electrolytic or hot-dip galvanized):
    The coating was removed with an aqueous solution which contained 20 g / l ammonium bichromate and 480 g / l 29% ammonia, and the layer weight was determined from the weights of the samples before and after the removal.
    Unit: g / m²
• (c) Alkali resistance of the coating:
  The phosphated steel sheet was immersed in 0.1N NaOH at 30 ° C for 5 minutes. The amounts of phosphorus before and after diving were measured and compared with an X-ray fluorescence analyzer. The alkali resistance was evaluated in each case from three test pieces according to the formula

• a) zur Ermittlung der auf der behandelten Metalloberfläche anhaftenden Menge von Nickel und ggf. Mangan mit Hilfe der Röntgenfluoreszenzmethode. Als Meßwert sind in der Tabelle mg/m² angegeben.
• b) Zur Ermittlung der Haftfestigkeit des nach der Phosphatierbehandlung aufgebrachten Lackes.

Further tests were carried out

• a) to determine the amount of nickel and possibly manganese adhering to the treated metal surface using the X-ray fluorescence method. The table shows mg / m² as the measured value.
• b) To determine the adhesive strength of the lacquer applied after the phosphating treatment.

To determine the resistance to hot saline solution, the lacquered sheet metal samples were first scratched to the metal surface and then immersed in a 5% saline solution at 55 ° C for 240 h. An adhesive strip was then applied to the scoring area and torn off, and the width of the paint removal from the scoring area was determined.
Unit: mm

In a further test, the painted sheets were immersed in demineralized water at 40 ° C. for 240 h. The sheet was then cross-cut in such a way that 100 fields each with a field size of 1 x 1 mm were created. The number of fields remaining on the metal surface was determined by pressing and peeling off an adhesive strip.

The table below shows that the process according to the invention is used to produce phosphate coatings of a fine, uniform quality which have excellent alkali resistance and therefore have only an extremely minor impairment in cathodic electrocoating. It can also be seen that the corrosion resistance after painting and the adhesion of the paint layer are excellent.

Claims (10)

1. Process for applying phosphate coatings on metal surfaces with the aid of phosphating solutions based on zinc phosphate, which additionally contain nickel and formic acid, characterized in that the metal surfaces are brought into contact with a phosphating solution which contains 0.3 to 5 g / l, preferably 1 up to 3 g / l, formic acid (calc. as HCOO) contains. 2. The method according to claim 1, characterized in that the metal surfaces are brought into contact with a phosphating solution which contains 0.5 to 2 g / l, preferably 0.7 to 1.5 g / l, of zinc. 3. The method according to claim 1 or 2, characterized in that the metal surfaces are brought into contact with a phosphating solution which contains 0.5 to 3 g / l of nickel. 4. The method according to claim 1, 2 or 3, characterized in that the metal surfaces are brought into contact with a phosphating solution containing 0.3 to 1.5 g / l, preferably 0.3 to 1.0 g / l, of manganese contains. 5. The method according to one or more of claims 1 to 4, characterized in that the metal surfaces are brought into contact with a phosphating solution which contains 10 to 25 g / l phosphate (calculated as PO₄). 6. The method according to one or more of claims 1 to 5, characterized in that the metal surfaces are brought into contact with a phosphating solution containing 0.5 to 2.0 g / l simple and / or complex fluoride (calc. As F) contains. 7. The method according to one or more of claims 1 to 6, characterized in that the metal surfaces are brought into contact with a phosphating solution which
2 to 15 g / l NO₃ ions
0.1 to 1.0 g / l ClO₃ ions
0.01 to 0.2 g / l NO₂ ions
0.3 to 2.0 g / l nitrobenzenesulfonate ions
contains individually or in any combination. 8. Application of the method according to one or more of claims 1 to 7 in diving. 9. Application of the method according to one or more of claims 1 to 7 to the production of phosphate coatings on steel, galvanized steel and / or galvanized alloy steel. 10. Application of the method according to one or more of claims 1 to 7 to the preparation of metal surfaces for electrocoating, in particular cathodic electrocoating.