EP0492713A1 - Process for rinsing conversion coatings - Google Patents

Abstract

Conversion coatings on metal surfaces, especially of steel, galvanised steel, alloy-galvanised steel and/or aluminium, are rinsed, in preparation for painting or the application of adhesives, with an aqueous solution which contains Ce(IV) and/or Ce(III) in a total quantity from 0.01 to 1.0 g/l and has a pH from 3 to 6. Preferably, a rinsing solution is used which contains Ce(III) and Ce(IV) in a weight ratio from 9 to 2:1 and, if appropriate, also molybdate and/or tungstate. Preferred applications of the process serve for preparing metal surfaces provided with conversion coatings for subsequent electrodip-painting, in particular cathodic electrodip-painting, or for rinsing metal surfaces which, as conversion coatings, have phosphate layers, layers based on Zr and/or Ti, F and, if appropriate, PO4 and produced by chromium-free acidic processes, or complex metal oxide layers.

Description

The invention relates to a method for passivating rinsing of conversion layers on metal surfaces, in particular made of steel, galvanized steel, alloy galvanized steel and / or aluminum as a preparation for painting or the application of adhesives using chromium-free, aqueous solutions and its application to the preparation for the subsequent electrodeposition, in particular the cathodic electrodeposition or the rinsing of metal surfaces which have certain conversion layers.

The process of phosphating is widely used industrially to prepare metal surfaces for subsequent painting. The phosphate layers produced with this cause, among other things, better adhesion of the paint films to the metal, increase the corrosion resistance and inhibit the penetration of paint from damage points in the paint film when exposed to corrosion. Other conversion layers have a similar effect, in particular the chromate layers produced by the processes of colorless, yellow or green chromating or the layers produced with solutions based on Ti or Zr. The protective properties of such conversion layers are further improved if they are subjected to an aqueous passivating rinse.

The passivating rinsing agents based on hexavalent and / or trivalent chromium offer good application properties. However, the toxicity of the tri- and especially the hexavalent chromium compounds is disadvantageous.

US Pat. No. 4,376,000 describes a chrome-free rinse aid based on polyvinylphenol. However, this must be used in a comparatively high concentration, which causes an undesirable wastewater pollution, in particular due to the high oxygen requirement required for the breakdown.

From US-A-3 695 942 it is known to use soluble zirconium compound for the aftertreatment of conversion layers. In addition to zirconium, the rinsing agents contain alkali and ammonium as cations. The presence of alkaline earth metal cations is expressly warned. They are used at a pH of 3 to 8.5. However, these rinsing agents do not reach the quality level of the chrome-containing agents.

US-A-3895970 describes acidic, aqueous rinsing agents for phosphate layers based on simple or complex fluorides and names chromium zirconium fluoride and zirconium fluoride from the group of zirconium compounds. With the exception of chrome zirconium fluoride, the products mentioned in this patent only meet moderate requirements. Chromium zirconium fluoride, however, has the disadvantage of toxicity already mentioned above.

According to an older proposal, phosphated metal surfaces are rinsed with aqueous solutions containing aluminum fluorozirconate with a molar ratio of Al: Zr: F of (0.15 to 0.67): 1: (5 to 7), the total concentration of Al + Zr + F is 0.1 to 2.0 g / l and the pH is set to 3 to 5. This rinse is characterized by a very good quality profile.

The object of the invention is now to provide a method for the passivating rinsing of conversion layers on metals before painting or applying adhesives To provide, which does not have the disadvantages of the known methods or represents a further improvement of the older proposal, is characterized by high corrosion protection and very good paint and adhesive adhesion and practically does not pollute the environment.

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 provided with conversion layers are rinsed with an aqueous solution which contains Ce (IV) and / or Ce (III) in a total amount of 0, Contains 01 to 1.0 g / l and has a pH of 3 to 6.

The method according to the invention is suitable for all types of conversion layers which can be produced on metals, in particular steel, galvanized and alloy-galvanized steel, aluminized steel, zinc, zinc alloys, aluminum and aluminum alloys. These include conversion layers based on phosphate, such as zinc, iron, manganese, calcium, zinc manganese and zinc calcium phosphate, as well as other mixed products with two or more divalent cations. It is particularly suitable for the phosphate layers which are formed using low-zinc phosphating processes with and without the addition of further cations, such as Mn, Ni, Co, Mg.

The method according to the invention is also suitable for rinsing conversion layers which have been produced on aluminum or aluminum alloy surfaces with solutions based on Ti or Zr. Solutions of this type can contain, for example, fluorides, phosphates, boron compounds and, if appropriate, passivating constituents, such as tannins, as further components. The method is also suitable for the aftertreatment of chromate coatings, for example of the type mentioned at the outset, and of conversion layers, for example on zinc or zinc alloys by means of a solution, the at least two different polyvalent metal ions and complexing agents contains and preferably has a pH greater than 11 have been obtained.

After the conversion layer has been created, water is rinsed before e.g. is aftertreated by dipping, spraying, flooding or rolling in accordance with the method according to the invention.

In all test results, the corrosion-protecting effect of the passivating rinsing according to the invention corresponds at least to the effect which is obtained by a passivating rinsing by means of the Cr (III) / Cr (VI) -containing solutions known for their excellent action.

A particularly advantageous embodiment of the invention provides that the metal surfaces provided with conversion layers are rinsed with an aqueous solution whose Ce concentration is 0.05 to 0.15 g / l.

A preferred embodiment of the invention provides that the metal surfaces provided with conversion layers are rinsed with a solution which contains Ce (III) and Ce (IV) in a weight ratio of 9 to 2 (Ce III): 1 (Ce IV). The use of a mixture of Ce (III) and Ce (IV) within the weight ratio range mentioned results in a higher corrosion-protective effect than if the Ce compound is used in only one oxidation stage.

A further preferred embodiment of the invention consists in rinsing the metal surfaces provided with conversion layers with a solution into which the Ce (III) and / or Ce (IV) has been introduced as nitrate and / or sulfate. Furthermore, it is advantageous to at least partially use the Ce compound as fluoride, hexafluorozirconate, hexafluorotitanate, hexafluorosilicate, hexafluoroaluminate, tetrafluoroborate or as a salt of Introducing carboxylic acids, hydroxycarboxylic acids and / or aminocarboxylic acids. Suitable carboxylic acids are, for example, acetic acid, oxalic acid and malonic acid, suitable hydroxycarboxylic acids, for example glycolic acid, lactic acid, tartaric acid, citric acid and suitable amino acids, for example nitrilotriacetic acid and ethylenediaminetetraacetic acid. The addition of Ce in the form of compounds of the aforementioned acid has a positive effect on the solubility, in particular of Ce (IV).

Another expedient embodiment of the invention provides that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which additionally contains molybdate and / or tungsten. These compounds are preferably added in the form of their alkali metal salts.

The pH of the rinse solution is expediently set using simple mineral acids, such as nitric acid or hydrofluoric acid. If alkaline additives are required to adjust the pH, volatile bases such as ammonia, ethanolamine, di- and triethanolamine can be used. According to a further advantageous embodiment of the invention, however, the pH is adjusted with alkali hydroxides, since alkali hydroxides, in particular sodium hydroxides, have the advantage over volatile bases that the post-rinse solutions are not destabilized.

The rinse solution to be used within the process according to the invention is usually prepared by diluting a concentrate.

A further advantageous embodiment of the invention provides for rinsing with demineralized water after the passivating rinsing. This rinse removes any adhering salts or salt solutions, which the Paint adhesion and corrosion protection could impair.

The passivating rinse solution is applied in a customary manner to the metal surfaces provided with a conversion layer, e.g. by dipping, flooding, spraying and roller application. Treatment times are between about 1 second to 2 minutes. The application temperature can range from room temperature to about 80 ° C. Temperatures between 20 and 50 ° C are usually preferred. For the preparation of the rinse solutions, it is recommended to use water that is as low in salt as possible, especially demineralized water. Water with a high salt content is not suitable for bath preparation.

The method according to the invention serves to prepare the metal surfaces covered with conversion layers for the painting or the application of adhesives. It increases the adhesion of the organic films, improves the resistance of the organic films to blistering when exposed to corrosion and inhibits the corrosive infiltration caused by damage in the film. The process is suitable for preparation before powder painting, painting with low-solvent high-solids paints and with paints that are essentially based on water as a solvent. A particularly advantageous application of the method according to the invention is the preparation of metal surfaces provided with conversion layers for the subsequent electrocoating, in particular the cathodic electrocoating.

The method is particularly suitable for rinsing metal surfaces that have phosphate layers as conversion layers, layers produced with chromium-free, acidic processes based on Zr and / or Ti, F and possibly PO₄ or complex metal oxide layers.

The method according to the invention is explained in more detail below, for example.

EXAMPLE 1

Degreased sheets of steel, electrolytically galvanized steel and AlMgSi were sprayed with a manganese-modified low-zinc phosphating process for 2 min at 55 ° C. The phosphating solution had the following composition: 0.7 g / l Zn 0.04 g / l Fe III 1.0 g / l Mn 13 g / l P₂O₅ 1.0 g / l Ni 2.1 g / l NO₃ 2.9 g / l Na 0.3 g / l F 0.15 g / l NH₄ 0.07 g / l NO₂

Fine-crystalline, uniformly covering phosphate layers with a basis weight of 2.5 to 3 g / m³ were produced on the three metal substrates. The sheets were then rinsed with water and then rinsed passively. The passivating rinse was carried out in immersion at 30 ° C and 1 min treatment time. The final treatment was a rinse with demineralized water.

The metal sheets were coated with a cathodic electrodeposition primer, a filler and a topcoat. Each layer of paint was baked separately. The total layer thickness was approx. 90 µm.

The sheets were scratched to the metal surface with a steel needle and subjected to various tests. The results are summarized in Tables 1 to 3 below.

The rinse aid according to the invention was prepared by dissolving 0.081 g Ce III (introduced as Ce (NO₃) ₃ · 6H₂O) and 0.020 g Ce IV (introduced as Ce (SO₄) ₂ · 4H₂O) per liter. 0.1 g / l fluoride in the form of HF was added to the bath to stabilize Ce (IV). The pH was then adjusted to 4.0 to 4.5 with ammonia. The slight turbidity that occurs during the preparation of the bath solution has no influence on the passivating properties of the rinse aid.

• 1.

  Cr VI/Cr III-Lösung:

  0,2 g/l CrO₃ und 0,037 g/l Cr III.

  Der pH-Wert war auf 3,5 bis 4,0 eingestellt worden.
• 2.

  Al-Zr-F-Lösung:

  0,014 g/l Al, 0,14 g/l Zr und 0,17 g/l F.

  Der pH-Wert war ebenfalls auf 3,5 bis 4,0 eingestellt worden.

The solutions used for comparison purposes included

• 1.

  Cr VI / Cr III solution:

  0.2 g / l CrO₃ and 0.037 g / l Cr III.

  The pH was adjusted to 3.5 to 4.0.
• 2nd

  Al-Zr-F solution:

  0.014 g / l Al, 0.14 g / l Zr and 0.17 g / l F.

  The pH was also adjusted to 3.5 to 4.0.

From the tables it can be seen that the method according to the invention has an at least as good as, in some cases even better, effect in terms of corrosion protection than, in particular, the recognized highly effective Cr VI / Cr III solution.

EXAMPLE 2

Cleaned and degreased aluminum sheets were immersed in a solution for 10 seconds to produce a conversion coating, the temperature of which was 50 ° C. and had the following composition: Ti 0.17 g / l F 1.24 g / l P₂O₅ 0.09 g / l NH₄ 0.91 g / l Tannin 0.11 g / l N / A 0.003 g / l Biocide 0.10 g / l

The sheets were then rinsed with water and rinsed passively. For this purpose, the sheets were immersed in the rinse solution at 35 ° C. for 5 seconds and then excess solution was removed by squeezing. After a drying time of 0.5 min in a convection oven at 60 ° C, the sheets were coated with a two-layer food lacquer, the first layer of which is an epoxy-phenol resin and the second layer of which is an organosol. The layer thickness of the total paint was between 10 and 15 µm.

Then, sheet blanks with a diameter of 60 mm were punched out of the treated, approximately 0.25 mm thick sheets and wells with a diameter of 26 mm and a height of 25 mm were deep-drawn.

These cells were subjected to a sterilization test by exposure to an aqueous solution of 3% sodium chloride, 1% citric acid and 0.5% lactic acid at 121 ° C. in a pressure vessel for 40 minutes. The defects found on the cells afterwards (paint detachments, bubbles) were rated on a scale from 1 (lacquer in the entire mantle area of the well = useless) to 15 (no lacquer defects = excellent).

The rinse solution (A) used contained 0.110 g / l Ce III (introduced as Ce (NO₃) ₃ · 6H₂O) and 0.020 g / l Ce IV (introduced as Ce (SO₄) ₂ · 4H₂O). The pH was adjusted to 4.0 to 4.5.

For comparison purposes, an aqueous solution of 0.6 g / l of polyvinylphenol and a pH of approximately 5 (rinsing solution B) and a solution containing 0.014 g / l of Al, 0.14 g / l of Zr, 0.17 Contained g / l F and 0.016 g / l NH₄ and had a pH of 3.5 to 4.0 (rinse solution C), rinsed.

A comparison of the table values shows that the method according to the invention delivers significantly better values, in particular compared to the comparative example with a rinse solution based on polyvinylphenol.

EXAMPLE 3

Cleaned and degreased sheets of hot-dip galvanized steel took 30 seconds to produce a conversion coating dipped in a solution whose temperature was 55 ° C and had the following composition: Co²⁺ 0.3 g / l Fe³⁺ 0.2 g / l NO₃- 1.3 g / l Sodium salt of hexahydroxyheptanoic acid 2.2 g / l NaOH 27.4 g / l

The sheets were then rinsed with water and rinsed passively. For this purpose, the sheets were immersed in the rinse solution at 30 ° C. for 5 seconds and then excess solution was removed by squeezing. After a drying time of 0.5 min in a forced air oven at 75 ° C, the pretreated sheets were coated with an epoxy primer and an acrylate top coat. The layer thickness of the total lacquer was approx. 25 µm.

The treated sheets were then subjected to the following tests:
The paint adhesion was determined in the T-Bend test, the sheets being bent through 180 ° and the different radii of curvature being stated as n times the sheet thickness (n = 0.1.2 ...) (Tn). The proportion of the chipped paint surface of the total curved surface is given as the tested size in%.

On further treated sheets, a metal needle was used to make cracks down to the metal surface and a cutting edge with sheet metal scissors. The sheets were then subjected to the salt spray test according to DIN 50021 SS for 1008 hours. The undercut of the paint (mm) starting from the scratch or the cut edge is specified as the tested size.

The rinse solution (A) used according to the invention contained 0.110 g / l Ce III (introduced as Ce (NO₃) ₃ · 6H₂O) and 0.320 g / l Ce IV (introduced as Ce (SO₄) ₂ · 4H₂O). Their pH was 4.0 to 4.5.

A rinsing solution (B) containing 0.014 g / l Al, 0.14 g / l Zr, 0.17 g / l F and 0.016 g / l NH und at a pH of 3.5 to 4 was used for comparison purposes , 0 and a rinse solution (C) containing 2.0 g / l Cr VI, 0.8 g / l Cr III, 0.2 g / l F and 0.3 g / l Zn.

The test results are given in the tables below. Paint adhesion in the T-Bend test Rinse solution chipped surface (%) at radius of curvature Tn T1 T2 T3 T4 A (invention) 100 50 10th <5 B (comparison) 100 55 15 5 C (comparison) 100 80 30th 5 Corrosion resistance in the salt spray test Rinse solution Infiltration (mm) after 1008 h at the Ritz on the edge A (invention) 0 - 1 6-8 B (comparison) <1 - 1 8 - 9 C (comparison) 1 - 3 9-10

A comparison of the table values shows that the method according to the invention in any case gave at least as good or better values, in particular as the highly effective comparison method with a rinsing solution based on Cr (VI) / Cr (III) recognized with regard to corrosion protection.

Claims (10)

Process for the passivating rinsing of conversion layers on metal surfaces, in particular made of steel, galvanized steel, alloy galvanized steel and / or aluminum as preparation for painting or applying adhesives using chromium-free, aqueous solutions, characterized in that the metal surfaces provided with conversion layers rinsed with an aqueous solution which contains Ce (IV) and / or Ce (III) in a total amount of 0.01 to 1.0 g / l and has a pH of 3 to 6. Process according to Claim 1, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution whose Ce concentration is 0.05 to 0.15 g / l. Process according to Claim 1 or 2, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which contains Ce (III) and Ce (IV) in a weight ratio of 9 to 2: 1. Method according to one or more of claims 1 to 3, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution into which Ce (IV) and / or Ce (III) has been introduced as nitrate and / or sulfate. Method according to one or more of claims 1 to 3, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution into which Ce (IV) and / or Ce (III) are at least partially via the anions fluoride, hexafluorozirconate, hexafluorotitanate , Hexafluorosilicate, hexafluoroaluminate, tetrafluoroborate or as a salt of carboxylic acids, hydroxycarboxylic acids and / or aminocarboxylic acids. Method according to one or more of claims 1 to 5, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution which additionally contains molybdate and / or tungsten. Method according to one or more of claims 1 to 6, characterized in that the metal surfaces provided with conversion layers are rinsed with an aqueous solution whose pH value using simple mineral acids, such as nitric acid and / or hydrofluoric acid, or using alkali metal hydroxides, preferably sodium hydroxide. Method according to one or more of claims 1 to 7, characterized in that, after the passivating rinsing, rinsing with demineralized water. Application of the method according to one or more of claims 1 to 8 to the preparation of metal surfaces provided with conversion layers for the subsequent electrodeposition, in particular the cathodic electrodeposition. Application of the method according to one or more of claims 1 to 9 to the post-rinsing of metal surfaces which have phosphate layers as conversion layers, chromium-free, acidic processes based on Zr and / or Ti, F and possibly PO₄ layers or complex metal oxide layers.