Method for passivating a metallic surface

Abstract

A method for passivating a metal surface of a light-weight metal part is disclosed, wherein a conversion layer is applied to the surface of the light-weight metal part in a passivation step. A passivation step is carried out wherein an aqueous passivation solution is used to create a calcium phosphate-containing conversion layer (5) on the metal surface of the part, said conversion layer comprising oxides and hydroxides from the material of the part and from the passivation solution and containing amino acids.

Claims

1. A method for passivating a metallic surface of a lightweight metal component, said method comprising: in a passivating step applying an aqueous passivation solution on the metallic surface of the lightweight metal component, thereby generating on the metallic component surface a conversion layer which contains calcium phosphate, amino acids, and further contains oxides and hydroxides of the lightweight metal component; and in a first coating step forming a lightweight metal KTL (cathodic dip painting) layer as an organic protective layer in a dip bath as paint particles dissolved in the dip bath are attracted by the lightweight metal component and adhere to the lightweight metal component.

2. The method of claim 1, further comprising at least one further coating step in which at least one further layer is applied.

3. The method of claim 2, wherein the at least one further layer is applied in a powder coating process with applied direct voltage.

4. The method of claim 1, wherein at least the metallic surface of the component is formed by a lightweight metal.

5. The method of claim 4, wherein the lightweight metal is magnesium, aluminum or alloys thereof.

6. The method of claim 1, wherein the passivation solution contains at least one of the following components as activators for activating the metal surface of the components: NaCl at a concentration between 5000 and 8000, and KCl with a concentration between 300 and 500.

7. The method of claim 1, wherein the passivation solution contains as a catalyst and layer former D-Ca-pantothenate at a concentration between 2 and 30 mg/l.

8. The method of claim 1, wherein the passivation solution contains L-Isoleucine as a layer adhesion agent at a concentration between 90 and 150 mg/l.

9. The method of claim 1, wherein the passivation solution contains at least one of the following components, which are integrated into the conversion layer as fragments for supporting formation of the conversion layer: NaH.sub.2PO.sub.4 at a concentration between 100 and 170 mg/l; CaCl.sub.2) at a concentration between 170 and 300 mg/l.

10. The method of claim 1, wherein the conversion layer contains carbonate containing components for supporting formation of the conversion layer, and wherein the passivation solution contains NaHCO.sub.3 for providing the carbonate containing components.

11. The method of claim 1, wherein the passivation solution contains Na-pyruvate at a concentration between 90-170 mg/l for supporting formation of the conversion layer.

12. The method of claim 1, wherein a pH value of the passivation solution is in a neutral to acid range.

13. The method of claim 1, wherein the passivation solution contains further the following components whose concentration is modeled according to their respective concentrations in human blood: NaCl at 6400 mg/l, KCl at 400 mg/l, NaH.sub.2PO.sub.4 at 124 mg/l, CaCl.sub.2 at 200 mg/l, NaHCO.sub.3 at 3700 mg/,l Na-Pyruvate at 110 mg/l, D-Ca-pantothenate at 4 mg/l, Myo-Inositol at 7.2 mg/l, L-Isoleucine at 105 mg/l.

14. The method of claim 1, wherein the passivation solution contains at least one of the following components for increasing coating properties: L-Arginine at 50 to 120 mg/l, L-Cysteine with 30 to 80, L-Histidine.HCl.H.sub.2O at 25 to 65 mg/l, L-Leucine at 70 to 140 mg/l, L-Lysin.HCl at 110 to 170 mg/l, L-Methionine at 20 to 50 mg/l, L-Penthylalanine at 40 to 80 mg/l, L-Threonine at 60 to 120 mg/l, L-Tryptophan at 13 to 20 mg/l, L-Tyrosine at 40 to 90 mg/l, L-Valine at 60 to 120 mg/l, L-Serine at 20 to 60 mg/l, Colin chloride at 2 to 10 mg/l, Folic acid at 2 to 10 mg/l, Nicotine amide at 2 to 10 mg/l, Pyridoxale.HCl at 2 to 10 mg/l, Riboflavine at 0.2 to 1 mg/l, Thiamine-HCl at 2 to 10 mg/l.

15. The method of claim 1, wherein the conversion layer of the component is at least partially covered with a layer in a subsequent coating step.

16. The method of claim 1, wherein the conversion layer has a floe-like layer morphology with crack structures, said layer morphology ensuring in the first coating step a sufficient residual conductivity between the dip bath and the lightweight metal component and/or increasing an adhesive connection between the conversion layer and the lightweight metal KTL layer by entering of a liquid starting component of the lightweight metal KTL layer into the cracks.

17. The method of claim 1, wherein the cathodic dip painting includes a dip method with applied direct voltage.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) In the following the invention and its advantageous embodiments and refinements and their advantages are explained in more detail by way of drawings.

(2) It is shown in:

(3) FIG. 1 the layer construction of a finished painted lightweight metal component which in this case exemplarily shows an outer part that can be arranged externally on the vehicle body;

(4) FIGS. 2 to 4 respective flow charts which illustrate the coating processes for producing the layer construction shown in FIG. 1; and

(5) FIGS. 5 to 7 respective strongly enlarged partial sectional views, which illustrate the coating process up to the application of the lightweight metal KTL layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 1 exemplarily shows in a strongly enlarged partial sectional view the layer construction 1 of a paint coating on the metal surface 25 of a vehicle body component 3. As an example the vehicle body component 3 is in this case made of a lightweight metal, for example aluminum, magnesium, or an alloy thereof. Accordingly the layer formation 1 has directly bordering the material surface 25 of the lightweight metal component 3 a conversion layer 5, which serves for passivation and corrosion protection. The conversion layer 5 is covered by a lightweight metal KTL-layer 6. On the lightweight metal KTL-layer a powder layer 7 is formed on which a conventional varnish formation 9 is applied. As shown in FIG. 1 the conversion layer 5 has a floe-like layer morphology in which cracks 13 are formed between individual floes 11. In a KTL coating process described below the cracks 13 ensure a sufficient residual conductivity between a KTL-dip bath and the lightweight metal material of the component 3. In addition the liquid starting component of the lightweight metal KTL layer 6 can enter the cracks 13 during the multi-step coating process and thereby increase the adhesive connection to the conversion layer 5.

(7) FIG. 1 and also the further FIGS. 2 to 7 have been prepared with the goal to facilitate understanding of the invention. Therefore the figures are strongly simplified representations, which do not reflect a realistic layer for formation 1. Thus the conversion layer 5 in actuality has a layer thickness in the m range.

(8) In the following a serial painting process that is performed in a painting plant is described by way of the flow chart shown in FIGS. 2 to 4, in which a passivation solution according to the invention is used: accordingly first a passivating step P (FIG. 2) is performed. In the passivating step P a degreasing, a grinding and/or pickling of he component 3 is performed. The thusly-cleaned component 3 is then subjected to a passivation according to the invention in which it is immersed in a dip bath containing of the passivation solution.

(9) The composition of the aqueous passivation solution is generally modeled after the composition of human blood. In this regard the passivation solution contains at least the following main components whose concentration is identical to that in human blood: NaCl with in particular 6400 mg/l KCl with in particular 400 mg/l NaH.sub.2PO.sub.4 with in particular 124 mg/l CaCl.sub.2 with in particular 200 mg/l NaHCO.sub.3 with in particular 3700 mg/l Na-Pyruvate with in particular 110 mg/l D-Ca-pantothenate with in particular 4 mg/l Myo-Inositol with in particular 7.2 mg/l L-Isoleucine with in particular 105 mg/l

(10) Hereby NaCl and KCl sin the passivation solution serve for activating the metal surface 25. The amino acids D-Ca-pantothenate and Myo-Inositol are mainly responsible for the coating process and in addition have a catalytic effect. The components NaH.sub.2PO.sub.4 and CaCl.sub.2 support the painting process by incorporation of Ca.sup.2+ and PO.sub.4.sup.3 ions into the conversion layer 5.

(11) The conversion layer according to the invention additionally contains carbonate containing layer components. These are provided in the passivation solution by the components NaHCO.sub.3 and CO.sub.2 (from the atmosphere). As a further auxiliary material of the layer formation the component Na-pyruvate serves.

(12) Important for the coating behavior are the following components of the passivation solution: L-Arginine with 84 mg/l L-Cysteine with 48 mg/l L-Histidine.HCl.H2O with 42 mg/l L-Leucine with 105 mg/l L-Lysin.HCl with 146 mg/l L-Methionine with 30 mg/l L-Penthylalanine with 66 mg/l L-Threonine with 95 mg/l L-Tryptophan with 16 mg/l L-Tyrosine with 72 mg/l L-Valine with 94 mg/l L-Serine with 42 mg/l.

(13) The above amino acids are also components of human blood whose concentration is retained substantially unchanged.

(14) Overall the passivation solution according to the invention is therefore a aqueous treatment liquid whose pH value is in the range of about 7 or in the acid range. The passivation is performed in the dip bath at a treatment temperature in the range of 18 to 25 C. The treatment time depends on the used pH value, the process temperature and optionally an additional polarization and the required target thickness of the coating. After passivation the component 3 is subjected to a rinsing/drying process.

(15) In the present application the component 3 that is coated with the conversion layer 5 in a further process step is provided (according to FIG. 3) in a coating station 17 with a lightweight metal KTL layer 6 (i.e., an organic protective layer). The lightweight metal KTL is performed conventionally in a dip method in which a direct voltage is applied between the vehicle body 1 and the dip bath, whereby the paint particles dissolved in the dip bath are attracted by the component 3 where they adhere uniformly. Additionally required pre- or post processing steps are not shown for reasons of clarity.

(16) In a subsequent drying station 18 the component 3 passes though a continuous furnace with ta predetermined transport speed in which the lightweight metal KTL layer 6 is burned in at process temperatures in the range of for example 180 C. Subsequently in the process step II a powder layer is applied on the component 3 in a coating station 20 in which the layer 7 is applied to the component 3 (FIG. 1). In the powder coating station 20 the paint particles are transported through an electric filed from the pointed heads under voltage to the component 3, which is at ground potential. Subsequent thereto a further burning in process is performed in a further drying station 19 in a continuous furnace.

(17) Subsequent to the component-coating process L (i.e., process steps I and II of FIG. 3) in a possible application the lightweight metal component 3 is joined to a not yet painted body in white 15 as a visible outer vehicle part. The body in white 15 is transported in a continuous process into a body painting plant (see FIG. 4). There a cataphoresis priming 25 is performed in the dip method in which a direct voltage is applied and the paint particles dissolved in the dip bath are attracted by the body in white 15 where they adhere and form a primer coat. Subsequently the primed body-in-white 15 is transported to a downstream continuous furnace 27 in which the primer coat is burned in. subsequent thereto the body-in-white 15 that is provided with the primer coat is transported to a further coating station 29 in which a KTL process is performed. Downstream of the KTL process 29 also a continuous furnace 31 is arranged in which the coating is burned in at high temperature. Subsequently in a further coating station 33 a conational automobile four layer paint construction 9 is applied which is then subjected to a burning in process 35.

(18) The vehicle body paining process shown in FIG. 4 is performed with an already pre-coated lightweight metal component 3. This means that the lightweight metal component 3 is eclectically insulated so that the KTL layer applied in the body in white painting process no longer adheres, whereas the convectional varnish construction 9 (i.e., a four layer construction) can be applied to the powder layer of the lightweight metal component 3 without problems.

(19) FIGS. 5 to 7 show in views corresponding to that of FIG. 1 the lightweight metal component 3 with cleaned and exposed metallic surface 25. FIG. 6 shows the lightweight metal component 3 after passivation and storage. According to this the conversion layer 5 is applied on the metallic surface 25 of the lightweight metal component, i.e., with the floe morphology according to the invention, i.e., with floe like individual fragments 11 and interposed cracks 13. FIG. 7 shows the lightweight metal component 3 after the lightweight metal KTL process in which the staring component of the lightweight KTL layer 6 permeates the crack structure 13 of the conversion layer 5, whereby the adhesive connection between the conversion layer 5 and the lightweight metal KTL layer 6 is significantly increased.