EP3129527B1 - Method for passivating a metal surface - Google Patents

Description

The invention relates to a method for passivating a metallic surface of a light metal component according to the preamble of claim 1.

The use of light metal components is common practice across industries. Especially in vehicle construction, for example, vehicle bodies in mixed construction, for example, made of light metal and sheet steel parts to achieve a weight reduction. By way of example, aluminum or magnesium materials or alloys thereof can be used as the light metal.

From the DE 196 30 289 C2 a method for painting vehicle bodies is known. Consequently, the body shell is subjected to pre-treatment before the actual painting process, in which bodies are first degreased in a spray and Volltauchzone. Subsequently, the cleaned vehicle bodies are coated in a phosphating bath with a zinc phosphate layer. This serves as additional corrosion protection and as a primer for the subsequent primer. Subsequent to this pretreatment step, a cataphoresis primer is applied in the dipping process, in which the paint particles dissolved in the dipping bath are attracted to the body panel under applied electrical direct voltage and there to form a primer stick to it. Subsequently, the primed vehicle body is conveyed in a downstream continuous furnace in which the primer is baked. Subsequently, the provided with the primer vehicle body is led to another coating station in which a top coat is done in the desired color by the customer. In this case, the paint particles can be transported by an electrostatic field of high voltage standing support heads to the grounded body. This topcoat is also followed by a continuous furnace in which curing of the topcoat takes place at high temperature. Subsequently, a clearcoat is applied in a further coating station, which is also cured at high temperature in a subsequent drying step.

In vehicle bodies manufactured in mixed construction, the light metal components (made of Mg or Al) are more susceptible to corrosion than the steel components. Especially paint undercuts and filiform corrosion are more common damage patterns. To protect the light metal components, a pickling passivation and the application of an anodic coating is common practice. However, the commercial coatings only partially provide the necessary protective effect against self-corrosion, filiform corrosion and / or paint infiltration. Especially when in contact with magnesium, the high potential difference should be regarded as promoting corrosion.

DE102010060700 A discloses a corrosion protection method that is suitable for use in mixed construction.

The commercially available coating systems for light metal components do not exhibit sufficient passivating behavior and are often too "noble" (that is to say too high corrosion potentials), especially with respect to magnesium. If magnesium alloys are electrochemically polarized by contact with more noble metals (eg aluminum), an exponential increase of the corrosion current occurs.

The published study entitled " Influence of artificial biological fluid composition on the biocorrosion of potential orthopedic Mg-Ca, AZ31, AZ91 alloys "appeared in Biomedical Materials (Vol. 4, Number 6, December 1, 2009, page 65011, XP020170659, ISSN: 1748-605X ) describes the treatment of magnesium surfaces with a nutrient medium to obtain conversion layers containing calcium phosphates, magnesium oxide, magnesium hydroxide and amino acids. The nutrient medium used was "Dulbecco's Modified Eagle's Medium" (DMEM for short). This nutrient medium contains inorganic salts and amino acids. From this study, a generic method for passivation of a metallic surface is known.

Also titled " In vitro studies of biomedical magnesium alloys in a simulated physiological environment: A review published study appeared in Acta Biomaterialia (Volume 7, Number 4, December 3, 2010, pages 1452-1459, XP028366341, ISSN: 1742-7061 ) describes the treatment of magnesium surfaces with the nutrient medium known as "Dulbecco's Modified Eagle's Medium".

The study entitled "High corrosion resistance of magnesium coated with hydroxyapatite directly synthesized in an aqueous solution" published a conversion layer for magnesium based on calcium phosphates is known. The study is in the Electrochimica Acta (Vol. 54, Number 27, November 30, 2009, pages 7085 to 7093, XP026600732, ISSN: 0013-4686 ) released.

The object of the invention is to provide a method for passivating the metallic surface of a light metal component, in which In particular with aluminum or magnesium, a sufficient passivation is achieved and a contact corrosion risk is reduced.

The object is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the subclaims.

The invention is based on the idea of orienting the composition of the passivation solution, at least in principle, to the composition of human blood. Surprisingly, it has been found that certain components of human blood produce a protective and passivating coating on metal surfaces, especially of light metal such as aluminum and / or magnesium. In a specific embodiment of the invention, concentrations of individual constituents can be simulated essentially unchanged in the passivation solution. Against this background, a special passivation step takes place in which a calcium phosphate-containing conversion layer which comprises oxides and hydroxides of the component material and of the passivation solution and contains amino acids is produced on the metallic component surface using an aqueous, in particular blood-like, passivation solution.

The light metal component formed with the passivated metallic surface can be used across industries. By way of example, the light metal component may optionally be used in the automotive sector, namely in a visible manner within the vehicle or as an externally visible outer part. By way of example, the light metal component can be realized as a vehicle-inner-side display frame, an aggregate part, a chassis part or a component of a seat frame structure.

The passivating anticorrosive primer (ie the conversion layer) produces a reduction of the self-corrosive currents by a factor of 10. In addition, an increase of the pitting potential by more than 0.5 V takes place with simultaneous reduction of the cathodic current densities. The conversion layer according to the invention is very favorably in contact with nobler materials (such as aluminum or steel). In addition, with the conversion layer according to the invention, there is a reduction in the contact corrosion currents with aluminum, steel, zinc, carbon fibers or CFRP. Furthermore, there is an increase in the penetration resistance by application of the coating (that is, the higher the penetration resistance, the lower the corrosion currents, the penetration resistance behaves inversely proportional to the corrosion currents). In addition, a passive behavior is generated in which much lower global corrosion currents occur. By comparison, in conventional conversion layers, a large number of finely divided local corrosion sites result. Overall, it should be emphasized that the conversion layer according to the invention generates low intrinsic corrosion currents and high passivity. In contact with aluminum and steel, only small contact corrosion currents result.

In the case of a component material made of aluminum, the passivation solution results in a compact calcium phosphate and aluminum hydroxide / oxide-containing coating with amino acids. The layer morphology is constructed in the form of lumps, with intermediate cracks which, for example, provide a sufficiently large residual conductivity in the case of a subsequent KTL deposition in a coating process. In addition, the liquid starting component of the primer can penetrate into the cracks, resulting in good adhesion between the conversion layer and the primer.

Alternatively, in the case of a component material made of magnesium, a compact calcium phosphate and magnesium hydroxide / oxide-containing coating is obtained, the layer morphology of which is also formed in a lump-shaped manner.

In the following, further optional features of the invention are described: Thus, the passivation solution may preferably have at least the following constituents as activators for activating the metal surface of the component: NaCl with a concentration between 5500 and 7500, in particular 6400 mg / l; and or KCl with a concentration between 300 and 500, in particular 400 mg / l.

Both NaCl and KCl act as a source of chloride and assist in the activation of film formation which results in increased release of material ions from the surface of the device, which are necessary for film formation.

In addition, the passivation solution can have at least the following amino acids as catalysts and layer formers: D-Ca pantothenate with a concentration between 2 and 5, in particular 4 mg / l, and / or L-isoleucine with a concentration between 80 and 120, in particular 105 mg / l.

The amino acid L-isoleucine acts specifically as a layering agent that supports the adhesion of the conversion layer on the metallic component surface.

To support the layer formation, Ca ²⁺ and / or PO ₄ ^3- ions are also incorporated as fragments in the conversion layer. In this case, the passivation layer may preferably contain calcium phosphates.

In addition, the conversion layer may contain carbonaceous constituents. To provide such carbonate-containing layer constituents, the passivation solution may contain NaHCO ₃ . The formation of carbonate also depends on possibly supplied CO ₂ .

As a further adjuvant to promote layer formation, the passivating solution may contain Na pyruvate, with a concentration between 90 and 150 mg / l, in particular 110 mg / l.

As stated above, an essential aspect of the invention is that certain components of human blood are transferred to the passivation solution in substantially unchanged concentrations. Accordingly, in one embodiment, the aqueous passivating solution may contain at least the following constituents, the concentrations of which are reproduced in their concentrations in human blood: NaCl in particular 6400 mg / l KCl with in particular 400 mg / l NaH ₂ PO ₄ in particular 124 mg / l CaCl ₂ in particular 200 mg / l NaHCO ₃ in particular 3700 mg / l Na-pyruvate in particular 110 mg / l D-Ca pantothenate in particular 4 mg / l Myo-inositol with in particular 7.2 mg / l L-isoleucine in particular 105 mg / l

Of essential importance for the coating behavior are at least one or more, in particular all of the following constituents of the passivation solution: L-Arginine HCL • in particular 84 mg / l L-cystine in particular 48 mg / l L-histidine • HCl • H ₂ O in particular 42 mg / l L-leucine in particular 105 mg / l L-lysine • HCl in particular 146 mg / l L-methionine in particular 30 mg / l L-phenylalanine in particular 66 mg / l L-threonine in particular 95 mg / l L-tryptophan in particular 16 mg / l L-tyrosine in particular 72 mg / l L-valine in particular 94 mg / l L-serine in particular 42 mg / l choline chloride in particular 4 mg / l folic acid in particular 4 mg / l nicotinamide in particular 4 mg / l • pyridoxal HCl in particular 4 mg / l riboflavin with in particular 0.4 mg / l Thiamine HCl in particular 4 mg / l

The passivation reaction according to the invention can be carried out at a pH of about 7. In this case, the coating reaction is slow. Alternatively, the coating reaction can also take place in the acidic range. The coating reaction can be accelerated by increasing the temperature, reducing the pH and / or by polarization and / or increasing the partial pressure of CO ₂ .

In a specific application, the light metal component may be a vehicle part, which is first pretreated with the passivation solution according to the invention to form the conversion layer. The conversion layer of the component can be covered with at least one further layer in a subsequent coating process. According to the invention, the coating process has a first coating step, in which a light metal KTL layer, ie an organic protective layer, is formed. This is done in a dipping process (ie light metal KTL) under applied DC voltage, whereby the paint particles dissolved in the immersion bath are attracted to the metallic component and remain there to form the light metal KTL layer. In a further coating step, a powder coating is then applied. This is done in a powder coating process with applied DC voltage. With regard to a process-reliable coating, the special clod-shaped layer morphology with the crack structures already mentioned above is of particular importance. This ensures namely in the dipping process and in the powder coating process sufficient electrical residual conductivity through the conversion layer.

Subsequent to the component coating process, in one possible application the light metal component, for example as a visible outer part, can be joined in a riveting operation to the shell body not yet painted. The body shell is then subjected to a conventional body painting process together with the light metal component mounted thereon. That is, there is a cataphoresis primer of the body shell in the dipping process in which under applied electrical DC voltage dissolved in the immersion paint particles are attracted to the body shell and remain there to form a primer. Subsequently, the primed body shell is conveyed into a downstream continuous furnace, in which the primer is baked. Subsequently, the green body provided with the primer is led to another coating station in which a KTL process takes place. The KTL process is also followed by a continuous furnace in which the KTL layer burns in at high temperatures. Subsequently, in a further coating station a conventional automotive paint system applied, which is also baked in a subsequent drying step at high temperature.

In the above body painting process, the light metal component mounted on the green body is already precoated with a layer structure, namely with the conversion layer, the light metal KTL layer and the powder coating. The light metal component is thus electrically insulated, so that the electroplated in the bodyshell painting process KTL layer no longer adheres, while the conventional automotive paint system can be easily applied to the already coated light metal component.

The advantageous embodiments and / or developments of the invention explained above and / or reproduced in the subclaims can be used individually or else in any desired combination-except, for example, in the case of clear dependencies or incompatible alternatives.

The invention and its advantageous embodiments and further developments and advantages thereof are explained in more detail below with reference to drawings.

Fig. 1

den Schichtaufbau eines fertig lackierten Leichtmetallbauteils, das hier beispielhaft ein außenseitig an der Fahrzeugkarosserie anbringbares Außenteil darstellen soll;

Fig. 2 bis 4

jeweils Ablaufpläne, die Beschichtungsprozesse zur Herstellung des in der Fig. 1 gezeigten Schichtaufbaus veranschaulichen; und

Fig. 5 bis 7

jeweils stark vergrößerte Teilschnittansichten, die den Beschichtungsprozess bis zum Auftragen der Leichtmetall-KTL-Schicht veranschaulichen.

Show it:

Fig. 1

the layer structure of a finished painted light metal component, which is intended to represent here an example externally attachable to the vehicle body outer part;

Fig. 2 to 4

each flowchart, the coating processes for the production of in the Fig. 1 illustrate layer structure shown; and

Fig. 5 to 7

respectively greatly enlarged partial sectional views illustrating the coating process to the application of the light metal KTL layer.

In the Fig. 1 is shown in a greatly enlarged partial sectional view of the example of the layer structure 1 of a paint coating on the metal surface 25 of a body part 3. By way of example, here is the body part 3 made of light metal, such as aluminum, magnesium or an alloy thereof. Accordingly, the layer structure 1 directly on the workpiece surface 25 of the light metal component 3, a conversion layer 5, which serves for passivation and corrosion protection. The conversion layer 5 is coated by a light metal KTL layer 6. On this a powder coating 7 is formed, on which a conventional automotive paint system 9 is applied. Like from the Fig. 1 Furthermore, the conversion layer 5 has a clod-shaped layer morphology, in which cracks 13 are formed between individual blocks 11. The cracks 13 provide in a later described KTL coating process for a sufficient residual conductivity between a KTL dip and the light metal material of the component 3. In addition, during the multi-stage coating process, the liquid starting component of the light metal KTL layer 6 penetrate into the cracks and thereby increase the adhesive bond to the conversion layer 5.

The Fig. 1 as well as the others Fig. 2 to 7 , are made with a view to a simpler understanding of the invention. Therefore, the figures are only roughly simplified representations that do not reflect a realistic layer structure 1. Thus, the conversion layer 5 actually has a layer thickness which lies in the μm range.

The following is based on the in the Fig. 2 to 4 In the flowchart shown, a series paint process carried out in a paint shop is described, in which a passivation solution according to the invention is used: Accordingly, a passivation step P (first) is carried out. Fig. 2 ). In the passivation step P, a degreasing, grinding and / or pickling of the component 3 is performed. The thus cleaned component 3 is then subjected to a passivation according to the invention, in which it is immersed in a dipping bath of the passivation solution.

The composition of the aqueous passivating solution is basically based on the composition of human blood. Against this background, the passivation solution contains at least the following main components whose concentrations are identical to the concentrations in human blood: NaCl with 6400 mg / l KCl with 400 mg / l NaH ₂ PO ₄ with 124 mg / l CaCl ₂ with 200 mg / l NaHCO ₃ with 3700 mg / l Na-pyruvate with 110 mg / l D-Ca pantothenate with 4 mg / l Myo-inositol with 7.2 mg / l L-isoleucine with 105 mg / l

Here, NaCl and KCl in the passivation solution serves to activate the metal surface 25. The amino acids D-Ca-pantothenate and myo-inositol are mainly responsible for the coating process and also have a catalyzing effect. The components NaH ₂ PO ₄ and CaCl ₂ support the coating process by incorporating the Ca ²⁺ and PO ₄ ^3- ions into the conversion layer 5.

The conversion layer 5 according to the invention also has carbonate-containing layer constituents. These are provided in the passivation solution by the component NaHCO ₃ and CO ₂ (from the atmosphere). Another component used for layer formation is the component Na pyruvate.

Of essential importance for the coating behavior are the following constituents of the passivation solution: L-arginine • HCl with 84 mg / l L-cystine with 48 mg / l L-histidine • HCl • H ₂ O with 42 mg / l L-leucine with 105 mg / l L-lysine • HCl with 146 mg / l L-methionine with 30 mg / l L-phenylalanine 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.

The above amino acids are also components of human blood whose concentrations are almost unchanged.

Overall, therefore, the passivation solution according to the invention is an aqueous treatment liquid whose pH is in the range of about 7 or in the acidic range. The passivation takes place in the immersion bath at a treatment temperature in the range of 18 to 25 ° C. The treatment time depends on the set pH value, the process temperature and optionally an additional polarization as well the required nominal thickness of the coating. After passivation, the component 3 is fed to a rinsing / drying process.

The coated with the conversion layer 5 component 3 is in the present application in the further process sequence (according to the Fig. 3 ) in a coating station 17 with a light metal KTL layer 6 (ie, an organic protective layer). The light metal KTL coating is carried out in common practice in the dipping process in which an electrical DC voltage is applied between the body 1 and the dip tank, whereby the paint particles dissolved in the dipping bath are attracted to the component 3 and adhere uniformly there. Additionally required pre- or post-treatment steps are omitted for the sake of easier understanding of the invention.

In a downstream drying station 18, the component 3 passes through a continuous furnace at a predetermined conveying speed, in which the light metal KTL layer 6 is baked at process temperatures in the range of, for example, 180 ° C. Subsequently, in process step II, a powder coating takes place in a coating station 20, in which layer 7 (FIG. Fig. 1 ) is applied to the component 3. In the powder coating 20, the paint particles are transported to the earthed component 3 by an electrostatic field from live pointing heads. Subsequently, in a further drying station 19 again a baking process in a continuous furnace.

Subsequent to the component coating process L (ie process steps I and II of the Fig. 3 ), the light metal component 3 is added in a possible exemplary application as a visible vehicle exterior part in a riveting to a not yet painted bodyshell 15. The body shell 15 is in a continuous process in a body painting plant (see Fig. 4 ). There is a cataphoresis primer 25 in the dipping process in which under applied electrical DC voltage dissolved in the immersion paint particles are attracted to the body shell 15 and adhere there to form a primer. Subsequently, the primed shell body 15 is conveyed to a downstream continuous furnace 27, in which the primer is baked. Subsequently, the green body 15 provided with the primer is led to a further coating station 29, in which a KTL process takes place. The KTL process 29 is also followed by a continuous furnace 31, in which the coating is baked at high temperature. Subsequently, in a further coating station 33, a conventional automotive four-layer paint system 9 is applied, which is subsequently subjected to a baking process 35.

The Indian Fig. 4 shown body painting process is performed with pre-coated light metal component 3. That is, the light metal component 3 is electrically insulated, so that the applied in the bodyshell painting process KTL layer no longer adheres, whereas the conventional automotive paint system 9 (ie, a four-layer structure) can be readily applied to the powder coating 7 of the light metal component 3.

In the Fig. 5 to 7 is in views according to the Fig. 1 the light metal component 3 shown in different process steps. So is in the Fig. 5 the light metal component 3 with cleaned and exposed metallic surface 25 shown. In the Fig. 6 the light metal component 3 is shown after passivation and outsourcing. Accordingly, the conversion layer 5 is applied to the metallic surface 25 of the light metal component, namely with the Schollenmorphologie invention, that is, with lumpy individual fragments 11 and intermediate cracks 13. In the Fig. 7 is this Light metal component 3 shown after completed light metal KTL process, in which the starting component of the light metal KTL layer 6 soaks through the crack structure 13 of the conversion layer 5, whereby the adhesive bond between the conversion layer 5 and the light metal KTL layer 6 is substantially increased.

Claims (12)

1. Method for passivating a metallic surface (25) of a light metal component (3), in which a conversion layer (5) is applied to the surface (25) of the light metal component (3) in a passivation step (P), wherein the calcium phosphate-containing conversion layer is produced on the metallic component surface (25) in the passivation step (P) using an aqueous passivation solution, which layer comprises oxides and hydroxides of the component material and the passivation solution and contains amino acids, wherein the conversion layer (5) of the component (3) is covered with at least one layer (6, 7, 9) in a subsequent coating process (L), wherein the pH of the passivation solution is in a neutral to acidic range, characterised in that the coating process (L) comprises a first coating step (I) in which a light metal CDC (Cathodic Dip Coating) layer (6), i.e. an organic protective layer, is formed in a dipping process under applied DC voltage, whereby the coating particles dissolved in the immersion bath are attracted by the metallic component (3) and adhere to it with the formation of the light metal CDC layer (6).
2. Method according to claim 1, characterised in that at least the metallic surface (25) of the component (3) is formed by a light metal, in particular magnesium, aluminium or alloys thereof.
3. Method according to claim 1 or 2, characterised in that the passivation solution has at least the following constituents as activators for activating the metal surface (25) of the component (3): NaCl with a concentration between 5000 and 8000, in particular 6400 mg/l; and/or KCl with a concentration between 300 and 500, in particular 400 mg/l.
4. Method according to claim 1, 2 or 3, characterised in that the passivation solution has at least the following amino acid as catalysts and layer formers: D-Ca-pantothenate with a concentration between 2 and 20, in particular 4 mg/l.
5. Method according to any one of the preceding claims, characterised in that the passivation solution has at least the following amino acid as a layering agent: L-isoleucine with a concentration between 90 and 150, in particular 105 mg/l.
6. Method according to any one of the preceding claims, characterised in that the passivation solution for aiding the layer formation contains at least the following constituents which are integrated into the conversion layer as fragments (that is, Ca²⁺ or PO₄ ^3-): NaH₂PO₄ with a concentration between 100 and 170, in particular 124 mg/l, and/or CaCl₂ with a concentration between 170 and 300, in particular 200 mg/l.
7. Method according to any one of the preceding claims, characterised in that the conversion layer (5) has carbonate-containing constituents in order to promote the formation of layers, and in that in particular for the preparation of the carbonate-containing constituents, the passivation solution contains NaHCO₃, in particular having a concentration between 3500 and 4500, in particular 3700 mg/l.
8. Method according to any one of the preceding claims, characterised in that the passivation solution contains Na pyruvate to aid the layer formation, and in that its concentration is between 90-170 mg/l, especially 110 mg/l.
9. Method according to any one of the preceding claims, characterised in that the aqueous passivation solution contains at least the following constituents, the concentrations of which are modelled after their concentrations in human blood: NaCl with in particular 6400 mg/l KCl with in particular 400 mg/l NaH₂PO₄ with in particular 124 mg/l CaCl₂ with in particular 200 mg/l NaHCO₃ 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. Method according to any one of the preceding claims, characterised in that the passivation solution for increasing the coating behaviour contains at least one or more, in particular all, of the following constituents: L-arginine•HCl with 50 to 120, in particular 84 mg/l L-cystine with 30 to 80, in particular 48 mg/l L-histidine•HCl•H₂O with 25 to 65, in particular 42 mg/l L-leucine with 70 to 140, in particular 105 mg/l L-lysine•HCl with 110 to 170, in particular 146 mg/l L-methionine with 20 to 50, in particular 30 mg/l L-phenylalanine with 40 to 80, in particular 66 mg/l L-threonine with 60 to 120, in particular 95 mg/l L-tryptophan with 13 to 20, in particular 16 mg/l L-tyrosine with 40 to 90, in particular 72 mg/l L-valine with 60 to 120, in particular 94 mg/l L-serine with 20 to 60, in particular 42 mg/l Choline chloride with 2 to 10, in particular 4 mg/l Folic acid with 2 to 10, in particular 4 mg/l Nicotinamide with 2 to 10, in particular 4 mg/l Pyridoxal•HCl with 2 to 10, in particular 4 mg/l Riboflavin with 0.2 to 1, in particular 0.4 mg/l Thiamine HCl with 2 to 10, in particular 4 mg/l
11. Method according to any one of the preceding claims, characterised in that the conversion layer (5) is formed with a floe-like layer morphology (11) having crack structures (13), and that in particular the layer morphology (11) in the first coating step (I) ensures a sufficient electrical residual conductivity between the immersion bath and the component material and/or an adhesive bond between the conversion layer (5) and the light metal CDC layer (6) increases by the liquid starting component penetrating the light metal CDC layer (6).
12. Method according to any one of the preceding claims, characterised in that the coating process (L) has at least one further coating step (II), in which at least one layer (7) is applied in a powder coating process with applied DC voltage.

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