WO2019115301A1

WO2019115301A1 - Anti-corrosion coating for metallic substrates

Info

Publication number: WO2019115301A1
Application number: PCT/EP2018/083560
Authority: WO
Inventors: Michael Grabowski
Original assignee: Audi Ag
Priority date: 2017-12-11
Filing date: 2018-12-05
Publication date: 2019-06-20
Also published as: DE102017011379A1

Abstract

The invention relates to amorphous anti-corrosion coatings based on calcium phosphate for metallic substrates, to aqueous passivation solutions, and to a method for producing coated molded bodies.

Description

Anti-corrosion coating for metallic substrates

DESCRIPTION: The invention relates to amorphous calcium phosphate based anti-corrosion coatings for metallic substrates, aqueous passivating solutions and a process for producing coated moldings.

The use of metal components, in particular of light metal components, is common practice across industries. Thus, vehicle bodies are today usually produced in mixed construction, for example, from light metal and sheet steel parts to achieve a weight reduction. As light metal, e.g. Aluminum or magnesium materials or alloys thereof used.

In vehicle bodies manufactured in mixed construction, there is a risk of contact corrosion between the light metal components (of magnesium or aluminum) and the steel components. Especially paint undercuts and filiform corrosion are common damage patterns. To protect the light metal components, washing / degreasing, pickling, passivation and the application of an organic coating (for example cathodic dip coating) are common practice. However, the commercial coating systems only partially provide the necessary protection against corrosion and / or paint infiltration. Especially when steel and magnesium come into contact, the high potential difference promotes corrosion.

The commercially available coating systems for light metal components do not exhibit sufficient passivating behavior, which means that the corrosion rate increases exponentially with increasing anodic Overvoltage increases. If magnesium alloys are electrochemically polarized by contact with more noble metals (eg aluminum, steel) (= high anodic overvoltage compared to the magnesium material), an exponential increase of the corrosion current or the corrosion rate (galvanic corrosion) occurs.

Various methods have already been proposed for passivating the metallic surface of a light metal component:

From DE 196 30 289 C2, a method for painting vehicle bodies is known, wherein the body shell is subjected to a pre-treatment before the actual painting process. Here, the cleaned vehicle bodies are coated in a phosphating bath with a zinc phosphate layer, which serves as corrosion protection and as a primer for the subsequent primer.

WO 2015/015524 A1 discloses a method for surface treatment of a surface of a component whose material is a magnesium alloy. The process can produce a nickel coating of uniform thickness.

DE 37 15 663 A1 discloses an anodizing solution for an anodic oxidation of magnesium or magnesium alloys.

JP 2009-228087 A discloses a method for coating a light metal component made from a magnesium alloy.

From DE 10 2014 005 444 A1, a method for passivating a surface of a metal component is known. In the process, a calcium phosphate-containing conversion layer is produced in a passivation step using an aqueous passivating solution on the surface of the metal component. The conversion layer comprises oxides and Hydroxides and carbonates of the component material and the cations contained in the passivation solution.

A conversion layer is produced which has a sheet-like layer morphology with crack structures. This leads to a reduction of the protective effect of the conversion layer. In addition, the passivation solution has a very complex composition and contains many different amino acids, vitamins and inorganic substances.

DE 10 2016 002 852 A1 relates to a method for passivating a surface of a metal component, in which a calcium phosphate-containing conversion layer is produced in a passivation step using an aqueous passivation solution on the surface, which has oxides and hydroxides and carbonates of the component material and the passivation solution , The passivation solution for the formation of the conversion layer is less complex than that of DE 10 2014 005 444 A1 and contains at least one substance acting as a chelate complexing agent.

The object of the invention is to provide an aqueous passivating solution and a method for passivating a metallic surface of a light metal component, in which good passivation, in particular in the case of aluminum or magnesium, is achieved by a well adhering, dense and crack-free conversion layer. Another object of the invention is to provide moldings with a correspondingly passivated surface.

According to the invention the object is achieved by a passivation solution having the features of claim 1, a method having the features of claim 4 and moldings having the features of claims 8 and 9. Embodiments emerge from the dependent claims.

The invention relates to an aqueous passivating solution for forming a calcium phosphate-containing conversion layer on a surface a metal component, wherein at least the surface of the metal component is formed by a light metal.

The passivation solution according to the invention contains

a) 0.4 to 5 mmol / l calcium cations;

b) 0 to 5 mmol / l magnesium cations;

c) 50 to 200 mmol / l chloride anions;

d) 0.2 to 5 mmol / l of phosphate anions;

e) 20 to 80 mmol / l carbonate anions;

f) 0 to 5 mmol / l sulfate anions; and

g) 5 to 30 mmol / l of at least one chelate complexing agent.

In one embodiment, the passivation solution a) according to the invention comprises 1, 2 to 2.4 mmol / l of calcium cations;

b) 0.5 to 1.1 mmol / l magnesium cations;

c) 80 to 155 mmol / l chloride anions;

d) 0.7 to 1.3 mmol / l of phosphate anions;

e) 30 to 60 mmol / l carbonate anions;

f) 0.5 to 1, 1 mmol / l sulfate anions; and

g) 5 to 15 mmol / l of at least one chelate complexing agent.

Unlike in the solutions disclosed in DE 10 2014 005 444 A1 or DE 10 2016 002 852 A1, in the passivation solution according to the invention the concentration of sodium ions is outside a range of 128 to 192 mmol / l; and / or the concentration of potassium ions is outside a range of 4 to 7 mmol / l.

In one embodiment, the concentration of sodium ions is less than 128 mmol / l. In a further embodiment, the passivation solution according to the invention is substantially free of sodium ions. In one embodiment, the concentration of potassium ions is less than 4 mmol / l. In another embodiment, the passivation solution according to the invention is substantially free of potassium ions. In a further embodiment, the passivation solution according to the invention contains neither Sodium or potassium ions in a substantial amount, ie in the passivation solution according to the invention, sodium and potassium ions are at most present in traces as an impurity.

This is based on the finding that sodium and potassium cations are irrelevant to the effectiveness of the coating process, and therefore are dispensable or can be replaced by other cations. However, sodium ions in a concentration of more than 192 mmol / l may also be present in the passivation solution according to the invention; and / or potassium ions in a concentration of more than 7 mmol / l, without this having an adverse effect. According to the invention, the composition of the passivation solution is defined by specifying the molar concentrations of the ions actually actively involved in the coating process.

One of the advantages of the passivation solution according to the invention is that anions (eg chloride) relevant for the coating process can be introduced into the coating electrolyte via a multiplicity of salts (eg NaCl, CaCl 2, KCl, SrCl 2, LiCl, etc.) or else via the corresponding acid ( eg HCl). In the event that certain cations (eg Na ⁺ , K ⁺ , Li ⁺ ) have no effect on the coating process, the use of an anion source with these same cations can be chosen variably, or the anions can be used over several salts (eg NaCl, KCl, CaCh) are introduced into the electrolyte. The corresponding ions can be transferred into the electrolyte in a variable manner (ie variable selection of the starting substances). Thus, it is irrelevant, for example, which chloride, carbonate or phosphate source is used. This results in a much greater flexibility in the preparation of the passivation solution according to the invention and in its composition in comparison with the passivation solutions of the prior art.

The aqueous passivation solution according to the invention contains 5 to 30 mmol / l, for example 5 to 15 mmol / l, of at least one chelate complexing agent. The chelate complexing agent can temporarily bind free ions (eg Ca ²⁺ ) and transport them to the metal surface, where this ion then enters the coating is installed. The complexing agent has a predominantly catalytic effect, ie it is not consumed itself. However, it can happen that individual molecules are "overgrown" by the layer and thus find themselves isolated in the coating.

As the chelating agent, any suitable substances can be used. In one embodiment, the chelating agent is an organic substance having optionally deprotonated functional groups with one or more lone pairs of electrons, for example OH end groups, COOH end groups, Nhh end groups or the like.

In one embodiment, the chelating agent is an organic acid or its salt. Examples include acetic acid, tartaric acid, citric acid, phytic acid, oxalic acid, EDTA, amino acids and vitamins. In a specific embodiment of the passivation solution, the chelating agent is an amino acid, for example L-arginine, L-glutamine or glutamic acid.

Examples of other chelating agents in addition to carboxylic acids are alcohols, phenols, naphthols, enols, thiols, sulfonic acids, phosphoric acid esters and phosphonic acids.

The invention also provides a method for passivating a surface of a metal component, wherein at least the surface of the metal component is formed by a light metal, in which a calcium phosphate-containing conversion layer is produced in a passivation step using an aqueous passivation solution according to the invention on the surface, the oxides and hydroxides and carbonates of the component material and constituents of the passivation solution.

At least the surface of the metal component used in the method is formed by a light metal. Suitable light metals include in particular magnesium, aluminum and alloys thereof. The passivation step with the passivation solution according to the invention is carried out in a manner known to the person skilled in the art, for example by immersing the metal component in the passivation solution according to the invention or spraying the metal component with the passivation solution according to the invention.

In one embodiment, the passivation process is carried out at acidic to neutral pH. In a specific embodiment of the method, the pH of the passivation solution is in the range from 1 to 7, in particular 1 to 5. The passivation reaction according to the invention proceeds relatively slowly at a pH of about 7. It can be accelerated by reducing the pH and / or by polarization and / or increasing the partial pressure of CO2.

In one embodiment of the method, the temperature in the passivation step is from 15 to 80 ° C, for example 37 ° to 65 ° C. The coating process can be accelerated by increasing the temperature.

In one embodiment of the method, the reaction time in the passivation step is from 5 to 15 minutes. At longer exposure time of the passivation solution thicker conversion layers are obtained.

By means of the passivation solution according to the invention, a conversion layer is produced which effects a reduction of the occurring contact corrosion currents upon contact of the passivated component with, for example, aluminum, steel, zinc, carbon fibers or CFRP. Furthermore, there is an increase in the penetration resistance of the passivated metal surface. The higher the penetration resistance of the conversion layer, the lower the resulting corrosion currents or the corrosion rate. In addition, the passive behavior of the conversion layer is increased by the closed, predominantly crack-free layer structure of the conversion layer. The passivation solution according to the invention and the process according to the invention lead to a special layer composition not yet described. The conversion layer is amorphous, while the calcium phosphates described in the literature from the medical and implant technology have a crystalline structure. In the case of amorphous materials, there is a "near order" of neighboring atoms, but unlike crystalline materials, there is no long-range order between the atoms. The amorphous nature of a material can be easily detected by X-ray diffraction (XRD) or LEED (Low-Energy Electron Diffraction). The coating has a compact morphology, which produces the good surface protection against corrosion.

In one embodiment, the composition of the conversion layer has the formula (Mg) x (Al) _y (Ca) z (PO 4) u (CO 3) v (0H) _w . The conversion layer thus contains magnesium, aluminum and calcium cations, as well as phosphate, carbonate and hydroxide anions. In one embodiment, the z / μ ratio for coatings on Mg alloys is 0.5 to 1.8. In another embodiment, the ratio z / u in coatings on Al alloys is 0.01 to 0.5.

The invention also provides moldings or metal components which have on their surface a conversion layer or passivation layer which can be produced by the process according to the invention or by using the passivation solution according to the invention.

One embodiment is a magnesium alloy shaped body having on its surface a conversion layer which

a) 1 to 10 mol% of magnesium cations;

b) 0 to 8 mol% of aluminum cations;

c) 4 to 18 mol% calcium cations;

d) 3 to 11 mol% of phosphate anions;

e) 1, 5 to 9 mol% of carbonate anions;

f) contains 2 to 15 mol% of oxygen and / or hydroxide anions. The mole fraction of the individual chemical constituents is determined by X-ray photoelectron spectroscopy (XPS analysis). The very surface-sensitive measuring method XPS determines the composition of the uppermost ~ 10 nm of the surface layer of the shaped body.

The missing in each case to 100 mol% portions of the surface layer go back to organic carbon on the surface (C1 s). The C1 s portion of the XPS spectrum is usually not explicitly stated in the listing.

With this analysis method, the big advantage is that it is also possible to analyze binding energy states or the electron configuration. In contrast to the EDX analysis, it is therefore possible to differentiate between impurities (CC bonds) and carbonates (CO3 ² ) in the case of carbon, for example. With XPS, a distinction can therefore be made quantitatively between certain binding energy states. This should be clarified using the example Mg2p. Mg2p stands for the 2p orbital of the Mg atom. The binding energies of the Mg2p orbital are sensitive to the chemical nature of the Mg compound in a compound. Depending on the binding partner, there may be a slight shift in the binding energies in the Mg2p orbital, which allow conclusions to be drawn about the binding partner. It can therefore be determined by measuring whether the Mg ^{2+ is present} as MgCO 3, MgO, Mg (OH) 2 or possibly even in metallic form. As a result, the exact composition or the chemical components involved can be determined very accurately and without errors. It does not have to be concluded indirectly from atomic concentrations on possible chemical components, as for example in the EDX analysis method.

In a further embodiment, the conversion layer contains 0 to 5 mol% of nitrogen, which is derived from NFIx groups from the occluded or overgrown organic chelating agent. If a nitrogen-containing chelate complexing agent is used, for example an amino acid, the nitrogen content of the layer allows a conclusion to be drawn regarding the amount of incorporated chelate complexing agent. In one embodiment, the average layer thickness of the conversion layer or passivation layer is 0.2 to 5 miti. In a further embodiment, the layer has a density of 2 to 3 g / cm ³ .

The magnesium alloy component has a compact coating containing calcium phosphate and magnesium hydroxide / magnesium oxide as well as carbonates and cations of the base alloy. The conversion layer is well adhering and dense.

Another embodiment is a molded article of an aluminum alloy having on its surface a conversion layer, which

a) 0 to 6 mol% of magnesium cations;

b) from 18 to 35 mole% of aluminum cations;

c) 0.05 to 1.5 mol% calcium cations;

d) 2 to 10 mol% of phosphate anions;

e) 0.3 to 6 mol% of carbonate anions;

f) contains 17 to 31 mol% of oxygen and / or hydroxide anions.

The molar fraction of the individual elements is determined by X-ray photoelectron spectroscopy (XPS analysis). In another embodiment, the conversion layer contains <0.5 mole% nitrogen, which may be derived from the enclosed or overgrown organic chelating agents from NH _X groups. If a nitrogen-containing chelate complexing agent is used, for example an amino acid, the nitrogen content of the layer allows a conclusion to be drawn regarding the amount of incorporated chelate complexing agent.

The missing in each case to 100 mol% portions of the surface layer go back to organic carbon on the surface (C1 s). The C1 s portion of the XPS spectrum is usually not reported. In one embodiment, the average layer thickness of the conversion layer or passivation layer is less than 300 nm. In another embodiment, the layer has a density of 2 to 3 g / cm ³ . The aluminum alloy component has a compact and completely crack-free coating comprising calcium phosphate and aluminum hydroxide / aluminum oxide as well as carbonates and cations of the base alloy. The conversion layer is well adhering and dense. The metal component with the passivated surface can be used in various industries, for example in the medical field, aerospace or in the electronics industry. Another example is the automotive sector. The molded article according to the invention or the component according to the invention can be used in a visible manner within the vehicle or as an externally visible outer part. For example, the metal component can be used as an aggregate part, a chassis part, a vehicle-inner-side display frame, or a component of a seat frame structure. In a specific application, the metal component may be a vehicle part, which is first pretreated with the inventive method to form the conversion layer or passivation. The conversion layer of the component can in a following

Coating process are covered with at least one other layer.

The downstream coating process may comprise a first coating step in which a KTL layer (cathodic dip coating = electrophoretic deposition), ie an organic protective layer, is formed. This is done in a dipping process (KTL) under applied DC voltage, whereby the paint particles dissolved in the dipping bath are attracted to the component and are coagulated there to form a water emulsified KTL at the cathode. In a subsequent baking step, the KTL is then crosslinked and hardened. In a further optional subsequent coating step, a powder coating is then applied. This is done in a powder coating process with applied DC voltage and a subsequent burn-in. With regard to a process-reliable coating, the layer morphology of the conversion layer is of particular importance. This ensures, on the one hand, a good adhesion of organic secondary coatings, and, on the other hand, a high corrosion protection effect. In addition, in the KTL coating process and in the powder coating process, sufficient electrical residual conductivity is ensured throughout the conversion layer, which is essential for the application of a coating method based on an electric potential.

Subsequent to the component coating process, the metal component, for example as a visible outer part or as a component integrated in the body structure, can be joined in a joining process to the shell body that has not yet been 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 primer of the body shell in the dipping process. 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 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 is applied, which is also baked in a subsequent drying step at high temperature.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the present invention. The invention will now be illustrated by means of exemplary embodiments and will be further described with reference to the examples. A series of magnesium alloy moldings having an Mg content of 85 to 99.5% or aluminum alloys having an Al content of 85 to 99% were treated with passivating solutions of various compositions according to the present invention to form a conversion layer on the surface of the molded bodies. Among others, various amino acids (for example L-arginine) were used as chelate complexing agents. The pFI value of the passivation solutions was in the range between 1 and 4. The treatment temperature was between 20 ° C and 60 ° C, the treatment time was in the range of 5 to 15 minutes. As alloys AZ31, AM50, AZ91, AE44 and ZXK were used. The layer composition ranges were determined by XPS analysis with a Physical Electronics 5600 analyzer with exciting monochromatized Al-Ka radiation (1486.6 eV, 300 W). The binding energies of the elements to be analyzed were determined with a transmission energy of 23.5 eV, with a resolution of <0.4 eV and an analysis angle of 45 °. The binding energies of all XPS spectra were referenced to the C1s peak (C-C / C-H) at 284.8 eV. The layer properties for the different substrates (Mg and Al alloys) are summarized in Table 1.

layer properties

Table 1: Investigation results of the passivation layers on Mg and

Al alloys The N concentration gives information about the layer contained in the layer

Amino groups (NH2) from the amino acids too. The amino acids have in the

Passivierungslösungen predominantly a catalytic effect and are not consumed itself. However, individual molecules may be different from the layer

"Overgrown" and can thus be found in the coating. This is evidenced by a certain N concentration of the layers found in the XPS analyzes of the coating.

Claims

CLAIMS:

Aqueous passivation solution for forming a calcium phosphate-containing conversion layer on a surface of a metal component, wherein at least the surface of the metal component is formed by a light metal containing

a) 0,

4 to 5 mmol / l calcium cations;

b) 0 to 5 mmol / l magnesium cations;

c) 50 to 200 mmol / l chloride anions;

d) 0.2 to 5 mmol / l of phosphate anions;

e) 20 to 80 mmol / l carbonate anions;

f) 0 to 5 mmol / l sulfate anions;

g) 5 to 30 mmol / l of at least one chelating agent;

with the proviso that the concentration of sodium ions in the solution is less than 128 mmol / l or more than 192 mmol / l and / or the concentration of potassium ions in the solution is less than 4 mmol / l or more than 7 mmol / l ,

An aqueous passivating solution according to claim 1, wherein the chelating agent is an organic substance having optionally deprotonated functional groups with one or more lone pairs of electrons, in particular an organic acid or its salt.

An aqueous passivating solution according to claim 2, wherein the chelating agent is an amino acid, especially L-arginine.

Method for passivating a surface of a metal component, wherein at least the surface of the metal component is formed by a light metal, in which a calcium phosphate-containing conversion layer is produced in a passivation step using an aqueous passivating solution on the surface, the oxides and / or hydroxides and / or carbonates, and cations of the component material and constituents of the passivation solution, characterized in that a passivation solution according to any one of the preceding claims is used.

5. The method of claim 4, wherein the light metal is magnesium, aluminum or an alloy thereof.

6. The method of claim 4 or 5, wherein the pH of the passivating solution is from 1 to 7.

7. The method according to any one of claims 4 to 6, wherein the temperature in the passivation step of 15 to 80 ° C.

8. molded body of a magnesium alloy having on its surface a conversion layer, which

a) 1 to 10 mol% of magnesium cations;

b) 0 to 8 mol% of aluminum cations;

c) 4 to 18 mol% calcium cations;

d) 3 to 11 mol% of phosphate anions;

e) 1, 5 to 9 mol% of carbonate anions;

f) 2 to 15 mol% of oxygen and / or hydroxide anions

contains, determined by XPS analysis, wherein the conversion layer was produced by a method according to any one of claims 4 to 7.

9. molded body made of an aluminum alloy having on its surface a conversion layer, which

a) 0 to 6 mol% of magnesium cations;

b) from 18 to 35 mole% of aluminum cations;

c) 0.05 to 1.5 mol% calcium cations;

d) 2 to 10 mol% of phosphate anions;

e) 0.3 to 6 mol% of carbonate anions;

f) 17 to 31 mol% of oxygen and / or hydroxide anions

10. Shaped body according to claim 8 or 9, wherein the conversion layer has a density of 2 to 3 g / cm ³ .