WO2009098326A1

WO2009098326A1 - Hexavalent-chromium-free anodizing with conductive polymers and nanoparticles

Info

Publication number: WO2009098326A1
Application number: PCT/ES2008/000056
Authority: WO
Inventors: Francisco J. Cano Iranzo; Oihana Zubillaga Alcorta; Gorka IMBULUZQUETA GARCÍA; Iñaki AZCARATE PEÑA
Original assignee: Fundacion Inasmet
Priority date: 2008-02-04
Filing date: 2008-02-04
Publication date: 2009-08-13

Abstract

The present invention describes a method for forming a coating on an aluminium surface that comprises: (i) placing the aluminium surface in contact with an anodizing composition; (ii) applying an electrical potential between the aluminium surface that acts as anode and a cathode; and (iii) producing a coating on the anode that comprises aluminium oxide, a conductive polymer and metal oxide nanoparticles, wherein the anodizing composition is an aqueous, acid composition that comprises a conductive polymer precursor monomer and metal oxide nanoparticles.

Description

HEXAVALENT CHROME-FREE ANODIZED WITH DRIVING POLYMERS AND NANOPARTICLES

FIELD OF THE INVENTION The present invention relates to a coating of an aluminum surface, in particular, a corrosion resistant coating, comprising aluminum oxide, metal oxide nanoparticles and a conductive polymer. The invention also relates to a new anodizing process for providing said corrosion resistant coating, as well as an anodizing composition for obtaining said coating.

BACKGROUND OF THE INVENTION

Anodizing is a treatment used to protect aluminum and its alloys against corrosion. Said coating also improves the adhesion of the organic coatings in the event that these are subsequently applied. Similarly, the anodizing layers can be used in various applications to increase wear resistance, abrasion or simply for decorative purposes. The anodizing process can be carried out in a wide variety of electrolytes and different process conditions (time, temperature, current density), imparting to the surface of the alloy the properties required for use in service.

The main anodizing processes are carried out in solutions of sulfuric acid and chromic acid. Other less used anodizing processes, for more specific purposes, are carried out in solutions of acids such as oxalic, phosphoric, boric acid, or others. The anodizing layer obtained by these processes is composed of a first oxide barrier layer in contact with the metal, and a second microporous oxide layer. This layer is usually sealed to close the pores and thus improve the resistance to

The corrosion and abrasion. In those applications with high requirements for corrosion resistance, anodizing and / or subsequent sealing are carried out using chromic acid. One of the main characteristics of coatings containing hexavalent chromium (Cr ^) is the self-repair capacity that is believed to be related to the migration of hexavalent chromium ions to defects created in the layer, inhibiting corrosion in said defects. However, hexavalent chromium is highly toxic and polluting, so its use requires extreme precautions. There is also the possibility that Cr ^ will be subsequently released from the treated products. Therefore, there is currently interest in the state of the art of developing alternative processes to anodizing and / or chromic sealing.

US 6,916,414 discloses an anodizing process for forming surface protective coatings comprising the use of anodizing solutions that may contain phosphate, permanganate, silicate, zirconate, vanadate, titanate, alkali metal fluorides and / or fluoride complexes. US Patent 5,374,347 describes sealed corrosion resistant coatings obtained by immersion of an anodized aluminum substrate in an aqueous solution of trivalent chromium. The corrosion properties of the coating thus obtained can be improved by subjecting the coating obtained to a subsequent treatment with an oxidizing agent such as a peroxide. This procedure has the disadvantage that it introduces a small amount of Cr ^(VI) into the coating.

The deposition of nanoparticles on previously anodized aluminum substrates is also collected in the state of the art. Thus, Kamada et al. they work in the deposition by electrophoresis of nanoparticles of SiO ₂ on previously anodized aluminum [(1) K. Kamada et al. lnsertion of SiO ₂ Nanoparticles into Pores of Anodized

Aluminum by Electrophretic Deposition in Aqueous System. Electrochemical Society and Solid State Letters. 2004, Vol. 7, No.8, pp. B25-B28. ISSN 0013-4651.)]. The same author describes a procedure in which the anodic oxidation and the injection into the anodizing layer of oxide nanoparticles by electrophoresis [(2) K. Kamada et al. Incorporation of oxide nanoparticles into barrier-type alumina film via anodic oxidation combined with electrophoretic deposition. Journal of Materials Chemistry. 2005, VoI 15, pp. 33883394.)]. The electrolytic bath is a solution of ammonium pentaborate with 0.05% oxide nanoparticles (SiO ₂ , TiO ₂ , SnO doped with Sb). On the other hand, Yu-Guo Guo et al. they start from an anodized aluminum oxide membrane in which acrylamide electropolymerize and then introduce Au and Pt nanoparticles [(3) Yu-Guo Guo et al. Highly Dispersed Metal Nanoparticles in Porous Anodic Alumina Films Prepared by a Breathing Process of Polyacrylamide Hydrogel. Chemical Materials 2003, VoI 15, pp. 4332-4336.). Therefore, the deposition of nanoparticles on previously anodized aluminum substrates, both in successive phases [(1), (3)] and simultaneously (2), is known from the state of the art.

On the other hand, it is known to use conductive polymers, such as polyaniline or polypyrrole, to protect metals against corrosion.

Said polymers can be used in the form of a continuous coating as described in US 6,762,238, or as corrosion inhibiting pigments (US 6,756,123; US 6,190,780).

US 6,328,874 describes an aluminum anodizing process simultaneous to the synthesis of a conductive polymer from the corresponding monomer on aluminum, giving rise to a coating composed of aluminum oxide and conductive polymer. The coating is then sealed in boiling water. Similarly, US Patent 5,980,723 describes a process in which aluminum oxide and the conductive polymer are formed at the same time. Sealing is done later depositing then another layer composed of conductive polymer.

Therefore there are numerous procedures in the state of the art to provide protective aluminum coatings. However, there is still a considerable need to develop alternative anodizing procedures for coating aluminum alloys that, avoiding the use of chromates, provide effective protective coatings against corrosion.

In this sense, the inventors of the present invention have discovered an anodizing process that provides an aluminum protective coating against corrosion, in which the anodizing and sealing are carried out in a single stage.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1: shows the results of an in-depth analysis of X-ray Photoelectron Spectrometry (XPS) of the external part of an anodized coating with polyaniline and TΪO2 nanoparticles on 3105 aluminum alloy; in the axis "y" the atomic percentage (%) of the elements C, N, O, Al, and Ti is shown and in "x" the distance in depth from the external surface in nm.

Figure 2: shows an image obtained by SEM-EDS electron microscope showing in section the anodizing coating with polyaniline and TΪO ₂ nanoparticles on 3105 aluminum alloy obtained according to Example 1, and where:

1. aluminum alloy substrate

2. aluminum oxide 3. polymer and nanoparticles Figure 3: shows an image of a test tube of the 3105 aluminum alloy with an anodized-polyaniline coating, with titanium oxide nanoparticles obtained according to Example 1, after 1000 hours of exposure in a neutral salt spray chamber.

Figure 4: shows the results of an XPS in-depth analysis of the external part of an anodized coating with polyaniline and ZrÜ ₂ nanoparticles on 2024T3 aluminum alloy; in the axis "y" the atomic percentage (%) of the elements C, N, O, Al, and Zr is shown and in "x" the distance in depth from the external surface in nm.

OBJECT OF THE INVENTION

An object of the present invention relates to a process for forming a coating on an aluminum surface comprising:

(i) contacting the aluminum surface with an anodizing composition;

(ii) apply an electric potential between the aluminum surface that acts as an anode and a cathode; and (iii) obtaining a coating on the anode comprising aluminum oxide, a conductive polymer and nanoparticles of a metal oxide,

where the anodizing composition comprises:

- Water;

- a precursor monomer of a conductive polymer;

- metal oxide nanoparticles; Y

- an acid.

Another object of the invention relates to the coating obtained by said method on the aluminum surface comprising oxide of aluminum, nanoparticles of a metal oxide and a conductive polymer.

Another additional object of the invention relates to an anodizing composition comprising water, a precursor monomer of a conductive polymer, nanoparticles of a metal oxide and an acid.

Another additional object of the invention relates to an aluminum surface coated with the coating of the invention.

Finally, one more object of the invention refers to an object, which comprises at least one aluminum surface, which comprises the coating of the present invention.

DESCRIPTION OF THE INVENTION The present invention relates in a first aspect to a method of anodizing an aluminum surface that confers a protective coating against corrosion. Said anodizing process, hereinafter the method of the invention, comprises the following steps: (i) contacting the aluminum surface with an anodizing composition;

(ii) apply an electric potential between the aluminum surface that acts as an anode and a cathode; Y

(iii) obtaining a coating on the anode comprising aluminum oxide, a conductive polymer and nanoparticles of a metal oxide,

where the anodizing composition comprises:

- Water;

- a precursor monomer of a conductive polymer; - nanoparticles of a metal oxide; Y

- an acid. The term "composition" in the present description refers to an aqueous solution in which each component thereof can be completely or partially dissolved or dispersed.

The process of the invention is performed on aluminum. Within the context of the invention, aluminum is understood as pure aluminum and its alloys. In principle, all alloys are treatable by the method of the invention. The aluminum surface can be a surface of any piece, and present any shape and dimensions, such as a test tube, or an iron, panel etc.

The process of the invention comprises contacting the aluminum surface with an anodizing composition described below. Said surface is cleaned and preconditioned according to any conventional method for aluminum and its alloys. The anodizing composition is placed in a suitable container and equipped with at least one entrance for the cathode and one entrance for the anode. According to the invention, the aluminum surface to be coated acts as an anode and occupies said position. As a cathode a conventional electrical conductive material is used. Once the anode and the cathode are in contact with the anodizing composition, an electrical potential is applied by means of a power supply of electricity typically between 0.5 and 50 V for a time generally between 1 and 350 min. The temperature at which the process takes place can be between 5 and 5O ⁰ C. The anodizing composition is stirred during the process of the invention.

Through the passage of electric current, the oxidation of aluminum is achieved,

The electrosynthesis of the conductive polymer and the deposition of nanoparticles at the same time on the anode, which are incorporated in the aluminum oxide-polymer coating that is being formed. You get from this an anodizing mode sealed by the conductive polymer and the nanoparticles in a single stage. The nanoparticles have a negative charge so that under the influence of the current they are directed towards the anode on which they are deposited.

The anodizing composition that is used to carry out the process of the invention constitutes an additional object of the invention, and comprises:

- Water;

- an acid.

The precursor monomer of a conductive polymer can be any monomer conventionally used to prepare a conductive polymer as any of those described in US Patent 6,328,874 B1. It should be noted only for illustrative purposes, among others, substituted or unsubstituted monomers of, aniline, aromatic heterocycles such as pyrrole, thiophene etc. A conductive polymer can be a homopolymer or a conductive copolymer, such as polyaniline, polypyrrole and polythiophene. The substituted monomers useful for the present invention are those polymerizable capable of producing an intrinsically conductive polymer. In general, a substituted monomer can be, among others, a monomer with one or more of the following substituents such as a hydrocarbon chain, a carboxyl group, carbonyl group, amino group, hydroxyl group and mixtures thereof, said substituents can be said or attached heteroatom like any of the different carbons in the monomer ring. In a particular embodiment, the precursor monomer is aniline. The monomer concentration is typically between 0.05 and 5 M.

In addition, the anodizing composition comprises an acid. With personality For illustrative purposes, oxalic acid, sulfuric acid, citric acid, hydrochloric acid, perchloric acid, nitric acid, sulfonic acids and their mixtures are among others. According to a particular embodiment the acid is oxalic acid, and according to another particular embodiment it is sulfuric acid. The acid concentration is generally between

0.01 and 5 M. The anodizing composition has a pH between 0.5 and 5.

The anodizing composition also comprises nanoparticles of a metal oxide. The metal oxide suitable for carrying out the process of

The invention can be selected from a wide group of metal oxides, among which we can mention, titanium oxide, zirconium oxide, aluminum oxide, silicon oxide and mixtures thereof. According to a particular embodiment, the nanoparticles are made of zirconium oxide. In another particular embodiment the nanoparticles are made of titanium oxide. The concentration of nanoparticles is generally between 0.1 and

100 grams / liter of anodizing composition.

The process of the invention has advantages over other procedures of the prior art. For example, the use of hexavalent chromium with the associated toxicological and environmental problems inherent is avoided, and nevertheless an effective coating with high protective properties against corrosion is obtained. In addition, the coating process is carried out in a single stage, unlike other prior art procedures, without the need for subsequent sealing.

In another aspect, the invention provides a coating, obtainable according to the process of the invention, on an aluminum surface comprising aluminum oxide, nanoparticles of a metal oxide and a conductive polymer. The coating of the invention is characterized in that the The presence of nanoparticles and conductive polymer is greater in the part furthest from the aluminum surface, and they act as a sealant of the microporous structure of the aluminum oxide.

The depth composition (expressed in% of each of the elements) of two examples of coating is represented in Figure 1 and Figure 4. Figure 1 (corresponding to example 1) shows an XPS spectrum (Photoelectron Spectrometry of X-rays; X-ray photoelectron spectroscopy) of the external part of said anodizing layer, and reveals a greater presence of titanium oxide (O, Ti) and polymer (C, N) near the external surface of the coating, which is It combines at lower levels closer to the aluminum surface, with the aluminum oxide. Figure 4 (corresponding to example 2) shows the composition of the outer part of a layer containing zirconium oxide (O, Zr) instead of titanium oxide, the presence of this oxide and the polymer (C,

N) also greater in the outer part of the layer, to be combined with the aluminum oxide as it approaches the aluminum surface.

The thickness of the anodizing coating layer is in the range of 0.5-50 microns, and that of the external part thereof, composed of polymer and nanoparticles in the range of 20-500 nm. In the coating the nanoparticle / polymer ratio is between 1 to 10, while in the external part thereof, the nanoparticle / polymer ratio can be between 0.1-100. That the nanoparticles and the polymer are concentrated in the outermost part of the coating does not rule out their presence throughout the entire section of the layer.

Figure 2 shows an SEM-EDS (Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy) image in section of a coating obtained according to Example 1 for the 3105 aluminum alloy in an anodizing composition of sulfuric acid, aniline and titanium oxide . In another additional aspect, the invention provides an aluminum surface comprising the coating of the present invention. As mentioned above the surface can have any size and dimensions, and be a surface of any article of any shape and size. By way of illustration, said aluminum surface is a test piece, an iron, a panel, among others.

The corrosion protection of aluminum, obtained with a coating of the invention, is illustrated in Figure 3 (corresponding to example 1), where an image of an aluminum alloy 3105 with an anodized-polyaniline nanoparticles coating after 1000 is shown. hours of exposure in neutral salt spray chamber. These tests were carried out in accordance with ASTM B-117. As can be seen from this image, good corrosion protection is observed with the coating that incorporates the nanoparticles, with no presence of white corrosion products or pitting.

Finally, in another aspect the invention provides any object, comprising at least one aluminum surface, comprising the coating of the present invention.

Below are illustrative examples of the invention that are set forth for a better understanding of the invention and in no case should it be considered a limitation of the scope thereof.

EXAMPLES

Example 1

An acid solution was first prepared in a vessel, starting from distilled water and sulfuric acid in a concentration of 0.5 mol / L. To this solution pure aniline was added until a concentration of

0.3mol / L, and stirred until a homogeneous solution. TO Then, zirconium oxide nanoparticles were incorporated into the homogeneous solution, so that their concentration reached 3g / L in the solution.

A test tube of aluminum alloy 3105 was introduced into said dispersion of nanoparticles.

A cathode made of titanium was then incorporated into the vessel and an electric current was established between the aluminum and the cathode, so that the aluminum acted as an anode, imposing a potential difference of 6 volts between the two electrodes for 1 hour.

A coating layer consisting of aluminum oxide, polyaniline and titanium oxide was obtained. The composition of the layer obtained is shown in Figure 1. This figure shows that the layer of TIO ₂ and polyaniline has a thickness of 25 nm, with a TiO ₂ / polymer ratio of 1.

A sectional image of the complete coating obtained in the conditions is shown in Figure 2. In this figure it is observed that the entire layer has a thickness of 4 microns.

The protection conferred to corrosion by this layer can be seen in Figure 3. In this figure it can be observed that there is no presence of corrosion, no white products of corrosion or pitting being detected.

Example 2

A solution was prepared in a vessel starting from distilled water and adding oxalic acid to a concentration of 0.1 mol / L. To this solution pure aniline was added to a concentration of 0.1mol / L, and stirred until a homogeneous solution was obtained. Then, zirconium oxide nanoparticles were incorporated into the solution until a concentration of 1g / L in the solution of acid and aniline, stirring to keep them dispersed.

In this dispersion, an aluminum test tube of the 2024T3 alloy was introduced.

A titanium cathode was then incorporated into the vessel, and a potential difference of 1OV between the cathode and the aluminum specimen, which functioned as an anode, was imposed by an electrical power source for 1 hour.

A coating layer consisting of aluminum oxide, polyaniline and zirconium oxide was obtained. The composition of the external part of the layer obtained is shown in Figure 4. In this figure it can be seen that the layer of ZrO ₂ and polyaniline has a thickness of 60 nm, with a ZKV polymer ratio of 5.

Claims

1. Method for forming a coating on an aluminum surface comprising: (i) contacting the aluminum surface with an anodizing composition;

(ii) apply an electric potential between the aluminum surface that acts as an anode, and a cathode; Y

(iii) obtaining a coating on the anode comprising aluminum oxide, a conductive polymer and metal oxide nanoparticles,

where the anodizing composition comprises:

- Water;

- an acid.

2. Method according to claim 1, wherein the aluminum surface is selected from pure aluminum or an aluminum alloy.

3. Method according to any of claims 1 or 2, wherein the applied electrical potential is between 0.5 and 50 V.

4. Method according to any of claims 1 to 3, wherein the electric potential is applied for a time between 1 and 350 min.

5. Method according to any of claims 1 to 4, which is carried out at a temperature between 5 and 5O ⁰ C.

6. Anodizing composition comprising: - Water;

- a precursor monomer of a conductive polymer;

- nanoparticles of a metal oxide;

- an acid.

7. Composition according to claim 6, wherein the precursor monomer of a conductive polymer is selected from aniline, pyrrole and thiophene, unsubstituted and substituted.

8. Composition according to claim 7, wherein the precursor monomer is aniline.

9. Composition according to any of claims 6 to 8, wherein the concentration of monomer is between 0.05 and 5 M.

10. Composition according to any of claims 6 to 9, wherein the acid is selected from oxalic acid, sulfuric acid, citric acid, hydrochloric acid, perchloric acid, nitric acid, sulfonic acids and mixtures thereof.

11. Composition according to claim 10, wherein the acid is oxalic acid.

12. Composition according to claim 10, wherein the acid is sulfuric acid.

13. Composition according to any of claims 6 to 12, wherein the acid concentration is between 0.01 and 5 M.

14. Composition according to any of claims 6 to 13, which has a pH between 0.5 and 5.

15. Composition according to any of claims 6 to 14, wherein the nanoparticles are of a metal oxide selected from titanium oxide, zirconium oxide, aluminum oxide, silicon oxide and mixtures thereof.

16. Composition according to claim 15, wherein the nanoparticles are made of zirconium oxide.

17. Composition according to claim 15, wherein the nanoparticles are made of titanium oxide.

18. Composition according to any of claims 6 to 17, wherein the nanoparticles are in a concentration between 0.1 and 100 grams / liter of anodizing composition.

19. Coating obtainable according to the method of any one of claims 1 to 5, on an aluminum surface comprising aluminum oxide, nanoparticles of a metal oxide and a conductive polymer.

20. Coating according to claim 19, which has a thickness between 0.5-50 microns.

21. Coating according to any of claims 19 or 20, which has a thickness of the outer part of the coating between 20 and 500 nm.

22. Coating according to any of claims 19 to 21, wherein the external part thereof has a higher concentration of nanoparticles and polymer than the internal part thereof.

23. Coating according to any of claims 19 to 22, in the that the nanoparticle / polymer ratio is between 1 to 10.

24. An aluminum surface comprising a coating according to any of claims 19 to 23.

25. An object comprising at least one aluminum surface according to claim 24.