Anti-corrosion surface multilayer structure for aluminum comprising cerium oxide particles and a method of forming such multilayer structure.

Field of the invention

The present invention relates to the field of multilayer structures for aluminum alloy surfaces and manufacturing methods for such multilayer structures and in particular to such structures comprising cerium oxide particles.

Background

Anodic aluminum oxidation, or anodization of aluminum alloys, hereinafter referred to as anodic aluminum oxidation, is extensively studied and has been an important industrial process for many years. It is used to form decorative layers and protective layers of oxide on an aluminum alloy surface. Aluminum surfaces and aluminum alloy surfaces are always covered by an aluminum oxide layer under typical atmospheric conditions. This native aluminum oxide layer is, however, thin, non-uniform and incoherent. Anodic oxidation is an electrochemical process for producing thicker oxide layers that has improved physical and chemical properties over said native layer. The use of anodic aluminum oxidation has greatly increased the number of applications for aluminum alloys. Today it is used in for example architectural sections for windows, doors and walls, interior fittings, truck and trailer flooring, frame systems, lighting, railings, fences, flexible assembly systems, special machinery elements, pneumatic installations, irrigation, heating and cooling pipes, furniture and office equipment.

There are two main types of layers formed from anodic aluminum oxidation: barrier type and porous type, they differ in thickness and in how thickness controlling parameters are used in the production process. The barrier type layer thickness is controlled solely by the applied voltage, whereas the porous type layer thickness depends on the current density, the oxidation time, the type of electrolyte and the electrolyte temperature.

Sealing is a process to close the pores of porous aluminum oxide layer formed from anodic oxidation. The process has been studied for many years and is utilized by the aluminum industry to produce hard, corrosion-resistant coatings on aluminum alloys. The current sealing processes involve either toxic and/or cancerogenic chemicals such as chromate or requires high temperature treatment such as the use of boiling water resulting in a high energy consumption.

US 8,197,613 discloses methods and compositions for forming a protective coating on alloys, reducing oxide spallation and corrosion of metals. In particular, it describes nanoparticle surface treatments and methods of using the surface treatments to form self- protective oxides on alloys and thereby reducing corrosion and damaging oxidation of stainless steel and other metal and alloy components.

US 6,248,184 discloses a process to use rare earth metal salt solutions for chemical sealing of porous anodic aluminum oxide in order to gain corrosion protection.

There is still a need for an energy efficient and/or non-hazardous production process for sealing a porous aluminum oxide layer, and a product not containing environmental and/or toxic compounds.

Summary

It is an object of the present invention to alleviate at least some of the disadvantages with the prior art and to provide an improved multilayer structure with regard to safety and energy consumption. This is achieved by the composite aluminum based multilayer structure as defined in claim 1 and the method as defined in claim 13.

The composite aluminum based multilayer structure for aluminum alloy surfaces according to the invention comprises an aluminum substrate having a porous aluminum oxide layer obtained from anodic oxidation of the aluminum substrate and cerium oxide particles. The porous aluminum oxide layer have an average pore density of 0.2-30 /nm ² , an average pore diameter of 2-100 nm and an average pore depth of 5-25 μm wherein the pores are at least partly filled with cerium oxide particles so that the porous anodic aluminum oxide layer comprises 50-90 %(vol/vol) aluminum oxide and 10-50 %(vol/vol) cerium oxide.

In one aspect the porous anodic aluminum oxide layer comprises 50-70 wt% aluminum oxide and 30-50 wt% cerium oxide, preferably 50-60 wt% aluminum oxide and 40-50 wt% cerium oxide.

In a further aspect the composite aluminum based multilayer structure comprises spherical shaped cerium oxide particles with an average particle size that is smaller than the average pore diameter of the pores of the porous anodic aluminum oxide layer. The spherical cerium oxide particles typically have an average particle size of 5-100 nm, preferably 5-50 nm, and even more preferably 5-20 nm.

In another aspect the porous anodic aluminum oxide surface layer has an average pore density of 1-25 /nm ² , an average pore diameter of 2-50 nm and an average pore depth of 10-25 μm, or preferably an average pore density of 5-20 /nm ² , an average pore diameter of 2-10 nm and an average pore depth of 5-10 μm, and the aluminum alloy belongs to the AL6000 series.

Further aspects include architectural sections for windows, doors and walls, interior fittings, truck and trailer flooring, frame systems, lighting, railings or fences, flexible assembly systems or special machinery elements, pneumatic components, irrigation, heating and cooling pipes, furniture and office equipment comprising the composite aluminum based multilayer structure of the invention.

In a further aspect there is a method of producing a composite aluminum based multilayer structure comprising the following sequential steps: providing an aluminum based multilayer structure comprising an aluminum alloy substrate with a porous aluminum oxide layer produced by anodic oxidation of the aluminum alloy substrate, the porous aluminum oxide layer having an average pore density of 0.2-30 /nm ² , an average pore diameter of 2- 100 nm and an average pore depth of 5-25 μm; immersing the aluminum based multilayer structure in an aqueous solution of cerium oxide particles at a concentration of 0.000015-0.15 g/L; and drying the immersed aluminum based multilayer structure in air.

In further aspects the aqueous solution of cerium oxide is composed of spherical cerium oxide particles at a concentration of 0.0015-0.15 g/L, preferably 0.0015-0.015 g/L. The average particle size of the cerium oxide particles is smaller than the average pore diameter of the pores of the porous anodic aluminum oxide layer, the average particle size is preferably 5-100 nm, more preferably 5-50 nm and even more preferably 5-20 nm in average particle size.

In a further aspect the immersion step is performed for at least 5 min and without any stirring, and the drying step is performed for at least 24 hours.

Thanks to the invention a multilayer sealing structure using materials that are not currently, or foreseen to be, listed as toxic and/or cancerogenic by governmental agencies like the US Environmental protection agency (EPA), such as cerium oxide, may be produced.

Thanks to the invention an aluminum based multilayered structure with high corrosion resistance may be produced in an environmentally friendly and energy effective way.

In one aspect afforded by the invention the immersion and drying times can be kept under 10 minutes and 24 hours, respectively.

In another aspect afforded by the invention the multilayered structure can be produced at room temperature. All aspects described herein are applicable to all embodiments of the invention and may be combined unless stated otherwise.

The invention will be further described in the following by way of examples and with references to the figures.

Description of drawings

Figure 1. Schematic view of the multilayer structure of the invention;

Figure 2. Schematic description of the method for producing the multilayer structure of the invention;

Figure 3a and b. SEM image of a multilayer structure with high cerium oxide concentration;

Figure 4. Graph showing corrosion resistance as a function of cerium oxide concentration;

Figure 5. Graph showing corrosion resistance as a function of drying time; and

Figure 6 a and b. TEM images of a multilayer structure of the invention.

Detailed description

The following terms are defined and used throughout the description and claims:

Aluminum is an alloy of aluminum with other elements, or pure aluminum.

Cerium oxide is a chemical compound or complex comprising the chemical element cerium (Ce) and the chemical element oxygen (0). The term "cerium oxide" denotes oxides of cerium including Ce2O3, Ce3O4 and CeO ₂ .

Sealing is a process to close a porous aluminum oxide layer formed after anodic oxidation of an aluminum alloy surface.

Sealants are chemicals, particles or any other matter used to seal a porous aluminum oxide layer formed after anodic oxidation of an aluminum surface.

Pores are empty space, voids, or cavities in a structure. It can also be described as openings in a surface.

Pore density is number of pores present on a defined area.

Spherical is a shape having three principal semiaxes: a, b and c, where a=b=c, or a=b≠c. %(vol/vol) is percent volume of a component in respect to the other components] present in the structure so that the total volume percent of all components added together makes 100.

%(wt/wt) is percent weight of a component in respect to the other components] present in the mixture so that the total percent of all components added together makes 100.

The composite aluminum based multilayer structure of the invention is schematically illustrated in Figure 1, it comprises a porous aluminum oxide layer 11 obtained from anodic oxidization of an aluminum substrate 10 and cerium oxide particles 13. The pores 12 are at least partly filled with cerium oxide particles 13. A portion of the cerium oxide particles 13' may additionally be located on the surface of the porous anodic aluminum oxide layer not occupied by a pore 12, referred to as the porous anodic aluminum oxide layer top surface 11'. The porous aluminum oxide layer 11 has a pore density of 0.2-30 /nm ² , an average pore diameter of 2-100 nm and an average pore depth of 5-25 μm. The cerium oxide particles 13 act as sealants of the porous aluminum oxide layer 11, i.e. as they enter the pores of the porous aluminum oxide layer 11 they enhance the corrosion protection of the porous aluminum oxide layer 11. Figure 6 and b show a composite aluminum based multilayer structure according to the invention at two magnifications. The figures show TEM images of cross-sections of a composite aluminum based multilayer structure according to the invention, wherein the pores 12 of an anodic oxide layer 11 on an AA 6060 sample are filled by cerium oxide particles 13. In the TEM images the pores 12 and the cerium oxide particles 13 within the pores 12 are clearly visible. The porous aluminum oxide layer 11 represent a distinct layer on top of the aluminum substrate 10. A portion of the cerium oxide particles 13' are located on the porous anodic aluminum oxide layer top surface 11'.

In one embodiment the pores 12 of the porous anodic aluminum oxide layer 11 has an average pore diameter of 2-100 nm, preferably 2-50 nm, and more preferably 2-10nm. The average pore depth of the pores 12 are 5-25 μm, preferably 10-25 μm, and more preferably 5-10 μm. The average pore density is 0.2-30 /nm ² , preferably 1-25 /nm ² , and more preferably 5-20 /nm ² .

The cerium oxide particles 13 may be of any shape and any average particle size that is smaller than the average pore diameter of the pores 12 of the porous anodic aluminum oxide layer 11. In one embodiment the cerium oxide particles 13 are spherical and between 5-100 nm in average particle size. The average particle size is preferable 5-50 nm and even more preferably 5-20 nm.

In one embodiment the porous aluminum oxide layer 11 is composed of 50-90 %(vol/vol) aluminum oxide and 10-50 %(vol/vol) cerium oxide. The porous aluminum oxide layer 11 is preferably composed of 50-60 %(vol/vol) aluminum oxide and 40-50 %(vol/vol) cerium oxide.

Cerium is known for its self-healing and corrosion protective properties. Moreover, cerium nanoparticles can be regarded as a green material for aluminum anodic oxide film sealing, as well as a safe biomedicine material. In an exemplary embodimentthe cerium oxide particles 13 are in the form of CeO ₂ particles.

The type of aluminum alloy used as the aluminum substrate 10 is any type that is generally used industrially such as for example pure aluminum, A1 7000 series, A16000 series, A1 2000 series. In one embodiment the aluminum alloy belongs to the A16000 series. A16000 series are universally used and one of the most versatile heat-treatable aluminum alloys, they offer good formability and medium strength.

In one embodiment the composite aluminum based multilayer structure protects the aluminum substrate 10 from corrosion. In certain embodiments the aluminum substrate 10 with the surface layer comprising porous aluminum oxide 11 and cerium oxide particles 13 are comprised in for examples architectural sections such as windows, doors and walls; interior fittings such as frame systems; lighting; ladders; railings and fences; truck and trailer flooring; in railways; for heating and cooling pipes; office equipment.

The method according to the invention to produce a composite aluminum based multilayer structure is schematically described in Figure 2, and comprises the following sequential steps:

20: providing an aluminum based multilayer structure comprising an aluminum substrate 10 with a porous anodic aluminum oxide layer 11;

21: immersing the aluminum based multilayer structure in an aqueous solution of cerium oxide particles 13 for a predetermined period of time, the immersion time to form an aluminum based multilayer structure with cerium oxide particles 13 within the pores of the porous anodic aluminum oxide layer 11; and

22: drying the immersed aluminum based multilayer structure in air for a predetermined drying time forming a composite aluminum based multilayer structure.

The step 20 of providing an aluminum substrate 10 with a porous anodic aluminum oxide layer 11 is performed by anodic oxidation of an aluminum substrate. Anodic oxidation is a well-known technique for persons skilled in the art and can for example by performed at 8 V for 30 min using Na2S04 as electrolyte.

The concentration of the aqueous cerium oxide particle solution is 0.000015-0.15 g/L, preferably 0.00015-0.15g /L, and even more preferably 0.0015-0.015 g/L. A too low cerium oxide particle concentration, such as below 0.00015 g/L leads to an insufficient corrosion resistance, this can be seen in Figure 4. A too high concentration of cerium oxide particles 13, such as above 0.015 g/L leads to the formation of a layer on top of the porous anodic aluminum oxide layer 11 that is brittle, and hence enables the formation of cracks in the layer exposing the porous anodic aluminum oxide layer 11 below. This can be seen in Figure 3a where the aluminum substrate 10 with the porous aluminum oxide layer 11 was immersed in a cerium oxide particle solution of 15 g/L for 1 hour and dried for 48 hours. The exposed aluminum oxide can be seen in Figure 3b, that shows a magnified SEM image of a crack seen in Figure 3a. The porous aluminum oxide layer 11 seen in Figure 3b, i.e. where the cracks are, is not resistant against corrosion. Therefore, the corrosion resistance is lowered when the concentration of cerium oxide particles 13 are too high as can be seen in Figure 4.

In one embodiment the porous anodic aluminum oxide layer 11 is prepared using anodic oxidation with an electrolyte such as Na2S04. Alternative electrolytes include but are not limited to sulfuric acid, organic acid, phosphoric acid, borate and tartrate baths.

The aluminum alloy substrate 10 is anodic oxidized in order to produce a porous anodic aluminum oxide layer 11 on the aluminum substrate 10 surface. In one embodiment the porous anodic aluminum oxide layer 11 have pores 12 with an average pore diameter of 2-100 nm, preferably 2-50 nm, and more preferably 2-10 nm. The porous anodic aluminum oxide layer 11 may have pores 12 with an average pore depth of 5-25 μm, preferably 10-25 μm, and more preferably 5-10 μm. The porous anodic aluminum oxide layer 11 may have an average pore density of 0.2-30 /nm ² , preferably 1-25 /nm ² , more preferably 5-20 /nm ² .

In an industrial process it is often advantageous to shorten all possible manufacturing steps. As can be seen in Figure 5 the corrosion resistance seems to increase rapidly for the first 20 h of drying time and it levels out, therefore the predetermined time for the drying step 22 should slightly above 20 h. The immersion step 21 should be performed for a time interval that is sufficiently long to obtain a sufficient corrosion resistance, e.g. a few minutes.

In one embodiment the immersing is performed so that the entire aluminum substrate 10 with the porous aluminum oxide layer 11 is immersed in the cerium oxide solution. Typically, the immersion time is a period of a few minutes. The person skilled in the art will realize that the time period will depend on the process and he or she will make necessary adjustments of the time period in order to obtain a sufficient corrosion resistance.

In one embodiment the drying time 22 after the immersion step 21 of the porous anodic aluminum oxide object in an aqueous cerium oxide solution is at least 24h.

It is to be noted that elements of different embodiments described herein may freely be combined with each other unless such a combination is expressly stated as unsuitable, as will be readily understood by the person skilled in the art.

Examples

All examples were conducted on recycled Aluminum Alloy 6060 (Hydro company).

Electrochemical impedance spectroscopy (EIS) is an experimental technique that measures the resistance and dielectric properties of a system, it can also describe electrochemical interface. The technique measures the system impedance over a range of frequencies. The obtained data is display as Nyquist and Bode plots which can be transformed into corrosion resistance plots using Electric Equivalent Circuit fitting.

Corrosion resistance of the samples was measured using EIS conducted in a Na2S04 solution (0.2 M, Sigma Aldrich) on a 1 cm ² area. A three-electrode electrochemical cell was used, with the sample as the working electrode, a saturated KC1 Ag/AgCl electrode as the reference electrode, and a platinum mesh as the counter electrode. EIS was performed at the open-circuit potential (OCP), with perturbation amplitude of 10 mV and a frequency range from 10 ⁴ Hz down to 10 ² Hz. The E1S instrument was an AUTOLAB electrochemical potentiostat and the results were fitted using the NOVA 1.1. software.

Effect of cerium oxide concentration and drying time.

Step 1. Preparation of porous anodic aluminum oxide

The aluminum alloy surfaces were ground with SiC paper (#300, #800 and #1200), after which they were rinsed with water. The ground and rinsed aluminum samples were anodic oxidized at 8 V, for 30 min in 0.2 M Na2S04 (Sigma Aldrich) electrolyte with a 3.14 cm ² exposure area. After the anodic oxidation step a porous anodic aluminum oxide layer had formed on top of the aluminum alloy surface. The formed pores had an average pore size of 400 nm ² , an average pore width of 25 nm and the pore density was 0.22 /nm ² .

Step 2. Immersion in cerium oxide particle solution

The porous anodic aluminum oxide samples prepared in step 1 were rinsed with water and afterwards immersed in cerium oxide (CeO ₂ , BYK Company, Germany) particle aqueous solutions. The cerium oxide particles had an average particle size of approximately 10 nm. The immersion time was 60 minutes. The cerium oxide particle solution concentrations were varied between 0.000015 and 15 g/L.

Step 3. Drying

The immersed samples prepared in step 2 were dried in air for a time period of 24 or 48 hours at room temperature (25 °C).

Results

As can be seen in Figure 4 higher corrosion resistance was achieved with concentrations between 0.0015 g/L and 0.015 g/L, independent of drying time. The samples prepared using 0.0015 g/L cerium oxide particle solution reach the highest measured corrosion resistance of 6xl0 ⁷ W-cm ² . Effect of immersion time.

Samples were prepared as described in steps 1-3 above, but with fixed cerium oxide particle concentration at 0.0015 g/L (step 2) and a fixed drying time of 46 hours (step 3). In this example the immersion time (step 2) was varied between 40 seconds and 6 hours. Results

A prolonged immersion time have only a minor or no effect on the corrosion resistance, all the tested time periods resulted in a corrosion resistance of 10 ⁷ W-cm ² .

Effect of air drying time.

Samples were prepared as described above in steps 1-3, but with fixed cerium oxide particle concentration at 0.0015 g/L (step 2) and fixed immersion time of 1 hour (step 3). In this example the air drying time (step 3) was varied between 2 hours and 48 hours.

Results

As can be seen in Figure 5 the corrosion resistance is increased with increased drying time up to 24 hours after which it reaches a plateau.