RS59341B1 - PROCESSING OF ANODIED ALUMINUM AND ALLOYS - Google Patents

Description

[0001] Description

[0002] Anodized aluminum has a low resistance to alkalis and acids. In applications such as automobiles and construction, there is an urgent need for anodized products whose surface, while retaining its characteristic appearance, can withstand contact with strong alkaline solutions and also preferably with strong acid solutions.

[0004] In the subject patent application when talking about aluminum it also includes common aluminum alloys.

[0006] The automotive industry has recently imposed the use of anodized parts on cars, alkali resistance tests that, in practice, aluminum that has been anodized cannot pass, even when conventionally "sealed". Anodized products are known to have porous surfaces. They particularly facilitate the attacks of alkaline or acid solutions on the surface. It is common to seal these porous surfaces to improve their resistance.

[0008] Processing to improve this resistance depends to a large extent on standards, such as those of the European Association for Anodized Aluminum, referred to here

[0009] "Qualanod" standards. However, products processed according to these standards, mostly related to building materials, show that they do not pass the tests in the automotive industry. These tests are still not unified, present in the same common goal which is that the material is successfully resistant even after the first contact with an acid, a strong alkaline solution with a pH value as high as 13.5.

[0011] Among the tests prescribed for the automotive industry can be mentioned:

[0012] FIAT production standard 9.57448

[0013] Volkswagen AG "Anorganische Schutzschicht Auf Aluminiumteilen-Oberflaeschenschutzanforderung TL 182

[0014] General Motors GMW 14665 “Anodic Coating of Aluminum

[0015] by oxidation" (General Motors Material Specification Finish-Worldwide Engineering Standards, January 2009)

[0016] Volvo TR 31804674.

[0018] Examples of the content of such a standard can be found in FIAT's documentation.

[0019] Products are exposed to a testing cycle that includes:

[0020] • Immersion for 10 minutes in an acidic solution that has a pH value of 1 (0.1 molar HCl solution)

[0021] • Rinsing with running water and drying with compressed air

[0022] • Keeping in the oven for 1 hour ± 5 minutes at 40°C ± 3°C

[0023] • Immersion for 10 minutes in an alkaline solution at a pH value of 13.5 (aqueous solution 12.7 g/l NaOH+4.64g/l Na<3>PO<4>)

[0024] • Rinsing with running water and drying with compressed air.

[0025] At the end of the test, there must be no aesthetic change, compared to the material that was not exposed to the test.

[0027] Several attempts noted in the prior art have been made to meet the needs of the automotive industry. All fall under one or more criteria that largely relate to the appearance of the product. This is the case when electrophoretic coating is used. This may enable it to pass resistance tests, but the coating is not visible and the required aesthetic quality is not fully met. When using sol-gels including silanes, another way to try to achieve a suitable resistance, the layer is not colorless and the transparency can be changed.

[0029] Taking the disadvantages of the prior art into account, there is still a need for improved anodized aluminum products, which allow meeting the very specific requirements listed above. The present invention aims to solve this problem. While starting with products for the automotive industry, the invention can also be applied to any type of anodized product including those intended for construction or domestic applications.

[0031] surprisingly, the inventors have shown that such a goal can be achieved according to the invention, by a special treatment of these anodized products, part of this treatment refers to the prior art. More precisely, in accordance with the invention, the processing of the anodized aluminum product is carried out as required in the first appended patent claim.

[0032] The anodized aluminum product that is processed first is the one obtained without any indication of previous defects, in order to obtain this anodization must follow the best known practice.

[0034] Conventional anodization is performed in an electrolytic solution that includes an inorganic acid. Of these acids, the most common is sulfuric acid. Other acids are also of interest, especially those that are less susceptible to dissolving the anodized oxide layer created. The anodizing conditions, such as voltage, temperature, which make the product free of defects but need precise control, especially when sulfuric acid solution is used, are well known from the prior art. For example, a low temperature corresponds to a low rate of formation of the oxide layer, but if the temperature is too high, the rate of dissociation increases. The temperature is usually not above 24°C.

[0036] At the end of the anodization step, the anodized solution is immediately removed and the product is rinsed to prevent further dissociation of the oxide layer.

[0038] The usual thickness that is chosen must be sufficient to allow the necessary resistance, both chemical and mechanical. It is usually above 5 µm and preferably above 10 µm. Preferably, the thickness is not more than 50 µm in order to prevent the possible formation of defects such as cracks.

[0040] Anodization leads to an oxide layer, the structure of which is obtained as a porous system that has poor resistance, especially to acid and alkaline solutions. These corrosive chemicals are usually prevented from penetrating the porous structure by sealing the pores. Conventional caulking is done by hydrating aluminum oxide, which swells and clogs the pores. Hydration can be done with hot water or steam. ("hot sealing"). This is done at a temperature above 95°C. Another option, called "cold sealing", starts with an impregnation step that involves bringing it into contact with a first solution, e.g. with nickel fluoride which is used for its properties of forming insoluble complexes with aluminum and plugging the pores, which produces a good seal. Other metal salts, especially chromium, can be substituted for those of nickel. With this cold impregnation, the temperature is not above 30°C. After impregnation, aging is followed by treatment with hot water. Impregnation also requires careful control of particularly appropriate concentrations of nickel and fluoride.

[0041] Any treatment that is performed to seal the pores of the oxide layer, corrosion resistance to strong acid and alkaline solutions is not sufficient to meet the levels required in the automotive industry. Further processing is necessary which forms the subject of the invention.

[0043] According to the invention, the first subsequent treatment is the use of silicate solutions in which the products are immersed under such conditions that result in further sealing or coating with "silicate polymers". This is followed by a second treatment involving the use of specific silane solutions that add a second coating.

[0045] Treatment with silicates has been proposed in the prior art, and the treated products should show increased resistance at least to corrosion without the need for another treatment.

[0046] Processing of this type occurs e.g. in EP1873278 which includes limiting conditions relating to the SiO<2>/Na<2>O ratio. But this treatment is not intended to result in products that can be resistant to a solution with a pH value higher than 12.5. When only this accessory is available, even if the products show a better resistance to high pH value, it does not allow simultaneous resistance to strong acid solutions as required to pass the tests prescribed for the automotive industry.

[0048] According to the invention, this type of processing using silicates should be further improved. In the derived patent claims, the invention provides conditions for performing this step, which makes both more effective control and prevents possible defects.

[0050] First, before starting the contact with the silicate solution, the treated products must be free of any powdery residue on the surface of the material that could alter the final aesthetic appearance.

[0052] [0020] The products are therefore immersed in a silicate solution. The silicate solution used is preferably one of alkali metal, sodium, potassium or lithium or a mixture thereof. These, particularly sodium and potassium, are readily available commercially and are widely used in many industrial areas, particularly as thickening agents. The solutions contain an amount of silicate that depends on the stability. When the concentration increases there is a limit above which the solution turns into a gel. The concentration that allows stability to be maintained for a long period of time also depends on the nature of the silicate including the molar ratio of SiO<2>/M<2>O (M stands for Na, K or Li). The higher this ratio, the less amount of silicate is possible when trying to prevent gel formation.

[0054] In accordance with the invention, a relatively high ratio of SiO<2>/M<2>O is desirable because this results in a product showing better resistance. The ratio is at least 2.5 and preferably at least 3.

[0056] The silicate solution preferably has a pH value above 5 and preferably above 6.

[0058] In order to apply the coating as quickly as possible, the concentration of the solution is as high as the solubility allows. This concentration for sodium silicate is preferably greater than 1g/l and preferably from 10 to 30g/l.

[0060] The coating also depends on the temperature of the solution. Faster precipitation of silicate is obtained with higher temperature. In order to control the process easier, the temperature is usually higher than 40°C and does not exceed 95°C. The best temperature range according to the invention is from 60 to 80°C.

[0062] The processing time depends on the previous parameters and is usually between 5 and 50 minutes.

[0064] Between each step of the process, the products are thoroughly rinsed and dried so that they do not affect the composition of the following baths. The next step according to the invention is treatment with a solution of one or more silanes or silanols obtained from the hydrolysis of these silanes.

[0066] The silanes used according to the invention are hydroxysilanes that can be hydrolyzed to give silanols that can react with hydroxyl groups leading to covalent bonding according to a known general mechanism:

[0068] Al-OH+R-SiOH → Al-O-Si-R H<2>O

[0070] The presence of aluminum oxide, partly hydrated by the sealing process, partly complexed by reaction with the silicate solution, makes the surface very susceptible to reaction with silanols.

[0071] Silanols are rarely available as such, for the reaction to be stable they must be in solutions that are more diluted. For this reaction, it is usually necessary to have silanols prepared when they are needed.

Silanes used in accordance with the invention preferably have the general formula:

[0075] R'(CH<2>)nSi(OR)<3>

[0077] where n is at most equal to 4

[0078] where -OR is one of:

[0080] -OCH<3>, -OC<2>H<5>, -OCOCH<3>

[0082] and R' is a radical selected from: vinyl, epoxy, methacrylate, primary amine, diamine, mercaptan. Of these silanes, the most preferred are those with the formula:

[0084] CH<2>=CHSi(OCH<3>)<3>

[0085] CH<2>=CHSi(OC<2>H5)<3>

[0086] CH<2>CH-CH<2>O-(CH<2>)<3>Si(OCH<3>)<3>

[0088]

[0090] CH<2>=C(CH<3>)-COO-(CH<2>)<3>Si(OCH<3>)<3>

[0091] H<2>N(CH<2>)<2>Si(OCH<3>)<3>

[0092] H<2>N(CH<2>)<2>NH(CH<2>)<3>Si(OCH<3>)<3>

[0093] N<2>HCN(O)H(CH2)<3>Si

[0094] HS(CH<2>)<3>Si(OCH<3>)<3>.

[0096] The silanes used can also be bis-silanes having the general formula:

[0098] (RO)<3>-Si(CH<2>)nR'(CH<2>)n-Si(OR)<3>

[0100] where R, R' and n have the same meaning as indicated above. Of the bis-silanes, the most preferred are:

[0102] (C<2>H<5>O)<3>Si(CH<2>)Si(OC<2>H<5>)<3>

[0103] (C<2>H<5>O)<3>Si(CH<2>)<3>-S<4>-(CH<2>)<3>Si(OC<2>H<5>)<3>

[0104] (C<2>H<5>O)<3>Si(CH<2>)<3>-NH-(CH<2>)<3>Si(OC<2>H<5>)<3>.

[0106] As stated above, in order to allow the reaction with the hydroxyl groups of the substrate, the hydrolysis of the silane in silanols is first carried out following the usual reactions:

[0108] R'-Si(OR)<3>+ H<2>O↔R'-Si(OR)<2>OH ROH

[0109] R'-Si(OR)<2>OH H<2>O↔R'-Si(OR)(OH)<2>+ROH

[0110] R'-Si(OR)(OH)<2>+ H<2>O↔R'-Si(OH)<3>+ ROH

[0112] These reactions are mutual and therefore the balance must be shifted to the right side. The reaction with the hydroxyl groups of the substrate is in competition with the self-condensation of silanols, which produces oligomeric siloxanols. In order to prevent an excess of this possible reaction, the silanol solutions are not necessarily diluted.

[0114] Preferably, the concentration in the silanes is less than 8% by mass of the solution and most preferably less than 5% by mass. At these concentrations, hydrolysis tends to complete. In order to limit processing, the concentration is preferably not less than 0.1% by weight.

[0116] Most useful organo-silanes have limited water solubility as such. Using a mixture of water and alcohol improves solubility to the required concentration. The presence of alcohol can also regulate the hydrolysis of the silane and can facilitate the drying of the final product. Depending on the silane, the alcohol content can be up to 15% by weight of the mixture.

[0118] Silane hydrolysis requires time and depends on various factors, especially the type of organosilane, the reaction medium and the pH value. Hydrolysis in water is faster than in a mixture of water and an organic solvent such as ethanol and tends to complete. Adjusting the pH can catalyze hydrolysis and increase its rate.

[0120] Hydrolysis is faster with the smallest alkoxy group of the silane, e.g. the hydrolysis of methoxy-silane is 6 to 10 times faster than that of ethoxy-silane of the same structure.

[0121] The treatment can be started when the concentration of silanol is sufficient, this corresponds to the phenomenon that the solution becomes clear, which shows a true dissolution where previously it was mainly a suspension.

[0123] Application on the surface can be, e.g. by dripping or spraying. The necessary contact time depends in particular on the temperature. For example, at a temperature of 15 to 35°C, this contact can be from 0.5 to 5 minutes.

[0125] Silanols react with hydroxyl groups bonded to the metal but can also enhance condensation reactions of the silanols themselves to form silane layers. The previous ones increase adhesion to the substrate, while silanol condensates help the formation of a layer whose thickness depends on the concentration of the silane solution. The usual thickness can be up to 400nm. The structure of the layer may be cross-linked depending on the particular silane (or mixture of silanes) used. Cross-linking is usually high with bis-silanes, resulting in better protection.

[0127] When the necessary contact is completed, the aluminum product is dried at a temperature that can be from room temperature to even 120°C, preferably from 40 to 120°C. The above-mentioned reactions are carried out during the drying step.

[0129] Anodized aluminum and aluminum alloys according to the invention show a particular resistance to corrosive action which is the subject of a test prescribed for the automotive industry. In particular, they have a mass loss of no more than 10mg/dm<2>surface and preferably no more than 5mg/dm<2>, and most preferably 1mg/dm<2>when exposed to acid followed by an alkaline test in accordance with corrosion test specifications 9.57448 of the Fiat Group. The mass loss is especially obtained for corrosion tests in which the acid has a pH value of 1 and the alkaline solution has a pH value of 13.5.

[0131] A product according to the invention, wherein its surface is polished so that, before being subjected to these corrosion tests mentioned above, it exhibits a high gloss when measured by a gloss meter. The loss of their shine after these tests is less than 10% and preferably less than 5%.

[0133] The invention is further disclosed in detail in the following examples.

[0134] Aluminum profile samples were prepared before anodic oxidation by cleaning the parts and polishing to a surface roughness R<a> of 0.6 µm.

[0136] Chemical cleaning or electrochemical surface treatment may be desired to obtain a different appearance, especially an engraved appearance or other patterns.

[0138] Before anodizing, the parts were thoroughly washed with running water.

[0140] The anodizing bath contained 200 ±10g/l sulfuric acid. This solution is maintained at about 15°C. The current density is about 1.8 A/dm<2>.

[0142] Anodization is performed until the oxide layer is 20 µm. At the indicated current density it must be about 45 minutes. The anodized product is rinsed thoroughly in deionized water to remove the adhering solution content.

[0144] A sealing process is then performed to close the pores of the oxide layer. The sealing is of the "cold sealing" type, including impregnation with nickel ions.

[0146] The example was performed with Ni2+ at 1.2g/l and fluoride ions at 0.250g/l.

[0148] The chosen temperature is 28°C and the treatment is maintained for 20 minutes.

[0150] In order to prevent the possible deposition of powdery material that changes the surface appearance, the solution is continuously filtered to remove the existing precipitate.

[0152] This sealing is a common treatment suggested by the standard

[0153] "QUALANOD" published by the European Association for Anodized Aluminum, is a standard for parts intended to be used in less corrosive conditions, such as primarily those for use in construction. That's not enough to pass tests like those required in the automotive industry, whatever the pore sealing ratio.

[0155] Once the sealing is complete and the samples are thoroughly washed and dried, they are ready for silicate treatment.

[0156] According to this treatment, the anodized samples are immersed in an aqueous solution of sodium silicate. The solution is prepared by diluting up to 1l, 10ml of sodium silicate solution containing by mass:

[0157] Na<2>O 9%

[0158] SiO<2>29%

[0159] The molar ratio of SiO<2>/Na<2>O is therefore 3.2.

[0161] The pH value is 8.0.

[0163] Samples are processed by immersion at a temperature of 70°C. Immersion time is 10 minutes.

[0165] Different parameters were chosen so that no precipitation occurs during the processing time. In addition, continuous filtration of the solution is performed.

[0167] The samples were washed with tap water and then in demineralized water and then dried.

[0169] A careful examination of the product is carried out in order to control that defects do not occur

[0170] such as dots or powdery phenomena on the surface.

[0172] The conditions listed in Table 1 for anodizing and sealing and silicate treatment are other possible examples that replace those indicated above.

[0174] Table 1.

[0175] It includes different conditions related to the steps: anodizing, sealing and silicate treatment.

[0177]

[0178]

[0179]

[0181] 1 - Black MLW from Clariant.

[0182] 2 - Chemical specificities for medium temperature sealing available on the market are based on a solution of about 10 g/nickel acetate and 0.2-05 g/l surfactant which acts as an "anti-blooming agent" (ie prevents the formation of powdery material on the surface of the anode layer). Temperature 85-90 °C, dripping time 1 minute / µm (i.e. 20 min)

[0183] 3 - The chemical specificities of hot sealing available on the market essentially include a surfactant mixture used at a concentration of 1-2 ml/l which acts as an "anti-blooming agent" (ie prevents the formation of powdery material on the surface of the anode layer). Temperature > 95 °C, drip time 2 minutes / micron (ie 40 min)

[0184] 4 - Additives known to stabilize the tin ion solution, which slows down its oxidation and/or precipitation.

[0186] In order to continue with the samples that have been anodized, sealed and treated with silicate solutions, they are exposed to treatment with silane. In this case, the selected silane composition is the one sold by the company CHEMETAL, under the name "OXILAN MM 0706". The solution contains 3% by weight of this silane composition in water with added ethanol (5% by weight).

[0188] These samples were immersed in the solution at room temperature for 2 minutes. After these samples were removed from the tank without rinsing, they were directly dried with hot air at 60°C for 20 minutes.

[0190] These samples are then tested according to the specification stated above. As a result, it was found that the mass loss is measured in each stage of preparation:

[0191] - anodized samples before the second treatment 692mg/dm<2> - after sealing 97.2mg/dm<2>

[0192] - after treatment with silicate 11.5mg:dm<2>

[0193] - after treatment with silane 0.7mg/m<2>

[0195] The product made in accordance with the invention was analyzed at each stage of preparation. This makes it possible to confirm that the product itself can be the subject of identification without using the process for making it. Possible testing includes X-ray photoelectron spectrometry (XPS). This method enables the identification of elements that enter the surface layer of the product. The method allows the analysis of a very thin layer (about 20-30Å), which prevents possible mixing with the elements of the layers below. However, when the considered layers show an average thickness lower than these values and/or a surface roughness that is significant, part of the measurement may indicate a limited amount of elements from the layers below.

[0197] The results are summarized in the following table.

[0200]

[0203] It is significant that each analysis is different according to the stage of preparation. This confirms that the method definitely allows the analysis of each upper layer regardless of the layers below. This particularly applies to the top layer of silane (4), including nitrogen (the starting silane is amino-silane). It also contains an amount of carbon that apparently exceeds the amount of the previous layers, because the silane contains organic parts. The anodized part (1) includes carbon and nitrogen corresponding to the black coating part. Sealing in (2) appears particularly with the presence of Ni and F. These elements also appear, but definitely in a lower concentration in the product of the silicate treatment step (3), which makes it clear that the silicate layer is thin and probably serves to further seal the pores. This layer also indicates that the Al content is lower than that of sub-layers (2).

[0204] Various other combinations of the silicate treatment conditions described above are suggested in the following Table 2. These are given as examples only, many others can be used in accordance with the invention, especially through different silane mixtures.

[0206] Table 2

[0208]

[0209]

[0211] Silanes 4-7 above are sold pre-hydrolyzed and therefore directly soluble in water.

Claims (20)

1. Patent claims 1. Chemical treatment applied to products made of aluminum or aluminum alloys, which have first been subjected to anodic oxidation and pore sealing according to known techniques, which includes two additional steps: - treatment with alkaline silicate solution; - treatment with silane solutions. 2. Processing according to claim 1, where the thickness of the anodic oxide layer is not less than 5 µm. 3. Processing according to patent claim 1, where the processing with an alkaline solution is carried out in a stable aqueous solution containing one or more silicates of sodium, potassium or lithium or their mixtures. 4. Processing according to patent claim 3 where the molar ratio SiO<2>/M<2>O is such that the solution has a pH value above 6, preferably above 8, where M represents any of Na, K or Li. 5. Processing according to patent claim 4 where the alkali silicate is sodium silicate, the molar ratio SiO<2>/Na<2>O is at least 3. 6. Processing according to any of the preceding patent claims where the solution contains from 0.1 g/l to the solubility limit, preferably from 10 to 30 g/l of alkaline silicate. 7. Processing according to any of the previous patent claims, where the processing with alkaline silicate is carried out at a temperature of 40 to 95 °C, preferably from 60 to 80 °C. 8. Processing according to one of the previous patent claims where the product is kept in an alkaline silicate solution for 5 to 50 minutes. 9. Processing according to any of the previous patent claims, where the processing of the product is carried out in an aqueous or aqueous-alcohol solution of silane or a mixture of silanes. 10. Processing according to claim 9, where the silanes are mono-silanes of the general formula: R'(CH<2>)nSi(OR)<3> where n is at most equal to 4 where -OR is one of: -OCH<3>, -OC<2>H<5>, -OCOCH<3> and R' is a radical selected from: vinyl, epoxy, methacrylate, primary amine, diamine, mercaptan. 11. Processing according to claim 10, where one or more of: CH<2>=CHSi(OCH<3>)<3> CH<2>=CHSi(OC<2>H5)<3> CH<2>CH-CH<2>O-(CH<2>)<3>Si(OCH<3>)<3> CH<2>=C(CH<3>)-COO-(CH<2>)<3>Si(OCH<3>)<3> H<2>N(CH<2>)<2>Si(OCH<3>)<3> H<2>N(CH<2>)<2>NH(CH<2>)<3>Si(OCH<3>)<3> N<2>HCN(O)H(CH2)<3>Si HS(CH<2>)<3>Si(OCH<3>)<3>. 12. Processing according to patent claim 9, where the silanes are bis-silanes of the general formula: (RO)<3>-Si(CH<2>)nR'(CH<2>)n Si(OR)<3> where R, R' and n have the same meaning as in claim 10. 13. Processing according to patent claim 12, where the bis-silanes are one of: (C<2>H<5>O)<3>Si(CH<2>)Si(OC<2>H<5>)<3> (C<2>H<5>O)<3>Si(CH<2>)<3>-S<4>-(CH<2>)<3>Si(OC<2>H<5>)<3> (C<2>H<5>O)<3>Si(CH<2>)<3>-NH-(CH<2>)<3>Si(OC<2>H<5>)<3>. 14. Processing according to one of the patent claims from 9 to 13, where the solution contains from 0.1 to 5% by mass of silane. 15. Processing according to one of claims 9 to 14, where the solution contains from 0.1 to 15% by mass of aliphatic alcohol. 16. Processing according to one of the patent claims from 9 to 15, where the time in contact with the solution is from 0.5 to 5mn, and the temperature is from 15 to 35°C. 17. Processing according to one of claims 9 to 16, where the product is dried at a temperature ranging from room temperature to 120°C. 18. An anodized aluminum or aluminum alloy product, the surface of which is sealed and further processed in silicate solutions and silane solutions, in accordance with the chemical treatment according to one of patent claims 1 to 17, which shows a mass loss of no more than 10mg/dm<2>surface, and preferably no more than 5mg/dm<2>when exposed to acid followed by an alkaline test in accordance with the corrosion test specifications. 19. The product according to patent claim 18, where the acid has a pH value of 1 and the alkaline solution has a pH value of 13.5. 20. The product according to patent claim 18 or 19, where its surface is polished before complete processing, including anodic oxidation, so that the loss of gloss when exposed to the corrosion test according to patent claim 19 or 20 is less than 10% and preferably less than 5%. Published and printed by: Institute for Intellectual Property, Belgrade, Kneginje Ljubice 5