Sealing composition for parts made of aluminum and its alloys

Abstract

Sealing composition for the treatment of aluminum or aluminum alloy parts, comprising, in aqueous solution: - a silicate salt, - a functional organosilane belonging to the family of alkoxysilanes of formula: (R1-O)3 – Si – R2 (I) or (R1-O)3 – Si – (CH2)n– O – R2 (II) in which R1 represents a linear or branched alkyl radical comprising from 1 to 5 carbon atoms; n is an integer which can take the values ​​from 0 to 5; R2 represents an alkyl radical, an amine radical or an epoxy radical; and - a phosphate salt. The composition can be used in a process for sealing the porous layer of aluminum or aluminum alloy parts in which the parts are immersed in a chromium (III) / zirconium (IV) bath and then in said sealing composition.

Description

Title of the invention: Composition for sealing parts made of aluminum and its alloys

[0001] The present invention belongs to the field of surface treatment of metal parts, and more particularly to treatments for protecting the surfaces of parts made of aluminum and aluminum alloys.

[0002] It relates to a sealing composition used to obtain a coating resistant to corrosion in aggressive environments. It also relates to a method for treating parts made of aluminum or aluminum alloy, using this composition.

[0003] Aluminum and its alloys are materials of choice widely used in the aeronautical field, because if they allow to reduce the mass of the structure of the aircraft and thus the energy consumption of the aircraft, they can also offer excellent mechanical performances. Obtaining alloys with high mechanical characteristics meeting the requirements of this industrial field requires implementing effective means of protection against corrosion to ensure their durability. Several processes are generally coupled to obtain optimal protection. A commonly used protection scheme includes the formation on the surface of the metal of an oxide layer by chemical or electrochemical conversion, before application of an external coating (often a primer and a finishing layer for protective and / or decorative purposes).

[0004] Chemical conversion treatments allow the development of a protective layer on the surface of the alloy, by chromating or phosphating. Chromatization involves Cr2O72 ions, forming a layer rich in hexavalent chromium or Cr(VI) and trivalent chromium or Cr(III), which provide good corrosion resistance. This technology has been widely used in the aeronautics industry, but raises pollution problems. It is now banned and subject to authorization by ECHA (the European Chemicals Agency), so the number of parts treated is decreasing day by day. In the following description, in accordance with conventional chemical notations, hexavalent chromium means chromium in the +6 oxidation state, and trivalent chromium means chromium in the +3 oxidation state.

[0005] Anodizing (or anodic oxidation) is an electrochemical conversion process that increases the thickness of the oxide layer naturally present on the surface of aluminum alloys. Among the oldest processes is chromic anodic oxidation (CAO), which is still used today, while particularly by aircraft manufacturers, despite the presence of hexavalent chromium in the baths. We also know sulfuric anodic oxidation (SAO), the most widespread process currently in the industrial sector, sulfotaric anodic oxidation (SAO) carried out in a bath containing both sulfuric acid and tartaric acid, and phosphoric anodic oxidation (PAO) used mainly in the United States for applications in aeronautics.

[0006] Whatever the anodic oxidation technique used, it results in the formation of a layer of aluminum oxide (or anodic layer) a few microns thick, whose marked porosity promotes the adhesion of organic, inorganic or hybrid coatings (such as paints, glues, sol-gels), but does not, however, provide good corrosion resistance. Indeed, due to its high porosity and despite its thickness, the anodic layer is very sensitive to the surrounding atmosphere and external aggressions, which leads to early degradation. In order to increase its resistance to corrosion, it is important to reduce its porosity by an additional sealing operation.

[0007] For years, sealing has been commonly carried out by immersing the part to be anodized in an aqueous solution containing hexavalent chromium via a sodium or potassium dichromate solution, which may also contain other additives, for a fixed time and at a fixed temperature. The mechanism at work is complex. It is generally assumed in the literature that the aluminum oxides formed during anodization are hydrated and converted into oxyhydroxide, a mineral similar to boehmite. The reaction is accompanied by swelling of the layer resulting in partial closure of the pores. The corrosion resistance and adhesion of the external coating are then generally good.

[0008] However, hexavalent chromium is an element harmful to the environment and to health due to its CMR (Carcinogenic, Mutagenic and Reprotoxic) character which must be eliminated from industrial processes. This is why, since 2007, with the REACH regulation (Registration, Evaluation and Authorisation of CHemicals or in French, system for the registration, evaluation and authorisation of chemical substances), new sealing processes have been developed with the aim of replacing those using Cr(VI)-based compounds.

[0009] In particular, processes have been proposed in which an impregnation step is carried out using a chromium (III) / zirconium (IV) solution, followed by a sealing step with near-boiling water. These impregnation solutions, commercially available, for example under the brands Surtec 650, Lanthane 613.3, SOCOSURF TCS, ... have been successfully tested according to standard qualification methods on alloys of various grades (2024T451 and 7175T73), but however relatively few sensitive to corrosion. These impregnation solutions combined with water sealing do not allow the expected anti-corrosion performance to be achieved on much more sensitive wrought alloys (for example type 2219T851, 2214T6, 2618AT8, 2219T6), nor on foundry alloys such as those type AS7G0.6T6, AS7U1GT6 and AU5NKZrT6.

[0010] Many other studies have been conducted over the past fifteen years to develop alternative techniques, as evidenced by the article "The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance", Ofoegbu SU et al., Coatings 2020, 10, 226; doi: 10.3390 / coatings 10030226. Examples include those based on the use of rare earths, in particular cerium salts; molybdate salts; hybrid solutions obtained from sol gel, triethanolamine; potassium permanganates; rare earths associated with strong oxidants which convert the oxides; or various electrophoretic solutions. Solutions based on PTFE (polyethylene terephthalate) polymers and copolymers exist but are not suitable for sealing all anodizations and cannot be coated.

[0011] None of these techniques has been able to meet a broad specification. None is suitable for the treatment of all grades of aluminum alloys and capable of being applied in complementarity with anodizing techniques complying with REACH regulations, to obtain excellent corrosion resistance associated with the adhesion of the coatings. Thus, despite the significant number of studies on the subject, no technology appears to date to be sufficiently efficient given the level of demand from manufacturers, particularly in the aeronautics sector.

[0012] As a result, sealing with potassium dichromate in addition to anodizing treatments by OAS or TSA, still remains the reference as an anti-corrosion treatment for aluminum alloy parts, and is still used for the manufacture of aluminum parts intended for use in areas where parts are subjected to high stresses with regard to corrosion (braking systems in extreme environments, underwater drilling, etc.).

[0013] There is therefore a pressing need to develop new technologies for the protection of aluminum alloys that do not use toxic elements. In particular, the development of sealing compositions free of hexavalent chromium is a major objective for the aeronautical, space, and other sectors.

[0014] By aluminum alloy is meant an alloy in which aluminum is clearly in the majority (by mass), with at least 80% aluminum, but generally more than 90% and often more than 95%. The alloying elements can be metals of any kind, in particular copper, magnesium, manganese, iron, nickel, titanium, chromium, silicon metal, etc., which are present in different concentrations and in varying combinations depending on the desired properties. The different types of alloys are classified into series and grades, the designations of which are standardized and well known to those skilled in the art.

[0015] According to a first aspect, the present invention aims to propose a sealing solution whose chemical composition complies with the REACH regulation, in particular which is free of chromium (VI). Such a composition must be able to be used in a sealing process compatible with the usual anodizing treatments for treating alloyed aluminum parts. Another objective of the invention is to have a composition for sealing aluminum alloys making it possible to achieve corrosion protection performances of more than 500 hours, and capable of going up to 1000 hours, after exposure to neutral salt spray (NSS). Another objective is to have a sealing composition providing good adhesion of paints used as external coating, including new non-chromated paints now available.Yet another goal is to have a sealing composition that can be used with all types of aluminum alloys, including those that have atypical behavior compared to most others.

[0016] According to a second aspect, the invention aims to propose a method for sealing aluminum alloys, which is efficient in terms of corrosion protection and coating adhesion, and in doing so, complies with environmental standards. The method according to the invention must avoid the formation of chromium (VI) both in the sealing composition and in residual form on the treated parts. Another objective of the invention is to have a universal treatment, i.e. on the one hand usable in addition to all the anodizing processes used in industry, and on the other hand suitable for a wide range of aluminum alloy grades. Aluminum alloys used in the fields of civil and military aeronautics, space and defense are particularly targeted. Finally, an objective of the invention is to have a method which is not difficult to master industrially.

[0017] It is to meet these needs and objectives that the composition which is the subject of the present application was developed. It essentially comprises three compounds whose combined action makes it possible to resolve the drawbacks mentioned above. The first two are silica compounds, one of which is mineral, the other organic, the third compound being a phosphate additive.

[0018] More specifically, the present invention relates to a sealing composition for the treatment of aluminum or aluminum alloy parts, comprising, in aqueous solution: - a silicate salt; - a functional organosilane belonging to the family of alkoxysilanes of formula: (Ri-O)3 - Si - R2 (I) or (RrO)3 - Si - (CH2)n- O - R2 (II) in which Ri represents a linear or branched alkyl radical containing from 1 to 5 carbon atoms; n is an integer which can take the values ​​from 0 to 5; R2 represents an alkyl radical, an amine radical or an epoxy radical; and - a phosphate salt.

[0019] The silicate salt has the role of forming a silica gel in the pores of the anodic layer where it then performs a sealing function. The transition from the soluble basic silicate form SiO32, to the form of a silica precipitate Si(OH)4 then SiO2, in the gel state, is the result of an acidification of the sealing composition in the vicinity of the anodic layer and the pores. Silica gel is a good sealing agent because it is insoluble and has good chemical inertness. However, due to its high hydrophilicity, it tends to become loaded with water and to swell when it is in contact with a humid environment, which causes the formation of blisters of the external coatings, and more generally raises adhesion problems.

[0020] On the other hand, unexpectedly, its use in a mixture with a second sealing agent chosen from organic silicon compounds, has proven capable of avoiding this phenomenon: the adhesion of the paints is significantly improved, without the corrosion resistance properties being negatively affected. This organic silicon compound (or organosilane) is more specifically chosen from the family of functional trialkoxysilanes.

[0021] It also appeared that the addition of a phosphate additive to these two sealing agents further strengthened the adhesion properties of the sealing composition. This phosphate compound is a phosphate salt whose action has been highlighted: it is capable of forming a precipitate with the aluminum ions present in the anodic layer, in a form of the A1PO4 type whose solubility is low (solubility product Ks = 10 19'2). This additional precipitation at the time of sealing is the explanation put forward for the marked reinforcement of the adhesion of the sealing layer during its formation.

[0022] This provides a sealing composition formed from three compounds acting in synergy, so that it offers properties of resistance to corrosion and adhesion at unmatched levels, without resorting to harmful and prohibited ingredients such as Cr(VI).

[0023] According to a preferred characteristic of the composition which is the subject of the invention, the silicate salt is chosen from sodium silicate, potassium silicate or lithium silicate. It can be provided indifferently in solid form or as a solution of defined concentration, optionally as a mixture.

[0024] According to a further preferred characteristic of the composition which is the subject of the invention, the silicate salt is sodium silicate present at a concentration which can range from 7 g / L to 20 g / L.

[0025] According to the invention, the composition comprises an alkoxysilane of formula (I) or (II), comprising three alkoxy substituents (-ORi), identical to each other, in which the group Ri is an alkyl radical, which may be linear or branched, and which comprises from 1 to 5 carbon atoms. Advantageously, Ri comprises 1 or 2 carbons, forming methoxyl or ethoxyl groups with oxygen. Thus, according to a preferred characteristic of the sealing composition, Ri is a methyl or ethyl radical.

[0026] The fourth silicon substituent is a functional group which can be present in two forms, namely a radical R2 linked directly to the silicon (compound (I)), or separated from it by an oxygen atom accompanied or not by a hydrocarbon chain (compound (II)).

[0027] According to a first variant, the sealing composition according to the invention comprises an organosilane of formula (I), in which R2 is an alkyl radical having from 1 to 10 carbon atoms, or a primary amine radical, or an epoxy radical. R2 may in particular be an ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, or aminoethyl, aminopropyl, aminobutyl radical. Preferably, the primary amine function is in the terminal position.

[0028] According to a second variant, the sealing composition according to the invention comprises an organosilane of formula (II), in which n can take the values ​​from 0 to 3 and R2 is an alkyl radical having from 1 to 5 carbon atoms or an epoxy radical. The oxygen atom can be linked directly to the silicon, in which case n = 0. The R2 group can be in particular a methyl, ethyl, propyl, butyl, or even glycidyl radical.

[0029] In a preferred embodiment, the composition according to the invention comprises an organosilane chosen from (3-glycidyloxypropyl)-trimethoxysilane, (3-aminopropyl)-triethoxysilane, tetraethoxysilane, octyl-trimethoxysilane or OTMS.

[0030] (3-Glycidyloxypropyl)-trimethoxysilane, abbreviated as GPTMS, is an organosilane of formula (II) in which Ri = - CH3, n = 3 and R2 = - C3H5O.

[0031] (3-Aminopropyl)-triethoxysilane, abbreviated as APTES, is an organosilane of formula (I) in which Ri = - CH2- CH3 and R2 = - (CH2)3 - NH2.

[0032] Tetraethoxysilane, abbreviated as TEOS, is an organosilane of formula (II) in which - CH2- CH3, n = 0 and R2 = - CH2- CH3.

[0033] Octyl-trimethoxysilane, also designated OTMS, is an organosilane of formula (I) in which Ri = - CH2- CH3 and R2 = - (CH2)7- CH3.

[0034] According to a preferred characteristic of the sealing composition, the organosilane is (3-glycidyloxypropyl)-trimethoxysilane present at a concentration ranging from 0.5 g / L to 1.5 g / L. It is introduced into the sealing composition through a commercially available solution with a purity greater than 95%.

[0035] In an advantageous embodiment, the composition according to the invention may comprise from 7 g / L to 15 g / L of sodium silicate, and from 0.7 g / L to 1.3 g / L of GPTMS. In a preferred embodiment, the composition according to the invention may comprise from 8 g / L to 12 g / L of sodium silicate, and from 0.8 g / L to 1.2 g / L of GPTMS.

[0036] Generally speaking, it will be ensured that, whatever the respective concentrations retained for the sealing agents, the silicate salt and the GPTMS are in a defined mass ratio, which can range from 20:1 to 1:1, and more commonly from 20:1 to 5:1.

[0037] The sealing composition comprises a phosphate salt capable of precipitating with the aluminum ions from the substrate. Different salts can be used, provided that their solubility in the sealing composition is sufficient to play their role without having repercussions on the process. In this respect, sodium or potassium phosphates can be used, while a lithium salt is to be excluded.

[0038] This is why, according to a preferred characteristic of the composition which is the subject of the invention, the phosphate salt is chosen from disodium phosphate (Na2HPO4), trisodium phosphate (Na3PO4), dipotassium phosphate (K2HPO4), tripotassium phosphate (K3HPO4), or a mixture of at least two of them. More preferably, the sealing composition according to the invention comprises disodium phosphate, at a concentration which can range from 0.1 g / L to 5 g / L. More preferably, the composition comprises from 0.3 g / L to 0.7 g / L of disodium phosphate.

[0039] The composition according to the invention may include a fourth compound, in this case lithium hydroxide (LiOH). The addition of a small amount of lithium hydroxide makes it possible to obtain excellent corrosion resistance for all the alloys tested. Indeed, for certain grades of aluminum alloys treated with a sealing composition devoid of this compound, slight defects may appear at the end of the corrosion tests. This is why, in a particular embodiment, the composition according to the invention additionally comprises lithium hydroxide at a concentration ranging from 0.005 g / L to 0.2 g / L.

[0040] To play their role effectively, the compounds of the sealing composition must be in a clearly basic medium. This is why said composition comprises at least one basic compound in sufficient quantity to bring its pH to a value ranging from 10 to 12. Preferably, a pH ranging from 10.5 to 11.5 will be chosen, and even more preferably the pH of the composition will be adjusted between 11 and 11.5. It goes without saying that the nature of the base and its concentration will be taken into account in order to define the quantity to be added to bring the pH of the composition of sealing to the desired value. The pH adjustment can be carried out using different bases known to those skilled in the art, alone or possibly in a mixture. Sodium hydroxide is conveniently used, but it is also interesting to use lithium hydroxide here, since it can be present elsewhere in the composition, as explained previously.

[0041] Thus, advantageously, the composition according to the invention comprises at least one basic compound chosen from sodium hydroxide, potassium hydroxide, lithium hydroxide, or a mixture thereof.

[0042] The sealing composition described above makes it possible to achieve unprecedented performance levels that fully meet the level required by the specifications of aircraft manufacturers, both in terms of corrosion resistance and coating adhesion. This result is based on the simultaneous use of two sealing agents, one of which forms a silica gel and the other creates a polymer network that prevents the silica from swelling, concomitantly with the introduction of a precipitating agent with the aluminum.

[0043] Parts made of aluminum and its alloys can advantageously undergo a surface treatment using this sealing composition, by a method which is also the subject of the present invention. The sealing method is carried out on parts whose surface comprises a porous aluminum oxide layer obtained beforehand, by an anodic oxidation operation, according to one of the techniques known in the state of the art.

[0044] The method of sealing the alloyed aluminum parts according to the invention then takes place in two essential stages. The first stage consists of impregnating the parts with a chromium (III) / zirconium (IV) solution, so that the porous layer takes on an acidic character. The second stage allows the actual sealing, by immersing the parts in a sealing composition as described above.

[0045] The present invention thus relates to a method for sealing the porous layer of aluminum or aluminum alloy parts which have undergone prior surface treatment (by anodization), comprising the steps consisting of: a) - immerse the parts in a chromium (III) / zirconium (IV) bath to impregnate the porous layer of the parts, then b) - immersing said parts in a sealing composition as described previously, to seal said porous layer.

[0046] Zirconium is present in the impregnation bath in the oxidation state +4, for example in the form of sodium hexafluorozirconate (Na2ZrF6) or potassium hexafluorozirconate (K2ZrF6), while chromium (III) is provided by solubilized chromium sulfate (Cr2(SO4)3) producing hydrated chromium (III) ions. Cr(H2O)63+, more simply denoted Cr3+. Solutions based on trivalent chromium and tetravalent zirconium, free of CMR compounds, are also available in manufacturers' catalogs.

[0047] According to a preferred embodiment of the sealing method according to the invention, the chromium (III) / zirconium (IV) bath is prepared from hexafluorozirconate and chromium oxide. This impregnation step is responsible for acidification of the surface of the parts.

[0048] The impregnation step can be carried out according to different protocols. According to the invention, this first impregnation step (step a) is generally carried out at a temperature of 30°C to 45°C for a treatment time of the order of 3 minutes to 20 minutes. The concentrations used are those recommended by the suppliers, in line with the manufacturers' requests. A draining and / or drying operation may be required.

[0049] During the second step of the claimed method (step b), the parts are immersed in a sealing composition as described above. According to a preferred embodiment, during this step, the parts remain for 15 min to 60 min in the sealing composition maintained at a temperature between 94°C and 98°C. In a preferred embodiment, the parts can remain in the sealing composition for 30 min at a temperature of 97°C.

[0050] The inventive method described above, with implementation of the sealing composition as defined above, thus provides a technology for sealing aluminum and aluminum alloy parts free of chromium VI and compatible with the anodizing treatments authorized by the REACH regulation. This technology makes it possible to achieve corrosion protection performances of more than 500 hours and up to 1000 hours after exposure to salt spray, as well as grade 0 dry and wet paint adhesion, in particular with the new non-chromated paints. In addition, it makes it possible to obtain the same performances with the different impregnation products used before sealing. To date, no solution providing such performances had been proposed.

[0051] The present invention will be better understood and relevant details will appear in light of the description which will be given of different embodiments. EXAMPLE 1: Sealing composition Cl

[0052] A sealing composition Cl of the following formula was prepared: - Na2SiO3: 10 g / L - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L - Na2HPO4: 0.5 g / L - LiOH: 0.2 g / L

[0053] On an industrial scale, a 360 L bath is prepared by putting in aqueous solution: - 9.5 kg of sodium silicate solution; - 3.6 kg of a commercial solution comprising 3-GPTMS and LiOH; - 0.18 kg of sodium phosphate in solid form.

[0054] The source of sodium silicate used is a 38-39% sodium silicate solution with a density of 1.38 and a %SiO2 / %Na2O ratio of between 2.9 and 3.2. The pH is between 11 and 11.5. EXAMPLE 2: C2 sealing composition

[0055] A sealing composition C2 of the following formula was prepared, using the same reagents as previously: - Na2SiO3: 15 g / L - (3-glycidoxypropyl)-trimethoxysilane: 1.3 g / L - Na2HPO4: 0.7 g / L - LiOH: 0.2 g / L EXAMPLE 3: C3 Sealant Composition

[0056] A sealing composition C3 of the following formula was prepared, using the same reagents as previously, except for LIOH which is absent: - Na2SiO3: 10 g / L - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L - Na2HPO4: 0.5 g / L

[0057] EXAMPLE 4: Impregnation and sealing treatment of parts

[0058] Impregnation step

[0059] The first step of the method according to the invention is an impregnation of the parts in an acid bath. In this way, the silica gel can form in the spores of the surface of the treated parts although the silicate salt of the sealing composition from which it is derived is basic.

[0060] Its mechanism is exposed in the following sequence of reactions, taking place in the vicinity of the layer of aluminum hydroxide A1(OH)3 present on the surface of the parts: 1) Recovery of hydroxides associated with Al3+ by Cr3+ and precipitation of Cr(OH)3 Cr3++ 3OH -> Cr(OH)3 (s) 2) Reaction between Al3+ and ZrF62, release of Zr4+ ions Al3++ ZrF62 -> A1F63 + Zr4+ 3) Recovery of hydroxides by ZrF62 ZrF62 (aq) + 4OH -> ZrO2-2H2O (s) + 6F (aq)

[0061] The consumption of hydroxide ions is responsible for the acidification of the layer at the pores.

[0062] The impregnation bath was prepared from a commercial product containing chromium (III) and zirconium (IV) (Lanthanum 613.3 distributed by COVENTYA). The impregnation is carried out at a pH of 3.9 at a temperature of 35°C + / - 5°C for 5 min to 15 min, generally for 10 min. The parts are then drained and dried, before carrying out the sealing step.

[0063] Sealing step

[0064] The actual sealing step was carried out by immersing the parts in the compositions C1, C2, or C3 described above, at a pH of 11 to 11.5, for 30 min, at a temperature of 97°C.

[0065] EXAMPLE 5: Performance of composition Cl with regard to corrosion

[0066] Tests were carried out to show the effectiveness of sealing by a composition according to the invention, as well as the influence of the impregnation step, on the level of performance in terms of corrosion resistance obtained on anodized layers of different grades of aluminum alloy.

[0067] The parts underwent anodization using one of the known techniques: conventional sulfuric anodic oxidation (OAS) or new generation (OAS NG), long cycle sulfo-tartaric anodic oxidation (TS CL) or short cycle (TSA CC). Impregnation with a commercial solution of Cr(III) / Zr(IV) distributed under the name Lanthane 613.3 was carried out on some of them. Finally, sealing with the composition Cl presented in example 1 was carried out.

[0068] Corrosion resistance performance was assessed by a salt spray test (defined by the international standard EN ISO 9227), by counting the number of pits per dm2 appearing on the part. The conformity criterion meeting the specifications is a number of pits less than 5 after 500 hours of exposure to salt spray. The results are presented in Table 1. They are noted according to the number of pits observed (pits), as compliant (++) for pits < 5, average (+ / -) for 5 < pits < 10, or non-compliant (—) for pits > 10.

[0069] [Tables 1] Anodizing Impregnation Solution Alloy 2024 2618 7175 AU5NKZr TSA CL Lanthanum 613.3 ++ ++ ++ ++ OAS NG Lanthanum 613.3 ++ ++ ++ + / - TSA CC ___ + / - ++ OAS ___ — — + / - —

[0070] It is noted that, in the absence of treatment with a sealing composition, the resistance to corrosion is very insufficient, or even poor. On the other hand, The results are good when the samples were pre-sealed with Lanthanum 613.3 solution, then sealed with the Cl composition. This highlights that the impregnation step is essential. The determining role of the treatment with a Cl sealing composition according to the invention is clearly demonstrated, on different types of anodization and on different aluminum substrates, including on alloys very sensitive to corrosion such as T AU5NKZR.

[0071] EXAMPLE 6: Performance of composition Cl with regard to adhesion

[0072] Tests were carried out to show the effectiveness of sealing by composition Cl according to the invention, on the performances obtained in terms of adhesion of a coating on anodized layers of different grades of aluminum alloy. As previously, the parts underwent anodization by one of the known techniques. They then received impregnation in a Cr(III) / Zr(IV) bath as explained in example 4, then sealing by composition CL. Finally, a coating was applied to the parts. The paints used are water-based and solvent-based paints, chromated and non-chromated.

[0073] Adhesion was evaluated for each test according to ISO 2409, in the dry process (without aging) and in the wet process (after 14 days of immersion in an aqueous solution). The results are rated on a scale of 0 to 5, where grade 0 corresponds to the best level of adhesion. They are presented in Tables 2a and 2b (dry adhesion and wet adhesion, respectively).

[0074] [Tables2a] Dry adhesion Alloy 2024 2618 7175 AU5NKZr TSA CL 0 0 0 0 OAS NG 0 0 0 0 TSA CC 0 0 0 0 OAS 0 0 0 0

[0075] [Tables2b] Wet adhesion Alloy 2024 2618 7175 AU5NKZr TSA CL 0 0 0 0 OAS NG 0 0 0 0 TSA CC 0 0 0 0 OAS 0 0 0 0

[0076] It is necessary to achieve grades 0 or 1 to comply with the criteria required by aircraft manufacturers. All samples are rated at grade 0, which shows that they have excellent coating adhesion, and this on the different substrates tested.

[0077] The two examples above show that the sealing composition C1 according to the invention achieves very good performance both from the point of view of corrosion resistance and paint adhesion. It thus makes it possible to surpass existing treatments and to meet strict specifications of the aeronautical industry. EXAMPLE 7: Performance of composition C2

[0078] The formula of composition C2 is identical to that of Cl in its components but differs in their concentration. Samples of the same aluminum alloys were treated in the same way as before. The pH, temperature conditions and treatment time are unchanged.

[0079] The performance of this composition C2 was studied according to identical protocols. The number of pits on the samples after 500 hours of exposure to salt spray remains less than 15: the corrosion resistance performance therefore remains very high and in accordance with the specifications. Paint adhesion is also satisfactory since a grade 0 is observed for the 4 alloys tested. The results are essentially the same as those obtained for composition CL EXAMPLE 8: Performance of composition C3

[0080] The formula of composition C3 is identical to that of Cl, its first three components being present at the same concentrations. The fourth component, lithium hydroxide, is not present. Samples of aluminum alloys underwent anodization by OAS, then impregnation with a chromium (III) solution, marketed under the name TCS / PACS by the company Socomore. Sealing was carried out in the same way as previously. The pH, concentration, temperature or treatment time conditions were adapted to comply with the conditions recommended by the suppliers of the impregnation solutions: TCS (temperature: 35-45°C, immersion time: 18-22 minutes); PACS (temperature: 15-30°C, immersion time: 4-6 minutes). The thickness of the coatings obtained is between 3 μm and 12 μm (depending on the alloys treated), which corresponds to the requirements of manufacturers for aeronautical applications.

[0081] Corrosion resistance and paint adhesion were tested by ISO 9227. The results are shown in Table 3.

[0082] [Tables3] Alloy Thickness ( pm) Sealing (grade) BS exposure test (pits / dm2) 24 h 48 h 96 h 192 h 384 h 480 h 576 h 2024 10.2 0 0 0 0 0 0 0 0 2618 8.2 0 0 0 0 0 0 0 3-4 7175 7.2 0 0 0 0 0 0 0 0 AU5NKZr 9.8 0 0 0 0 0 0 < 10 < 10

[0083] The performance of the sealing composition remains correct and in accordance with the required level, despite a small decrease in corrosion resistance for the 2618 and AU5NKZr alloys around 500 hours of BS exposure (salt spray). This decrease is limited since it remains below 10 pits after 500 hours of BS exposure. Paint adhesion is also good with a grade 0 for the four alloys tested. The results obtained are thus of an excellent general level, demonstrating the possibility of implementing the compositions according to the invention in addition to the various anodizing techniques. EXAMPLE 9: C4 Sealant Composition

[0084] A sealing composition C4 was prepared, based on the model of composition Cl of Example 1, in which LiOH and Na2HPO4 were replaced by Li2HPO4. Indeed, the active species being HPO42 and Li+, the use of this compound could be equivalent from a chemical point of view, although its cost is very expensive compared to that of LiOH and Na2HPO4. In addition, its solubility is very low (less than 0.25 g / L) so that the concentration introduced cannot be equivalent to those used in composition CL

[0085] The formula of composition C4 is as follows: - Na2SiO3: 10 g / L - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L - Li2HPO4: 0.25 g / L

[0086] Aluminum alloy parts were treated with different types of anodizing and impregnation, followed by sealing with composition C4. Salt spray tests were carried out. The corrosion resistance performance of the coating obtained with this composition is described in Table 4. The part is compliant (C) if it has less than 5 pits after 500 h of BS exposure; otherwise, it is non-compliant (NC).

[0087] [Tables4] Anodizing Impregnation solution Alloy 2024 2618 7175 AU5NKZr OAS fine TCS / PACS (Socomore) CCC NC TSA CL Lanthanum 613.3 (Coventya) CCC NC OAS Lanthane 613.3 (Coventya) NC C c NC OAS Surtec 650 (SURTEC) NC cc C

[0088] It is noted that the performances are not up to the previous results. In particular, the corrosion resistance is not consistent on two alloys (grades 2024 and AU5NKZr). It is also noted that different types of anodization and impregnation are concerned before sealing with composition C4. This therefore does not have the desired universal character.

Claims

Claims

1. Sealing composition for the treatment of aluminum or aluminum alloy parts, characterized in that it comprises, in aqueous solution: - a silicate salt, - a functional organosilane belonging to the family of alkoxysilanes of formula: (RrO)3 - Si - (CH2)n- 0 - R2 (II) in which Ri represents a linear or branched alkyl radical containing from 1 to 5 carbon atoms; n is an integer which can take the values ​​from 0 to 3; R2 represents an alkyl radical having from 1 to 5 carbon atoms or an epoxy radical; and - a phosphate salt.

2. Sealing composition according to claim 1, characterized in that the silicate salt is chosen from sodium silicate, potassium silicate, lithium silicate.

3. Sealing composition according to claim 1 or 2, characterized in that the silicate salt is sodium silicate present at a concentration ranging from 7 g / L to 20 g / L.

4. Sealing composition according to one of the preceding claims, characterized in that Ri is a methyl or ethyl radical.

5. Sealing composition according to one of claims 1 to 4, characterized in that it comprises an organosilane chosen from (3-glycidyloxypropyl)-trimethoxysilane, tetraethoxysilane.

6. Sealing composition according to the preceding claim, characterized in that the organosilane is (3-glycidyloxypropyl)-trimethoxysilane present at a concentration ranging from 0.5 g / L to 1.5 g / L.

7. Sealing composition according to one of the preceding claims, characterized in that it comprises from 7 g / L to 15 g / L of sodium silicate, and from 0.7 g / L to 1.3 g / L of (3-glycidyloxypropyl)-trimethoxysilane.

8. Sealing composition according to one of the preceding claims, characterized in that the phosphate salt is chosen from disodium phosphate, trisodium phosphate, phosphate dipotassium, tripotassium phosphate, or a mixture of at least two of them.

9. Sealing composition according to the preceding claim, characterized in that it comprises from 0.1 g / L to 5 g / L of disodium phosphate.

10. Sealing composition according to any one of the preceding claims, characterized in that it comprises lithium hydroxide at a concentration ranging from 0.005 g / L to 0.2 g / L.

11. Sealing composition according to one of the preceding claims, characterized in that it further comprises at least one basic compound in sufficient quantity to bring the pH of the composition to a value ranging from 10 to 12.

12. Sealing composition according to the preceding claim, characterized in that said at least one basic compound is chosen from sodium hydroxide, potassium hydroxide, lithium hydroxide, or a mixture thereof.

13. Method for sealing the porous layer of aluminum or aluminum alloy parts having undergone a prior surface treatment, characterized in that it comprises the steps of: a) - immersing the parts in a chromium (III) / zirconium (IV) bath to impregnate the porous layer of the parts, then b) - immersing said parts in a sealing composition according to one of the preceding claims to seal said porous layer.