Composition for sealing parts made of aluminium and alloys thereof

Abstract

A sealing composition for treating parts made of aluminium or aluminium alloy, 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 that 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 method for sealing the porous layer of parts made of aluminium or aluminium alloy in which the parts are immersed in a chromium (III) / zirconium (IV) bath and then in said sealing composition.

Description

[0001] SEALANT FOR ALUMINUM AND ITS ALLOY PARTS

[0002] The present invention belongs to the field of surface treatment of metal parts, and more particularly to treatments for the protection of the surfaces of aluminum and aluminum alloy parts.

[0003] 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.

[0004] Aluminum and its alloys are materials of choice widely used in the aeronautics field, because while they allow for a reduction in the mass of aircraft structures and therefore their energy consumption, they can also offer excellent mechanical performance. Obtaining alloys with high mechanical characteristics that meet the requirements of this industrial field requires the implementation of effective means of protection against corrosion to ensure their durability. Several processes are generally combined to obtain optimal protection. A commonly used protection scheme involves the formation of an oxide layer on the surface of the metal by chemical or electrochemical conversion, before applying an external coating (often a primer and a top coat for protective and / or decorative purposes).

[0005] Chemical conversion treatments allow the development of a protective layer on the surface of the alloy, by chromating or phosphating. Chromation involves C^O? ions. 2- , 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 standard chemical notations, hexavalent chromium refers to chromium in the +6 oxidation state, and trivalent chromium refers to chromium in the +3 oxidation state.

[0006] 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), still used today, particularly by aircraft manufacturers, despite the presence of hexavalent chromium in the baths. Other known processes include sulfuric anodic oxidation (SAO), the most widespread process currently used in the industrial sector, sulfotartaric anodic oxidation (SAO), carried out in a bath containing both sulfuric and tartaric acid, and phosphoric anodic oxidation (PAO), used mainly in the United States for aeronautical applications.

[0007] Regardless of 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 premature degradation. In order to increase its resistance to corrosion, it is important to reduce its porosity by an additional sealing operation.

[0008] For years, sealing was commonly achieved 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 set time and at a set 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. Corrosion resistance and adhesion of the external coating are then generally good.

[0009] However, hexavalent chromium is an element harmful to the environment and 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. 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 which are nevertheless relatively insensitive 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 of type AS7G0.6T6, AS7U1 GT6 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 / coatingsl 0030226. Examples include those based on the use of rare earths, particularly cerium salts; molybdate salts; hybrid solutions obtained from sol-gel, triethanolamine; potassium permanganates; rare earths associated with strong oxidants that 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] Also disclosed from CN106400085 is a sealing agent for post-treatment of anodic oxidation film of aluminum and aluminum alloy, which consists of solution A, solution B and water. Solution B is composed in particular of a pH adjusting agent, a silane, a surfactant, a catalytic accelerator and an appropriate amount of water.

[0012] None of these techniques has been able to meet a broad specification. None is suitable for processing all grades of aluminum alloys and suitable for being applied in conjunction with anodizing techniques that comply with REACH regulations, to achieve excellent corrosion resistance combined with coating adhesion. Thus, despite the significant number of studies on the subject, no technology currently appears to be sufficiently efficient given the level of demand from manufacturers, particularly in the aeronautics sector.

[0013] 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 subject to high stresses with regard to corrosion (braking systems in extreme environments, underwater drilling).

[0014] 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 aeronautics, space, and other sectors.

[0015] An aluminum alloy is an alloy in which aluminum is clearly 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, including copper, magnesium, manganese, iron, nickel, titanium, chromium, silicon metal, etc., which are present at different contents and in variable 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.

[0016] According to a first aspect, the present invention aims to provide 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 aluminum alloy 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 likely to go up to 1000 hours, after exposure to neutral salt spray (NSM). 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 exhibit atypical behavior compared to most others.

[0017] 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.

[0018] 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.

[0019] More specifically, the present invention relates to a sealing composition for the treatment of aluminum or aluminum alloy parts, comprising, in aqueous solution:

[0020] - a silicate salt;

[0021] - a functional organosilane belonging to the alkoxysilane family of formula:

[0022] (Ri-O)3- Si - R2(I) or (Ri-O)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

[0023] - a phosphate salt.

[0024] 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 passage from the basic silicate form SiO3 2-soluble, in the form of a silica precipitate Si(OH)4 then SiO2, in the gel state, is the result of acidification of the sealing composition in the vicinity of the anodic layer and 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 swell when in contact with a humid environment, which causes the formation of blisters in the external coatings, and more generally raises adhesion problems.

[0025] 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.

[0026] 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 an AIPO4 type form whose solubility is low (solubility product Ks = 10 192). This additional precipitation at the time of clogging is the explanation put forward for the marked strengthening of the adhesion of the clogging layer during its formation.

[0027] This provides a sealing composition made up of three compounds acting synergistically, so that it offers unrivalled levels of corrosion resistance and adhesion properties, without resorting to harmful and prohibited ingredients such as Cr(VI).

[0028] 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, possibly as a mixture.

[0029] 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. According to the invention, the composition comprises an alkoxysilane of formula (I) or (II), comprising three alkoxy substituents (-OR1), identical to each other, in which the group Ri is an alkyl radical, which can 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.

[0030] The fourth silicon substituent is a functional group which can appear in two forms, namely a radical R2 linked directly to silicon (compound (I)), or separated from it by an oxygen atom accompanied or not by a hydrocarbon chain (compound (II)).

[0031] 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.

[0032] 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.

[0033] 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.

[0034] (3-Glycidyloxypropyl)-trimethoxysilane, abbreviated as GPTMS, is an organosilane of formula (II) in which Ri = - CH3, n = 3 and R2 = - C3H5O.

[0035] (3-Aminopropyl)-triethoxysilane, abbreviated as APTES, is an organosilane of formula (I) in which Ri = - CH2- CH3 and R2 = - (CH2)3- NH2.

[0036] Tetraethoxysilane, abbreviated as TEOS, is an organosilane of formula (II) in which Ri = — CH2— CH3, n = 0 and R2 = — CH2— CH3.

[0037] Octyl-trimethoxysilane, also referred to as OTMS, is an organosilane of formula (I) in which Ri = - CH2- CH3 and R2 = - (CH2)7- CH3. 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 by means of a solution of purity greater than 95%, commercially available.

[0038] 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.

[0039] 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.

[0040] The sealing composition comprises a phosphate salt capable of precipitating aluminum ions from the substrate with the ions. Different salts can be used, provided that their solubility in the sealing composition is sufficient to perform their role without affecting the process. For this purpose, sodium or potassium phosphates can be used, while a lithium salt should be excluded.

[0041] 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 (NasPO^, 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.

[0042] 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.

[0043] To play their role effectively, the compounds of the sealing composition must be in a clearly basic environment. 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 sealing composition 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 advantageous to use lithium hydroxide here, since it can be present elsewhere in the composition, as explained above.

[0044] 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.

[0045] According to a preferred characteristic of the composition according to the invention, it is free of anionic surfactant and non-ionic surfactant. In particular, said composition is free of sulfonate and sulfate ester and non-ionic surfactant of polyoxymethylene or polyol type.

[0046] 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.

[0047] 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.

[0048] The process for sealing 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.

[0049] The present invention thus relates to a method for sealing the porous layer of parts made of aluminum or aluminum alloy having undergone a prior surface treatment (by anodization), comprising the steps consisting 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 as described previously, to seal said porous layer.

[0050] Zirconium is present in the impregnation bath in the +4 oxidation state, 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)e 3+ , more simply noted Cr 3+. Solutions based on trivalent chromium and tetravalent zirconium, free of CMR compounds, are also offered in manufacturers' catalogs.

[0051] 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 acidifying the surface of the parts.

[0052] 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 around 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.

[0053] In 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.

[0054] The inventive process described above, with the implementation of the sealing composition as defined above, thus provides a technology for sealing aluminum and aluminum alloy parts that is 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, particularly with 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 has been proposed.

[0055] The present invention will be better understood and relevant details will appear in light of the description which will be given of various embodiments.

[0056] EXAMPLE 1: Sealing composition C1

[0057] A C1 sealing composition of the following formula was prepared:

[0058] - Na2SiOs: 10 g / L

[0059] - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L

[0060] - Na2HPO4: 0.5 g / L

[0061] - LiOH: 0.2 g / L

[0062] On an industrial scale, a 360 L bath is prepared by putting in aqueous solution:

[0063] - 9.5 kg of sodium silicate solution;

[0064] - 3.6 kg of a commercial solution comprising 3-GPTMS and LiOH;

[0065] - 0.18 kg of sodium phosphate in solid form.

[0066] 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: Sealing composition C2

[0067] A C2 sealant composition of the following formula was prepared, using the same reagents as before:

[0068] - Na2SiO3: 15 g / L

[0069] - (3-glycidoxypropyl)-trimethoxysilane: 1.3 g / L

[0070] - Na2HPO4: 0.7 g / L

[0071] - LiOH: 0.2 g / L

[0072] EXAMPLE 3: C3 Sealant Composition

[0073] A C3 sealant composition of the following formula was prepared, using the same reagents as before, except for LIOH which is absent:

[0074] - Na2SiO3: 10 g / L

[0075] - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L

[0076] - Na2HPO4: 0.5 g / L

[0077] EXAMPLE 4: Impregnation and sealing treatment of parts

[0078] Impregnation stage

[0079] The first step of the process 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 even though the silicate salt of the sealing composition from which it is derived is basic.

[0080] Its mechanism is exposed in the following sequence of reactions, taking place in the vicinity of the layer of aluminum hydroxide AI(OH)3 present on the surface of the parts: 1) Recovery of hydroxides associated with Al 3+ by Cr 3+ and precipitation of Cr(OH)3

[0081] Cr 3+ + 3OH ^ Cr(OH)3(s)

[0082] 2) Reaction between Al 3+ and ZrFe 2 , release of Zr ions 4+

[0083] AI 3+ + ZrF6 2 - AIF6 3 ' + Zr 4+

[0084] 3) Recovery of hydroxides by ZrF6 2-

[0085] ZrF6 2' (aq) + 4OH' ZrO2-2H2O (s) + 6F (aq)

[0086] The consumption of hydroxide ions is responsible for the acidification of the layer at the pore level.

[0087] The impregnation bath was prepared from a commercial product containing chromium (III) and zirconium (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.

[0088] Sealing step

[0089] The actual sealing step was carried out by immersing the parts in compositions C1, C2, or C3 described above, at a pH of 11 to 11.5, for 30 min, at a temperature of 97°C.

[0090] EXAMPLE 5: Performance of composition C1 with regard to corrosion

[0091] 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.

[0092] The parts were anodized using one of the known techniques: conventional sulfuric anodic oxidation (ASO) or new generation (NGA OAS), long cycle sulfo-tartaric anodic oxidation (LCS TS) or short cycle (CCS TS). Impregnation with a commercial Cr(l ll) / Zr(IV) solution distributed under the name Lanthane 613.3 was carried out on some of them. Finally, sealing with composition C1 presented in example 1 was carried out.

[0093] Corrosion resistance performance was assessed by a salt spray test (defined by international standard EN ISO 9227), counting the number of pits per dm 2appearing 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 rated according to the number of pits observed (pits), as compliant (++) for pit < 5, average (+ / -) for 5 < pit < 10, or non-compliant (-) for pit > 10.

[0094] TABLE 1 It is found that, in the absence of treatment with a sealing composition, the corrosion resistance is very insufficient, even poor. On the other hand, the results are good when the samples were pre-sealed with the Lanthanum 613.3 solution, then sealed with the composition C1. This highlights that the impregnation step is essential. The determining role of the treatment with a sealing composition C1 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 AU5NKZR.

[0095] EXAMPLE 6: Performance of composition C1 with regard to adhesion

[0096] Tests were carried out to show the effectiveness of sealing by composition C1 according to the invention, on the performance obtained in terms of adhesion of a coating on anodized layers of different grades of aluminum alloy.

[0097] As before, the parts were anodized using one of the known techniques. They were then impregnated in a Cr(l I l) / Zr(IV) bath as described in example 4, then sealed with composition C1. Finally, a coating was applied to the parts. The paints used are water-based and solvent-based, chromated and non-chromated paints.

[0098] Adhesion was assessed for each test according to ISO 2409, in the dry (without aging) and wet (after 14 days of immersion in an aqueous solution) processes. 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).

[0099] TABLE 2a TABLE 2b

[0100] Achieving grades 0 or 1 is necessary to meet the criteria required by aircraft manufacturers. All samples are rated at grade 0, which shows that they have excellent coating adhesion to the various substrates tested.

[0101] The two examples above show that the C1 sealing composition according to the invention achieves very good performance both in terms of corrosion resistance and paint adhesion. It thus makes it possible to surpass existing treatments and meet strict specifications of the aeronautical industry.

[0102] EXAMPLE 7: Performance of composition C2

[0103] The formula of composition C2 is identical to that of C1 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.

[0104] The performance of this C2 composition was studied using identical protocols. The number of pits on the samples after 500 hours of exposure to salt spray remains below 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 C1.

[0105] EXAMPLE 8: Performance of composition C3

[0106] The formula of composition C3 is identical to that of C1, 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 and 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 manufacturers' requirements for aeronautical applications.Corrosion resistance and paint adhesion were tested by ISO 9227. The results are shown in Table 3.

[0107] TABLE 3

[0108] The performance of the sealing composition remains correct and in line 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.

[0109] EXAMPLE 9: C4 Sealant Composition

[0110] A sealing composition C4 was prepared, based on the model of composition C1 of Example 1, in which LiOH and Na2HPO4 were replaced by U2HPO4. Indeed, the active species being HPC 2- 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 C1.

[0111] The formula for composition C4 is:

[0112] - Na2SiOs: 10 g / L

[0113] - (3-glycidoxypropyl)-trimethoxysilane: 1 g / L

[0114] - U2HPO4: 0.25 g / L

[0115] 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 fewer than 5 pits after 500 h of BS exposure; otherwise, it is non-compliant (NC).

[0116] TABLE 4

[0117] It is noted that the performance is not up to the previous results. In particular, the corrosion resistance is not consistent with two alloys (grades 2024 and AU5NKZr). It is also noted that different types of anodization and impregnation are involved 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 alkoxysilane family of formula: (Ri-O)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 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, dipotassium phosphate, 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-. Sealing composition according to one of the preceding claims, characterized in that it is free of anionic surfactant and non-ionic surfactant. 14-, 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 consisting 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.

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