METHOD FOR SEALING ALUMINUM ALLOYS USING A TUNGSTEN SALT AND PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY OBTAINED BY THIS METHOD

Abstract

The present invention relates to a part (1) made of aluminum or an aluminum alloy, comprising - a body (10) made of aluminum or an aluminum alloy, - an inner layer (12) comprising aluminum oxide arranged on the body (10), - an intermediate layer (14) comprising chromium and zirconium, and - an outer layer (16) comprising tungsten oxide, the intermediate layer (14) being arranged between the inner layer (12) and the outer layer (16). It also relates to a method for post-anodization sealing of a body (10) made of aluminum or an aluminum alloy. Figure for abstract: None

Description

Title of the invention: METHOD FOR SEALING ALUMINUM ALLOYS USING A TUNGSTEN SALT AND PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY OBTAINED BY THIS METHOD Technical field of the invention

[0001] The present invention falls within the scope of research into new solutions for anti-corrosion protection on aluminium alloys, in particular for aeronautical applications, with or without the application of a paint system. Technical background

[0002] Aluminum alloys are materials of choice for the transportation industry, and more particularly for the aeronautics industry, due to their excellent mechanical properties / weight ratio and their relatively low manufacturing cost. However, these alloys are likely, depending on the environment in which they are found, to be affected by several types of localized corrosion, causing degradation of the part and possibly leading to its shrinkage or failure. Many strategies have been implemented to overcome this weakness, and among them, the formation or deposition of a protective layer on the surface of aluminum alloy parts is the most used. This is particularly the case for protective layers obtained by the anodizing process of aluminum alloy parts.Anodizing is an electrolytic process that replaces the natural oxide (native oxide), a few nanometers thick, that covers aluminum, with an oxide layer that can be up to several micrometers thick. The oxide layers produced by anodizing have a thickness that can range from two microns to around fifteen microns, in order to provide long-term protection against corrosion. Anodizing, also called anodic oxidation, therefore consists of forming a porous layer of aluminum oxides / hydroxides, called an anodic layer, on the surface of the part, by applying a current to the part immersed in an electrolytic bath containing a strong acid electrolyte, the part constituting the anode of the electrolytic system. The layer thus formed on the surface of the part, after a sealing treatment, makes it possible to reinforce the corrosion resistance of the part.Anodizing treatments are now commonly used in the aeronautics industry, mainly to improve the corrosion resistance of parts, and therefore their lifespan, but also to facilitate the adhesion of organic layers (paints). However, the anodizing process is directly impacted by European regulations (REACH), which, since . September 2017, prohibits (or restricts to authorization) the use of certain key components in surface treatments, in particular, hexavalent chromium. However, hexavalent chromium is present in the anodizing treatment of the OAC type (Chromic Anodic Oxidation as described, for example, in www.a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / a nodisation-chromique), but also in the usual surface preparation pretreatments, aimed at cleaning / stripping the surfaces of the parts before the anodizing treatment, and finally in the final treatments known as sealing, the objective of which is to close the pores of the anodic layer formed during the anodizing treatment.

[0003] Different processes have therefore been proposed to replace OAC and OAS (Anodic Sulfur Oxidation) treatments clogged with hexavalent chromium, impacted by the European REACH regulation:

[0004] - the OAS NG (new generation sulfuric anodic oxidation as described by example in "a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anod isation-sulfurique-version-5-2") was proposed to replace the OAS;

[0005] - OAST (anodic sulfo-tartaric oxidation as described, for example, in "a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anod isation-sulfo-tartrique-oast-tartric-sulfuric-anodizing-tsa") was proposed to replace the OAC.

[0006] OAC can also be replaced by OAS NG FE (new generation thin thickness sulfuric anodic oxidation) which is an OAS NG type anodization whose anodization parameters (Voltage, Immersion time) have been adapted to obtain an anodization layer whose thickness is between 2 and 7 pm.

[0007] Although current conventional anodizing solutions, such as for example OAS NG followed by hot water sealing, implement treatment ranges compatible with the European REACH regulation, they nevertheless remain little or not satisfactory in terms of anti-corrosion protection on certain grades of so-called "difficult" aluminum alloys. As a non-limiting example of so-called "difficult" aluminum alloys, we can cite alloys 2214, 2618A or AU5NKZr. These alloys have particular microstructures due to their chemical composition, which give them either foundry-type defects or precipitates such as intermetallics rich in copper or iron or nickel, etc. Thus, when the anodic layer forms on the surface of these alloys, layer defects may reside, resulting in certain local fragilities sensitive to corrosion.

[0008] For these alloys, it is therefore necessary to optimize the anodizing ranges in order to improve anti-corrosion performance.

[0009] Patent FR 3106838B1 proposes a post-anodization sealing process for aluminum or aluminum alloy which improves the corrosion resistance of the part without using hexavalent chromium impacted by the European REACH regulation. This process, which is also suitable for so-called "difficult" aluminum alloys, includes a step of impregnating the aluminum or aluminum alloy in an aqueous bath containing a hexaflurozirconate salt and a trivalent chromium salt followed by a sealing step carried out in an aqueous solution comprising an alkali metal or alkaline earth metal silicate and possibly followed by the application of a paint.Despite the improvement in anti-corrosion performance provided by this process, there is a real need to further optimize current anodizing processes for aluminum or aluminum alloy parts, including so-called "difficult" aluminum alloys, in order to improve the anti-corrosion performance of these alloys, while complying with the requirements of the European REACH regulation.

[0010] The present invention aims to remedy the drawbacks of current anodizing processes for parts made of aluminum or aluminum alloy, including so-called "difficult" aluminum alloys, in terms of corrosion resistance of said alloys and which allows good adhesion of the paints.

[0011] Furthermore, the present invention aims to provide a part made of aluminum or aluminum alloy whose corrosion resistance is improved while complying with the requirements of the European REACH regulation. Summary of the invention

[0012] The present invention aims precisely to meet these needs, by providing a part (1) based on aluminum or an aluminum alloy, characterized in that it comprises:

[0013] - a body (10) made of aluminum or an aluminum alloy,

[0014] - an internal layer (12) comprising aluminum oxide disposed on the body (10),

[0015] - an intermediate layer (14) comprising chromium and zirconium, and

[0016] - an outer layer (16) comprising tungsten oxide, the inter layer medial (14) being disposed between the inner layer (12) and the outer layer (16).

[0017] The invention also relates to a method for post-anodization sealing of a body (10) made of aluminum or an aluminum alloy, comprising at least the following steps:

[0018] A) a step of impregnating the anodized aluminum or aluminum alloy, in an aqueous bath of demineralized water containing

[0019] - a hexafluorozirconate salt selected from the group consisting of hexafluoro- ammonium zirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), hexa- potassium fluorozirconate (K2ZrF6), and

[0020] - a trivalent chromium salt selected from the group consisting of CrF3,xH2O, CrCl3,xH2 O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3(OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O,

[0021] at a temperature between 20 and 80°C;

[0022] B) a sealing step carried out in an aqueous solution of deionized water having a conductivity less than or equal to 100 pS / cm containing between 1 and 200 g / L, preferably between 5 and 100 g / L, of an alkali metal or alkali-earth metal tungstate, at a temperature between 60 and 100°C;

[0023] C) a post-clogging rinsing step with deionized water having a conductivity less than or equal to 100 pS / cm and at a temperature between 15 and 75°C.

[0024] Intermediate rinses, in particular with demineralized water, can be carried out:

[0025] - between steps A) and B), and / or

[0026] - before and / or after treatment of the part by anodization.

[0027] Since anodizing layers have a very porous structure, when chemical and / or corrosion resistance is of prime importance, the anodizing layer must be sealed. This involves the aluminum oxide layer being transformed into an aluminum hydroxide complex where the pores are closed. Therefore, sealing, in addition to anodizing, is crucial for the quality of the anodizing layer because: • sealing of pores leads to increased corrosion resistance; • fouling is avoided; • leaching of dyes out of the pores is avoided.

[0028] The post-anodization sealing process of the invention makes it possible to obtain a coating having very high anti-corrosion properties on so-called difficult aluminum alloys such as, for example, 2618A and 2214, but also on the most common aluminum alloys in the aeronautical field, such as 2024 or 7175 for example.

[0029] The sealing process of the invention can be applied to different anodizations known to those skilled in the art, among which we can cite OAC (Chromic Anodic Oxidation), OAD (Hard Anodic Oxidation), OAS (Sulfuric Anodic Oxidation), OAST (Sulfo-Tartaric Anodic Oxidation), OAS NG FE (New Generation Fine Thickness Anodic Sulfuric Oxidation), OAS NG (New Generation Sulfuric Anodic Oxidation), TSA (Tartaric Sulfuric Anodizing).

[0030] The invention also relates to a method for manufacturing a part (1) based on of aluminum or an aluminum alloy according to the invention, comprising at least the following steps:

[0031] i) subjecting a body (10) made of aluminum or an aluminum alloy to an anodizing step, having possibly previously undergone a surface preparation step (degreasing, then pickling);

[0032] ii) treatment of the anodized body comprising an internal layer (12) by a post-anodization sealing process according to the invention, to successively obtain an intermediate layer (14) comprising chromium and zirconium, and an external layer (16) comprising tungsten oxide; and optionally

[0033] iii) application of a layer of paint (18).

[0034] The invention further relates to a part according to the invention, possibly comprising a layer of paint and intended for the aeronautical sector. Brief description of the figures

[0035] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

[0036] [Fig. 1] schematically represents the steps for carrying out a surface treatment of test pieces in 2618 T6 and 2024 aluminum alloys, implementing the post-anodization sealing process of the invention.

[0037] [Fig.2] represents the treatment range and the operating conditions of the tests carried out. SOCOCLEAN A3432 from SOCOMORE is a degreaser compatible with aluminum and its alloys. SOCOSURF A1858-A1806 from SOCOMORE is a two-component bath for deoxidizing or bleaching aluminum and its alloys after degreasing or alkaline pickling. The SOCOSURF TCS bath from SOCOMORE impregnates the pores of the oxide layer obtained after OAS (Sulfuric Anodic Oxidation) anodizing with Cr(III) and Zirconium. The porous layer is finally sealed with an aqueous solution of sodium tungstate.

[0038] [Fig.3] is a schematic cross-sectional representation of a part according to one of the embodiments of the invention. Detailed description of the invention

[0039] A first object of the invention relates to a part (1) based on aluminum or an aluminum alloy, characterized in that it comprises:

[0040] - a body (10) made of aluminum or an aluminum alloy,

[0041] - an inner layer (12) comprising aluminum oxide disposed on the body (10),

[0042] - an intermediate layer (14) comprising chromium and zirconium (CrIU / Zr), and

[0043] - an outer layer (16) comprising tungsten oxide, the inter layer medial (14) being disposed between the inner layer (12) and the outer layer (16).

[0044] As indicated, the part (1) according to the invention comprises a body (10) made of aluminum or an aluminum alloy.

[0045] The aluminum alloy may be chosen from the 2000, 6000 and 7000 series, in particular chosen from the group consisting of 2014, 2017, 2024, 2214, 2219, 2618, AU5NKZr, 7175, 5052, 5086, 6061, 6063, 7010, 7020, 7050, 7050 T7451, 7055, 7068, 7085, 7075, 7175 and 7475, aluminum casting alloys chosen from the group consisting of AS7G06, AS7G03, AS10G and AS9U3, so-called difficult aluminum alloys chosen from the group consisting of 2618A, 2214 and AU5NKZr.

[0046] The part (1) comprises a body (10) on which an internal layer (12) is arranged. The layer (12) comprises aluminum oxide. The internal layer (12) has a thickness of between 2 and 30 μm, preferably between 5 μm and 25 μm. The internal layer (12) is obtained by anodic oxidation of the body (10) by one of the aforementioned anodizing processes known to those skilled in the art.

[0047] The part (1) according to the invention comprises an intermediate layer (14) comprising chromium and zirconium. The thickness of the intermediate layer (14) is between 1 μm and 10 μm, for example between 3 μm and 5 μm. The intermediate layer (14) is obtained at the end of the impregnation step (A) of the body (10) comprising the internal layer (12) under the conditions described below.

[0048] The part (1) according to the invention also comprises an outer layer (16). The intermediate layer (14) is arranged between the inner (12) and outer (16) layers. The outer layer (16) comprises tungsten oxide. The outer layer (16) has a thickness of between 10 and 500 nm, preferably 200 nm. The outer layer (16) is obtained at the end of the sealing step (B) of the body (10) successively comprising an inner layer (12) and an intermediate layer (14) under the conditions described below.

[0049] According to one embodiment of the invention, the part (1) further comprises a layer of paint (18). The layer of paint (18) is arranged on the external layer (16) comprising tungsten oxide. The layer of paint (18) has a thickness of between 10 μm and 100 μm, preferably 50 μm. A part (1) according to this embodiment of the invention is illustrated in [Fig. 3]. In the case where several layers of paint are deposited, the total thickness of the layers is between 10 and 100 μm, preferably 50 μm.

[0050] The part (1) according to the invention has good corrosion resistance and good adhesion to paint. Indeed, the combination of the intermediate layer (14) comprising chromium and zirconium (CrIU / Zr) and the external layer (16) comprising tungsten oxide, provides good corrosion resistance of the part (1). In addition, the outer layer 16 promotes the adhesion of the paint layer (18).

[0051] The present invention relates to a method for post-anodization sealing of a body (10) made of aluminum or an aluminum alloy, comprising at least the following steps:

[0052] A) a step of impregnating the anodized aluminum or aluminum alloy, in an aqueous bath of demineralized water containing

[0053] - a hexafluorozirconate salt selected from the group consisting of hexafluoro- ammonium zirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexa-fluorozirconate (K2ZrF6), and

[0054] - a trivalent chromium salt selected from the group consisting of CrF3,xH2O, CrCl3,xH2 O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3(OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O,

[0055] at a temperature between 20 and 80°C;

[0056] B) a sealing step carried out in an aqueous solution of deionized water having a conductivity less than or equal to 100 pS / cm containing between 1 and 200 g / L, preferably between 5 and 100 g / L of an alkali metal or alkali-earth metal tungstate, at a temperature between 60 and 100°C;

[0057] C) a post-clogging rinsing step with deionized water having a conductivity less than or equal to 100 pS / cm and at a temperature between 15 and 75°C.

[0058] The deposition of tungstate on the surface of an anodization impregnated with CrIII / Zr makes it possible to improve the resistance of the aluminum or aluminum alloy part to corrosion as well as the adhesion of the paint to said part.

[0059] Furthermore, due to its relative hardness, tungstate can affect other properties of an aluminum or aluminum alloy part, such as friction, surface hardness, etc.

[0060] As already indicated, intermediate rinses, in particular with demineralized water, can be carried out

[0061] - between steps A) and B), and / or

[0062] - before and / or after treatment of the part by anodization.

[0063] The optimized sealing process of the invention can be suitable for any type of aluminum alloy including so-called "difficult" alloys, in particular aluminum alloys of the 2000, 6000 and 7000 series, previously anodized by different processes including OAC, OAD, OAS, OAST, OAS NG FE, OAS NG, TSA.

[0064] Furthermore, the post-anodization sealing process of the invention is compatible with the requirements associated with the European REACH regulation and leads to good anti-corrosion protection on so-called “difficult” aluminum alloys (e.g. example, 2618A, 2214 and AU5NKZr). This object process may or may not be followed by an application of paint.

[0065] Thus, the post-anodization sealing process of the invention makes it possible to obtain a coating (external layer (16)) having very high anti-corrosion properties on aluminum alloys of the 2000, 6000 and 7000 series and difficult aluminum alloys such as 2618A and 2214, but also on the most common aluminum alloys in the aeronautical field, such as 2024 and 7175.

[0066] In the impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L. The concentration of trivalent chromium salt in this step is between 0.1 and 50 g / L.

[0067] The trivalent chromium salt may be, for example, one of the following commercial products: Surtec 650 from the company SURTEC, Lanthane 613.3 from the company COVENTYA, TCS from the company SOCOMORE.

[0068] In the impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L, for example equal to 2 g / L.

[0069] The temperature of the bath in step A) may be between 20 and 80°C. According to one embodiment, the temperature of the bath in step A) is between 20 and 60°C. According to another embodiment of the invention, the temperature of the bath in step A) is between 35 and 45°C.

[0070] The pH of the bath in step A) is between 3 and 5, preferably between 3.5 and 4.5, for example between 3.7 and 4.2.

[0071] The duration of the impregnation, in the bath in step A) is between 1 and 40 minutes, preferably between 5 and 30 minutes, for example between 5 and 20 minutes.

[0072] After step A) and before step B), the method of the invention may optionally comprise a step of immersion in a bath of lanthanum salt and hydrogen peroxide. This immersion step is well known to those skilled in the art for reinforcing the corrosion resistance properties of the body (10).

[0073] The alkali metal or alkaline earth metal tungstate in step B) is selected from the group consisting of lithium tungstate (Li2WO4), sodium tungstate (Na2 WO4), potassium tungstate (K2WO4), calcium tungstate (CaW04), zirconium tungstate (Zr(WO4)2), ammonium tungstate potassium tungstate ((NH4)i0H2(W2O 7)0)-

[0074] The alkali metal or alkaline earth metal tungstate may be, for example, one of the following commercial products: lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), calcium tungstate (CaW04), ammonium tungstate potassium tungstate ((NH4)ioH2(W207)6) of the Sigma Aldrich brand of the Merck company, and zirconium tungstate (Zr(WO4)2) of the Fisher company Scientific.

[0075] The sealing of step B) is carried out in an aqueous solution of deionized water having a conductivity less than or equal to 100 pS / cm, preferably between 1 and 100 pS / cm, for example between 1 and 50 pS / cm.

[0076] The concentration of alkali metal or alkaline earth metal tungstate in the sealing step B) is preferably between 1 and 50 g / L, for example between 5 and 50 g / L.

[0077] In one embodiment of the invention, the temperature of the aqueous solution of step B) is between 80 and 100°C. In another embodiment of the invention, the temperature of the aqueous solution of step B) is between 80 and 98°C.

[0078] The longer the duration of the sealing step B), the more tungstate will be deposited. The duration of the sealing step B) is between 1 and 40 minutes, preferably between 5 and 35 minutes, for example between 5 and 30 minutes.

[0079] The pH of the sealing solution is less than 12, for example between 7 and 11.5. In one embodiment of the invention the pH of the sealing solution is between 8 and 9.

[0080] The sealing is followed by a rinsing step C) in deionized water having a conductivity less than or equal to 100 pS / cm, preferably between 1 and 100 pS / cm, more preferably between 10 and 100 pS / cm, for example between 10 and 50 pS / cm.

[0081] Post-clogging rinsing is preferably carried out at a temperature between 10 and 75°C, for example between 15 and 60°C.

[0082] The pH of the water in step C) is between 4.5 and 8.5, preferably between 5 and 8, for example between 5.5 and 7.5.

[0083] The duration of the post-clogging rinse is between 10 seconds and 10 minutes, preferably between 10 seconds and 5 minutes, for example between 30 seconds and 2 minutes.

[0084] It has been found, quite unexpectedly, that the combination of the steps of impregnation A) + sealing B) + post-sealing rinsing C), as described below, is essential to guarantee good anti-corrosion performance of aluminum or aluminum alloy.

[0085] Furthermore, the application of the alkali metal or alkaline earth metal tungstate must be carried out after the impregnation step A), in order to obtain the desired anti-corrosion properties and performances of the aluminum or aluminum alloy.

[0086] Intermediate rinses, in particular with demineralized water, can be carried out between the steps described above.

[0087] Before subjecting the body (10) made of aluminum or an aluminum alloy to the anodizing step, said body may be subjected to a surface preparation step by degreasing and / or pickling in order to remove the grease, dirt and oxides present on its surface.

[0088] This preliminary surface preparation step may include one or more of the following operations: - solvent degreasing, to dissolve greases present on the surface of the aluminum or aluminum alloy. This operation can be carried out by dipping, spraying, or any other method known to those skilled in the art; - alkaline degreasing, to dissolve greases present on the surface of the aluminum or aluminum alloy. This operation can be carried out by dipping, spraying, or any other technique known to those skilled in the art; - alkaline pickling, to dissolve the oxides naturally formed on the surface of the aluminium or aluminium alloy. This operation can be carried out by dipping, spraying, or any other technique known to those skilled in the art. At the end of this operation, the aluminium or aluminium alloy is covered with a powdery layer formed from oxidation products of the intermetallic compounds, which should be removed by an acid pickling step; - acid pickling, to dissolve the oxides naturally formed on the surface of the aluminium or aluminium alloy, and / or the oxidation layer formed on the surface of the part during the alkaline pickling step. This operation can be carried out by dipping, spraying, or any other technique known to those skilled in the art.

[0089] The preliminary step of preparing the surface of the body (10) made of aluminum or an aluminum alloy by degreasing and / or pickling to remove the grease, dirt and oxides present on its surface can be carried out under the conditions described, for example, in application WO 2013 / 117759.

[0090] Intermediate rinses, in particular with demineralized water, are preferably carried out between the successive steps above, and before the treatment of the part by anodization.

[0091] Before applying the sealing method of the invention, the body (10) made of aluminum or an aluminum alloy, optionally subjected to a surface preparation step by degreasing and / or pickling by one or more of the operations described above, is anodized. Any type of anodization on aluminum known to those skilled in the art may be suitable.

[0092] Different processes have been proposed to replace OAC and OAS (Anodic Sulfur Oxidation) treatments clogged with hexavalent chromium, impacted by the European REACH regulation:

[0093] - the OAS NG (New Generation Sulfuric Anodic Oxidation as described for example in "a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anod isation-sulfurique-version-5-2") was proposed to replace the OAS;

[0094] - OAST (Anodic SulfoTartaric Oxidation as described, for example, in "a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anod isation-sulfo-tartrique-oast-tartric-sulfuric-anodizing-tsa") was proposed to replace the 0AC; and

[0095] - OAS NG FE: New Generation Sulfuric Anodic Oxidation Fine E thickness, which is an OAS NG type anodization whose anodization parameters (Voltage, Immersion time) have been adapted to obtain an anodization layer whose thickness is between 2 and 7 pm.

[0096] In the context of the present invention, the anodizing processes OAST, OAS NG FE, OAS NG are preferred.

[0097] The surface treatment method of the invention significantly improves the corrosion resistance properties of aluminum or aluminum alloy parts and complies with the requirements of the European REACH regulation.

[0098] Furthermore, the method of the invention makes it possible to achieve better conditions with regard to paint adhesion. In addition, due to its chemical nature, it is likely that tungstate can lead to a functionalization of the surface such as an improvement in surface hardness or an improvement in friction properties.

[0099] The invention also relates to a method for manufacturing a part (1) based on aluminum or an aluminum alloy according to the invention, comprising at least the following steps:

[0100] i) subjecting a body (10) made of aluminum or an aluminum alloy to an anodizing step, having possibly previously undergone a surface preparation step (degreasing, then pickling);

[0101] ii) treatment of the anodized body comprising an internal layer (12) by a post-anodization sealing process according to the invention, to successively obtain an intermediate layer (14) comprising chromium and zirconium, and an external layer (16) comprising tungsten oxide; and optionally

[0102] iii) application of a layer of paint (18).

[0103] The invention further relates to a part according to the invention, possibly comprising a layer of paint and intended for the aeronautical sector. EXAMPLES Example 1:

[0104] Surface treatment method of aluminum alloy part

[0105] Rolled parts made of 2618 T6, 7175 T73 aluminum alloy and 2024 T351 aluminum alloy machined on one of the two faces with dimensions of 120x60x2 mm are treated according to the method described below.

[0106] Surface preparation steps for the part are first carried out successively: - alkaline degreasing, by soaking the part in a bath of SOCOCLEAN A3432 at 11% vol / vol (from the company SOCOMORE) at a temperature of 45°C, for 10 minutes; - rinsing with tap water or demineralized water; - acid pickling, by soaking the part in a solution of SOCOSURF A1858 at 42% vol / vol - A1806 at 10% vol / vol (from the company SOCOMORE) at a temperature of 50°C, for 10 minutes; - rinse with tap water or demineralized water.

[0107] The pickled and rinsed parts are then subjected to an anodizing process known to those skilled in the art, during which the parts are immersed in an aqueous bath comprising sulfuric acid at a concentration of between 160 g / L and 220 g / L, for example equal to 180 g / L. This bath is brought to and maintained at a temperature of 18°C. A direct voltage is applied to the immersed parts according to the following voltage profile: voltage rise from a value of 0V, at a speed of 0.4 V / min until reaching a so-called plateau voltage value of 6V. The voltage is maintained at the plateau value for 50 minutes. An anodic layer with a thickness of 4 to 7 μm is formed on the surface of the parts.

[0108] The thickness of the anodic layer formed on the part is measured by eddy current according to the ISO2360 standard.

[0109] The anodized parts are then subjected to one or more rinses, preferably with demineralized water, then to the impregnation and sealing operations in accordance with the invention under the conditions and in the order indicated below: - step A): a step of impregnating said parts, successively, in an aqueous bath of SOCOSURF TCS at 34% vol / vol (from the company SOCOMORE), at a temperature of 40°C for 10 minutes and at a pH of 3.9, and an aqueous bath of SOCOSURF PACS at 10% vol / vol (from the company SOCOMORE) at a temperature of 25°C for 5 minutes at a pH between 4.5 and 5.5, then - step B): sealing by immersion of the parts at the end of the impregnation step A) in an aqueous sodium tungstate solution at 20 g / L at a temperature of 98°C for 10 minutes and at a pH below 12, for example, at a pH of 8 to 9, then - step C): post-sealing rinsing by immersing the parts at the end of these operations in deionized water with a conductivity of less than 100 pS / cm, at a temperature of 20°C for 1 minute.

[0110] Between each step, a rinse with demineralized water is carried out. [YES] Corrosion resistance results evaluated on anodized and sealed alloys by conventional sealing methods and by the sealing method of the invention:

[0112] For comparison, the aluminum alloy parts anodized according to the method indicated above are then subjected to sealing operations such as hydrothermal sealing (CrIII / Zr + Water) and silicate sealing (CrlIVZr + Silicate) according to the methods known to those skilled in the art (WO2021 / 152241 for silicate sealing), and compared to the parts sealed by the process of the invention (CrlIV / Zr + sodium tungstate). The parts thus treated are subjected to a salt spray (SSP) resistance test in accordance with standard NF EN ISO 9227. The number of pits at 500 hours of salt spray (SSP) is reported in Table 1 below.

[0113] [Tables 1] Configuration Alloy 500h BS OASfine+CrIII / Zr+tungstate (according to the invention) 2618 1 / 0 0 / 1 / 2 2024 1 / 0 1 / 1 / 3 7175 0 / 1 / 1 OASfine+CrIII / Zr+Si (Reference 1) 2618 2 / 1 2024 2 / 1 / 0 / 0 7175 2 / 1 / 1 OASfine+CrIII / Zr+H2O (Reference 2) 2618 2 / 2 / 3 7175 3 / 3 / 8 2024 3 / 3 / 6

[0114] In view of the results, two observations can be identified:

[0115] 1 - Comparing the Reference 2 configuration with the others, in view of the number of pitting by test piece, it is visible that tungstate and silicate sealants improve the corrosion resistance of the anodizing layer.

[0116] 2 - Comparing Reference 1 with the Tungstate configuration in view of the number of pitting per test piece, the corrosion resistance of tungstate-sealed anodization is equivalent to the resistance of silicate-sealed layers.

[0117] Thus, the anodizing treatment followed by sealing according to the invention based on tungstate obtains anti-corrosion performances which are much better than hydrothermal sealing and comparable to those of a silicate-based sealing as described in WO2021 / 152241. Extreme surface analysis (XPS and EDX)

[0118] X-ray photoelectron spectrometric (XPS) analyses were performed on a THERMO K-alpha+ instrument with monochromatized Al Kalpha source. Processing software: Advantage.

[0119] X-ray photoelectron spectrometric (XPS) analyses were carried out on:

[0120] - the surface of a sample having undergone anodization sealed with CrIII / Zr+tungstate (named 109 - NaW in Table 2 below);

[0121] - the surface of a sample having undergone anodization sealed with CrIII / Zr+H2O (named 110 - HWS in the table below);

[0122] in order to have a reference on the presence of tungstate on the surface compared to another type of clogging.

[0123] [Tables2] Samples Al(III)* Cr(ni)* F Na O Si W Zr 109-NaW 2.19 2.69 1.48 1.1 74.47 1.07 7.19 7.42 110-HWS 3.54 3.36 4.68 - 74.27 1.71 - 12.44

[0124] Al(III)* and Cr(III)* mean that these elements are present in oxide or hydroxide form.

[0125] As shown by the elemental analysis of the extreme surface (50 nm surface), an enrichment in tungsten in oxidized form (presence of oxygen) is detectable: W = 7.19 and O = 74.47. This confirms the presence of tungstate on the surface.

[0126] Deposition of tungstate on the surface of a Cr(III) / Zr impregnated anodization improves the corrosion resistance of a chrome-free anodization. Paint adhesion tests

[0127] Paint adhesion tests were carried out on parts that had been treated using a sealing process according to the invention. The painted parts were characterized after immersion in various aggressive fluids (water, Skydrol). The adhesion rating for this type of test according to ISO 2409 ranges from 0 (no degradation) to 5 (total degradation) Dry After immersion in water (2 weeks at room temperature 20+5°C) Skydrol (2 weeks at 70°C) Alloy primer Primer+topcoat primer Primer+topcoat primer Primer+topcoat 2024 0 0 0 0 0 0 2618 0 0 0 0 0 0

[0128] All the results show very good adhesion of the paint tested with tungstate sealing.

Claims

Claims

1. Part (1) based on aluminum or an aluminum alloy, characterized in that it comprises: - a body (10) made of aluminum or an aluminum alloy, - an internal layer (12) comprising aluminum oxide arranged on the body (10), - an intermediate layer (14) comprising chromium and zirconium, and - an external layer (16) comprising tungsten oxide, the intermediate layer (14) being arranged between the internal layer (12) and the external layer (16).

2. Part according to claim 1, characterized in that it comprises a layer of paint (18) arranged on the external layer (16).

3. Part according to one of claims 1 or 2, characterized in that the aluminum alloy is - an aluminum alloy selected from the group consisting of 2014, 2017, 2024, 2214, 2219, 2618, AU5NKZr, 7175, 5052, 5086, 6061, 6063, 7010, 7020, 7050, 7050 T7451, 7055, 7068, 7085, 7075, 7175 and 7475, - an aluminum casting alloy selected from the group consisting of AS7G06, AS7G03, AS10G and AS9U3, and - a so-called difficult aluminum alloy selected from the group consisting of 2618A, 2214 and AU5NKZr.

4. Part according to any one of claims 1 to 3, characterized in that the external layer (16) has a thickness of between 10 and 500 nm.

5. Part according to one of claims 2 and 3, characterized in that the layer of paint (18) has a thickness of between 10 and 100 μm.

6. Method for post-anodization sealing of a body (10) made of aluminum or an aluminum alloy, comprising at least the following steps: A) a step of impregnating the body (10) made of anodized aluminum or aluminum alloy comprising an internal layer (12) comprising aluminum oxide, in an aqueous bath of demineralized water containing - a hexafluorozirconate salt selected from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6), and - a trivalent chromium salt selected from the group consisting of CrF3,xH2O, CrCl3,xH2O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3 (OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O, at a temperature between 20 and 80°C; B) a sealing step carried out in an aqueous solution of deionized water having a conductivity less than or equal to 100 pS / cm containing between 1 and 100 g / L of an alkali metal or alkaline earth metal tungstate, at a temperature between 60 and 100°C; C) a post-sealing rinsing step with deionized water having a conductivity less than or equal to 100 pS / cm and at a temperature between 15 and 75°C.

7. 7. Method according to claim 6, characterized in that in the impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L.

8. Method according to one of claims 6 or 7, characterized in that in the impregnation step A), the concentration of trivalent chromium salt in this step is between 0.1 and 50 g / L.

9. A method according to any one of claims 6 to 8, characterized in that the alkali metal or alkaline earth metal tungstate in step B) is selected from the group consisting of lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2 WO4), calcium tungstate (CaW04), zirconium tungstate (Zr(WO4)2), ammonium tungstate ((NH4)iOH2(W2O7)6).

10. Method according to any one of claims 6 to 9, characterized in that after step A) and before step B), the method comprises a step of immersion in a bath of lanthanum salt and hydrogen peroxide.

11. Method for manufacturing a part (1) based on aluminum or an aluminum alloy according to any one of claims 1 to 5, comprising at least the following steps: i) subjecting a body (10) made of aluminum or an aluminum alloy to an anodization step, having optionally previously undergone a surface preparation step (degreasing, then pickling); ii) treating the anodized body comprising an internal layer (12) by a post-anodization sealing process according to any one of claims 6 to 9, to successively obtain an intermediate layer (14) comprising chromium and zirconium, and an external layer (16) comprising tungsten oxide; and optionally iii) applying a layer of paint (18).

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