Method for post-anodisation sealing of aluminium and aluminium alloys without using chromium

Abstract

The invention relates to a method for post-anodisation sealing of aluminium or aluminium alloy without using chromium. The invention also relates to a method for treating the surface of a part made of aluminium or aluminium alloy intended for use in the aviation sector, including at least the following steps: i) subjecting the part to an anodisation step; ii) treating the anodised part by a post-anodisation sealing method according to the invention; and optionally iii) applying one or more coat(s) of paint. The invention also relates to a part made of aluminium or aluminium alloy treated by a post-anodisation sealing method according to the invention, optionally including one or more coat(s) of paint.

Description

[0001]DESCRIPTION TITLE: METHOD FOR POST-ANODIZING SEALING OF ALUMINUM AND ALUMINUM ALLOYS WITHOUT USING CHROMIUM Technical field of the invention The present invention falls within the scope of the search for new solutions for the anti-corrosion protection of aluminum or aluminum alloys, in particular for aeronautical applications, with or without the application of a paint system. In particular, the method of the invention makes it possible to obtain a coating having very high anti-corrosion properties on aluminum or aluminum alloys, without using chromium. Technical background The technical background includes in particular documents US-A1-2002 / 117 236, US-A1-2006 / 191 599, US-B-6,663,700, US-A1-2016 / 047 057 and WO-A1-2013 / 117767.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 susceptible, depending on the environment in which they are found, to be affected by several types of localized corrosion, causing the 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 the alloys is the most used. This is particularly the case for protective layers obtained by the anodizing process of aluminum alloys.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 OAC type anodizing treatment (Chromic Anodic Oxidation as described, for example, in www.a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anodisation-chromique, but also in the usual surface preparation pre-treatments, 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. Different processes have therefore been proposed to replace the OAC and OAS (Sulfuric Anodic Oxidation) treatments sealed with hexavalent chromium, impacted by the European REACH regulation: • OAS NG (new generation sulfuric anodic oxidation as described for example in www.a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anodisation-sulfurique-version-5-2) has been proposed to replace OAS; • OAST (anodic sulfo-tartaric oxidation as described, for example, in www.a3ts.org / actualite / commissions-techniques / fiches-techniques-traitement-surface / anodisation-sulfo-tartrique-oast-tartric-sulfuric-anodizing-tsa has been proposed to replace OAC. OAC can also be replaced by OAS NG FE (new generation thin anodic sulfuric oxidation), which is an OAS NG type anodization whose anodization parameters (Voltage, Immersion time) have been adapted to obtain an anodization layer with a thickness between 2 and 7 µm. Although current conventional anodizing solutions, such as OAS NG followed by hot water sealing, implement treatment ranges compatible with the European REACH regulation, they nevertheless remain unsatisfactory or unsatisfactory 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. For these alloys, it is therefore necessary to optimize the anodizing ranges in order to improve anti-corrosion performance. 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, comprises 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. Despite the improvement in anti-corrosion performance provided by this process, there is a risk of long-term obsolescence of trivalent Cr. Indeed, the Cr impregnation step. III / Zr, which is mandatory to allow the silicate sealing step to take place, may be subject to obsolescence (due to chromium) in the future with the tightening of environmental regulations. To prevent the long-term obsolescence of trivalent Cr and to be able to comply with the future tightening of environmental regulations, it is necessary to optimize the sealing process for anodized aluminum or aluminum alloys, in particular by avoiding the use of chromium. There is therefore a real need for a sealing process for aluminum or aluminum alloys, including so-called "difficult" anodized aluminum alloys, which does not use chromium.In particular, there is a real need for a post-anodization sealing process as described above, which provides good anti-corrosion performance of aluminum or aluminum alloy including so-called "difficult" aluminum alloys, while complying with the requirements of current and future REACH regulations.Summary of the invention The present invention aims precisely to meet these needs, in particular, in terms of corrosion resistance of aluminum alloys, in particular of the 2xxx, 6xxx and 7xxx series, aluminum casting alloys such as AS7G06, AS7G03, AS10G or AS9U3, aluminum alloys resulting from processes such as additive manufacturing, and so-called difficult aluminum alloys, by providing a method for post-anodization sealing of aluminum or aluminum alloy, comprising at least the following steps: A) a step of impregnating the anodized aluminum or aluminum alloy, in an aqueous bath of demineralized water containing • a hexafluorozirconate salt chosen from the group consisting of hexafluorozirconate of. aammonium ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6), and • a di-, tri-, tetra-, or hepta-valent manganese salt selected from the group consisting of lithium permanganate (LiMnO4), sodium permanganate (NaMnO4), potassium permanganate (KMnO4), ammonium permanganate (NH4MnO4), manganese chloride (MnCl2(H2O)x, where the value of x is 0, 2, or 4), or • a hexafluorozirconate salt selected from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6), and • a tungsten salt selected from the group consisting of lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), calcium tungstate (CaWO4), zirconium tungstate (Zr(WO4)2), ammonium tungstate potassium tungstate ((NH4) 10H2(W2O7)6), 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 µS / cm containing between 1 and 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 and 100°C; C) a post-sealing rinsing step in deionized water having a conductivity less than or equal to 100 µS / cm and at a temperature between 15 and 75°C. In the impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L. The concentration of manganese salt in this step is between 0.1 and 50 g / L. Intermediate rinses, in particular with demineralized water, are preferably carried out - between steps A) and B), and / or - before and / or after the part is treated by anodization.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 the pores leads to an increase in corrosion resistance; • fouling is prevented; •the leaching of dyes out of the pores is avoided. 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. The sealing process of the invention can be applied to different anodizations known to those skilled in the art, among which we can cite OAST, OAS NG FE, OAS NG. It may or may not be followed by an application of paint. The process of the invention demonstrates the possibility of using other types of chromium-free impregnation layers allowing the silicate to be deposited on the surface, such as, for example, an Mn / Zr impregnation.Knowing that a sulfuric anodization is composed of a layer of aluminum oxide, the inventors sought to know a) if it was possible to deposit silicate on a chromium-free impregnation layer, for example a Mn / Zr impregnation layer instead of Cr. III / Zr, and b) if so, whether this deposition would improve the corrosion resistance of the chromium-free impregnation layer, for example a Mn / Zr impregnation layer, as is the case for a Cr impregnation layer III / Zr. The invention also relates to a method for surface treatment of an aluminum or aluminum alloy part intended to be used in the aeronautical sector comprising at least the following steps: i) subjecting said part to an anodization step, having possibly previously undergone a surface preparation step (degreasing, then pickling); ii) treatment of the anodized part by a post-anodization sealing method according to the invention; and possibly iii) application of one or more layers of paint. Another subject of the invention is the use of a post-anodization sealing method according to the invention, in the surface treatment of aluminum or aluminum alloy parts intended for the aeronautical sector. The invention also relates to an aluminum or aluminum alloy part treated by a post-anodization sealing method according to the invention,possibly comprising one or more layers of paint and intended for the aeronautical sector. Brief description of the figures 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: [Fig.1] Figure 1 represents a theoretical diagram comparing the deposition of the species on the anodizing layer (OA) by the sealing process described in FR3106838B1 and by the sealing process of the invention. Detailed description of the invention The present invention aims precisely to meet the needs of the state of the art, in particular, in terms of corrosion resistance of aluminum alloys, in particular of the 2xxx, 6xxx and 7xxx series, foundry alloys, and aluminum alloys resulting from processes such as additive manufacturing and so-called difficult aluminum alloys,by providing a method for post-anodization sealing of aluminum or aluminum alloy, comprising at least the following steps: A) a step of impregnating the anodized aluminum or aluminum alloy, 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 di-, tri-, tetra-, or hepta-valent manganese salt selected from the group consisting of lithium permanganate (LiMnO4), sodium permanganate (NaMnO4), potassium permanganate (KMnO4), ammonium permanganate (NH4MnO4), manganese chloride (MnCl2(H2O)x, where the value of x is 0, 2 or 4), or • a hexafluorozirconate salt selected from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6),and • a tungsten salt chosen from the group consisting of lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), calcium tungstate (CaWO, 4) , zirconium tungstate (Zr(WO4)2), ammonium tungstate, potassium tungstate ((NH4) 10H2(W2O7)6), 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 µS / cm containing between 1 and 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 and 100°C; C) a post-sealing rinsing step in deionized water having a conductivity less than or equal to 100 µS / cm and at a temperature between 15 and 75°C. The di-, tri-, tetra-, or hepta-valent manganese salt may be, for example, one of the following commercial products: Bonderite M-ED160 / 161. The tungstate salt may be, for example, one of the following commercial products: lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), calcium tungstate (CaWO 4) , ammonium tungstate potassium tungstate ((NH4) 10H2(W2O7)6) of the Sigma Aldrich brand of the Merck company, and zirconium tungstate (Zr(WO4)2) of the Fisher Scientific company. Intermediate rinses, in particular with demineralized water, are preferably carried out - between steps A) and B), and / or - before and / or after the treatment of the part by anodization. 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 2xxx, 6xxx and 7xxx series, previously anodized by different processes, for example, by the OAST (sulfo-tartaric anodic oxidation), OAS NG FE (new generation thin thickness sulfuric anodic oxidation) or OAS NG (new generation sulfuric anodic oxidation) processes.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 (for example, 2618A, 2214 and AU5NKZr). This process may or may not be followed by an application of paint. Thus, the post-anodization sealing process of the invention makes it possible to obtain a coating having very high anti-corrosion properties on aluminum alloys of the 2xxx, 6xxx and 7xxx series and difficult aluminum alloys, but also on the most common aluminum alloys in the aeronautical field, such as 2024 and 7175 and so-called "difficult" aluminum alloys such as 2618A and 2214.The method of the invention is more particularly suitable for parts made of aluminum and aluminum alloy of the 2xxx, 6xxx and 7xxx 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 type AS7G06, AS7G03, AS10G and AS9U3, aluminum alloys resulting from processes such as additive manufacturing. 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. The concentration of di-, tri-, tetra-, or hepta-valent manganese salt, or tungstate salt in this step is between 0.1 and 50 g / L, for example equal to 1 g / L. The temperature of the bath in the step. A)may be between 20 and 80°C, preferably between 20 and 60°C, more preferably between 35 and 60°C, for example between 35 and 45°C. 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. 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. The impregnation step A) is followed by a step B) which is a sealing step. The sealing of step B) is carried out in an aqueous solution of deionized water having a conductivity less than or equal to 200 µS / cm, preferably between 1 and 100 µS / cm, for example between 1 and 50 µS / cm. The temperature of the aqueous solution of step B) is preferably between 80 and 100°C, for example between 80 and 98°C.The alkali metal or alkaline earth metal silicate may be selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, calcium silicate and magnesium silicate. In the sealing step B), the concentration of alkali metal or alkaline earth metal silicate in the solution is preferably between 1 and 500 g / L, for example between 5 and 100 g / L. 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. The pH of the sealing solution is between 9 and 12, preferably between 10 and 11.5 minutes, for example between 10.5 and 11.4. The sealing is followed by a rinsing step C) in deionized water having a conductivity less than or equal to 100 µS / cm, preferably between 1 and 100 µS / cm, more preferably between 10 and 100 µS / cm, for example between 10 and 50 µS / cm.The post-sealing rinsing is preferably carried out at a temperature between 10 and 75°C, for example between 15 and 60°C. 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. The duration of the post-sealing rinsing is between 10 seconds and 10 minutes, preferably between 10 seconds and 5 minutes, for example between 30 seconds and 2 minutes. It has been found, quite unexpectedly, that the combination of the steps of impregnation + sealing + post-sealing rinsing, as described below, is essential to guarantee good anti-corrosion performance of the aluminum or aluminum alloy. Intermediate rinses, in particular with demineralized water, can be carried out between the steps described above.Boiling water sealing, which has the advantage of not using harmful substances and can significantly improve the corrosion resistance of anodizing layers when properly controlled: sealing must be carried out in demineralized water with a minimum temperature above 75°C, preferably above 90°C, more preferably above or equal to 96°C and a pH between 5.5 and 6.5. The quality of the water used is important for the success of the operation, as some impurities are known to be harmful at very low levels (e.g., Ca. 2+ , Cu 2+ , Fe 2+ , F-, Cl-, SiO3-, PO4 3-). In particular, a particularly harmful effect of silicate, phosphate and chloride ions will be observed. The treatment time is approximately 2.5 min / μm (close to the anodizing time). This operation is a partial thermohydration of the alumina which crystallizes into monohydrated alumina (Böhmite). Before submitting the aluminum or aluminum alloy to the anodizing step, the aluminum or aluminum alloy may be subjected to a surface preparation step by degreasing and / or pickling in order to remove grease, dirt and oxides present on its surface. This preliminary surface preparation step may include one or more of the following operations: - solvent degreasing, to dissolve grease 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 oxides naturally formed 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.At the end of this operation, the aluminum or aluminum alloy is covered with a powdery layer formed from oxidation products of the intermetallic compounds, which must be removed by an acid pickling step; - acid pickling, to dissolve the oxides naturally formed on the surface of the aluminum or aluminum 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. The preliminary step of surface preparation of the aluminum or aluminum alloy by degreasing and / or pickling to remove grease, dirt and oxides present on its surface can be carried out under the conditions described, for example, in application WO 2013 / 117759.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. Before applying the sealing method of the invention, the aluminum or 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. In this respect, mention may be made of. • OAS: Sulfuric Anodic Oxidation (sealing based on Chromium VI, process impacted by European REACH regulations), • OAC: Chromic Anodic Oxidation (Based on Chromium VI, process impacted by European REACH regulations), •OAST: SulfoTartaric Anodic Oxidation, • OAST: SulfoTartaric Anodic Oxidation, • OAS NG FE: New Generation Fine Thickness Sulfuric Anodic Oxidation, • OAS NG: New Generation Sulfuric Anodic Oxidation. In the context of the present invention, the anodizing processes OAST LC, OAS NG FE, OAS NG are preferred. The surface treatment process of the invention significantly improves the corrosion resistance properties of metal or metal alloy parts, in particular aluminum or aluminum alloy parts, and complies with the requirements of the European REACH regulation. The process of the invention is of great interest in any type of industry where it is sought to improve the corrosion resistance properties of metal or metal alloy parts, in particular aluminum or aluminum alloy parts, such as in aeronautics, automotive,in the oil industry etc. The method according to the invention may comprise one or more of the following features and / or steps, taken in isolation from one another or in combination with one another: - the aluminum alloy is an aluminum alloy of the 2xxx, 6xxx and 7xxx 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 type AS7G06, AS7G03, AS10G and AS9U3, alloys of aluminum from processes such as additive manufacturing; - in impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L; - in impregnation step A), the concentration of di-, tri-, tetra-, or hepta-valent manganese salt, or tungstate is between 0,1 and 50 g / L; - the alkali metal or alkaline earth metal silicate is chosen from the group consisting of lithium silicate, sodium silicate, potassium silicate,calcium silicate and magnesium silicate; - the concentration of alkali metal or alkaline earth metal silicate in the solution is between 5 and 100 g / L; - the rinsing step C) is carried out in deionized water having a conductivity of between 1 and 100 µS / cm. The process of the invention makes it possible to obtain corrosion resistance similar to chromated anodization without the use of chromium. It makes it possible to find an innovative sealing solution for the anodization of aluminum alloys thus meeting the specifications of corrosion resistance and paint adhesion without using chromium. The invention also relates to a method for surface treatment of an aluminum or aluminum alloy part intended to be used in the aeronautical sector comprising at least the following steps: i) subjecting said part to an anodization step, having possibly previously undergone a surface preparation step (degreasing,then pickling); ii) treatment of the anodized part by a post-anodization sealing process according to the invention; and optionally iii) application of one or more layers of paint. Another subject of the invention is the use of a post-anodization sealing process according to the invention, in the surface treatment of aluminum or aluminum alloy parts intended for the aeronautical sector. The invention also relates to an aluminum or aluminum alloy part treated by a post-anodization sealing process according to the invention,possibly comprising one or more layers of paint and intended for the aeronautical sector. Anodizing treatment followed by a paint application: certain aeronautical parts have a paint treatment after anodizing in order to reinforce the anti-corrosion protection. The invention is compatible with different painting systems. Other advantages and characteristics of the invention will become apparent upon reading the examples below given for illustrative purposes. EXAMPLES Example 1: Process of, Anodizing aluminum alloy parts 2618 T6 aluminum alloy parts measuring 120x100x5 mm are treated using the methods described below. The surface preparation steps for the part are first carried out successively: - alkaline degreasing, by soaking the part in a solution of SOCOCLEAN A3431 at 11% by volume, at a temperature of 45°C, for 10 minutes; - rinsing with tap water or demineralized water; - acid pickling by soaking the part in a mixture of SOCOSURF A1858 at 42% by volume and SOCOSURF A1806 at 10% by volume at a temperature of 50°C for 10 minutes; - rinsing with tap water or demineralized water. The stripped and rinsed parts are then subjected to a new generation sulfuric anodizing process (standard thickness or thin thickness (FE)). The operating parameters for anodizing are indicated in Table 1 below. [Table 1] The anodized parts according to the invention are then subjected to the sealing process in accordance with the invention under the conditions and in the order indicated below in: - step A): a step of impregnating said parts, successively, in an aqueous bath of BONDERITE M-ED 160 / 161 (15g / L ED160 and 18g / L ED161) at a temperature of 40°C for 10 minutes and at a pH of 3.9, then - step B): sealing by immersing the parts at the end of an aqueous solution of deionized water having a conductivity of less than 100 µS / cm with 80g / L of a sodium silicate, a temperature of 100°C and for 10 minutes; - step C): a post-sealing rinse by immersing the parts after the three previous sealing operations in deionized water with a conductivity lower than 100 µS / cm, at a temperature of 20°C for 1 minute. Between each step, a rinse with demineralized water is carried out. These conditions are indicated in [Table 2]. [Table 2] Corrosion resistance results evaluated on the anodized and sealed alloys by conventional sealing methods and by the process of the invention: For comparison, the aluminum alloy parts anodized according to the method indicated in [Table 1], are then subjected to one or more sealing operations such as hydrothermal sealing (Mn / Zr + Water) according to the methods known to those skilled in the art and compared to the parts anodized and sealed by the process of the invention (Mn / Zr + Silicate). 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 3 below. [Table 3] obtain much better anti-corrosion performance than hydrothermal sealing. In view of these results, the positive impact of silicates is detected and allows to have interesting results close to a chromated impregnation. Extreme surface analysis (XPS) X-ray photoelectron spectrometric analyses (XPS) were carried out on a THERMO K-alpha+ instrument with a monochromatized Al Kalpha source Processing software: Advantage. The surface of a sample having undergone anodization sealed with Mn / Zr+Si (named 2618-T652-047-104 in Table 4 below). [Table 4] As shown by the elemental analysis of the extreme surface (50 nm surface), an enrichment in silicon in oxidized form (presence of oxygen) is detectable. This confirms the presence of silicate on the surface. The deposition of silicate on the surface of an anodization impregnated with Mn / Zr makes it possible to improve the corrosion resistance of an anodization without chromium. The invention proves the possibility of depositing silicate on an impregnation other than Cr III / Zr thus allowing interesting corrosion performances to be achieved without the use of chromium.

Claims

CLAIMS 1. Method for post-anodization sealing of aluminum or aluminum alloy, comprising at least the following steps: A) a step of impregnating the anodized aluminum or aluminum alloy, in an aqueous bath of demineralized water containing • a hexafluorozirconate salt chosen from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6), and • a di-, tri-, tetra-, or hepta-valent manganese salt chosen from the group consisting of lithium permanganate (LiMnO4), sodium permanganate (NaMnO4), potassium permanganate (KMnO4), ammonium permanganate (NH4MnO4), manganese chloride (MnCl2(H2O) x, where the value of x is 0, 2 or 4, or • a hexafluorozirconate salt selected from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6), and • a tungsten salt selected from the group consisting of lithium tungstate (Li2WO4), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), calcium tungstate (CaWO 4), zirconium tungstate (Zr(WO4)2), ammonium tungstate potassium tungstate ((NH4)10H2(W2O7)6), 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 µS / cm containing between 1 and 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 and 100°C; C) a post-sealing rinsing step in deionized water having a conductivity less than or equal to 100 µS / cm and at a temperature between 15 and 75°C.

2. Method according to claim 1, characterized in that the aluminum alloy is an aluminum alloy of the 2xxx, 6xxx and 7xxx 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, alloys of aluminum foundry type AS7G06, AS7G03, AS10G and AS9U3, aluminum alloys resulting from processes such as additive manufacturing 3. Method according to one of claims 1 or 2, characterized in that in the impregnation step A), the concentration of hexafluorozirconate salt is between 0.5 and 50 g / L.

4. Method according to any one of claims 1 to 3, characterized in that in the impregnation step A), the concentration of di-, tri-, tetra- or hepta-valent manganese salt, or of tungstate salt is between 0.1 and 50 g / L.

5. Method according to any one of claims 1 to 4, characterized in that the sealing of step B) is carried out in an aqueous solution of deionized water having a conductivity between 1 and 100 µS / cm. 6.

7. Method according to any one of claims 1 to 6, characterized in that the concentration of alkali metal or alkaline earth metal silicate in the solution is between 5 and 100 g / L.

8. Method according to any one of claims 1 to 7, characterized in that the rinsing step C) is carried out in deionized water having a conductivity of between 1 and 100 µS / cm. 9.Method for surface treatment of an aluminum or aluminum alloy part intended to be used in the aeronautical sector comprising at least the following steps: i) subjecting said part to an anodization step, having possibly previously undergone a surface preparation step (degreasing, then pickling); ii) treatment of the anodized part by a post-anodization sealing method according to any one of claims 1 to 8; and optionally iii) application of one or more layers of paint.

10. Use of a post-anodization sealing method according to any one of claims 1 to 8, in the surface treatment of aluminum or aluminum alloy parts intended for the aeronautical sector.

11. Part made of aluminum or aluminum alloy treated by a post-anodization sealing process according to any one of claims 1 to 8, optionally comprising one or more layers of paint, intended for the aeronautical sector.

Images