EP4097277A1

EP4097277A1 - Method for the surface treatment of aluminium-based parts

Info

Publication number: EP4097277A1
Application number: EP21706344.5A
Authority: EP
Inventors: Jean-Arthur DREVET; Valentin DESPREZ
Original assignee: Safran Aerosystems SAS
Current assignee: Safran Aerosystems SAS
Priority date: 2020-01-31
Filing date: 2021-01-21
Publication date: 2022-12-07
Also published as: WO2021152240A1; US20230064264A1; JP2023511727A; CA3165749A1; FR3106837B1; KR20220134574A; BR112022014784A2; MX2022009223A; CN115053022A; FR3106837A1

Abstract

The invention relates to a method for the surface treatment of a part made of aluminum or aluminum alloy, comprising an anodisation step and a step of sealing with a silicate salt. The invention also relates to the use of a surface treatment method according to the invention for manufacturing an aluminium or aluminium alloy part intended, in particular, for use in the aviation sector.

Description

DESCRIPTION

TITLE: SURFACE TREATMENT PROCESS OF ALUMINUM-BASED PARTS

Technical field of the invention

The present invention is part of the search for new solutions aimed at improving the biocorrosion resistance properties of aluminum or aluminum alloy parts.

Technical background

Biocorrosion encompasses all corrosion phenomena in which microorganisms and in particular bacteria act directly or indirectly through their metabolism. This is an electrochemical phenomenon of the dissolution of a metal that affects all industries where microorganisms, especially bacteria, can thrive. Most metals and alloys are sensitive to biocorrosion: iron, steels, unalloyed or weakly alloyed, stainless steels, copper, aluminum and their alloys. Biocorrosion is considered a serious problem in many industries such as aeronautics and automobiles, in oil fields and in marine environments. The economic loss directly associated with biocorrosion can reach billions of dollars each year. The prevention of biocorrosion of metal or metal alloy parts has therefore aroused considerable interest in recent decades.

One of the most widespread techniques aimed at improving the biocorrosion resistance properties of metal or metal alloy parts, in particular aluminum or aluminum alloy parts, is anodization. Anodization is an electrolytic process aimed at replacing the natural oxide (native oxide), of a few nanometers which covers the aluminum, with an oxide layer of up to several micrometers. The oxide layers produced by anodization can have a thickness close to 10 μm, in order to provide protection against long-term corrosion. Depending on requirements, the anode layer thickness can also vary from a few microns to 20-30 microns. Anodization, also called anodic oxidation, therefore consists in forming on the surface of the part a layer of porous aluminum oxides / hydroxides called anodic layer, by applying a current to the part immersed in an electrolytic bath containing an electrolyte Of type strong acid, 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. This clogged anodic layer can also constitute a support for the adhesion of paint systems. In general, the anodic layers developed by anodization improve the corrosion resistance of the part but their high porosity makes them very sensitive to aggressive environments. The porous structure does not constitute an effective barrier against aggressive species such as microorganisms and it is the barrier layer which mainly provides protection. Thus, a suitable sealing treatment makes it possible to increase the resistance to biocorrosion of the anode layers.

Current anodizing surface treatments, namely, OAC (Chromic Anodic Oxidation), TSA (Tartaric sulfuric Anodizing) as described for example in https://www.a3ts.org/actualite/commissions-techniques/fiches-techniques -surface-treatment / anodizing-sulpho-tartaric-oast-tartric-sulfuric-anodizing-tsa /. Fine OAS (Fine Anodic Sulfuric Oxidation), OAS (Anodic Sulfuric Oxidation) as described for example in https://www.a3ts.org/actualite/commissions-techniques/fiches- techniques-treatment-surface / anodization-sulfuric-version -5-2 /. BSAA (Boric sulfuric Acid Anodizing) as described for example in https://www.anoplate.com/finishes/boric-sulfuric-acid-anodize-bsaa/. PSAA (Phosporic sulfuric Acid Anodizing) as described for example in http://www.metroplating.co.uk/phosphoric-acid-anodislng.php. etc. sealed with standard market products do not withstand the environment defined above. To date, none of these surface treatments resist the biocorrosion that can occur in the low points of aircraft tanks, for example. The corrosive acidic environment, linked to the release of acidic substances by microorganisms in the presence of a salm environment, attacks anodized aluminum. This is directly linked to the failure of anodizations at pH <4.

In addition, the chromic anodic oxidation (OAC) and sulfuric anodic oxidation clogged with hexavalent chromium (OAS) processes, used to protect aluminum alloys against corrosion, are impacted by the REACH regulations. Another solution implemented against biocorrosion is to paint the exterior surfaces of the equipment in contact with these critical areas of the tanks. This generates additional costs and cycle times. There is therefore a real need for a surface treatment process aimed at improving the biocorrosion resistance properties of aluminum or aluminum alloy parts and which complies with the REACH regulation.

Summary of the invention

The present invention aims to remedy the drawbacks of the processes for anodizing aluminum or aluminum alloy parts, described above, in particular, in terms of resistance to biocorrosion of the treated part.

The object of the present invention is precisely to meet these needs, in particular, in terms of resistance to biocorrosion of the part treated, by providing a process for the surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps:

A) an anodization step; and

B) a step of plugging the anode layer formed on said part at the end of step A) the plugging being carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1MOhms, more preferably equal to or greater than 10MOhms, and from 1 to 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 ° C and 100 ° C.

In one embodiment of the invention, the anodization step A) is an anodization during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration of between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a direct voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a speed less than 1 V / min until a voltage value called plateau between 5 and 13 V.

Another embodiment of the invention consists in carrying out, after the plugging step B), a post-plugging rinsing (step B1)) in deionized water having a resistivity equal to or greater than 0.01MOhms, preferably equal to or greater than 0.1MOhms, more preferably equal to or greater than 10MOhms, at a temperature between 15 and 35 ° C.

Another embodiment of the invention consists in carrying out, prior to the silicate salt plugging step (step B)), a step of immersing Al) of said part, - in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (N03) 3, xH ₂ O, (CH3CC> 2) 2Cr, xH ₂ O,

(CH ₃ CO ₂ ) 7Cr ₃ (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)); then optionally - in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO) ₄ ), sodium permanganate (NaMnO ₄ ) (step A1-2)).

In another embodiment, after the silicate salt sealing according to step B), the surface treatment process may further comprise a final hydrothermal sealing (step C)) in deionized water with a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10 MOhms, at a temperature between 97 and 100 ° C. The surface treatment process of the invention significantly improves the biocorrosion resistance properties of aluminum or aluminum alloy parts and meets the requirements of the REACH regulation.

The method of the invention is of great interest in any type of industry where it is sought to improve the properties of resistance to biocorrosion of aluminum or aluminum alloy parts, as in aeronautics, automobiles, oil industry etc.

Another object of the invention relates to a method of manufacturing an aluminum or aluminum alloy part intended for use in the aeronautical sector comprising (i) - a step of surface treatment of said part by a method according to the invention, and possibly

(ii) - a step of applying one or more coat (s) of paint, varnish, dry lubricants, or sealants.

The invention also relates to the use of a surface treatment process according to the invention, for the manufacture of aluminum or aluminum alloy parts intended for the aeronautical sector.

The invention also relates to a part made of aluminum or an aluminum alloy anodized and sealed by a surface treatment process according to the invention, comprising one or more layer (s) of paints, varnishes, dry lubricants, or mastics, said part being intended for the aeronautical sector.

Brief description of the figures

Other characteristics and advantages of the invention will become apparent on reading the detailed description which follows, for the understanding of which reference is made to the appended drawings in which:

[Fig.l] FIG. 1 represents the diagram of the assembly for carrying out the biocorrosion test of the parts treated by the method of the invention and by the methods of the state of the art, according to § 4.7.19 of the MIL-C-27725B standard.

Detailed description of the invention

The object of the present invention is to meet the needs of the state of the art, in particular, in terms of resistance to biocorrosion of the treated part, by providing a process for the surface treatment of an aluminum or alloy part. aluminum, comprising at least the following steps:

A) an anodization step; and

According to a preferred embodiment of the invention, the anodization step A) is an anodization during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration of between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a direct voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a speed less than 1 V / min until a voltage value is reached said plateau between 5 and 13 V.

Once the so-called plateau voltage value has been reached, the applied voltage is maintained at said plateau value for an adequate period of time to obtain on the surface of said part an anode layer with a thickness of between 2 and 7 μm. The voltage applied to said submerged part can be maintained at the plateau value for a period of between 20 and 80 minutes.

The so-called plateau voltage value can be between 6 and 10V.

This anodization is a fine OAS.

In the process of the invention, the anodization step A) can also be an anodization of the TSA (sulfo-tartaric anodic oxidation), OAS (sulfuric anodic oxidation), PSAA (phosphoric acid anodic sulfuric acid oxidation), BSAA ( sulfuric acid anodic oxidation), or OAC (chromic anodic oxidation).

The method of the invention is suitable, more particularly, for parts made of aluminum and an aluminum alloy chosen from the group consisting of 2014, 2017A, 2024, 2214, 2219, 2618, AU5NKZr, 7175, 5052, 5086, 6061, 6063, 7010, 7020, 7050, 7050 T7451, 7055 T77, 7068, 7085 T7651, 7075, 7175 and 7475, AS7G06, AS7G03, AS10G, AS9U3, AS7G06 and AS 10G obtained by a different production method, namely, by additive manufacturing .

As indicated, the voltage profile applied to the part includes a voltage rise, from a starting value of 0V, at a rate less than 1V / min, preferably 0.3V / min to 0.7V / min, up to 'to reach a so-called plateau voltage value of between 5 and 13V, preferably between 6 and 10V. The voltage applied to said part immersed in said bath is then maintained at said plateau value for an adequate period of time to obtain on the surface of said part an anodic layer of aluminum oxides / hydroxides, with a thickness of between 2 and 7 μm. , for example, of thickness equal to about 3 μm.

According to one embodiment of the invention, the voltage applied to said submerged part is maintained at the plateau value for a period of between 20 and 80 minutes, preferably between 30 and 60 minutes.

Without wishing to be bound by any theory whatsoever, the inventors have found, unexpectedly, that during anodization by the preferred fine OAS anodization process described above, the slower the voltage rise, the greater the resistance to resistance. biocorrosion of the anodic layer formed on the surface of the part is important. Likewise, the lower the applied voltage, the greater the resistance to biocorrosion of said anode layer. These two parameters make it possible to advantageously create a layer that is less porous, more dense and therefore more resistant to biocorrosion. In the anodization step A) according to the preferred embodiment of the invention, the concentration of sulfuric acid in the bath is preferably between 160 g / L and 220 g / L, for example equal to 190 g / L.

In the anodizing step A) according to the preferred embodiment of the invention, the temperature of the bath may be between 10 and 25 ° C, preferably between 14 and 21 ° C, for example equal to 18 ° vs.

In the method of the invention, the anodization step A) is, directly or indirectly, followed by a step B) which is a step of sealing the anode layer formed on said part during step A) . As indicated above, the sealing of step B) is carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, and more preferably equal to or greater than 10MOhms), and from 1 to 500g / L of an alkali metal or alkaline earth metal silicate.

The alkali metal or alkaline earth metal silicate can be selected from the group consisting of lithium silicate, sodium silicate, potassium silicate, calcium silicate and magnesium silicate.

The quality of the water in the sealing bath is important because it has an impact on the resistance of the anodic layer formed on the part against biocorrosion. Purer water such as, for example, water with a resistivity equal to or greater than 10MOhms is likely to provide better resistance over time than water with a resistivity of less than 10 MOhms. According to a preferred variant, the deionized water is an assembly water, that is to say a water used to fill an active bath during the assembly / filling thereof, said water having a resistivity equal to or greater than 0, 01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10MOhms.

In sealing step B), the concentration of alkali metal or alkaline earth metal silicate in the solution is preferably between 15 and 40 g / L, for example equal to 23 g / L.

The temperature of the sealing solution in step B) can be between 60 ° C and 100 ° C, preferably between 97 ° C and 100 ° C, for example equal to 98 ° C.

The duration of the sealing step B) is between 1 and 40 minutes, preferably between 15 and 25 minutes, for example 20 minutes. In accordance with one embodiment of the invention, prior to the silicate salt plugging step (step B)), a step of immersion A1) of said part,

- in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr , xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)) ; then possibly

- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ) , sodium permanganate (NaMnO ₄ ) (step A1-2)); can take place.

The trivalent chromium salt can be, for example, one of the following commercial products: Surtec 650 from the company SURTEC, Lanthanum 613.3 from the company COVENTYA, TCS from the company SOCOMORE, Bonderite MNT 65000 from the company HENKEL.

The oxidizing compound can be, for example, the product PACS from the company SOCOMORE.

In the immersion step A1), steps A1-1) and A1-2) can take place successively in the following order: step A1-1) then step A1-2). The immersion step A1), can also be the step A1-1) alone without being followed by the step A1-2).

The temperature of the aqueous bath containing the trivalent chromium salt and that of the aqueous bath containing the oxidizing compound in step A1-1) and A1-2) as described above are between 20 and 80 ° C, preferably between 20 and 60 ° C. The temperatures of the two baths can be identical or different.

The immersion time in each bath of step A1) can be the same or different. It can be between 5 and 40 minutes, preferably between 5 and 20 minutes.

The pH of the bath containing a trivalent chromium salt can be between 3 and 4.5, preferably between 3 and 4, for example equal to 3.5. The concentration of trivalent chromium salt in the bath is preferably between 0.5 and 500 g / L.

The pH of the bath containing an oxidizing compound is between 3 and 6.

The concentration of oxidizing compound in the bath is preferably between 0.1 and 500 g / L. According to another embodiment of the invention, the process further comprises a final hydrothermal sealing after the sealing with the silicate salt according to step B) which will be called step C). The final hydrothermal sealing C) is carried out in deionized water with a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, and more preferably equal to or greater than 10MOhms, and at a temperature T> 96 ° C, for example, between 97 and 100 ° C.

In the final hydrothermal sealing C), the part is immersed in deionized water having a resistivity of, advantageously, equal to or greater than 10MOhms. The immersion of the part in this step can be 10 to 30 minutes, preferably 15 to 25 minutes.

According to one embodiment of the invention, the process for surface treatment of an aluminum or aluminum alloy part, according to the invention, comprises the following steps:

A) an anodizing step during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a DC voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a rate of less than 1 V / min until a so-called plateau voltage value of between 5 and 13 V is reached; and

According to another embodiment of the invention, the method of surface treatment of an aluminum or aluminum alloy part, according to the invention, comprises the following steps:

A) an anodizing step during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a direct voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a rate of less than 1 V / min until a so-called plateau voltage value of between 5 and 13 V is reached;

Al) a step of immersing said part, - in an aqueous bath containing a trivalent chromium salt chosen from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO3) 3, xH ₂ O, (CH3CC> 2) 2Cr, xH ₂ O,

(CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)); then

- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ) , sodium permanganate (NaMnO ₄ ) (step A1-2)); and

B) a plugging step carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10MOhms, and from 1 to 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 ° C and 100 ° C.

A) an anodizing step during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a DC voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a rate of less than 1 V / min until a so-called plateau voltage value of between 5 and 13 V is reached;

A1) a step of immersing said part,

- in an aqueous bath containing a trivalent chromium salt chosen from the group consisting of CrF3, xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr, xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) ₃ , xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)); then

- in an aqueous bath containing an oxidizing compound selected from the group consisting in hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ), sodium permanganate (NaMnO ₄ ) (step Al-2) );

B) a plugging step carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10MOhms, and from 1 to 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 ° C and 100 ° C; and

C) a final hydrothermal plugging in deionized water with a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, and more preferably equal to or greater than 10MOhms, at a temperature between 97 and 100 ° vs.

According to yet another embodiment of the invention, the method of surface treatment of an aluminum or aluminum alloy part, according to the invention, comprises the following steps:

A1) a step of immersing said part,

- in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr , xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)) ; and

B) a plugging step carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10MOhms, and from 1 to 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 ° C and 100 ° C. In another embodiment of the invention, the method of surface treatment of a part made of aluminum or of an aluminum alloy, according to the invention, comprises the following steps:

A) an anodizing step during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a DC voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a rate of less than 1 V / min until a so-called plateau voltage value of between 5 and 13 V is reached; Im) a step of impregnating said anodized part at the end of step A) in a bath of organic or inorganic dyes, then optionally

A1) a step of immersing said part,

- in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr , xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)) ; then

- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ) , sodium permanganate (NaMnO ₄ ) (step A1-2)); and then possibly

Step Im) can be carried out by any technique known to those skilled in the art. For example, it can be carried out in a dye bath suitable for the surface treatment available from companies such as Clariant. By way of example, mention may be made of the organic dye Sanodal Blue (from the company Clariant) with a concentration of 3 g / L to which must be added 2 g / L of sodium acetate, pH between 5 and 6 at a temperature between 40 and 65 ° C, preferably equal to 50 ° C, for a period of time included in 5 and 35min, preferably equal to 20 minutes. The active principle of the organic dye is the Anthraquinone molecule.

In all the embodiments, before subjecting the part to the surface treatment method of the invention and therefore prior to the anodizing step A), the part can be subjected to a surface preparation step by degreasing and / or stripping 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 part. This operation can be carried out by soaking, sprinkling, or any other method known to those skilled in the art; alkaline degreasing, to dissolve grease present on the surface of the part. This operation can be carried out by soaking, sprinkling, or any other technique known to those skilled in the art; alkaline pickling, to dissolve the oxides naturally formed on the surface of the part. This operation can be carried out by soaking, sprinkling, or any other technique known to those skilled in the art. At the end of this operation, the part is covered with a powdery layer formed of oxidation products of intermetallic compounds, which should be removed by an acid pickling step; acid pickling, to dissolve the oxides naturally formed on the surface of the part, and / or the oxidation layer formed on the surface of the part during the alkaline pickling step. This operation can be carried out by soaking, sprinkling, or any other technique known to those skilled in the art.

These steps are described in detail, 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.

The surface treatment process of the invention significantly improves the biocorrosion resistance properties of aluminum or aluminum alloy parts and meets the requirements of the REACH regulation.

The method of the invention is of great interest in any type of industry where it is sought to improve the properties of resistance to biocorrosion of parts in aluminum or aluminum alloy, such as in aeronautics, automotive, oil industry etc.

Another object of the invention relates to a process for manufacturing an aluminum or aluminum alloy part intended for use in the aeronautical sector comprising

(i) - a step of surface treatment of said part by a method according to the invention, and optionally

The application of one or more coat (s) of paint, varnish, dry lubricants, or sealants can be done by any process known to those skilled in the art. In addition, those skilled in the art will know how to choose paints, varnishes, dry lubricants and sealants suitable for use in the aeronautical sector.

The invention also relates to a part made of aluminum or aluminum alloy anodized and sealed by a surface treatment process according to the invention, comprising one or more layer (s) of paints, varnishes, dry lubricants, or sealants, said part being intended for the aeronautical sector.

EXAMPLES

Example 1:

Aluminum alloy part surface treatment process

Rolled 2024 T3 aluminum alloy parts machined on one of the two sides with dimensions of 120x60x2 mm are processed according to the methods described below.

The part surface preparation steps are first of all carried out successively: alkaline degreasing, by soaking the part in a bath of ALUMAL CLEAN 101 (from the company COVENTYA) at a temperature of 60 ° C., for 20 minutes; rinsing with tap water or demineralized water; acid pickling: by soaking the part in a solution of ALUMAL DEOX 411 (from the company COVENTYA); rinsing with tap water or demineralized water. The pickled and rinsed parts are then subjected to an anodizing process in accordance with the invention, 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 190 g / L. This bath is brought to and maintained at a temperature of 18 ° C. A DC voltage is applied to the submerged 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 held at the plateau value for 50 minutes. An anodic layer 2 to 4 μm thick is formed on the surface of the parts.

As comparative examples, identical parts that have been subjected to the same surface preparation operations are anodized using conventional chromic anodization (OAC) and fine sulfuric anodization (OAS fine) methods. The operating conditions for these anodizations are indicated in [Table 1].

[Table 1]

The thickness of the anode layer formed on the part is measured by eddy current according to the ISO2360 standard.

The anodized parts according to the invention are then subjected to one or more rinses, preferably with demineralized water, then to the sealing operations in accordance with the invention under the conditions and in the order indicated below: step A1. l) and step A1.2): a step of immersing said parts, successively, in an aqueous bath containing 29% Vol / Vol of trivalent Chromium salt (Chromium potassium sulfate ΙΠ of chemical formula KCr (SO ₄ ) ₂ , at a temperature of 40 ° C for 20 minutes and at a pH of 3.9, then in an aqueous bath containing 7% Vol / Vol of H2O2, at a temperature of 25 ° C for

5 minutes and at a pH of 4.2; step B): plugging by immersion of the parts at the end of the two preceding operations in an aqueous solution of deionized assembly water having a resistivity of 10 MOhms with 23 g / L of a sodium silicate, a temperature of 98 ° C and during

20 minutes.

Between each sealing step, rinsing with demineralized water is carried out for 1 minute, at a temperature of around 20 ° C.

By way of comparison, parts anodized according to the conventional OAC and OAS fine methods are also subjected to one or more conventional sealing operations such as hot sealing with hexavalent chromium salts (for OAC), hot hydrothermal sealing with pre-clogging (or impregnation) prior to trivalent chromium salts and in an oxidizing bath, according to the conditions indicated in [Table 2]

[Table 2]

Resistance to biocorrosion

At the end of these sealing operations, a sealed anodic layer is obtained on each treated part. Once treated, the parts are subjected to the immersion test in a medium representative of biocorrosion which follows the protocol of § 4.7.19 of standard MIL-27725B. The assembly diagram for performing the biocorrosion test according to § 4.7.19 of standard MIL-C-27725B with the different parts is shown in [Fig. 1]

The results are evaluated visually by removing the pieces from the medium in order to observe any signs of degradation of the treatment and / or attacks on the substrate by the medium (lower phase). This test was carried out in a comparative manner with historical treatments (OAC) and alternative treatments of the more recent state of the art (compliant with REACH). The visual degradation can be confirmed by a measurement of ohmic resistivity of the layer which, when it is not infinite, shows a deterioration of the layer which can go as far as the substrate. Method of measuring resistance with an Ohmmeter:

A multimeter can be used to measure resistance. It must then be used in Ohmmeter mode.

Using the multimeter in Ohmmeter mode:

Choice of terminals: the COM terminal and the terminal with the Ω symbol.

Connection: the multimeter is connected directly at two points of the specimen under test on the area which has been in contact with the lower phase of the two-phase medium.

The caliber: the highest caliber is chosen and then it is reduced until the smallest of the caliber greater than the measured value is found.

[Table 3] summarizes the results of tests of the resistance to biocorrosion of different surface treatments as a function of the number of days of immersion in the two-phase medium.

[Table 3]

Pitting corrosion is localized corrosion which results in the formation, on the surface of the aluminum alloy part, of irregularly shaped cavities. They occur when the aluminum alloy part is contacted with an aqueous solution containing halide ions, most commonly chloride ions. Based on the results shown in [Table 3], it is clear that the surface treatment according to the invention makes it possible to at least double the resistance to the test simulating biocorrosion compared to conventional surface treatments.

The behavior observed on the part which has had the treatment according to the invention is equivalent to the behavior obtained on painted parts (by anaphoresis, for example) which would undergo the same tests.

Example 2:

Speed of voltage rise in the fine OAS anodization step

12 test pieces were degreased and pickled following the same conditions as for Example 1. They were subsequently anodized with different times of voltage rise (5 minutes and 15 minutes) and different plate voltages (6, 10 and 13 Volts respectively for durations of 50, 40 and 30 minutes). Then these test tubes have were successively immersed in the baths of steps A1-1), A1-2) under the same conditions as those described in Example 1. The test pieces 1, 3, 6, 7, 9 and 11 were then immersed for 30 minutes in a final hydrothermal sealing bath in deionized water with a resistivity equal to or greater than 10 MOhms, at a temperature of 98 ° C. (step C) previously).

The test pieces 2, 4, 5, 8, 10, 12 were sealed and then rinsed according to the same conditions as those described in Example 1 (step B)) with associated post rinsing). The thicknesses of the clogged anode layers were measured by eddy current according to the ISO2360 standard. The conditions and measurements of the thicknesses of the layers are given in [Table 4].

[Table 4]

The resistance of the specimens to biocorrosion was then tested according to the same conditions and types of control of Example 1. The controls were carried out after 14 and 17 days of immersion. The results are shown in [Table 5].

After 17 days of immersion, the test results reveal that when the sealing is carried out in a bath with the silicate salt according to the invention, the appearance of the test piece remains unchanged or very slightly discolored. Conversely, generalized corrosion is noted when the sealing carried out is hydro thermal for the test specimens anodized at 13 and 10 Volts. The effect is much less marked for specimens 1 and 3 anodized with

6 Volts. It is also noted that for the anodized test pieces at 6 and 10 volts clogged with silicate salt, a voltage rise of 15 minutes provides better results than for a voltage rise of 5 minutes.

Consequently, we deduce that in view of these results, a slower rise in voltage (15 minutes to be preferred compared to 5 minutes) with a plateau voltage preferably equal to 10 or 6 volts, with clogging with salt. of silicate according to the invention provides the best results to withstand the biocorrosion simulation test

[Table 5]

Claims

1. A method of surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps:

A) an anodization step; and

2. Method according to claim 1, characterized in that the anodization step A) is an anodization during which said part is immersed in an aqueous bath comprising sulfuric acid at a concentration of between 150 and 250 g / L and at a temperature between 14 and 21 ° C, and a direct voltage is applied to said submerged part according to a voltage profile comprising a voltage rise at a rate of less than 1 V / min until a value of so-called plateau voltage between 5 and 13 V.

3. Method according to claim 1, characterized in that the anodization step A) is an anodization of the TSA type (sulpho-tartaric anodic oxidation), OAS (sulfuric anodic oxidation), PSAA (anodic oxidation of phosphoric sulfuric acid), BSAA (Boric Sulfuric Acid Anodic Oxidation), or OAC (Chromic Anodic Oxidation).

4. Method according to one of claims 1 to 3, characterized in that the aluminum alloy is selected from the group consisting of 2014, 2017A, 2024, 2214, 2219, 2618, AU5NKZr, 7175, 5052, 5086, 6061, 6063, 7010, 7020, 7050, 7050 T7451, 7055 T77, 7068, 7085 T7651, 7075, 7175 and 7475, AS7G06, AS7G03, AS10G, AS9U3, AS7G06 and AS10G obtained by additive manufacturing.

5. Method according to any one of claims 1 to 4, characterized in that 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.

6. Method according to any one of claims 1, 2, 4 to 5, characterized in that the voltage applied to said part immersed in said bath is then maintained at said plateau value for a period adequate to obtain the surface of said. part an anodic layer of thickness between 2 and 7μm.

7. Method according to any one of claims 1, 2, 4 to 6, characterized in that the voltage applied to said submerged part is maintained at the plateau value for a period of between 20 and 80 minutes.

8. Method according to any one of claims 1, 2, 4 to 7, characterized in that the so-called plateau voltage value is between 6 and 10V.

9. Method according to any one of claims 1 to 8, characterized in that the plugging step B) is followed by a post-plugging rinsing step Bl) in deionized water having a resistivity equal to or greater than O .OlMOhms, preferably equal to or greater than 0.1MOhms, more preferably equal to or greater than 10MOhms, at a temperature between 15 and 35 ° C.

10. Surface treatment method according to any one of claims 1 to 9, characterized in that it comprises, prior to the silicate salt plugging step (step B)), an immersion step Al) of said part, - in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr, xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al -1)); then possibly

- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ) , sodium permanganate (NaMnO ₄ ) (step A1-2)).

11. The method of claim 10, characterized in that the temperature of the aqueous bath containing the trivalent chromium salt (step Al-1)) and that of the aqueous bath containing the oxidizing compound (step Al-2)) are between 20 and 80 ° C, preferably between 20 and 60 ° C.

12. Method according to any one of claims 1 to 11, characterized in that it further comprises a final hydrothermal sealing (step C)) in deionized water with a resistivity equal to or greater than 0.01 MOhms. , preferably equal or greater at 0.1MOhms, and more preferably equal to or greater than 10MOhms, at a temperature between 97 and 100 ° C, said step C) taking place after the sealing with the silicate salt according to step B).

13. Surface treatment method according to any one of claims 10 to 12, characterized in that it comprises, in the immersion step A1), the concentration of trivalent chromium salt in the aqueous bath (step A1 -1)) is between 0.5 and 500 g / L, and that of the oxidizing compound in the aqueous bath (step A1-2)) is between 0.1 and 500 g / L.

14. A method of surface treatment of an aluminum or aluminum alloy part, according to any one of claims 1 to 13, comprising the following steps:

15. A method of surface treatment of an aluminum or aluminum alloy part, according to any one of claims 1 to 13, comprising the following steps:

A1) a step of immersing said part, - in an aqueous bath containing a trivalent chromium salt chosen from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr, xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) ₃ , xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step A1-1)); then

16. A method of surface treatment of an aluminum or aluminum alloy part, according to any one of claims 1 to 13, comprising the following steps:

A1) a step of immersing said part,

- in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF ₃ , xH ₂ O, CrCl ₃ , xH ₂ O, Cr (NO ₃ ) 3, xH ₂ O, (CH ₃ CO ₂ ) ₂ Cr , xH ₂ O, (CH ₃ CO ₂ ) 7Cr3 (0H) ₂ , xH ₂ O, Cr ₂ (SO ₄ ) 3, xH ₂ O, CrK (SO ₄ ) ₂ , xH ₂ O (step Al-1)) ; then - in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H2O2), ammonium fluoride (NH4F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ) , sodium permanganate (NaMnO ₄ ) (step A1-2)); B) a plugging step carried out in an aqueous solution of deionized water having a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, more preferably equal to or greater than 10MOhms, and from 1 to 500 g / L of an alkali metal or alkaline earth metal silicate, at a temperature between 60 ° C and 100 ° C; and C) a final hydrothermal sealing in deionized water with a resistivity equal to or greater than 0.01 MOhms, preferably equal to or greater than 0.1 MOhms, and more preferably equal to or greater than 10MOhms, at a temperature between 97 and 100 ° C.

17. A method of surface treatment of an aluminum or aluminum alloy part, according to any one of claims 1 to 13, comprising the following steps:

A1) a step of immersing said part,

18. A method of surface treatment of an aluminum or aluminum alloy part, according to claim 15, comprising, before step A1), a step of impregnating (step Im)) of said anodized part in a bath of organic or mineral dyes.

19. A method of manufacturing an aluminum or aluminum alloy part intended for use in the aeronautical sector comprising

(i) - a step of surface treatment of said part by a method according to any one of claims 1 to 18, and optionally

20. Use of a surface treatment process according to any one of claims 1 to 18, for the manufacture of aluminum or aluminum alloy parts intended for the aeronautical sector.

21. Part made of aluminum or anodized aluminum alloy and sealed by a surface treatment process according to any one of claims 1 to 18, comprising one or more layer (s) of paints, varnishes, dry lubricants, or sealants, said part being intended for the aeronautical sector.