FR3160707A1 - METHOD FOR TREATING THE SURFACE OF A PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY COMPRISING A STEP OF CHEMICAL CONVERSION OF THE ANODIZED AND SEALED PART, ITS USE AND PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY OBTAINED BY THIS METHOD - Google Patents

Abstract

The present invention relates to a method for surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps: A) an anodizing step; B) a sealing step in an aqueous solution of deionized water of at least one alkali metal or metal silicate; D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and E) a step of chemical conversion of the unprotected areas with trivalent chromium or hexavalent chromium to form a conductive protective coating thereon. It also relates to the use of this method in the aeronautics, aerospace, and/or automotive sectors. Figure for abstract: None

Description

METHOD FOR TREATING THE SURFACE OF A PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY COMPRISING A STEP OF CHEMICAL CONVERSION OF THE ANODIZED AND SEALED PART, ITS USE AND PART BASED ON ALUMINUM OR AN ALUMINUM ALLOY OBTAINED BY THIS METHOD Technical field of the invention

The present invention relates to the field of surface treatment and is part of the search for new solutions aimed at improving the corrosion resistance properties, the electrical conduction properties, and the resistance to acidic environments of aluminum or aluminum alloy parts, in particular for applications in the fields of aeronautics, aerospace and the automobile industry.

Technical background

Aluminum alloys are materials of choice for the transportation industry, and more specifically 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 potentially leading to its shrinkage or failure. Many strategies have been implemented to overcome this weakness, including chemical conversion. Chemical conversion is usually carried out by placing the part in contact with a bath containing hexavalent chromium, for example, made from a solution of Alodine 1200 marketed by Henkel. This treatment produces a coating on the surface of the part that increases corrosion resistance and maintains electrical conductivity. This coating also allows good adhesion of paints.

However, since September 2017, the European REACH regulation (English acronym for Registration, Evaluation, Authorisation and Restriction of Chemicals) has prohibited (or restricted to authorisation) the use of certain key components in surface treatments, in particular, hexavalent chromium.

Historically, the industrial process, impacted by the REACH regulation (because it includes the use of CrVI in both anodizing and conversion to Alodine 1200 (manufactured by Henkel) follows the following pattern:

- anodization of aluminum by Chromic Anodic Oxidation (ACO), then local deprotection by mechanical operation (manual by scouring, counterboring, milling, turning etc.), then

- application of a chemical conversion treatment (with Alodine 1200 or Bonderite M-CR 1200) by partial or total immersion which may or may not include the use of acid pickling.

At the end of these operations, the treated part therefore has a double, separate treatment. Historically, Alodine 1200 (although very acidic and therefore degrading) had little impact on the corrosion resistance of the OAC layer sealed with Chrome VI. The process described in FR3106837, compliant with the REACH regulation, guarantees corrosion resistance of less than 5 pits/dm² of corrosion after more than 1500 hours of salt spray according to ISO9227

It follows the following steps:

Anodization of aluminum by Sulfuric Anodic Oxidation (thin thickness for example) followed by sealing with silicate salts then local deprotection by mechanical operation (manual by canvas, counterboring, milling, turning etc.), then application of a chemical conversion treatment (with Alodine 1200 or Bonderite M-CR 1200 or with a conversion compliant with the REACH regulation such as, for example, TCS PACS manufactured by Socomore) by partial or total immersion which includes or does not include the use of acid pickling before the chemical conversion treatment itself.

By September 2024, OAC (Chromic Anodic Oxidation) and Alodine 1200S (chemical conversion) must be substituted in accordance with the requirements of the REACH regulation. Therefore, the two processes that follow one another on the same part in the industrial process must be substituted individually. However, the compatibility of the chemical treatment with Alodine 1200 in relation to its potential impacts on the anodizing treatment is a challenge to be overcome.

Treatments using baths containing trivalent chromium have been developed. However, these treatments do not guarantee sufficient corrosion resistance.

There is therefore a real need for a surface treatment process aimed at improving the corrosion resistance properties, resistance to acidic environments, and electrical conduction properties of aluminum or aluminum alloy parts and which complies with the REACH regulation.

In particular, there is therefore a real need for a surface treatment process as described above, which can guarantee compatibility between two successive treatments: one to provide strong protection against corrosion (anodization) with resistances greater than 500 hours (up to 1500 hours on 2024T3 aluminum alloy, for example) and the other to provide light protection against corrosion (classic requirement of 168 hours of resistance in salt spray) while guaranteeing low contact resistivity in accordance, for example, with MIL-DTL-5541 and MIL-DTL-81706 (chemical conversion).

The present invention aims precisely to meet these needs, in particular, in terms of corrosion resistance, resistance in acidic environments, compatibility between two subsequent treatments (anodization and chemical conversion) and the electrical conduction properties of aluminum or aluminum alloy parts of the treated part, by providing a method for surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps:
A) an anodizing step;
B) a step of sealing the anodic layer formed on the part at the end of step A)
the sealing 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 0.1 MOhms, more preferably equal to or greater than 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C; and

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating there.

It should be noted that at the end of step B) , the anodized and sealed coating is non-conductive over the entire part.

Thus, the method of the invention makes it possible to obtain a part which is effectively protected in particular from corrosion by a coating comprising conductive zones (deprotected zones having undergone chemical conversion also called conductive protective layer) and non-conductive zones (protected zones not altered by the chemical conversion step E) also called non-conductive protective layer). The part then has, on non-conductive zones, less than five pits per square decimeter (dm ² ) after exposure of 1500 hours to neutral salt spray, in accordance with the requirements of standard NF EN ISO 9277:2017.

On conductive areas, the part may have fewer than five pits per square decimeter (dm ² ) after a minimum exposure of 168 hours to neutral salt spray, in accordance with the requirements of standard NF EN ISO 9277:2017.

It should be noted that the non-conductive protective layer (anodized and sealed which was not deprotected in step D) ) is not altered or modified by the chemical conversion step E) . The non-conductive layer also does not pollute the chemical conversion bath. Thus, the chemical conversion step to trivalent chromium or hexavalent chromium E) only takes place on the unprotected areas which were previously deprotected during step D) .

Another subject of the invention relates to a method for manufacturing an aluminum or aluminum alloy part intended to be used in the aeronautical, aerospace and/or automotive sector comprising

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

(ii) - a step of applying one or more layers of paint, varnish, dry lubricants, or mastics.

The invention also relates to the use of a surface treatment method according to the invention, for the manufacture of aluminum or aluminum alloy parts intended for the aeronautical, aerospace and/or automotive sector.

The invention further relates to a part made of aluminum or aluminum alloy having undergone a surface treatment according to the invention, comprising one or more layers of paints, varnishes, dry lubricants, or mastics, said part being intended for the aeronautical, aerospace and/or automotive sector.

Detailed description of the invention

The present invention relates to a method for surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps:
A) an anodizing step;
B) a step of sealing the anodic layer formed on the part at the end of step A)
the sealing 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 0.1 MOhms, more preferably equal to or greater than 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C; and

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating there.

The method of the invention comprises the virtuous and sequential combination of anodization, sealing with one or more alkali metal or alkaline earth metal silicate salt(s) carried out in an aqueous medium, followed by partial mechanical deprotection of the anodized and sealed part (in order to locally remove on areas of interest and return to raw aluminum) then a chemical conversion treatment (with or without chromium VI), by total or partial immersion of the part, which may or may not include an acid pickling step.

The anodizing steps A) and sealing B) with one or more silicate salt(s) allow the part to resist the effect of the chemical conversion treatment E) which follows even if this includes acid pickling which naturally has/would tend to dissolve/leach the sealing of the sealed anodic layers which can drastically reduce the corrosion resistance of the anodic layer.

Other anodizations alternative to OAC with sealing without silicate salt(s), such as for example a fine OAS impregnated with CrIII with or without PACS and sealed with water, do not guarantee these virtuous combinatorial aspects or in any case compatible between two treatments which follow one another: one to strongly protect against corrosion (anodization) with resistances greater than 500h (up to 1500h on 2024T3 aluminum alloy, for example) and the other to slightly protect against corrosion (classic requirement of 168h of resistance in salt spray) while guaranteeing a low contact resistivity in accordance.

It should be noted that at the end of step B) , the anodized and sealed coating is non-conductive over the entire part.

According to a preferred embodiment of the invention, the anodizing step A) is an anodizing during which the part is immersed in an aqueous bath comprising sulfuric acid at a concentration of between 150 and 250 g/L and at a temperature of between 14 and 21°C, and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed of less than 1 V/min until a so-called plateau voltage value of between 5 and 13 V is reached.

Once the so-called plateau voltage value is reached, the applied voltage is maintained at said plateau value for a suitable duration to obtain on the surface of said part an anodic layer with a thickness of between 2 and 7µm.

The voltage applied to said immersed 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.

Before subjecting the aluminum or aluminum alloy part to the anodizing step, said body may be subjected to a surface preparation step by degreasing and/or pickling in order to eliminate grease, dirt and oxides present on its surface.

• dégraissage au solvant, pour dissoudre des graisses présentes à la surface de l’aluminium ou de l’alliage d’aluminium. Cette opération peut être réalisée par trempage, aspersion, ou tout autre méthode connue de l’homme du métier ;
• dégraissage alcalin, pour dissoudre des graisses présentes à la surface de l’aluminium ou de l’alliage d’aluminium. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier ;
• décapage alcalin, pour dissoudre les oxydes naturellement formés à la surface de l’aluminium ou de l’alliage d’aluminium. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier. A l’issue de cette opération, l’aluminium ou l’alliage d’aluminium est recouvert d’une couche pulvérulente formée de produits d’oxydation des composés intermétalliques, qu’il convient d’éliminer par une étape de décapage acide ;
• décapage acide, pour dissoudre les oxydes naturellement formés à la surface de l’aluminium ou de l’alliage d’aluminium, et/ou la couche d’oxydation formée à la surface de la pièce lors de l’étape de décapage alcalin. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier.

This preliminary surface preparation step may include one or more of the following operations:

• solvent degreasing, to dissolve greases present on the surface of 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 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 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 should 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 part made of aluminum or an 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, particularly with demineralized water, are preferably carried out between the successive stages above, and before the part is treated by anodization.

In the process of the invention, the anodizing step A) can also be an anodizing of the OAST (Sulfotartaric Anodic Oxidation), OAS (Sulfuric Anodic Oxidation), PSAA (Sulfuric Phosphoric Acid Anodic Oxidation), BSAA (Sulfuric Boric Acid Anodic Oxidation), OAS NG FE (New Generation Thin Thickness Sulfuric Anodic Oxidation), OAS NG (New Generation Sulfuric Anodic Oxidation), OAD (hard anodic oxidation), or OAC (Chromic Anodic Oxidation). These are processes well known to those skilled in the art.

The method of the invention is suitable for all types of aluminum alloys including so-called "difficult" alloys, in particular aluminum alloys of 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, aluminum alloys so-called difficult ones selected from the group consisting of 2618A, 2214 and AU5NKZr.

• d’eau déionisée ayant une résistivité égale ou supérieure à 0,01 MOhms, de préférence égale ou supérieure à 0,1MOhms, et plus préférentiellement égale ou supérieure à 10MOhms), et
• de 1 à 500g/L d’au moins un silicate de métal alcalin ou de métal alcalinoterreux.

In the process of the invention, the anodizing step A) is, directly or indirectly, followed by a step B) which is a step of sealing the anodic layer formed on said part during step A) . As indicated above, the sealing of step B) is carried out in an aqueous solution

• 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 10 MOhms), and
• from 1 to 500g/L of at least one alkali metal or alkaline earth metal silicate.

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 or a mixture thereof.

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 to corrosion. Purer water, such as, for example, water with a resistivity equal to or greater than 10 MOhms, is likely to provide better resistance over time than water with a resistivity lower than 10 MOhms. According to a preferred variant, the deionized water is assembly water, i.e. water used to fill an active bath during 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 10 MOhms.

In the sealing step B) , the concentration of alkali metal or alkaline earth metal silicate(s) in the solution is preferably between 15 and 100 g/L, for example equal to 80 g/L.

The temperature of the sealing solution in step B) may 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.

According to one embodiment of the invention, prior to the silicate salt sealing step (step B ), a step A1) of immersion 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 ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, Cr ₂ (SO ₄ ) ₃ ,xH ₂ O, CrK(SO ₄ ) ₂ ,xH ₂ O (step A1-1) );

then possibly

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

can take place.

Trivalent chromium salt can be, for example, one of the following commercial products: SURTEC 650 from SURTEC, Lanthane 613.3 from COVENTYA, TCS from SOCOMORE, BONDERITE MNT 65000 from HENKEL.

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

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

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

The pH of the bath containing a trivalent chromium salt may 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.

These steps between anodizing and sealing allow the creation of strong chemical bonds between Cr, Zr and silicon (from the silicate(s)) via metal oxides. These give the whole a very robust corrosion resistance and a strong chemical inertia with respect to agents, particularly acids, which may be in contact with the anodizing (via industrial processes such as chemical conversion or environmental stresses such as acid attacks in service/use).

According to another embodiment of the invention, the method further comprises a final hydrothermal sealing after the sealing with 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 10 MOhms, 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 advantageously equal to or greater than 10 MOhms. The immersion of the part in this step can be 10 to 30 minutes, preferably 15 to 25 minutes.

Step D) is a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas. This step leads to a partial deprotection of the anodized and sealed part (in order to locally remove the anodization on areas of interest and return to raw aluminum) then to carry out a chemical conversion treatment (with or without Chrome VI) during step E) , by total or partial immersion of the part, which includes or does not include an acid pickling step.

Mechanical deprotection therefore makes it possible to expose specific areas of the part where conductive parts of the corrosion protection are desired. The number of unprotected areas is not limited to one. In step D) , the non-conductive corrosion protection layer is removed only in areas where electrical continuity between the part and external elements is desired.

As indicated, deprotection by localized removal of material is carried out by a mechanical operation such as, for example, by milling, counterboring, milling, turning, etc. These are processes, well known in the field of machining and widely described in the literature. The person skilled in the art is able to apply these processes to the part, during step D) .

Step D) is followed by step E) which is a step of chemical conversion to trivalent or hexavalent chromium of the unprotected areas to form a conductive protective coating.

Conversion step E) can be applied to bare surfaces of the substrate which have not undergone the anodizing and sealing steps, i.e. to areas of the substrate which have been masked during steps A) and B) .

The chemical conversion step to trivalent chromium or hexavalent chromium is known per se. Typically, the implementation conditions are provided with the technical data sheets by the manufacturers of chemical conversion baths.

It is possible to provide for carrying out conversions following step E) in parallel without steps A) and B) , i.e. on raw non-anodized areas. However, this does not fall within the scope of the present invention.

As a non-limiting example, the chromating bath may be a bath marketed under the brand name SURTEC 650 or Lanthane 613.3, SOCOSURF TCS, SOCOSURF TCS-PACS.

After immersion in a chemical conversion bath, the part can be rinsed with demineralized water and dried.

As a non-limiting example, this may involve rinsing by immersion followed by rinsing by spraying with demineralized water.

By way of non-limiting example, the drying step can be carried out at room temperature (20 ± 5°C) under compressed air and/or in an oven at a temperature less than or equal to 60°C. It is understood that compressed air can be used at room temperature (20 ± 5°C) and then the part can be placed in an oven at a temperature less than or equal to 60°C until the part is dry.

In certain embodiments, after step D) , the unprotected area may be cleaned. By way of non-limiting example, the cleaning may be by mechanical brushing or by ultrasound.

In certain embodiments, before step E) , the non-conductive protective layer and the unprotected area may be degreased with a solvent and/or an alkaline solution. This step makes it possible to degrease the part when the non-conductive protective layer and/or the unprotected area have “fingerprint” type dirt which may result from successive handling of the part during the previous steps. The degreasing step, if any, is not carried out using an acid solution.

As a non-limiting example, the solvent may be ethanol or methyl ethyl ketone (butanone-2 also called MEK for MethylEthyl Ketone in English).

By way of non-limiting example, the alkaline solution may be a solution marketed under the name Sococlean A3432.

According to one embodiment of the invention, the method 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 of between 150 and 250 g/L and at a temperature of between 14 and 21°C; and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed of less than 1 V/min until a so-called plateau voltage value of between 5 and 13 V is reached;

B) a step of sealing the anodic layer formed on said part at the end of step A)

the sealing 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 0.1 MOhms, more preferably equal to or greater than 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C;

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating there.

According to another embodiment of the invention, the method 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 of between 150 and 250 g/L and at a temperature of between 14 and 21°C, and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed 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 immersion 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 ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, Cr ₂ (SO ₄ ) ₃ ,xH ₂ O, CrK(SO ₄ ) ₂ ,xH ₂ O (step A1-1) );
Then
- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H ₂ O ₂ ), ammonium fluoride (NH ₄ F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ), sodium permanganate (NaMnO ₄ ) (step A1-2 );

B) a sealing 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 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C;

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating.

According to another embodiment of the invention, the method 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 of between 150 and 250 g/L and at a temperature of between 14 and 21°C, and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed 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 immersion 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 ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, Cr ₂ (SO ₄ ) ₃ ,xH ₂ O, CrK(SO ₄ ) ₂ ,xH ₂ O (step A1-1) );
Then
- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H ₂ O ₂ ), ammonium fluoride (NH ₄ F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ), sodium permanganate (NaMnO ₄ ) (step A1-2 );

B) a sealing 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 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C;

C) 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 10 MOhms, at a temperature between 97 and 100°C;

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating there.

According to yet another embodiment of the invention, the method 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 of between 150 and 250 g/L and at a temperature of between 14 and 21°C, and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed 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 immersion 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 ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, Cr ₂ (SO ₄ ) ₃ ,xH ₂ O, CrK(SO ₄ ) ₂ ,xH ₂ O (step A1-1) );

B) a sealing 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 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C;

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating there.

In another embodiment of the invention, the method 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 of between 150 and 250 g/L and at a temperature of between 14 and 21°C, and

a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed 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 mineral dyes; then optionally

A1) a step of immersion 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 ) ₇ Cr ₃ (OH) ₂ ,xH ₂ O, Cr ₂ (SO ₄ ) ₃ ,xH ₂ O, CrK(SO ₄ ) ₂ ,xH ₂ O (step A1-1) ),
Then
- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H ₂ O ₂ ), ammonium fluoride (NH ₄ F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO ₄ ), sodium permanganate (NaMnO ₄ ) (step A1-2 ); then

B) a sealing 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 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C;

D) a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and

E) a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating.

In one embodiment of the invention, the method for surface treatment of an aluminum or aluminum alloy part, according to the invention, comprises, at the end of step A) and prior to the immersion step A1) , a step Im) of impregnation of said anodized part in a bath of organic or mineral dyes.

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 surface treatment available from a company such as Clariant. For example, mention may be made of the organic dye Sanodal Blue (from Clariant) with a concentration of 3g/L to which 2g/L of sodium acetate must be added, pH between 5 and 6 at a temperature between 40 and 65°C, preferably equal to 50°C, for a duration of between 5 and 35min, preferably equal to 20 minutes. The active ingredient of the organic dye is the anthraquinone molecule.

In all embodiments, before subjecting the part to the surface treatment method of the invention and therefore prior to the anodizing step A) , the part 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.

• dégraissage au solvant, pour dissoudre des graisses présentes à la surface de la pièce. Cette opération peut être réalisée par trempage, aspersion, ou tout autre méthode connue de l’homme du métier ;
• dégraissage alcalin, pour dissoudre des graisses présentes à la surface de la pièce. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier ;
• décapage alcalin, pour dissoudre les oxydes naturellement formés à la surface de la pièce. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier. A l’issue de cette opération, la pièce est recouverte d’une couche pulvérulente formée de produits d’oxydation des composés intermétalliques, qu’il convient d’éliminer par une étape de décapage acide ;
• décapage acide, pour dissoudre les oxydes naturellement formés à la surface de la pièce, et/ou la couche d’oxydation formée à la surface de la pièce lors de l’étape de décapage alcalin. Cette opération peut être réalisée par trempage, aspersion, ou toute autre technique connue de l’homme du métier.

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 dipping, spraying, 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, 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 part. 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 part 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 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 dipping, spraying, 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, particularly with demineralized water, are preferably carried out between the successive stages above, and before the part is treated by anodization.

Another subject of the invention relates to a method for manufacturing an aluminum or aluminum alloy part intended to be used in the aeronautical, aerospace and/or automotive sector comprising

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

(ii) - a step of applying one or more layers of paint, varnish, dry lubricants, or mastics.

The invention also relates to the use of a surface treatment method according to the invention, for the manufacture of aluminum or aluminum alloy parts intended for the aeronautical, aerospace and/or automotive sector.

The invention further relates to a part made of aluminum or aluminum alloy having undergone a surface treatment according to the invention, comprising one or more layers of paints, varnishes, dry lubricants, or mastics, said part being intended for the aeronautical, aerospace and/or automotive sector.

EXAMPLES

The method of the invention applies to aluminum and its alloys. The tests described below were carried out on 2024T3, a copper-plated aluminum grade considered to be one of the most sensitive to corrosion and representative of uses in aeronautics.

The tests aim to test the resistance to salt spray of the new anodizations without Chrome VI applied to Aluminum after having carried out a 50% immersion in a bath of BONDERITE M-CR 1200 or Alodine 1200.

These tests are part of the technical validation of the OAS fine anodizing treatment (Fine Anodic Sulfur Oxidation) to replace chromic anodizing (OAC - BF5 Chromic Anodizing). The latter involves the use of Chromium VI-based substances and will be banned from use in the European Union by September 2024.

In comparative ways, different fine OAS were tested:

- Fine OAS with anodization between 10 and 15V over 30 to 40 minutes with impregnation with CrIII (TCS) then PACS and sealing with boiling water (without additive) with a water conductivity >0.5MOhms,

- Optimized fine OAS sealed with SuperSeal 2S silicate salt: 10V 15min SS produced according to examples 1 and 2 of patent FR 3106837,

- Optimized fine OAS sealed with SuperSeal 2S silicate salt: 6V 15min SS produced according to examples 1 and 2 of patent FR 3106837.

OAC/BF5 class 2 (sealed at 50g/L with potassium dichromate) served as the reference treatment. OAC:BF5 is the subject of a patent belonging to SGI on improved OAC.

The objective of the tests carried out is to compare the results and to validate non-regression of alternative treatments to OAC/BF5.

These treatments were carried out on 2024T3 rolled specimens from the same material batch. Thus, only the influence of the treatment was evaluated for comparison. Note that the 2024T3 aluminum grade was deliberately chosen to reveal corrosion as quickly as possible following salt spray tests.

Theory

The Bonderite M-CR 1200 or Alodine 1200 bath is a chemical conversion bath used to treat the metallization areas of equipment. This treatment can be carried out by total immersion, partial immersion or by local application with a brush using a sample of the bath. This bath is composed of chromic acid and other compounds containing fluorine and zirconium. Its pH is between 1.4 and 1.8. Its acidity therefore affects the anodized layers which also undergo contact with this environment when a partially deprotected anodized part is immersed in the bath. Indeed, the anodic layers dissolve according to variable kinetics (depending on the temperature, concentration of the bath, type of anodization) for pH values below 4.

The immersion time is between 30 seconds and 3 minutes in order to achieve a shade on an unprotected, raw aluminum area between straw yellow and light brown. Only one complete recovery is allowed, thus leaving the possibility of a total cumulative immersion time in the Bonderite M-CR 1200 bath of 1 to 6 minutes.

In the context of this presentation, it was decided to place ourselves in the most critical case, namely to half immerse (half the height of the test piece) all of the test pieces with the different treatments mentioned in paragraph 1 for a period of 6 minutes.

Prior to the partial immersion of the test pieces in the Bonderite M-CR 1200 bath, a counterbore was made on a diameter of approximately 6 mm to deprotect an anodized area in order to visually mark the imprint of the Alodine 1200 treatment.

Test protocol

5 test pieces were immersed to 50% of their height for 6 minutes, respecting an identical operating range. Degreasing was limited to acetone given the cleanliness of the test pieces. As the counterboring was carried out less than 24 hours before immersion, no chemical (acid etching) or mechanical reactivation was carried out.

After 6 minutes of partial immersion of the specimens, it is noted that Bonderite M-CR 1200 caused a change in appearance on all the specimens as indicated in Table 1.

Bonderite M-CR 1200 is supplied by Henkel with a concentration of 15 to 20g/L in the bath, the bath is at room temperature (20 ± 5°C). The 6 minutes represent the maximum immersion time in the conversion bath in production: 3 minutes of immersion for a change of appearance, if it is missed, 3 additional minutes of immersion).

Table 1:

The readings of the values on the different anodizations are indicated in Table 2.

It is noted that the average thicknesses are lower after immersion for all treatments. However, the uncertainty linked to the eddy current measurement method must be taken into account for these thickness levels which remain relatively low.

24 hours after the immersion step, the test pieces were placed in the salt spray to undergo a corrosion resistance test according to ISO 9227.

Corrosion resistance results evaluated on anodized and sealed alloys by conventional sealing processes and by the sealing process of the invention:

Table 3 shows the number of corrosion pits depending on the duration of exposure to salt spray. The tests were carried out on rolled 2024 T3 aluminum alloy specimens machined on one of the two faces, measuring 120x60x2 mm, anodized with or without sealing, placed in salt spray to undergo a corrosion resistance test for 1080 hours according to ISO 9227.

It is observed that only the thin OAS specimen sealed with boiling water shows numerous pits after 360 hours of exposure. The phenomenon gradually becomes more widespread to reach 62 pits after 504 hours of exposure.

It can be deduced that if we compare the fine OAS specimen with silicate salt and the fine OAS sealed with boiling water, the silicate salt provides significant protection against the effect of immersion in a Bonderite M-CR 1200 bath.

Fine silicate salt OAS and BF5 cl2 remain intact after 504 hours of salt spray exposure. Salt spray tests extended up to 1080 hours demonstrated a non-regression of fine silicate salt OAS compared to the historical BF5 cl2 treatment.

Reference documents

- 2QP1127: Generic Qualification Plan for REACH compliant surface treatments on aluminum alloys

- REACH European regulation (regulation no. 1907/2006)

- French patent no. 3106837

- Table of electrolytic potentials of different materials - Rafale ind C protection plan Page 16/17 of 04/16/96

- ITC/CO/DV/104: Implementation of the special chemical conversion process using Bonderite M-CR 600/1200/1500

Conclusion

Silicate salts therefore provide better safety in the face of a range of subsequent conversions using acid (hot) baths and without chromium VI.

Beyond corrosion resistance, the process of the invention, in particular sealed anodization with silicate salt(s), guarantees significant resistance to various acid environments (chromic, sulfuric, nitric, acetic acid). It also improves the adhesion of paints, varnishes, dry lubricants, or mastics.

An anodization sealed with boiling water without silicate shows a clear regression in this property.

The stageB)is compatible with conversions with and without Chrome VI, respectively Alodine 1200 and Socosurf TCS/PACS.

An anodization sealed with silicate salt(s) is equivalent or even improved (with Alodine 1200) compared to the BF5/OAC on this property.

Claims (18)

Method for surface treatment of an aluminum or aluminum alloy part, comprising at least the following steps:
HAS)an anodizing step;
B)a step of sealing the anodic layer formed on the part at the end of the stepHAS)
the sealing 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 0.1 MOhms, more preferably equal to or greater than 10 MOhms, and from 1 to 500 g/L of at least one alkali metal or alkaline earth metal silicate, at a temperature between 60°C and 100°C; and
D)a step of partial mechanical deprotection of areas of the anodized and sealed coating to form unprotected areas; and
E)a step of chemical conversion to trivalent chromium or hexavalent chromium of the unprotected areas to form a conductive protective coating. Method according to claim 1, characterized in that the anodizing step A) is an anodizing 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 of between 14 and 21°C, and
a direct voltage is applied to said immersed part according to a voltage profile comprising a voltage increase at a speed of less than 1 V/min until a so-called plateau voltage value of between 5 and 13 V is reached. Method according to claim 1, characterized in that the anodizing step A) is an anodizing of the OAST (Sulfotataric Anodic Oxidation), OAS (Sulfuric Anodic Oxidation), PSAA (Sulfuric Phosphoric Acid Anodic Oxidation), BSAA (Sulfuric Boric Acid Anodic Oxidation), OAS NG FE (New Generation Thin Anodic Oxidation), OAS NG (New Generation Sulfuric Anodic Oxidation), OAD (hard anodic oxidation), or OAC (Chromic Anodic Oxidation). Method according to one of claims 1 to 3, characterized in that the aluminum alloy is 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, the aluminum casting alloys chosen from the group consisting of AS7G06, AS7G03, AS10G and AS9U3, the so-called difficult aluminum alloys chosen from the group consisting of 2618A, 2214 and AU5NKZr. A 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. Method according to any one of claims 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 suitable duration to obtain on the surface of said part an anodic layer with a thickness of between 2 and 7µm. Method according to any one of claims 2, 4 to 6, characterized in that the voltage applied to said immersed part is maintained at the plateau value for a period of between 20 and 80 minutes. Method according to any one of claims 2, 4 to 7, characterized in that the so-called plateau voltage value is between 6 and 10V. Process according to any one of claims 1 to 8, characterized in that the concentration of alkali metal or alkaline earth metal silicate in the solution of step B) is between 15 and 100 g/L. Surface treatment method according to any one of claims 1 to 9, characterized in that it comprises, prior to the silicate salt sealing step (step B ), a step A1) of immersion of said part,
- in an aqueous bath containing a trivalent chromium salt selected from the group consisting of CrF3,xH2O, CrCl3,xH2O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, ( _{CH3CO2 ) 7 Cr3} (OH) ₂ , _xH2O , _Cr2 ( _SO4 ) ₃ , _xH2O , CrK( _SO4 ) ₂ , _xH2O (step A1-1) );
then possibly
- in an aqueous bath containing an oxidizing compound selected from the group consisting of hydrogen peroxide (H ₂ O ₂ ), ammonium fluoride (NH ₄ F), potassium fluoro-zirconate (K ₂ ZrF ₆ ), potassium permanganate (KMnO4), sodium permanganate (NaMnO4) (step A1-2 )). Method according to claim 10, characterized in that the temperature of the aqueous bath containing the trivalent chromium salt (step A1-1 )) and that of the aqueous bath containing the oxidizing compound (step A1-2 )) are between 20 and 80°C. Surface treatment method according to one of claims 10 or 11, characterized in that it comprises, at the end of step A) and prior to the immersion step A1) , a step Im) of impregnation of said anodized part in a bath of organic or mineral dyes. Method according to any one of claims 1 to 12, 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, at a temperature between 97 and 100°C, said step C) taking place after the sealing with silicate salt according to step B) . Surface treatment method according to one of claims 10 or 11, 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. Method for surface treatment of an aluminum or aluminum alloy part, according to any one of claims 1 to 14, characterized in that in step D) , the deprotection is carried out by a mechanical operation, the mechanical operation being grinding, countersinking, milling, turning. Process for manufacturing a part made of aluminum or aluminum alloy 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 15, and optionally
(ii) - a step of applying one or more layers of paint, varnish, dry lubricants, or mastics. Use of a surface treatment process according to any one of claims 1 to 15, for the manufacture of aluminum or aluminum alloy parts intended for the aeronautical, aerospace and/or automotive sector. Part made of anodized aluminum or aluminum alloy and sealed by a surface treatment process according to any one of claims 1 to 15, comprising one or more layers of paints, varnishes, dry lubricants, or mastics, said part being intended for the aeronautical, aerospace and/or automotive sector.