EP2812467A1

EP2812467A1 - Method for anodizing parts made of an aluminum alloy

Info

Publication number: EP2812467A1
Application number: EP13703427.8A
Authority: EP
Inventors: Pierre Bares; Céline GAZEAU; Cédric STEPHAN; David PEDELMAS; Claude Rossignol; Sylvain BRUET; Olivier BRUCELLE; Paul DEDIEU; Philippe COMBES; Laurent Arurault; Viviane TURQ
Original assignee: Mecaprotec Industries SA
Current assignee: Mecaprotec Industries SA
Priority date: 2012-02-10
Filing date: 2013-02-11
Publication date: 2014-12-17
Anticipated expiration: 2033-02-11
Also published as: TR201902209T4; CA2864107A1; BR112014019652A8; FR2986807A1; FR2986807B1; US9879355B2; CA2864107C; EP2812467B1; BR112014019652A2; ES2711541T3; US20160047057A1; MX368584B; MA35901B1; MX2014009607A; TN2014000339A1; BR112014019652B1; WO2013117759A1; BR112014019652B8

Abstract

The invention relates to a method for anodizing a part made of aluminum or of an aluminum alloy, by immersing the part in an aqueous bath essentially comprising sulfuric acid at a concentration of 150 to 250 g/L and at a temperature of 5 to 25°C, then applying, to the part, a DC voltage according to a voltage profile comprising a voltage increase at a rate of 1 to 32 V/min, and then maintaining the voltage at a so-called plateau voltage value of 12 to 20 V for a duration sufficient for obtaining, at the surface of the part, an anode layer having a thickness of 3 to 7 mum and/or a layer weight of 20 to 150 mg/dm2.

Description

PROCESS FOR ANODIZING ALUMINUM ALLOY PARTS

The present invention is in the field of surface treatment of aluminum or aluminum alloy parts, aimed at improving their corrosion resistance properties. More particularly, it relates to a method of anodizing an aluminum part or one of its alloys, as well as a more general method of surface treatment of such a part implementing said method of anodization followed by a clogging step.

Aluminum alloy parts intended to be used in the aeronautical sector, or in other sectors in which they may be exposed to risks of corrosion that may be problematic, generally receive, before their implementation. , a surface treatment to protect them against corrosion.

One of the most widespread techniques for this purpose is anodization, also called anodic oxidation, which consists in forming on the surface of the part a porous aluminum oxide / hydroxide layer, called anodic layer, by applying a current to the piece immersed in an electrolytic bath containing a strong acid electrolyte, the piece constituting the anode of the electrolytic device. The anodic layer thus formed on the surface of the workpiece, after having been subjected to a clogging after-treatment, protects the workpiece against corrosion. This anodic layer is also a support for the attachment of conventional paint systems.

The electrolytic baths currently used for the anodizing of aluminum alloy parts, which provide the most advantageous performance in terms of, in particular, protection against corrosion of the part, mechanical hanging of paint coatings on the surface of the workpiece, and fatigue abatement, are formed with hexavalent chromium. However, chemicals containing hexavalent chromium are harmful to health and the environment. In order to avoid the use of hexavalent chromium-based substances for the anodizing of aluminum alloy parts, it has been proposed by the prior art to use anodizing processes employing other strong acids in the process. electrolytic bath, and in particular sulfuric acid. None of these baths, however, has satisfactory performance in terms of both protection of the part against corrosion, adhesion of conventional paint systems on the workpiece, and fatigue reduction of the workpiece. These performances prove to be notably insufficient compared to the requirements imposed in the aeronautical field. The present invention aims to remedy the disadvantages of anodizing processes of aluminum alloy parts of the prior art, in particular to those described above, by proposing such a process that does not implement any harmful substance, particularly to hexavalent chromium base, while having at least equivalent performance to the processes of the prior art using hexavalent chromium, in particular in terms of resistance to corrosion of the treated part, fatigue fatigue of the part and adhesion of conventional paint systems on its surface.

It has now been discovered by the present inventors that a sulfuric type anodizing process, implemented under particular conditions, made it possible to achieve these objectives.

It is thus proposed according to the present invention a method of anodizing a piece of aluminum or aluminum alloy, wherein the part is immersed in an aqueous bath comprising essentially sulfuric acid at a concentration of between 150 and 250 g / L and maintained at a constant temperature of between 5 and 25 ° C. By essentially comprising sulfuric acid is meant that the bath contains no other active electrolytic substance, especially strong acid, in an amount sufficient to intervene in the anodization. The bath contains in particular no phosphoric, boric, chromic or tartaric acid, or only in the form of traces.

This process according to the invention is characterized by the application to the piece immersed in the bath of a DC voltage according to a voltage profile comprising a rise in voltage, from a starting value of 0 V, at a speed of between 1 and 32 V / min, then maintaining the voltage at a value a so-called plateau voltage of between 12 and 20 V for a period of time sufficient to obtain an anodic layer of aluminum oxides / hydroxides at the surface of the part, of a thickness of between 3 and 7 μητι, preferably of between 3 and 7 μητι. and 5 μητι, and / or a layer weight of between 20 and 150 mg / dm ² .

Such an anodic layer exhibits properties of adhesion to paint and corrosion resistance after clogging equivalent to those of the anode layers obtained by prior art chromic anodizing processes, while not using any substance. based on hexavalent chromium.

This result is also advantageously obtained with a thickness of the weak anodic layer, that is to say less than or equal to 7 μητι, preferably less than or equal to 5 μητι, when the so-called standard sulfuric anodization processes proposed by the prior art require, to achieve acceptable performance, which remain however less than that of the method according to the invention, to form on the part an anode layer of much greater thickness, typically between 8 and 12 μηη. In this, the method according to the invention has an additional advantage, which is to overcome the problems of resizing and fatigue reduction generated by standard sulfuric anodization processes of the prior art.

In particular embodiments of the invention, the voltage profile applied to the part comprises a rise in voltage at a speed of between 1 and 32 V / min until reaching the so-called plateau voltage value between 12 and 20 V, then maintaining the voltage at said plateau voltage value for a period of time sufficient to obtain an anodic layer of aluminum oxides / hydroxides of thickness between 3 and 7 on the surface of the part. μητι, preferably between 3 and 5 μη-ι, and / or weight of layer between 20 and 150 mg / dm ² .

In different embodiments, the voltage profile applied to the part comprises a plurality of voltage rise phases, at least one of which is carried out at a speed of between 1 and 32 V / min, and which can be separated. two by two by a bearing during which the voltage is temporarily maintained at a fixed value, before the implementation of the final phase of maintaining the voltage at the plateau voltage value between 12 and 20 V.

It is within the competence of those skilled in the art to determine the duration of maintaining the voltage at the plateau value, in order to obtain the desired anodic layer thickness on the part, particularly as a function of the characteristics of the particular alloy and the conditions for subsequent implementation of the part.

In embodiments of the invention, the voltage is maintained at the plateau value for a period of between 5 and 30 minutes, depending on the aluminum alloy and the thickness of the desired anodic layer. .

According to an advantageous characteristic of the invention, in terms of protective performance of the part against corrosion, the rate of rise in voltage is between 1 and 6 V / min, preferably equal to 3 V / min.

Preferably, the plateau voltage value is between 14 and 16 V. It is within the competence of those skilled in the art to determine the optimum voltage value within this range, depending in particular on the characteristics of the alloy. constituting the room.

The concentration of sulfuric acid in the bath is preferably from 180 to 220 g / l, for example equal to 200 g / l.

In embodiments of the invention, the bath temperature is between 15 and 25 ° C, preferably between 18 and 20 ° C, and for example equal to 19 ° C. All these preferential parameters ensure the best performance of the bath from the point of view of the properties of the anodic layer formed on the surface of the part.

The part may be subjected to a surface preparation step by degreasing and / or pickling prior to immersion in the bath, so as to remove grease, dirt and oxides present on its surface.

This preliminary surface preparation step can comprise one or more of the following operations:

- Solvent degreasing, to dissolve fats present on the surface of the room. This operation may be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by soaking in methoklone or acetone, at a temperature below 42 ° C, for a period of between 5 seconds and 3 minutes; - alkaline degreasing, to dissolve fats present on the surface of the room. This operation may be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by dipping in a mixture of TURCO 4215 NCLT (Henkel), at 40 to 60 g / L, and TURCO 4215 additive (Henkel), at 5 to 20 g / L, at a temperature of between 50.degree. and 70 ° C for a period of between 10 and 30 minutes;

- alkaline pickling, to dissolve naturally formed oxides on the surface of the workpiece. This operation may be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by soaking in a sodium hydroxide solution at 30 to 70 g / l, at a temperature between 20 and 60 ° C, for a period of between 10 seconds and 2 minutes. 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, for dissolving 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 may be carried out by dipping, spraying, or any other technique known in itself. It may for example be carried out by dipping in a 15 to 25% v / v solution of SMUT-GO NC (Henkel) at a temperature of between 10 and 50 ° C for a period of between 1 and 10 minutes; or by soaking in a solution of ARDROX 295GD (Chemetall) at 15 to 30% v / v, at a temperature between 10 and 30 ° C, for a period of between 1 and 10 minutes. Interleaved rinses, especially with water, are preferably made between the successive steps above, and before the treatment of the piece by anodizing.

Another aspect of the invention is a more general method of surface treatment of an aluminum or aluminum alloy part, according to which the part is subjected to an anodizing process corresponding to one or more of the characteristics above, then to a clogging step of the anodic layer then formed on the workpiece.

The sealing step of the porous anodic layer may be of any type known to those skilled in the art. It can for example be a hydrothermal clogging, hot clogging with hexavalent chromium salts or nickel salts, etc. Clogging processes not involving any substance harmful to the environment and / or health are particularly preferred in the context of the invention.

In advantageous embodiments of the invention, this sealing step comprises immersing the part in an aqueous bath containing a trivalent chromium salt and an oxidizing compound, with a temperature of between 20 ° and 80 ° C., preferably between 20 and 60 ° C, more particularly between 35 and 45 ° C, and / or the immersion of the piece in water at a temperature between 98 and 100 ° C, and pH for example between 4 , 5 and 8. In the present description, conventional trivalent chromium is understood to mean chromium in the +3 oxidation state. Hexavalent chromium means chromium in the +6 oxidation state.

The oxidizing compound may be of any type known in itself for post-anodizing clogging baths of aluminum or its alloys. Compounds having no adverse effect on the environment are particularly preferred in the context of the invention. Nonlimiting examples of such oxidizing compounds are fluoride-based substances, such as ammonium fluoride or potassium fluoro-zirconate K ₂ ZrF ₆ , of permanganate, such as potassium permanganate, hydrogen peroxide H 2 O 2. etc. The concentration of oxidizing compound in the bath may especially be between 0.1 and 50 g / l.

The trivalent chromium salt and the oxidizing compound present in the bath may consist of two different compounds, or of one and the same compound capable of ensuring on its own the two functions of corrosion inhibition and oxidation, for example by trivalent chromium fluoride CrF ₃ .

The trivalent chromium salt can be brought in any conventional form in itself for post-anodization sealing treatments of aluminum, especially in the form of fluoride, chloride, nitrate, acetate, acetate hydroxide, sulfate, potassium sulfate, etc. of trivalent chromium, for example 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, etc.

In preferred embodiments of the invention, the trivalent chromium salt present in the bath is a fluoride. This is, for example, chromium trifluoride CrF ₃ .

In particular embodiments of the invention, the immersion step in the aqueous bath meets one or more of the following operating parameters:

the temperature of the bath is between 20 and 80 ° C, preferably between 20 and 60 ° C, more preferably between 35 and 60 ° C, and preferably between 35 and 45 ° C, for example equal to 40 ° C;

the bath pH is between 3 and 4.5, preferably between 3 and 4, for example equal to 3.5;

the immersion time in the bath is between 5 and 40 min, preferably between 10 and 30 minutes, for example equal to 15 or

20 minutes.

The concentration of trivalent chromium salt in the bath is preferably between 0.5 and 50 g / l.

The immersion of the piece in water at a temperature between 98 and 100 ° C, can be carried out with an immersion time of between 10 and 60 minutes, in accordance with the operating parameters of the so-called traditional hydrothermal sealing processes.

In particular embodiments of the invention, the sealing step comprises immersing the part successively in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, and in water at a temperature of between 98 and 100 ° C. These steps can be carried out in any order, and in particular be separated by one or more interleaved water rinses.

For example, the clogging step may include immersing the workpiece in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, and then, after rinsing (s), in water at a temperature of 98 ° C. at 100 ° C. Otherwise, the clogging step may include immersing the piece in water at a temperature of 98 to 100 ° C, and then, after rinsing (s), in the aqueous bath containing a trivalent chromium salt and an oxidizing compound.

The features and advantages of the invention will appear more clearly in the light of the following examples of implementation, provided for illustrative and not limiting purposes of the invention, with the support of FIGS. 1A to 1E, which show micrographs of anodic layers formed on the surface of aluminum parts by FIG. 1A, chromic anodizing (OAC), FIG. 1B, standard sulfuric anodization (OASstandard), FIG. 1C, sulfo-tartaric anodization (OAST), FIG. 1D, sulfoboric anodization (OASB) and FIG. 1E, anodizing according to an implementation mode of the invention. EXAMPLE 1

1 .1 / Methods of anodizing aluminum alloy parts

Parts of 2024 T3 aluminum alloy laminated 120x80x2 mm dimensions are treated by anodization according to the methods below.

Steps for surface preparation of the part are firstly carried out successively:

- alkaline degreasing, by soaking the part in a mixture of TURCO 4215 NCLT at 50 g / L and TURCO 4215 additive at 10 g / L, at a temperature of 60 ° C, for 20 min;

- rinsing with water; - acid pickling, by soaking the piece in a solution of

SMUT-GO NC at 19% v / v, at a temperature of 20 ° C, for 5 min;

- rinsing with water.

Parts are then subjected to an anodizing process according to an embodiment of the invention, as follows. A bath is prepared by diluting a solution of sulfuric acid in water to obtain a sulfuric acid concentration of 200 g / L, excluding any other compound. This bath is heated and maintained at a temperature of 19 ° C.

The part is immersed in the bath, and it is applied a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 3 V / min, to a value called 16 V tray. The tension is maintained at the plateau value for 16 minutes. On the surface of the part, an anodic oxide / aluminum hydroxide layer with a thickness of approximately 4 to 5 μηη is formed.

By way of comparative examples, identical parts having been subjected to the same surface preparation operations are anodized according to the conventional methods of chromic anodization (OAC), standard sulfuric anodization (standard OAS), sulfo-tartaric anodizing (OAST) and sulfoboric anodizing (OASB).

The operating parameters for standard OAS, OAST, OASB and OAC are shown in Table 1 below.

Table 1 - Operating Parameters Implemented for the Various Anodizing Processes of the Standard OAS, OAST, OASB and OAC OAS

The different pieces thus obtained are subjected to the following tests. 1 .21 Morphological analysis of the anodic layer

A morphological analysis of the anodic layer formed on the surface of each of the pieces thus treated is carried out by electron microscopy with field effect (SEM-FEG). The micrographs are shown in the figures 1 to 1 E. FIG. 1 E, corresponding to the anode layer obtained by a method according to an embodiment of the invention, shows a homogeneous morphology in the thickness of the layer, with the absence of micro-precipitates from the substrate within the layer. From the micrographic observations, the pore diameters were measured for each of the anode layers and the results are shown in Table 2 below.

Table 2 - Pore diameter of the anodic layer formed on the 2024 T3 aluminum alloy parts rolled according to the anodizing process implemented

It can be seen from this table that the morphology of the anodic layer formed on the parts by the process according to an embodiment of the invention is similar to that of a layer obtained by chromic anodizing, compared to the other processes of the invention. anodizing using sulfuric acid proposed by the prior art.

1 .3 / Fatigue fatigue tests

The various anodized parts are subjected to a fatigue test in order to evaluate the fatigue reduction related to the formation of the anodic layer on their surface. The parameters of the fatigue test are as follows:

- solicitation: rotary bending

- temperature: 20 ° C

- R = -1

- Frequency: 100 Hz

Kt = 1.035

- type of test specimens: FFRT16

- number of test pieces: 12 The results of this test, in terms of fatigue limit and abatement with respect to non-anodized parts, for parts processed by the method according to an embodiment of the invention and by various conventional methods, are shown in the Table. 3 below.

Table 3 - Fatigue load evaluated by fatigue test for 2024 T3 aluminum alloy parts according to the anodizing process used

These results clearly demonstrate that the fatigue reduction generated by the process according to an embodiment of the invention is significantly lower than that generated by conventional anodizing processes, whether anodizing. sulfuric acid (OAS) standard but even chromic anodizing (OAC) for an equivalent anodic layer thickness. The parts treated by the anodizing method according to an embodiment of the invention have a better resistance to efforts than those treated by anodizing processes of the prior art. In particular, in comparison with the standard sulfuric anodizing, they allow a lightening of the structures in which they are implemented. These parts may also advantageously be substituted for parts processed by chromic anodization already implemented, especially in aircraft, without the need to resize.

1 .4 / Adhesion tests of paint coatings

Parts anodized by the method according to an embodiment of the invention, as indicated above, are subjected to tests adhesion of conventional paint systems.

Two paint systems are tested: a water-based epoxy system (P60 + F70) and a solvent-based polyurethane system (PAC33 + PU66). The tests are carried out according to the ISO 2409 standard, for dry adhesion, after drying of the paint system, and for wet adhesion: after drying of the paint system, the samples are immersed in demineralized water for 14 hours. days and then dried before undergoing the adhesion test according to the standard.

The results are shown in Table 4 below.

Table 4 - Results of adhesion tests of two paint systems on rolled 2024 T3 aluminum alloy parts treated by a method according to an embodiment of the invention

By way of comparison, similar tests are carried out on parts treated with standard sulfuric anodization (standard OAS) as indicated above. The results of these tests are shown in Table 5 below.

Table 5 - Adhesion test results of two paint systems on laminated 2024 T3 aluminum alloy parts treated by a standard sulfuric anodizing process These results show that the parts treated by the process according to one embodiment of the invention have an adhesion to the paint systems, whether of the water-soluble or solvent-based type, equivalent to those treated by conventional OAST anodizing processes. and OASB, which also show, in a known manner, Grade 0 results in the adhesion tests above. This adhesion, for one or the other of the two paint systems, is much greater than that obtained by the standard sulfuric anodizing process proposed by the prior art.

1 .5 / Resistance to corrosion after clogging

The parts treated by the process according to an embodiment of the invention, by OAC, OAST or OASB, as indicated above, are subjected to the clogging process C1 according to an embodiment of the invention. following invention:

immersion in an aqueous bath of composition: CrF ₃ : 6 g / L and K ₂ ZrF ₆ : 1 g / L, in water, at a pH of 3.5 and a temperature of 40 ° C. for 15 minutes ,

and then immersion in water at a pH of 6.5, at a temperature of 98 ° C., for 40 minutes.

By way of comparative examples, anodized parts are also subjected to the following conventional sealing methods: hydrothermal clogging, hot clogging with hexavalent chromium salts, hot clogging with nickel salts, according to the operating conditions indicated in Table 6 below. Clogging Clogging at hydrothermal clogging chromium salts nickel salts

VI

Composition H ₂ OK ₂ Cr ₂ O ₇ : (CH ₃ COO) ₂ Ni:

30 mg / L 10 g / L

PH 6.5 6 5.5

Temperature (° C) 98 98 98

Immersion time 40 20 30

(Min.)

Table 6 - Operating Parameters Used for Different Clogging Processes

On each treated part, an anodic layer clogged.

The parts thus treated are subjected to a salt spray test according to the ISO 9227 standard.

First approximate average results, obtained on a small number of pieces, are shown in Table 7 below.

Table 7 - Salt spray resistance of 2024 T3 aluminum alloy rolled parts treated by anodizing and clogging, the anodization being carried out by a method according to an embodiment of the invention or by methods of anodizing of the prior art

More precise average results concerning the appearance of first corrosion pits (specifically the 1 ^st pitting ( "1 ^st") and the spread of corrosion ( "G ^is")), obtained on a larger number of parts, are shown in Table 8 below -after.

Table 8 - Salt spray resistance of 2024 T3 aluminum alloy rolled parts treated by anodization and clogging, the anodization being carried out by a method according to an embodiment of the invention or by methods of anodizing of the prior art

These results clearly demonstrate that the anodizing method according to an embodiment of the invention, followed by a clogging step, of any type whatsoever, makes it possible to give the treated part a corrosion resistance. at least equivalent to that obtained by conventional anodizing processes followed by the same clogging.

In particular, the method of anodizing according to an embodiment of the invention has anticorrosion performance equivalent to chromic anodization (OAC) in combination with hydrothermal sealing or hot clogging with hexavalent chromium salts, and much better than dilute sulfo-tartaric (OAST) or sulfoboric (OASB) anodizations.

This ability of the anode layer formed by the process according to the invention to be clogged during a post-treatment to provide him with Corrosion resistance properties could be explained by its pore size greater than 10 nm, which facilitates its hydration during hydrothermal sealing, for example, resulting in pore sealing and protection against corrosion by effect. barrier layer. Finally, it is observed that the particular combination of the anodizing method according to an embodiment of the invention, with the clogging method C1 according to an embodiment of the invention, makes it possible to obtain results. in terms of corrosion resistance of the treated part, which are significantly higher than those obtained for any other combination anodizing / clogging.

EXAMPLE 2

Various parameters of the anodizing process according to the invention are varied with respect to Example 1 above.

2.1 / Variations of concentration in sulfuric acid

Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process according to the invention. by immersion in a bath at 19 ° C containing sulfuric acid at a concentration of 150 or 250 g / l, excluding any other compound. It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 6 V / min, up to a so-called plateau value of 16 V. The tension is maintained at the plateau value for 16 minutes.

The anode layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100 ° C for 40 min. On the surface of each part is formed an anodic layer of aluminum oxide / hydroxide with a thickness of approximately 3.5 to 4.5 μηη.

As a comparative example, the same process of anodizing treatment and clogging is applied to a similar room, but using a concentration of sulfuric acid in the bath of 100 g / l only.

The parts thus treated are subjected to a salt spray test in accordance with the ISO 9227 standard. The results obtained are shown in Table 9 below.

Table 9 - Salt spray resistance of aluminum alloy parts

2024 T3 laminated treated by anodization then clogging, for different concentrations of sulfuric acid anodizing bath

These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention implementing a concentration of sulfuric acid in the bath of between 150 and 250 g / l. This efficiency is notably superior to the comparative method implementing a sulfuric acid concentration of 100 g / l, lower than that recommended by the present invention.

2.2 / Variations of speed of rise in tension

Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process according to the invention by immersion in a bath at 19 ° C. containing sulfuric acid at a concentration of 200 g / l, excluding any other compound. It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, to a so-called plateau value of 16 V. The voltage is then maintained at the value of plateau for 16 minutes. Different speeds of rise in voltage are tested: 1 V / min, 20 V / min, 32 V / min.

The anode layer is then sealed by immersing the part in a water bath at a temperature between 98 and 100 ° C for 40 min.

On the surface of each part, an anodic oxide / aluminum hydroxide layer with a thickness of approximately 4 to 4.5 μηη is formed.

The parts thus treated are subjected to a salt spray test in accordance with the ISO 9227 standard. The results obtained are shown in Table 10 below.

Table 10 - Salt spray resistance of laminated 2024 T3 aluminum alloy parts treated by anodizing and clogging, for different voltage rise rates

These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention implementing a rise in voltage at a speed included between 1 and 32 V / min.

2.3 / Variations of plateau voltage value

Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process according to the invention. by immersion in a bath at 19 ° C containing sulfuric acid at a concentration of 200 g / l, to the exclusion of any other compound. It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, with a speed of 3 V / min, up to a plateau value of 14 V or The voltage is then maintained at the plateau value for 16 minutes.

The anode layer is then sealed by the clogging method C1 described in Example 1 above.

On the surface of each part, an anodic oxide / aluminum hydroxide layer approximately 4 to 5 μηη thick is formed.

The parts thus treated are subjected to a salt spray test according to ISO 9227. The results obtained are shown in Table 1 1 below.

Table 1 1 - Salt spray resistance of laminated 2024 T3 aluminum alloy parts treated by anodizing and clogging, for different plateau voltage values These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention implementing the final maintenance of the voltage to a plateau value of between 14 or 16 V.

2.4 / Temperature variations of the anodizing bath

Aluminum alloy parts similar to those of Example 1, having previously been subjected to surface preparation steps as indicated in Example 1 above, are subjected to an anodizing process according to the invention. by immersion in a bath containing sulfuric acid at a concentration of 200 g / l, to the exclusion of any other compound. Several bath temperatures are tested, more particularly 6 ° C, 12 ° C and 25 ° C.

It is then applied to each piece a DC voltage according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 3 V / min, up to a so-called plateau value of 16 V. The tension is maintained at the plateau value for a period of between 10 and 60 minutes, depending on the temperature value of the bath. This time is fixed to obtain on the surface of each piece an anodic layer thickness of oxide / aluminum hydroxide thickness of about 4 to 5 μηη.

As a comparative example, the same method of treatment by anodization and clogging is applied to a similar room, but using an anode bath temperature of 30 ° C.

The parts thus treated are subjected to a salt spray test according to ISO 9227. The results obtained are shown in Table 12 below. Salt spray resistance (h)

Temperature of the anodizing bath Apparition of the 1 ^st Generalization

(° C) pitting corrosion corrosion

6 1272 2304

12 1224 2280

25 1320 2424

30 624 1536

Table 12 - Salt spray resistance of aluminum alloy parts

2024 T3 laminated treated by anodizing then clogging, for different temperatures of the anodizing bath

These results show the effectiveness, in terms of corrosion resistance of the treated parts, anodizing processes according to the invention implementing an anode bath temperature of between 5 and 25 ° C. This efficiency is notably greater than the comparative method implementing a bath temperature of 30 ° C, higher than that recommended by the present invention. The above description clearly illustrates that by its different characteristics and advantages, the present invention achieves the objectives it has set for itself. In particular, it provides a method of anodizing aluminum alloy parts which avoids the use of substances based on hexavalent chromium, while exhibiting performance, in particular in terms of corrosion resistance of the treated part, fatigue reduction and adhesion of paint coatings on the surface of the part, which are at least equivalent to those of chromic anodizing processes, and superior to those sulfuric anodizing processes proposed by the prior art .

Claims

1. A method of anodizing an aluminum or aluminum alloy part, according to which:

said part is immersed in an aqueous bath essentially comprising sulfuric acid at a concentration of between 150 and 250 g / l and at a temperature of between 5 and 25 ° C., characterized in that:

to said part immersed in said bath is applied a DC voltage according to a voltage profile comprising a rise in voltage at a speed of between 1 and 32 V / min, and then maintaining the voltage at a so-called plateau plateau voltage between 12 and 20 V for a period of time sufficient to obtain on the surface of said part an anodic layer with a thickness of between 3 and 7 μηη and / or a layer weight of between 20 and 150 mg / dm ² .

2. Method according to claim 1, characterized in that the voltage is maintained at said plateau value for a period of between 5 and 30 minutes.

3. Method according to one of claims 1 to 2, characterized in that the rate of rise in voltage is between 1 and 6 V / min, preferably equal to 3 V / min.

4. Method according to any one of claims 1 to 3, characterized in that the plateau voltage value is between 14 and 16 V.

5. Method according to any one of claims 1 to 4, characterized in that the concentration of sulfuric acid in the bath is between 180 and 220 g / l, preferably equal to 200 g / l.

6. Method according to any one of claims 1 to 5, characterized in that the bath temperature is between 15 and 25 ° C, preferably between 18 and 20 ° C.

7. Method according to any one of claims 1 to 6, characterized in that said part is subjected to a degreasing step and / or pickling prior to its immersion said bath.

8. Method according to any one of claims 1 to 7, characterized in that the voltage profile applied to the part comprises a rise in voltage at a speed between 1 and 32 V / min until said voltage value is reached plateau.

9. A method of surface treatment of a piece of aluminum or aluminum alloy, wherein said part is subjected to an anodizing process according to any one of claims 1 to 8, then to a step of sealing the anode layer formed on said piece.

Surface treatment method according to claim 9, characterized in that said sealing step comprises immersing said workpiece in an aqueous bath containing a trivalent chromium salt and an oxidizing compound, and / or immersion of said piece in water at a temperature between 98 and 100 ° C.

11. Surface treatment method according to claim 10, characterized in that the temperature of the aqueous bath containing a trivalent chromium salt and an oxidizing compound is between 20 and 80 ° C, preferably between 20 and 60 ° C.

12. A surface treatment method according to any one of claims 10 to 1 1, characterized in that said sealing step comprises the immersion of said piece successively in said aqueous bath containing a trivalent chromium salt and an oxidizing compound, then in water at a temperature between 98 and 100 ° C.