Method of anodising aluminum alloy pieces

ANODIZATION PROCESS FOR PARTS IN ALUMINUM WING

SHORT

The invention relates to a process for anodizing an aluminum or aluminum alloy part by immersing the part in an aqueous bath comprising essentially sulfuric acid at a concentration of between 150 and 250 g / L and at a temperature between 5 and 5 25 ٥c, then application to the part of a DC voltage according to a voltage profile comprising a voltage rise at a speed between 1 and 32 v / min, then maintaining the voltage at a so-called plateau voltage value of between 12 and 20 V for an adequate period of time to obtain on the surface of the part an anode layer with a thickness of between 3 and 10 7 μm and / or a weight of layer between 20 and 150 mg / dm٩

My

35901Β1 أ ٠ DEC 20 "

ANODIZING PROCESS FOR ALUMINUM ALLOY PARTS

The present invention relates to the field of surface treatment of aluminum or aluminum alloy parts, aimed at improving their corrosion resistance properties. More particularly, it relates to a process for anodizing a part made of aluminum or one of its alloys, as well as a more general process for the surface treatment of such a part using said anodizing process followed by a plugging step.

Aluminum alloy parts intended for use in the aeronautical sector, or in other sectors in which they are likely to be exposed to risks of corrosion which may prove to 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 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 the strong acid type, the part constituting the anode of the electrolytic device. The anode layer thus formed on the surface of the part, after having been subjected to a post-sealing treatment, protects the part against corrosion. This anode layer also constitutes a support for the adhesion of conventional paint systems.

The electrolytic baths currently used for the anodization of aluminum alloy parts, which provide the most advantageous performance in terms in particular of protection against corrosion of the part, mechanical adhesion of paint coatings at the surface of the part, and fatigue reduction, are forms based on hexavalent chromium. Chemicals containing hexavalent chromium are, however, harmful to health and the environment.

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In order to avoid the use of substances based on hexavalent chromium for the anodization of aluminum alloy parts, it has been proposed by the prior art of anodization processes using other strong acids in the electrolytic bath, and in particular sulfuric acid. None of such baths, however, exhibits satisfactory performance in terms both of protection of the part against corrosion, of adhesion of conventional paint systems on the part, and of fatigue reduction of the part. These performances prove in particular insufficient compared to the requirements imposed in the aeronautical field.

The present invention aims to remedy the drawbacks of the processes for anodizing aluminum alloy parts of the prior art, in particular those explained above, by proposing such a process which does not use any harmful substance, in particular to based on hexavalent chromium, while exhibiting performance at least equivalent to the methods of the prior art using hexavalent chromium, in particular in terms of corrosion resistance of the part treated, fatigue reduction 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 special 2٥ conditions, made it possible to achieve these objectives.

There is thus proposed according to the present invention a process for anodizing an aluminum or aluminum alloy part, according to which the part is immersed in an aqueous bath comprising essentially sulfuric acid at a concentration of between 150 and 250 g / L and 25 maintained at a constant temperature between 5 and 25 ^٠vs. By essentially comprising sulfuric acid is meant the fact that the bath does not contain any other active electrolytic substance, in particular strong acid, in an amount sufficient to intervene in the anodization, the bath in particular does not contain phosphoric acid, boric, chromic or tartaric, or only in trace amounts.

This method according to the invention is characterized by the application to the part

Μα 35901Β1 immersed in the bath of a direct voltage according to a voltage profile comprising a rise in voltage, from a starting value of 0 V, at a speed between 1 and 32V / min, then maintaining the voltage at a a so-called plateau voltage value of between 12 and 20 V for a period 5 adequate to obtain on the surface of the part an anodic layer of aluminum hydrides, with a thickness of between 3 and 7 μm, preferably of between 3 and 5 pm, and / or layer weight between 20 and 150 mg / dm².

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

This result is also advantageously obtained with a low thickness of the anode layer, that is to say less than or equal to 7 μm, preferably less than or equal to 5 μm, when the so-called standard sulfuric anodizing processes proposed. by the prior art require, in order to achieve acceptable performances, which however remain lower than those of the method according to the invention, to form on the part an anode layer 20 of much greater thickness, typically between 8 and 12 pm. In this, the method according to the invention has an additional advantage, which is to overcome the problems of resizing and reduction in fatigue generated by the standard sulfuric anodizing methods 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 between 1 and 32 V / min until the so-called plateau voltage value between 12 and 20 V, then maintaining the voltage at said plateau voltage value for an adequate period of time to obtain on the surface of the part an anodic layer, of aluminum oydes / hydroyides, with a thickness of between 3 and 7 pm, preferably between 3 and 5 µm, and / or weight of

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35901 Bl layer between 20 and 150 mg / dm2.

In different embodiments, the voltage profile applied to the part comprises a plurality of voltage rise phases, at least one of which is performed at a speed between 1 and 32 ν / min, and 5 which can be separated in pairs by a level 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 tension at the plateau value, in order to obtain the desired anode layer thickness on the part, depending in particular on the characteristics of the particular alloy and conditions for the subsequent use 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 desired anode layer thickness.

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

Preferably, the plateau voltage value is between 14 and 16 V. It is up to a person skilled in the art to determine the optimum voltage value within this range, in particular as a function of the characteristics of the alloy constituting the part.

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

In embodiments of the invention, the temperature of the bath is between 15 and 25 ٥c, preferably between 18 and 20 ٥c. and for example equal to 19 ٥c.

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All these preferred parameters ensure the best performance of the bath from the point of view of the properties of the anodic coating formed on the surface of the part.

The part can be subjected to a surface preparation step by degreasing and / or pickling prior to its immersion in the bath, so as to remove the 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 technique known in itself. It can for example be carried out by soaking in metlioklone or acetone, at a temperature below 42 ٥c, for a period of between 5 seconds and 3 minutes:

- 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 in itself. It can for example be carried out by soaking in a mixture of TURCO 4215 NCIT (Henkel), at 40 to 60 g / L, and TURCO 4215 additive (Henkel), at 5 to 20 g / L, at a temperature of between 50 and 70 ٥c, for a period of between 10 and 30 minutes;

- 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 in itself. It can for example be carried out by soaking in a solution of sodium hydroxide at 30 to 70 g / L, at a temperature between 20 and 60 ٥c, 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;

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<img file="MA35901B1_D0001.tif" />

- 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 per se. It can for example be carried out by soaking in a solution of SMUT-GO NC (Henkel) at 15 to 25 ٠/٥٧/٧, at a temperature between 10 and 50 ٥c, for a period of between 1 and 10 minutes; or by soaking in a solution of ARDROX 295GD (Chemetall) at 15 to 30 ٠/٥٧/٧, at a temperature between 10 and 30 ٥c, for a period of between let 10 minutes.

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

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 above characteristics, then to a sealing step of the anode layer then formed on the part.

The step of sealing the porous anode layer can be of any type known to those skilled in the art. It can be, for example, hydrothermal sealing, hot sealing with hexavalent chromium salts or nickel salts, etc. The sealing processes which do not use any substance harmful to the environment and / or to 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, at a temperature of between 20 and 80 ٥c, preferably between 20 and 60 ٥c, more particularly between 35 and 45 ٥c, and / or immersing the part in water at a temperature between 98 and 100 ٥c, and with a pH for example between 4.5 and 8.

35901Β1

In the present description, in a conventional manner in itself, the term “trivalent chromium” is understood to mean clirome in the +3 oxidation state. By hexavalent chromium is meant chromium in the +6 oxidation state.

The oxidizing compound can be of any type known per se for post-anodization sealing baths of aluminum or its alloys. Compounds exhibiting no harmful effect on the environment are particularly preferred in the context of the invention. Non-limiting examples of such oxidizing compounds are substances based on fluorides, such as ammonium fluoride or potassium fluoro-zirconate KaZrFe, permanganate, such as potassium permanganate, Η2Ο2 dihydrogen peroxide, etc. The concentration of oxidizing compound in the bath can in particular be between 0.1 and 50 g / L.

The trivalent chromium salt and the oxidizing compound present in the bath can be constituted by two different compounds, or by one and the same compound capable of providing on its own the two functions of corrosion inhibition and oxidation, for example by trivalent chromium fluoride CrF3.

The trivalent chromium salt can be supplied in any form which is conventional in itself for post-anodizing aluminum sealing treatments, in particular in the form of fluoride, chloride, nitrate, acetate, acetate, hydroxide, sulfate, potassium sulfate, etc. ., of trivalent chromium, for example CrFsxHO, CrCxHO, Cr (NO3) 3, xH2٠, (CH3CO2) 2Cr, xH2O, (CH3C٠2) 7Cr3 (OH) 2, xH2O, Cr2 (SO4) 3, xH2O, CrK (SO4) 2, xH2O, etc.

In preferred embodiments of the invention, the trivalent chromium salt present in the bath is a fluoride. This is for example chromium trifluoride CrF3٠

In particular embodiments of the invention, the step of immersing 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 6O٥C, more preferably between 35 and 60 ٥c, and

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35901 B1 preferably between 35 and 45 ٥c, for example equal to 40 ٥c;

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

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

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

The immersion of the part in water at a temperature of between 98 and 100 ٥c can be carried out with an immersion time of between 10 and 60 minutes, in accordance with the operating parameters of traditional so-called 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. temperature between 98 and 100 ٥c. These steps can be carried out in any order, and in particular be separated by one or more intermediate water rinses.

For example, the sealing step may comprise immersing the part in the aqueous bath containing a trivalent chromium salt and an oxidizing compound, and then, after possible resurfacing (s), in water. at a temperature of 98 to 100 ٥c. Alternatively, the sealing step may comprise immersing the part in water at a temperature of 98 to 100 ٥c, then, after optional rinsing (s), in the aqueous bath containing a trivalent chromium salt and an oxidizing compound.

The characteristics and advantages of the invention will emerge more clearly in the light of the examples of implementation below, provided purely by way of illustration and in no way limiting the invention, with the support of the figures.

IA at 1Ε, which show micrographs of anode layers formed on the surface of aluminum parts by, figure IA, chromic anodization (OAC),

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35901-1 Figure IB, standard sulfuric anodization (OASstandard), Figure IC, sulfo-tartaric anodization (OAST), Figure ID, sulfo-boric anodization (OASB) and Figure IE, anodization according to an embodiment of the invention.

EXAMPLE 1

1.1 / Anodizing processes for aluminum alloy parts

Rolled 2024 lam3 aluminum alloy parts with dimensions of 120χ80χ2 mm are treated by anodizing according to the methods below.

The workpiece surface preparation steps are first of all carried out successively:

- alkaline degreasing, by soaking the part in a mixture of TURCO 4215 NCIT at 50g / L and TURCO 4215 additive at 10g / L, at a temperature of 60 ٥c, for 20 min;

- water rinses:

- acid pickling, by soaking the part in a solution of

SMUT-GO NC at 19 ٥/٥ v / v, at a temperature of 2O٥C, for 5 min;

- water rinses.

Parts are then subjected to an anodizing process in accordance with 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 brought to and maintained at a temperature of 19 ٥c.

The part is immersed in the bath, and a DC voltage is applied to it according to the following voltage profile: voltage rise, from an initial value of ov, at a speed of 3 V / min, up to a so-called plateau value of 16 V. The voltage is maintained at the plateau value for 16 minutes.

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An anodic layer of aluminum oxide hydroxide with a thickness of about 4 to 5 μm is formed on the surface of the part.

As comparative examples, identical parts which have been subjected to the same surface preparation operations are anodized according to the conventional chromic anodizing (OAC), standard sulfuric anodizing (OAS standard), sulfuric anodizing methods. tartaric (OAST) and sulfo-boric anodization (OASB).

The operating parameters for standard IOAS, IOAST, IOASB and! OAC are shown in Table 1 below.

<td></td><td>OAS standard</td><td>OAST</td><td>OASB</td><td>OAC</td>

<td>Bath composition</td><td>H2SO4: 2٠٠ g / L</td><td>H2SO4: 40 g / L C4H6O6: 80 g / L</td><td>H2SO4: 45 g / L Η3ΒΟ3: 8 g / L</td><td>CO: 60 g / L C2H2O4: 2 g / L</td>

<td>Bath temperature (٥C)</td><td> 16-29</td><td> 36-39</td><td> 25-28</td><td> 38-42</td>

<td>Voltage rise (V / min)</td><td> 3,4</td><td> 2,8</td><td> 5,3</td><td> 4</td>

<td>Tension and plateau time</td><td>17٧ 40 mins</td><td>14 V 25 min</td><td>15V 23 min</td><td>20V 50 min</td>

<td>Thickness of the anodic layer formed on the part (pm)</td><td>8 to 10</td><td>2 to 5</td><td>1 to 3</td><td>3 to 5</td>

Table 1 - operating parameters implemented for the various anodizing processes of the prior art OAS standard, OAST, OASB and OAC The various parts thus obtained are subjected to the following tests.

1.2 / Morphological analysis of the anode layer

A morphological analysis of the anodic layer formed on the surface of each of the parts thus treated is carried out by field effect electron microscopy (ΜΕΒ-FEG). micrographs are shown in the figures

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35901Β1

IA to IE. FIG. IE, corresponding to the anode layer obtained by a method in accordance with an embodiment of the invention, shows a homogeneous morphology in the thickness of the layer, with the absence of micro-precipitates originating from the substrate at the within the diaper. From the micrographic observations, the pore diameters were measured for all of the anode layers and the results are shown in Table 2 below.

<td>Anodizing process</td><td>٥AC</td><td>OAST</td><td>OASB</td><td>Anodization according to the invention</td>

<td>Pore ​​diameter of the anodic layer (nm)</td><td> 20-30</td><td> 5-10</td><td> 5-10</td><td> 10-20</td>

Table 2 - Diameter of the pores of the anodic layer formed on the rolled 2024 Τ3 aluminum alloy parts according to the anodizing process used

It can be seen in this table that the morphology of the anodic layer formed on the parts by the method in accordance with one embodiment of the invention is similar to that of a layer obtained by chromic anodization, compared to other methods of anodization using 15 sulfuric acid proposed by the prior art.

1.3 / Fatigue reduction tests The various anodized parts are subjected to a fatigue test in order to evaluate the fatigue reduction linked to the formation of the 2٥ anodic layer on their surface, the parameters of the fatigue test are The following :

- stress: rotary bending

- temperature: 20 ٥c

- R = -1

- Frequency: 100 Hz

- Kt = 1.035

- type of test specimens: FFRT16

- number of test pieces: 12 t

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The results of this test, in terms of fatigue limit and reduction with respect to non-anodized parts, for parts treated by the method according to one embodiment of the invention and by various conventional methods, are shown in the Table. 3 below.

<td></td><td>Untreated part</td><td>Part processed by OkC</td><td>Standard OAS treated part</td><td>Part treated by anodization according to the invention</td>

<td>Fatigue limit (10 cycles) 90 ٥/٥ survival (MPa)</td><td> 147</td><td> 120</td><td> 107</td><td> 136</td>

<td>Abatement</td><td>Reference</td><td> ٥/٥ 22,5-</td><td> ٥/٥ 37-</td><td> ٥/٠ 8-</td>

Table 3 - Fatigue reduction evaluated by a fatigue test for 2024 Τ3 aluminum alloy parts according to the anodizing process used

These results clearly demonstrate that the fatigue reduction generated by the process according to one embodiment of the invention is significantly lower than that generated by the conventional anodizing processes, whether it is standard sulfuric anodization (CAS) but even also chromic anodization (OAC), for an equivalent anodic layer thickness. The parts treated by the anodization process in accordance with one embodiment of the invention exhibit in particular better resistance to forces than those treated by the anodization processes of the prior art. In particular, in comparison with standard sulfuric anodization, they allow a lightening of the structures within which they are implemented. These parts can also advantageously be substituted for parts treated by chromic anodization already used, in particular in aircraft, without it being necessary to resize them.

4 / Adhesion tests for paint coatings

Parts anodized by the method in accordance with one embodiment of the invention, as indicated above, are subjected to tests.

35901Β1 adhesion of conventional paint systems.

Two paint systems are tested: a water-based epoxy-based system (Ρ60 + F70) and a solvent-based polyurethane-based system (PAC33 + PU66). Tests are carried out in accordance with ISO 2409, 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 days, then dried before undergoing the adhesion test according to the standard.

The results are shown in Table 4 below.

<td colspan="2">Painting system</td><td>Dry adhesion</td><td>Wet adhesion</td>

<td rowspan="2">Solvent base</td><td>PAC33</td><td>Grade 0</td><td> -</td>

<td>PAC33 + PU66</td><td>Grade 0</td><td>Grade 0</td>

<td rowspan="2">Water-soluble base</td><td>Ρ60</td><td>Grade 0</td><td> -</td>

<td>P6O + F7O</td><td>Grade 0</td><td>Grade 0</td>

Ta_4- Results of adhesion tests of two paint systems on 2024 d'aluminium3 laminated aluminum alloy parts treated by a method in accordance with one 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.

<td colspan="2">Painting system</td><td>Dry adhesion</td><td>Wet adhesion</td>

<td rowspan="2">Solvent base</td><td>PAC33</td><td>Grade 0</td><td> -</td>

<td>PAC33 + PU66</td><td>Grade 0</td><td>Grade 1</td>

<td rowspan="2">Water-soluble base</td><td>Ρ60</td><td>Grade 1</td><td> -</td>

<td>P6O + F7O</td><td>Grade 1</td><td>Grade 2</td>

Table 5 - Results of adhesion tests of two paint systems on aluminum alloy 2024 Τ3 laminated parts treated by a standard sulfuric anodizing process t

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These results show that the parts treated by the process in accordance with one embodiment of the invention exhibit adhesion to paint systems, whether of the water-based or solvent-based type, equivalent to those treated by the conventional OAST anodizing processes. and OASB, which also exhibit, in a known manner, results expressed as Grade 0 in the above adhesion tests. This adhesion, for either of the two paint systems, is much greater than that obtained by the standard sulfuric anodizing process proposed by the prior art.

5 / Corrosion resistance after clogging

The parts treated by the process in accordance with an embodiment of the invention, by OAC, OAST or OASB, as indicated above, are subjected to the sealing process C1 in accordance with an implementation mode of the following invention:

- immersion in an aqueous bath of composition: CrF₃ : 6 g / l and K₂ZrF₆ : 1 g / L, in water, at a pH of 3.5 and a temperature of 40 ٠c, for 15 minutes,

- then immersion in water at a pH of 6.5, at a temperature of 98 ٥c, for 40 minutes.

By way of comparative examples, anodized parts are also subjected to the following different conventional sealing processes: hydrothermal sealing, hot sealing with hexavalent chromium salts.

hot sealing with nickel salts, according to the operating conditions indicated in Table 6 below.

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<td></td><td>Hydrothermal sealing</td><td>Sealing with chrome salts M \</td><td>Clogging with nickel salts</td>

<td>Composition</td><td>HO</td><td>KCO: 30 mg / L</td><td>(CHaCOONi: 10g / L</td>

<td>pH</td><td> 6,5</td><td> 6</td><td> 5,5</td>

<td>Temperature (٥c)</td><td> 98</td><td> 98</td><td> 98</td>

<td>Immersion time (min.)</td><td> 40</td><td> 20</td><td> 30</td>

Table 6- operating parameters implemented for different sealing processes

A sealed anodic layer is obtained on each treated part.

The parts thus treated are subjected to a salt spray resistance test in accordance with the ISO 9227 standard.

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

<td></td><td colspan="4">Resistance to salt spray (appearance of 1^time corrosion pitting) (h)</td>

<td rowspan="2">Type of anodization</td><td colspan="4">Type of clogging</td>

<td>Hydrothermal sealing</td><td>Sealing with chromium VI salts</td><td>Clogging with nickel salts</td><td>Clogging</td>

<td>OAC</td><td> 300</td><td> 1500</td><td> -</td><td> -</td>

<td>OAST</td><td> 96</td><td> 1 300</td><td> 450</td><td> 550</td>

<td>OASB</td><td> 96</td><td> 1000</td><td> 336</td><td> 450</td>

<td>Anodization according to the invention</td><td> 300</td><td> 1500</td><td> 850</td><td> 1 600</td>

Table 7 - Resistance to salt mist of rolled aluminum alloy parts 2024 Τ3 treated by anodization then sealing, the anodization being carried out by a method in accordance with an embodiment of the invention or by methods of 'anodizing of the prior art

More precise average results concerning the appearance of t

35901Β1 first corrosion pits (more precisely of the 1 ج ٢ ه corrosion pitting ("م") and the generalization of corrosion ("G٥n")), obtained on a larger number of parts, are shown in Table 8 below .

<td></td><td colspan="8">Resistance to salt spray (h)</td>

<td rowspan="2">Type of anodization</td><td colspan="8">Type of clogging</td>

<td colspan="2">Hydrothermal sealing</td><td colspan="2">Sealing with chromium salts \ | \</td><td colspan="2">Clogging with nickel salts</td><td colspan="2">Clogging</td>

<td></td><td>Era</td><td>G٥n</td><td>.,time</td><td>G٥n</td><td>م</td><td>G °</td><td>1st</td><td>G٥n</td>

<td>CAC</td><td> 336</td><td> 1056</td><td> 1320</td><td> 2136</td><td> -</td><td> -</td><td> -</td><td> -</td>

<td>OAST</td><td> 72</td><td> 192</td><td> 1176</td><td> 1368</td><td> 336</td><td> 840</td><td> 480</td><td> 1344</td>

<td>OASB</td><td> 48</td><td> 168</td><td> 912</td><td> 1056</td><td> 288</td><td> 744</td><td> 384</td><td> 1128</td>

<td>Anodization according to the invention</td><td> 312</td><td> 1008</td><td> 1296</td><td> 2064</td><td> 792</td><td> 1344</td><td> 1488</td><td> 2520</td>

Table 8 - Resistance to salt spray of aluminum alloy parts

2024 Τ3 laminates treated by anodization and then sealing, the anodization being carried out by a process in accordance with an embodiment of the invention or by anodization processes of the prior art

These results clearly demonstrate that the anodization process in accordance with one embodiment of the invention, followed by a sealing step of any type whatsoever, makes it possible to give the treated part resistance to heat. corrosion at least equivalent to that obtained by conventional anodizing procedures followed by the same clogging.

In particular, the anodization process according to one embodiment of the invention exhibits anticorrosion performance equivalent to a cliromic anodization (OAC) in association with a hydrothermal sealing or a hot sealing with hexavalent chromium salts, and much better than dilute sulfo-taitric (OAST) or sulfo-boric (OASB) anodizations.

This ability of the anodic layer formed by the 2 conforme process according to the invention to be clogged during a post-treatment to provide it with

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35901Β1 corrosion resistance properties could be explained in particular by its morphology with pores larger than 10 nm, which facilitates its hydration during hydrothermal clogging for example, resulting in clogging of the pores and protection against corrosion by barrier layer effect.

Finally, it is observed that the particular combination of the anodizing process in accordance with an embodiment of the invention, with the sealing process C1 in accordance with an implementation 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 anodizing / sealing combination.

EXAMPLE 2

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

2.1 / Variants of sulfuric acid concentration

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 1. invention by immersion in a bath at 19 ٥c containing sulfuric acid at a concentration of 150 or 250 g / l, to the exclusion of any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage rise, from an initial value of 0 V, at a speed of 6 v / min٠ up to a so-called plateau value of 16 V. The voltage is held 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 ٥c, for 40 min.

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An anodic coating of aluminum oxide / hydroxide with a thickness of about 3.5 to 4.5 μm is formed on the surface of each part.

By way of comparative example, the same process of treatment by anodization then sealing is applied to a similar part, but using a concentration of sulfuric acid in the bath of only 100 g / l.

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

<td></td><td colspan="2">Salt spray kills (h)</td>

<td>Concentration of sulfuric acid in the anodizing bath (g / l)</td><td>Appearance of 1®٢® corrosion pitting</td><td>Widespread corrosion</td>

<td> 100</td><td> 120</td><td> 288</td>

<td> 150</td><td> 264</td><td> 888</td>

<td> 250</td><td> 264</td><td> 864</td>

- - Resistance to salt spray of aluminum alloy parts

2024 Τ3 laminate treated by anodizing then sealing, for different sulfuric acid concentrations in the anodizing bath

These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes in accordance with the invention employing a concentration of sulfuric acid in the bath of between 150 and 250 g / l. This efficiency is in particular much higher than the comparative process using a sulfuric acid concentration of 100 g / l, lower than that recommended by the present invention.

? / Voltage rise speed variants

Aluminum alloy parts similar to those of Example 1,

MY

35901Β1 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 ٥c containing sulfuric acid at a concentration of 200 g / l, excluding any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage rise, from an initial value of 0 V, up to a so-called plateau value of 16 V. The voltage is then maintained at the value of tray for 16 minutes. Different voltage rise rates 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 ٥c, for 40 min.

An anodic layer of aluminum oxide / hydroxide with a thickness of about 4 to 4.5 μm is formed on the surface of each part.

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

<td></td><td colspan="2">Resistance to salt spray (h)</td>

<td>Speed ​​of voltage rise (V / min)</td><td>Appearance of the 1^timecorrosion pitting</td><td>Widespread corrosion</td>

<td> 1</td><td> 312</td><td> 984</td>

<td> 20</td><td> 288</td><td> 960</td>

<td> 32</td><td> 288</td><td> 984</td>

Table 10 - Resistance to salt spray of 2024 aluminum alloy parts Τ3 laminates treated by anodization then sealing, for different 20 voltage rise speeds

These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes in accordance with the invention implementing a rise in voltage at a speed included.

MY

35901Β1 between let 32 ​​ν / min.

2.3 / Variants of the plate tension 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 invention by immersion in a bath at 19 ٥c containing sulfuric acid at a concentration of 200 g / l, excluding any other compound. A DC voltage is then applied to each part according to the following voltage profile: voltage rise, from an initial value of 0 V, with a speed of 3 ν / min, up to a so-called plateau value of 14 V or of 16 V. The voltage is then held at the plateau value for 16 minutes.

The anode layer is then sealed by the sealing process C1 described in Example 1 above.

An anodic layer of aluminum oxide / hydroxide of about 4 to 5 μm thick is formed on the surface of each part.

The parts thus treated are subjected to a salt spray resistance test in accordance with the ISO 9227 standard. The results obtained are shown in Table 11 below.

<td></td><td colspan="2">Resistance to salt spray (h)</td>

<td>Plateau voltage value (V)</td><td>Appearance of 1٥٢® corrosion pitting</td><td>Widespread corrosion</td>

<td> 14</td><td> 1176</td><td> 2376</td>

<td> 16</td><td> 1320</td><td> 2544</td>

Table 11 - Resistance to salt spray of aluminum alloy parts

2024 Τ3 laminates treated by anodization then sealing, for different values ​​of plate tension

MY

35901Β1

These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes according to the invention implementing the final maintenance of the voltage at a plateau value of between 14 or 16 V.

2.4 / Temperature variants 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 invention by immersion in a bath containing sulfuric acid at a concentration of 200 g / l, excluding any other compound. Several bath temperatures are tested, more particularly 6٥c, 12 ٥c and 25 ٥c.

A DC voltage is then applied to each part according to the following voltage profile: voltage rise, from an initial value of ov, at a speed of 3V / min, up to a so-called plateau value of 16 V. The voltage 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 so as to obtain on the surface of each part an anodic layer of aluminum oxide / hydroxide with a thickness of about 4 to 5 μm.

The anode layer is then sealed by the sealing process C1 described in Example 1 above.

By way of comparative example, the same process of treatment by anodization then sealing is applied to a similar part, but using an anodizing bath temperature of 30 ٥c.

The parts thus treated are subjected to a salt spray resistance test in accordance with the ISO 9227 standard. The results obtained are shown in Table 12 below.

t

35901Β1

<td></td><td colspan="2">Resistance to salt spray (h)</td>

<td>Anodizing bath temperature (٥c)</td><td>Appearance of the 1st corrosion pitting</td><td>Widespread corrosion</td>

<td> 6</td><td> 1272</td><td> 2304</td>

<td> 12</td><td> 1224</td><td> 2280</td>

<td> 25</td><td> 1320</td><td> 2424</td>

<td> 30</td><td> 624</td><td> 1536</td>

Table 12 Resistance to salt spray of aluminum alloy parts

2024 Τ3 laminates treated by anodizing then sealing, for different temperatures of the anodizing bath

These results show the effectiveness, in terms of corrosion resistance of the treated parts, of the anodizing processes in accordance with the invention using an anodizing bath temperature of between 5 and 25 ٥c. This efficiency is in particular much higher than the comparative method using a bath temperature of 30 ° C., higher than that recommended by the present invention.

the above description clearly illustrates that by virtue of its various characteristics and their advantages, the present invention achieves the objectives which it had set for itself. In particular, it provides a process for 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 of the sulfuric anodizing processes proposed by the prior art.