FR3087208A1 - PROCESS FOR THE SURFACE TREATMENT OF ALUMINUM PARTS - Google Patents

Abstract

The invention relates to a method of surface treatment of an aluminum alloy part, which comprises a first stage of treatment of the part by micro-arc oxidation, the part being immersed in an aqueous electrolytic medium containing a silicate salt. at a concentration greater than or equal to 2 g / l and an aluminate salt at a concentration greater than or equal to 1 g / l, and being crossed by an electric current; then a second stage of treatment of the part by bringing the part into contact successively with a first aqueous composition containing a metal salt inhibiting corrosion, with the exception of a hexavalent chromium salt, and an oxidizing compound, then a second aqueous composition containing a rare earth salt inhibiting corrosion and an oxidizing compound. The part thus treated has a high resistance to corrosion.

Description

The present invention is in the field of wet surface treatment of aluminum or aluminum alloy parts, with a view to improving their resistance to corrosion.

More particularly, the present invention relates to a method of surface treatment of an aluminum or aluminum alloy part, as well as a part obtained at the end of such a method.

Aluminum alloys are commonly used, particularly in the aeronautics and space industries, due to their lightness properties and low cost. For many applications, prior to their use, the parts formed in these alloys must however be subjected to a surface treatment so as to improve their resistance to corrosion, in order to prolong their service life when used in environments in which they are exposed to corrosive conditions.

Anodization and chemical conversion treatments, which create an oxide layer on the surface of the part that protects the material constituting it from the external environment, are the main surface treatment processes used for this purpose.

Many such methods use hexavalent chromium and are used to form thin anticorrosion layers, of thickness typically between 5 and 10 μm, on the surface of the parts. However, such processes are associated with significant environmental problems and harmfulness to living organisms, so that the prior art has sought to propose alternative processes not using hexavalent chromium.

Most of the solutions that have been proposed for this purpose are of the anodic oxidation and clogging type with solutions based on trivalent chromium. The layers which these techniques make it possible to form on the surface of the parts may have, depending on the alloys, a high corrosion resistance. However, the resistance of these layers to wear, in particular by abrasion, is insufficient for certain applications.

It has also been proposed by the prior art to carry out the surface treatment of light alloy parts, in particular based on aluminum or magnesium, by the micro-arc oxidation technique, also commonly called oxidation by electrolytic plasma. or microplasma oxidation. Micro-arc oxidation processes make it possible to create a semi-crystalline oxide layer on the surface of the parts ensuring good wear resistance properties and isolating the parts from the external environment, thus protecting them from corrosion.

Micro-arc oxidation consists in imposing a potential difference between the part to be treated and a counter-electrode which are both immersed in an alkaline electrolytic medium. During the first moments of the treatment, a gaseous evolution takes place and the formation on the surface of the part of a surface layer of insulating oxide. When the voltage reaches a critical threshold value, called the dielectric breakdown value, the dielectric breakdown of this insulating layer is observed, which results in the appearance of sparks allowing the oxidation process to continue. As the process proceeds, there is a growth of the layer at the interface of the part and the electrolyte, and therefore an increase in the resistivity of the part, which results in less discharges. numerous but of greater intensity, called micro-arcs. With the formation of micro-arcs, a localized plasma is created near the surface of the room. In the end, a coating is obtained on this surface divided into several sublayers: a porous surface sublayer and a dense and hard internal sublayer. The electrolytic medium used in this technique typically consists mainly of sodium silicate and potash. The important electrical parameters are the frequency of the electrical signal, the current density and the processing time.

This process is the subject of growing interest on the part of manufacturers, in particular because it makes it possible to improve the surface properties, in particular of resistance to wear and to corrosion, of the treated part, in a single treatment step, unlike anodization which requires prior steps of surface preparation, and in weakly concentrated electrolytes and without dissolving action, which meet the increasingly stringent requirements of international legislation concerning aggressive and polluting products.

Micro-arc oxidation processes are therefore increasingly used to form on the surface of parts made of light alloys, based on magnesium or aluminum, a thick protective ceramic layer, typically of the order of 150 μm. thickness for aluminum alloys. After rectification, the treated parts have good anti-corrosion and tribological properties. However, the implementation of such a solution to replace the current chromic anodization treatments, which make it possible to form on the parts surface protective layers of much smaller thickness, requires a resizing of the parts to be treated.

In an industrial problem, it thus turns out to be advantageous to use, as a replacement for chromic anodization techniques on light alloys, in particular of aluminum, the micro-arc oxidation process under conditions allowing surface formation of parts of the layers of thin thickness, in particular of the order of 5 to 10 μm.

The corrosion resistance of aluminum alloy parts on which a protective layer of thin thickness is formed by the micro-arc oxidation technique is however insufficient to meet the requirements of industrial fields such as the field of aeronautics.

It has now been discovered by the present inventors that the implementation of the micro-arc oxidation technique, carried out under specific conditions, and associated with a specific post-treatment, makes it possible to form on the surface parts of alloy d aluminum a thin layer which has significant corrosion resistance properties, the parts thus treated can advantageously replace, without the need for resizing, the parts treated by current chromic anodizing techniques.

Thus, the present invention aims to provide a method of surface treatment of aluminum or aluminum alloy parts which can replace the existing chromic anodization methods, this method making it possible to form on the surface of these parts a protective layer of thin thickness. having good corrosion resistance properties, regardless of the aluminum alloy concerned. The invention also aims to ensure that this process does not use harmful or polluting substances.

An additional objective of the invention is that this process can be carried out in a simple manner, using current materials, and at limited cost.

To this end, there is proposed according to the present invention a method of surface treatment of an aluminum or aluminum alloy part, which comprises:

a first step of treatment of the part by micro-arc oxidation, the part being immersed in an aqueous electrolytic medium, preferably alkaline, containing a silicate salt at a concentration greater than or equal to 2 g / l and an aluminate salt at a concentration greater than or equal to 1 g / l, and being crossed by an electric current,

- then a second stage of treatment of the part by bringing the part into contact successively with:

• a first aqueous composition containing a corrosion inhibiting metal salt, excluding a hexavalent chromium salt, and an oxidizing compound, • then a second aqueous composition containing a corrosion inhibiting rare earth salt and an oxidizing compound.

The first stage of treatment of the part by micro-arc oxidation is carried out in a conventional manner in itself, and has the effect of forming a semi-crystalline protective layer on the surface of the part, which improves the corrosion resistance of the room.

Conventionally, the conditions used, in particular the electrical parameters applied, in particular the frequency of the electrical signal, the current density and the processing time, are chosen to reach the value of the dielectric breakdown voltage of the alloy d particular aluminum constituting the part.

Preferably, this first step is also carried out under conditions making it possible to form on the part a protective ceramic surface layer with a thickness of less than 100 μm, preferably less than 30 μm, and preferably between 2 and 10 μm, in particular between 5 and 10 pm.

The second stage of treatment of the process according to the invention advantageously achieves the clogging of the protective layer formed during the first stage of treatment, by bringing the part into contact with the first aqueous composition, then the passivation of this layer clogged, by bringing the part into contact with the second aqueous composition.

Both the first aqueous composition and the second aqueous composition used are advantageously devoid of hexavalent chromium, that is to say chromium with an oxidation state of +6.

The method according to the invention advantageously applies to unpainted parts.

The part obtained at the end of the implementation of the method according to the invention has particularly good corrosion resistance properties, compatible with industrial requirements, in particular in the aeronautical field. For example, subjected to the salt spray test, it does not show any puncture after 500 to 700 hours of exposure to salt spray.

It also has good resistance to wear, especially to abrasion.

The method according to the invention makes it possible to form a fine protective layer on the surface of the part, so that there is no need to resize the part to use it to replace a part treated with the prior art chromic anodization methods.

This protective layer is also advantageously homogeneous, in thickness as in composition, and its composition and its porosity are controlled. It has good adhesion to paints and electrical conductivity properties.

These particularly advantageous results, which are verified whatever the aluminum alloy used, and whether the treated part is laminated or machined, are obtained by a process which does not use any substance harmful to living organisms or the environment. , such as hexavalent chromium. The electrolytic medium used for the first stage of treatment by micro-arc oxidation is in particular weakly concentrated, non-aggressive and non-polluting.

In addition, the method according to the invention does not require steps for pretreatment of the part, a simple chemical degreasing, for example with acetone, being sufficient before the implementation of the first stage of treatment of the part by micro-arc oxidation. Thus, the method according to the invention does not require any use of dangerous or toxic products for the surface preparation of the part, and it is simple to implement.

It also does not require any step to rectify the part after or during its production.

The method according to the invention can also respond to one or more of the characteristics described below, implemented in isolation or in each of their technically operating combinations.

As explained above, the micro-arc oxidation technique is classic in itself, and well known to those skilled in the art. Its principle will therefore not be described in detail in the present description, only the specific parameters of the invention will be described below.

We can, however, give the following general outline. For the implementation of the micro-arc oxidation step, the part to be treated is immersed in the electrolytic medium, where it forms a first electrode. A second electrode, called a counter electrode, is immersed in the electrolytic medium. An electrical current is then imposed between the part and the counterelectrode, so as to apply to them a voltage typically varying between 100 and 400 V.

The counter-electrode is preferably chosen to have a shape complementary to that of the part to be treated, which makes it possible to treat parts of complex shape more effectively.

The distance between the part and the counter-electrode is preferably less than or equal to 10 cm.

As explained above, the key electrical parameters of the micro-arc oxidation step are the density of the electric current, the frequency of the electric signal, that is to say the frequency of repetition of the electric current pulses, and processing time.

The shape of the electric current can be of any type, for example direct, alternating, pulsed bipolar, etc.

The shape of the electric current applied can for example be such as that described in document WO 01/81658.

In preferred embodiments of the invention, during the first stage of treatment of the part by micro-arc oxidation, one or more, preferably all, of the following parameters are applied:

- the density of the electric current flowing through the room is between 5 and 50 A / dm ² ;

the part is subjected to an electric current in a pattern repeated periodically at a frequency between 100 and 1000 Hz,

- And / or the treatment of the part in the electrolytic medium is carried out for a period of between 5 and 30 minutes.

Such characteristics advantageously ensure the best performance for the treated part, in terms of resistance to corrosion and also to wear.

During the first stage of treatment of the part by micro-arc oxidation, the temperature of the electrolytic medium is preferably maintained between 10 and 30 ° C, for example at 20 ° C ± 2 ° C.

The best performance is also ensured by the particular choice of active components in the electrolytic medium: quite surprisingly, the use in the electrolytic medium, in solution in water, of a mixture of silicate salt and of aluminate salt, in sufficient concentrations, makes it possible to obtain corrosion resistance properties of the treated part which are much superior to those of the parts treated by micro-arc oxidation in electrolytic media of different composition, but close .

In the present description, the term “silicate salt” means a single salt or a mixture of different silicate salts.

Likewise, by aluminate salt is meant a single salt or a mixture of different aluminate salts.

In addition to the silicate salt and the aluminate salt, the aqueous electrolytic medium used according to the invention preferably contains one or more alkali metal hydroxides, for example sodium hydroxide, preferably in a concentration of between 1 and 5 g / L

The pH of the aqueous electrolytic medium is preferably between 11 and 14.

In preferred embodiments, apart from the alkali metal hydroxide (s), the silicate salt and the aluminate salt are the majority compounds of the aqueous electrolytic medium.

Preferably, the aqueous electrolytic medium essentially contains the silicate salt and the aluminate salt, with one or more alkali metal hydroxides. This means that it does not contain any other substance, except in the form of traces.

The silicate salt can be any conventional silicate salt in itself. It is preferably chosen from alkali metal salts. It is for example sodium silicate Na ₂ SiO ₃ .

The concentration of silicate salt in the aqueous electrolytic medium is preferably between 2 and 20 g / l.

The aluminate salt can also be any conventional aluminate salt in itself. It is preferably chosen from alkali metal salts. Examples include sodium aluminate NaAIO2.

The concentration of aluminate salt in the aqueous electrolytic medium is preferably between 1 and 10 g / l.

During the second stage of processing the part of the process according to the invention, the part is brought into contact successively with:

• a first aqueous composition containing a corrosion inhibiting metal salt, excluding a hexavalent chromium salt, and an oxidizing compound, • then a second aqueous composition containing a corrosion inhibiting rare earth salt and an oxidizing compound.

Here again, quite surprisingly, it has been discovered by the present inventors that the implementation of such a second treatment step makes it possible to confer on the treated part corrosion resistance properties which are particularly high, and much better than those obtained when, after the first step of micro-arc oxidation treatment, the parts are treated by the other conversion / clogging / passivation techniques proposed by the prior art, and what these techniques use chromium hexavalent, trivalent chromium, lanthanum, etc.

In the present description is meant, by corrosion inhibiting metal salt, a single salt or a mixture of such salts.

As regards the first composition, the corrosion inhibiting metal salt is preferably a corrosion inhibiting transition metal salt, and preferably a trivalent chromium salt, that is to say chromium with an oxidation state +3, or a mixture of such salts.

Any counter-ion can be used in the context of the invention. Trivalent chromium salts which are particularly preferred in the context of the invention are the trivalent chromium sulfates and fluorides.

The concentration of corrosion inhibiting metal salt, in particular trivalent chromium salt, in the first aqueous composition according to the invention is preferably between 0.5 and 50 g / L

In the present description, the term “oxidizing compound” means any single oxidizing compound or any mixture of oxidizing compounds. An oxidizing compound is defined in the present description as a compound capable of oxidizing the surface layer of the treated part.

For the first aqueous composition as for the second aqueous composition, any oxidizing compound conventionally used in itself for the chemical conversion of aluminum alloys can be used in the context of the present invention.

For the first aqueous composition as for the second aqueous composition, the oxidizing compound is chosen in particular from the group consisting of:

- fluoride-based substances, such as ammonium fluoride or potassium fluorozirconate,

- permanganate-based substances, such as potassium permanganate,

- and hydrogen peroxide,

- or any of their mixtures.

The concentration of oxidizing compound in the first aqueous composition can in particular be between 0.1 and 50 g / l, or between 5 and 500 ml / l for the hydrogen peroxide used in aqueous solution at about 3035% v / v.

The concentration of oxidizing compound in the second aqueous composition can in particular be between 0.1 and 50 g / l, or between 5 and 500 ml / l for the hydrogen peroxide used in aqueous solution at about 3035% v / v.

The pH of the first aqueous composition is preferably between 1 and 7, and preferably between 2 and 5.

The temperature of the first aqueous composition is also preferably between 10 and 80 ° C, and preferably between 30 and 50 ° C.

The duration of contacting the part with the first aqueous composition is preferably between 1 and 60 minutes, and preferably between 10 and 20 minutes.

The contacting of the part with the first aqueous composition can be carried out in any conventional manner in itself. It is preferably carried out by immersing the part in a bath of the first aqueous composition.

In the present description is meant, by rare earth salt inhibiting corrosion, a single salt or a mixture of such salts. Rare earths are defined here in a classic way, as including the fifteen lanthanides, scandium and yttrium.

The rare earth salt inhibiting corrosion contained in the second aqueous composition according to the invention is preferably chosen from lanthanum salts and cerium salts, preferably cerium in the +3 oxidation state, or the any of their mixtures.

Any counter-ion can be used in the context of the invention.

Rare earth salts which are particularly preferred in the context of the invention are nitrates, in particular cerium nitrates and lanthanum nitrates.

The concentration of corrosion-inhibiting rare earth salt in the second aqueous composition according to the invention is preferably between 1 and 50 g / L

The pH of the second aqueous composition is preferably between 1 and 7, and preferably between 2 and 5.

The temperature of the second aqueous composition is also preferably between 15 and 40 ° C, and preferably between 20 and 30 ° C.

The duration of bringing the part into contact with the second aqueous composition is preferably between 1 and 60 minutes, and preferably between 2 and 10 minutes.

The contacting of the part with the second aqueous composition can be carried out in any conventional manner in itself. It is preferably carried out by immersing the part in a bath of the second aqueous composition.

Between bringing the part into contact with the first aqueous composition and bringing the part into contact with the second aqueous composition, the method according to the invention may comprise a rinsing step, in particular with water, and optionally a step drying room.

The characteristics of the second processing step of the method according to the invention are in particular such as those described in document WO 2013/117767.

The first aqueous composition used according to the invention may for example be the composition sold under the name Socosurf TCS by the company Socomore.

The second aqueous composition used according to the invention may for example be the composition sold under the name Socosurf

PACS, also by Socomore.

As explained above, the method according to the invention advantageously does not require any prior step of surface preparation of the part.

It may however include a preliminary step of degreasing the part, for example with acetone.

Between the first stage of treatment of the part by micro-arc oxidation and the second stage of treatment of the part, the method according to the invention may also comprise an intermediate stage of rinsing the part, in particular with water, and optionally a part drying stage.

In particular embodiments of the invention, the method can also include a final step of rinsing the treated part, in particular with water, and optionally a step of drying the part.

According to another aspect, the present invention relates to an aluminum or aluminum alloy part obtained by a method according to the invention, meeting one or more of the characteristics described above.

This part, called treated part, comprises in particular a protective surface layer of oxides with a thickness of less than 100 μm, preferably less than 30 μm, and preferably between 2 and 10 μm, in particular between 5 and 10 μm.

The roughness Ra of this surface layer is in particular between 0.5 and 3 μm.

The characteristics and advantages of the invention will appear more clearly in the light of the following implementation examples, provided for illustrative purposes only and in no way limitative of the invention, with the support of FIGS. 1 to 8, in which:

- Figure 1 schematically shows a device for the implementation of the first step of micro-arc oxidation treatment of a surface treatment process of a part according to the invention;

- Figure 2 shows a bar graph illustrating the number of pitting corrosion as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to conditions in accordance with the invention ("OMA1") or not in accordance with the invention ("OMAcl", "OMAc2 >>," OMAc3 >>, "OMAc4 >>); in this figure, "Level 4 >> indicates a total puncture of the surface of the room, and" Level 1 >>, "Level 2 >>and" Level 3 >> respectively indicate the different successive stages existing between a total absence of pitting on the workpiece surface and total pitting on the workpiece surface;

- Figure 3 shows a bar diagram illustrating the number of pitting corrosion as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to conditions according to the invention ( "OMA1 >>) or after such 1 ^st treatment step followed by a 2 ^nd process step according to the invention (">> OMA1 + PT1); in this figure, Level 4 indicates a total pitting of the surface of the part, and Level 1, Level 2 and Level 3 respectively indicate the different successive stages existing between a total absence of pitting on the surface of the part and a total pitting of the surface of the room;

- Figure 4 shows a bar diagram illustrating the number of pits of corrosion as a function of the time of exposure to salt spray of different parts (2024 T3 alloy rolled, 2024 T3 alloy machined or AS7G06 alloy) subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to conditions according to the invention ("OMA1 >>) or after such a 1 ^st treatment step followed by a 2 ^nd treatment step according to the invention (" OMA1 + PT1 >>);

- Figure 5 shows a bar diagram illustrating the number of bites as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to non-conforming conditions to the invention ( "OMAcl") or after such 1 ^st treatment step followed by a 2 ^nd process step under conditions according to the invention ( "OMAcl PT1 +");

- Figure 6 shows a bar diagram illustrating the number of bites as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to non-conforming conditions to the invention ( "OMAc5") or after such 1 ^st treatment step followed by a 2 ^nd process step under conditions according to the invention ( "OMAc5 PT1 +");

FIG. 7 shows a bar diagram illustrating the number of bites as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to conditions in accordance with the invention ( "OMA1") or after such a 1 ^st process step followed by a 2 ^nd step of treatment with surface treatment compositions of the prior art not according to the invention ( "Alodine 120 >>, "Surtec 650 >>," La 613.3 / post DIP >>, "Iridite NCP >>);

- And Figure 8 shows a bar diagram illustrating the number of bites as a function of the time of exposure to salt spray of parts subjected to a salt spray test after a 1 ^st treatment step by micro-arc oxidation according to conforming conditions to the invention (“OMA1”) followed by a 2 ^nd step of treatment with either a first aqueous composition according to the invention (“TCS”), or a second aqueous composition according to the invention (“PACS”).

A method of surface treatment of an aluminum or aluminum alloy part according to the invention comprises a first step of treatment of the part by micro-arc oxidation.

Beforehand, it may include a degreasing step of the part carried out in a conventional manner, for example degreasing with acetone at room temperature.

The first stage of treatment of the part by micro-arc oxidation implements a conventional device in itself, an example of which is illustrated diagrammatically in FIG. 1.

This device comprises a tank 10 containing an aqueous electrolytic medium 11. According to the invention, this aqueous electrolytic medium 11 is alkaline and it contains a silicate salt and an aluminate salt, in respective concentrations in accordance with the invention.

The part to be treated 12, made of aluminum or aluminum alloy, is immersed in the aqueous electrolytic medium 11. It constitutes there an electrode, in particular an anode. A counter electrode 13, in particular forming a cathode, is also immersed in the aqueous electrolytic medium 11.

The device further comprises an electric current generator 14, associated with means for controlling the conventional electric signal generated in themselves, and an electric circuit 15 connecting this electric current generator 14 to the part 12 and to the counter-electrode 13.

It also includes a circuit for regulating the temperature of the aqueous electrolytic medium 11, which is not shown in the figure.

The operating conditions of the first stage of treatment of the part by micro-arc oxidation being conventional in themselves, they will not be described in more detail in the present description. With the exception of the particular parameters which will be indicated below for the examples of implementation of the method according to the invention, these operating conditions are in particular such as those described in document WO 01/81658.

At the end of this first treatment step, the part can be rinsed, in particular with distilled water, possibly dried, then brought into contact, in particular immersed, successively, in:

- a first aqueous composition containing a corrosion-inhibiting metal salt, excluding a hexavalent chromium salt, and an oxidizing compound,

- then a second aqueous composition containing a rare earth salt inhibiting corrosion and an oxidizing compound.

The operating conditions of this second treatment step are in particular such as those described in document WO 2013/117767.

At the end of the second treatment step, the part can be rinsed, in particular with distilled water, and dried.

EXAMPLES

For the examples below, a micro-arc oxidation device is used comprising 2 treatment tanks of 70 and 115 I respectively of useful volume, a bipolar pulsed current generator CIRTEM 307 (70 A / 1000 V), a CIRTEM log 228 control software and a chiller for regulating the temperature of the aqueous electrolytic medium between 10 and 30 ° C.

The temperature of the aqueous electrolytic medium is thus permanently maintained between 10 and 30 ° C.

The second stage of treatment of the part is carried out in conventional tanks in themselves.

The corrosion resistance of the parts is determined by the salt spray test (BS) described in standard ISO 9227.

The treated parts are rectangular, 120 x 80 mm, made of aluminum alloy 2024 T3, AS7G06 or 2219 T62.

Example 1 - 1 ^st processing step

This example is made on parts made of rolled 2024 T3 alloy.

After manual degreasing with acetone, parts are treated by micro-arc oxidation in accordance with the invention (OMA1) or according to conditions not in accordance with the invention (OMAcl, OMAc2, OMAc3, OMAc4).

The operating conditions applied (electrical parameters and substances dissolved in water to form the aqueous electrolytic medium) are indicated in Table 1 below.

Operating conditions OMA1 OMAcl OMAc2 OMAc3 OMAc4 Aqueous electrolytic medium Na ₂ SiO ₃ (g / l) 2-20 15 (liquid) 8 - 1 NaAIO ₂ (g / l) 1-10 - - 0.5 0.5 Na ₂ Ti ₃ O ₇ (g / l) - - - 0.2 0.2 NaOH (g / l) 1-5 - 2 0.5 0.5 KOH (g / l) - 4 - - - PH 11-14 11.2 12.4 11.7 11.9 Conductivity (mS / cm) 5-10 6.8 5.5 1.6 1.9 Electrical parameters Current density (A / dm ² ) 5-50 20 10 5 5 Frequency of electrical pulses (Hz) 100- 1000 400 250 400 250 Processing time (min) 5-30 10 6 40 20

Table 1 - Operating conditions of the 1 ^st stage micro-arc oxidation treatment

A uniform layer of brown color with a thickness between 2 and 100 μm is obtained on the part treated in accordance with the invention (0MA1), depending on the exact parameters applied.

After rinsing with distilled water and drying with compressed air, the parts thus treated are subjected to the salt spray test.

The results obtained are shown in FIG. 2. As can be observed, the part treated in accordance with the invention (OMA1) has better resistance to corrosion than the parts treated under operating conditions not in accordance with the invention. , for which the electrolytic medium does not combine a silicate salt and an aluminate salt, but contains either one or the other, or else for which the electrolytic medium combines a silicate salt and an aluminate salt , but in concentrations not in accordance with the present invention.

Example 2 - Surface treatment method according to the invention

This example is made on parts made of rolled 2024 T3 alloy.

After degreasing the parts with acetone, the micro-arc oxidation step according to the invention described in Example 1 (OMA1) is implemented.

After rinsing the parts with distilled water and drying with compressed air, parts are either subjected to the salt spray test, or subjected to the second treatment step comprising the following steps, in accordance with the invention:

- immersion in a bath of the product sold under the name Socosurf TCS at 40 ° C for 15 min,

- rinsing with distilled water, by soaking for 1 min,

- immersion in a bath of the product sold under the name Socosurf PACS at room temperature for 5 min,

- rinsing with distilled water, by soaking for 1 min,

- and drying with compressed air.

The parts thus obtained are subjected to the salt spray test.

The results obtained for the parts after microarcs oxidation ( "OMA1") and to the parts after microarcs oxidation and ^2nd stage treatment according to the invention ( "OMA + PT1"), are shown in Figure 3 .

The parts treated by the complete process according to the invention have a corrosion resistance which is much higher than that of the parts obtained at the end of the 1 ^st treatment step of this process. The implementation of the 2 ^nd processing step of the process according to the invention increases the corrosion resistance of the treated part from 24 h to more than 600 h.

Example 3 - Repeatability test - reproducibility

The operating conditions described in Example 2 are applied to various parts of rolled or machined 2024 T3 aluminum alloy or of AS7G06 aluminum alloy (foundry alloy).

The results obtained are shown in FIG. 4. There is observed, for all the parts treated by the complete process according to the invention (“OMA1 + PT1”), good resistance to corrosion, the best results being obtained for the parts. in laminated 2024 T3 alloy and for parts in alloy AS7G06, for which a resistance to salt spray is observed greater than or equal to 500 h. For parts in rolled 2024 T3 alloy, the resistance to salt spray is on average between 600 and 700 h. Such performance advantageously satisfies industrial requirements in the aeronautical field.

Example 4 - comparative examples

By way of comparison, the following tests are carried out on parts of laminated 2024 T3 alloy (unless otherwise indicated).

For each example, the operating protocol is as described in example 2 above (OMA1 + PT1), with the exception of the steps which are detailed below.

1 / OMAd + PT1

At the end of the OMAcl micro-arc oxidation step, described in Example 1 above (commercial solution "If liquid"), a uniform brown layer of thickness 10 μιτι is obtained on the part. . The part is subjected to the 2 ^nd processing step carried out under conditions in accordance with the invention PT1.

The results obtained for the salt spray test implemented on parts thus treated are shown in FIG. 5. It is observed that the 2 ^nd treatment step allows an improvement in the corrosion resistance of the parts. However, this improvement, which brings the resistance to salt spray to approximately 50 h, is not as significant as in the case where the 1 ^st treatment step is carried out in accordance with the invention (OMA1).

2 / OMAc5 + PT1

This test is carried out on aluminum alloy parts 2219 T62.

The OMAc5 micro-arc oxidation step is carried out with the following operating parameters:

- electrolytic medium: Na2SiOg 30 g / l, KOH 15 g / l, pH 13.35

- electrical parameters: current density 10 A / dm ² , frequency 100 Hz, treatment time 5 min

This electrolytic medium highly concentrated in silicate salt and these operating parameters make it possible to form thick protective layers on the surface of the parts.

At the end of this step, a uniform brown layer 16 μm thick is thus obtained on the part. The part is then subjected to the 2 ^nd processing step PT 1.

The results obtained for the salt spray test implemented on parts thus treated are shown in FIG. 6. It is observed that the 2 ^nd treatment step allows an improvement in the corrosion resistance of the parts. However, this improvement is minimal, the maximum salt spray resistance obtained being 226 h.

3 / OMA1 + other clogging treatments

These tests are carried out on 2024 T3 aluminum alloy parts.

At the end of the 1 ^st stage of treatment of the parts by micro-arc oxidation according to the invention (OMA1), the parts, having on their surface a protective layer of thickness 8 μm, are rinsed with distilled water and dried with compressed air.

Parts are then subjected to the following sealing treatments, not in accordance with the invention, commercially available and commonly used for the surface treatment of aluminum alloy parts:

- immersion in an Alodine 1200 bath at room temperature for 2 min (commercial solution based on hexavalent chromium)

- immersion in a Surtec 650 bath at 37 ° C for 10 min (commercial solution based on trivalent chromium)

- immersion successively in a Lanthanum 613.3 bath (solution sold by the company Coventya) at 40 ° C for 15 min then in a Post DIP bath (solution sold by the company Coventya) at room temperature for 1 min

- or immersion in an Iridite NCP bath (solution sold by the company McDermid) at 30 ° C for 15 min.

After rinsing with distilled water and drying with compressed air, the parts are subjected to the salt spray test.

The results obtained are shown in FIG. 7. Whatever the treatment of the prior art used, it can be seen that none allows a part to be obtained which exceeds 300 h of resistance to salt spray, which is turns out to be much lower than the performance obtained by the method according to the invention.

4 / OMA1 + TCS or PACS

In this test, the parts are treated in accordance with the invention for the 1 ^st treatment step (OMA1), then, for the second treatment step, the pieces are immersed only either in a bath of the 1 ^st composition aqueous according to the invention (TCS, 40 ° C for 15 min) or in a bath of the ^2nd aqueous composition according to the invention (PACS, room temperature for 5 min).

After rinsing with distilled water and drying with compressed air, the parts are subjected to the salt spray test.

The results obtained are shown in FIG. 8. It is observed that the two comparative methods used are largely insufficient to obtain a satisfactory corrosion resistance performance of the treated parts.

Claims (16)

1. Method for surface treatment of a part (12) made of aluminum or aluminum alloy, characterized in that it comprises: a first step for treating said part (12) by micro-arc oxidation, said part (12) being immersed in an aqueous electrolytic medium (11) containing a silicate salt at a concentration greater than or equal to 2 g / l and an aluminate salt at a concentration greater than or equal to 1 g / l, and being crossed by an electric current, - then a second step of processing said part (12) by bringing said part (12) into contact successively with: • a first aqueous composition containing a corrosion inhibiting metal salt, excluding a hexavalent chromium salt, and an oxidizing compound, • then a second aqueous composition containing a corrosion inhibiting rare earth salt and an oxidizing compound. 2. Method according to claim 1, according to which the concentration of silicate salt in the electrolytic medium (11) is between 2 and 20 g / l. 3. Method according to any one of claims 1 to 2, according to which the concentration of aluminate salt in the electrolytic medium (11) is between 1 and 10 g / l. 4. Method according to any one of claims 1 to 3, according to which the pH of the electrolytic medium (11) is between 11 and 14. 5. Method according to any one of claims 1 to 4, according to which, during the first step of treatment of said part by micro-arc oxidation, the density of electric current passing through the part (12) is between 5 and 50 A / dm ² . 6. Method according to any one of claims 1 to 5, according to which, during the first stage of treatment of said part by micro-arc oxidation, said part (12) is subjected to an electric current according to a pattern repeated so periodic at a frequency between 100 and 1000 Hz. 7. Method according to any one of claims 1 to 6, according to which the first step of treatment of said part by micro-arc oxidation is carried out for a period of between 5 and 30 minutes. 8. Method according to any one of claims 1 to 7, according to which the corrosion inhibiting metal salt in the first aqueous composition is a trivalent chromium salt. 9. Method according to any one of claims 1 to 8, according to which the oxidizing compound in the first aqueous composition is chosen from fluoride-based substances, permanganate-based substances and hydrogen peroxide, or any of their mixtures. 10. Method according to any one of claims 1 to 9, according to which, for the second stage of treatment of said part, the temperature of the first aqueous composition is between 10 and 80 ° C, preferably between 30 and 50 ° C. 11. Method according to any one of claims 1 to 10, according to which, for the second stage of treatment of said part, the duration of contacting of said part (12) with the first aqueous composition is between 1 and 60 minutes, preferably between 10 and 20 minutes. 12. Method according to any one of claims 1 to 11, according to which the rare earth salt inhibiting corrosion in the second aqueous composition is chosen from lanthanum salts and cerium salts, or any one of their mixtures . 13. Method according to any one of claims 1 to 12, according to which the oxidizing compound in the second aqueous composition is chosen from substances based on fluoride, substances based on permanganate and hydrogen peroxide, or any of their 5 mixes. 14. Method according to any one of claims 1 to 13, according to which, for the second stage of treatment of said part, the temperature of the second aqueous composition is between 15 and 40 ° C, preferably between 20 and 30 ° C. 15. Method according to any one of claims 1 to 14, according to which, for the second step of treating said part, the duration of contacting of said part (12) with the second aqueous composition is between 1 and 60 minutes, preferably between 2 and 10 minutes. 16. Part made of aluminum or aluminum alloy obtained by a process according to any one of claims 1 to 15.