Anti-corrosion treatment process for a magnesium alloy part, corresponding anti-corrosion substance and treated part

Abstract

The present invention relates to a method for the anti-corrosion treatment of a magnesium alloy part, the method comprising bringing a portion of the part into contact, at least locally, with an applicator, with an anti-corrosion substance for a predetermined time. The anti-corrosion substance contains permanganate ions (MnO4-), dihydrogen phosphate ions (H2PO4-), and an epoxy gel. After the predetermined time, the method comprises rinsing the portion of the part with water. Abstract figure: Figure 1

Description

Title of the invention: Method for anti-corrosion treatment of a magnesium alloy part, corresponding anti-corrosion substance and treated part

[0001] The present invention relates to a process for the anti-corrosion treatment of a part made of magnesium alloy, an anti-corrosion substance enabling the implementation of the process and a part obtained after treatment according to the process of the invention.

[0002] Magnesium alloys are mixtures of magnesium and other alloying metals, generally at least one metal selected from aluminium, zinc, silicon, manganese, copper, zirconium, neodymium, gadolinium. Some alloys may also include rare earth elements.

[0003] Furthermore, one of the remarkable characteristics of magnesium is its density, 1.7 g / cm³ (grams per cubic centimeter); magnesium-based alloys are therefore used when weight reduction is an important consideration, for example in aircraft or rotorcraft components and equipment. Magnesium alloys have a hexagonal crystal lattice structure, which is more rigid than a cubic structure. Moreover, magnesium alloys are generally used as cast alloys.

[0004] However, these magnesium alloys can oxidize in the presence of water or water vapor, which can alter their mechanical characteristics and degrade the static or fatigue resistance of the parts thus formed.

[0005] Document EP1277853 A1 describes a process for the anti-corrosion treatment of a cast plate made of a magnesium alloy AZ91D (containing 90% magnesium, 9% aluminum, and 1% zinc). The plate is immersed in a degreasing agent solution at 40°C (degrees Celsius) for 10 minutes (10 min), then washed with deionized water for one minute. The plate is then immersed in a potassium hydroxide pickling solution for 15 minutes at 60°C, and then washed with deionized water. Finally, it is immersed in one liter (IL) of an anti-corrosion bath consisting of an aqueous manganese phosphate solution containing 100 grams (100 g) of ammonium dihydrogen phosphate and 20 g of potassium permanganate, the pH of which is adjusted to 3.5 with orthophosphoric acid. The bath takes place at 40°C for 15 minutes. The plate is then rinsed with deionized water and dried.

[0006] The process described in the aforementioned document does indeed comply with certain regulations, such as the REACH regulation, aimed at improving the protection of human health and the environment against the risks associated with chemical substances, because the solutions used do not contain, in particular, hexavalent chromium. This This process is suitable for treating parts in a bath. However, when localized treatments are required on painted or equipped parts, immersing the part or using a liquid solution carries risks of damaging the paint or equipment. Therefore, in these cases, there is a need for a suitable local touch-up and repair process that avoids the risk of runs or leaks onto parts, whether or not they have been previously treated.

[0007] Furthermore, the aforementioned document does not provide information on the thickness of the anti-corrosion layer formed. The thickness of the formed layer is of great importance when treating parts with precise dimensions that must be met for the part to be used in a mechanism, for example, in an engine. The characteristics of this layer are also of great importance when dealing with a part subjected to varying stresses. The influence of the treatment layer on the fatigue resistance of the treated alloy is therefore also significant.

[0008] An object of the present invention is therefore to propose an anti-corrosion treatment process which is easily industrialized, applicable locally on a portion of the part, and which makes it possible to guarantee the fatigue resistance, the anti-corrosion performance and the compatibility with the paint systems of the parts thus treated.

[0009] Another object of the present invention is to propose a method which makes it possible to obtain a coating suitable for providing anti-corrosion protection of less than 5 micrometers (5pm) thick, the dimensions of which must be respected so that the part can perform its function in a mechanism or forms a housing suitable for housing and guiding a mechanism, for example in rotation or translation.

[0010] Another object of the invention is to provide an anti-corrosion substance that does not contain chromium.

[0011] Another object of the present invention is to provide a corrosion-treated magnesium alloy part which has a corrosion-resistant treatment layer whose thickness is less than 5 pm.

[0012] Another object of the present invention is to provide a part made of magnesium alloy treated with anti-corrosion coating in which the anti-corrosion treatment layer has optimal adhesion to paints, in particular according to ISO 2409.

[0013] The invention therefore relates to a method of anti-corrosion treatment of a part made of magnesium alloy, the method comprising bringing a portion of the part into contact, at least locally, with an applicator element, with an anti-corrosion substance for a predetermined period.

[0014] According to the invention, such a process is remarkable in that the anti-corrosion substance contains: - permanganate ions MnO4, - dihydrogen phosphate ions (H2PO4), and - an epoxy gel,

[0015] and in that, after the predetermined time, the process includes rinsing the portion of the part with water.

[0016] Furthermore, the applicator allows the anti-corrosion substance to be applied by localized deposition on the portion of the part. The anti-corrosion substance can then impregnate the area of ​​the part to be treated so that the permanganate ions can react with the magnesium of the alloy forming the part.

[0017] Such an applicator organ can be, for example, chosen from a brush, a roller, a pad, a cloth or a sponge.

[0018] Epoxy gel, for its part, makes the anti-corrosion substance viscous, allowing it to adhere to the part at ambient temperature and thus providing a stable carrier medium for the chemical ingredients dissolved in the anti-corrosion substance, while limiting the risks of dripping and seepage. Such an anti-corrosion substance is therefore less fluid than an aqueous solution, and epoxy gel is a medium compatible with the dissolved chemical components, namely permanganate ions (MnO4) and dihydrogen phosphate ions (H2PO4).

[0019] In addition, such an epoxy gel may have a pH between 6 and 8.

[0020] Furthermore, the pH of the anti-corrosion substance may have, before contact with the part, a value equal to or greater than 3.2 and equal to or less than 4.2 and preferably equal to or greater than 3.4 and equal to or less than 4.0.

[0021] In practice, the anti-corrosion substance can be obtained by mixing the epoxy gel with a powder containing potassium permanganate KMnO4 and potassium dihydrogen phosphate KH2PO4 in solid form, after mixing the anti-corrosion substance having a molar concentration of permanganate ions [MnO4] greater than or equal to 0.06 mol / L (mol per liter) and less than or equal to 0.31 mol / L, and a molar concentration of dihydrogen phosphate ions [H2PO4] greater than or equal to 0.47 mol / L and less than or equal to 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L.

[0022] The aforementioned molar concentrations and the pH value allow for obtaining a particularly thin layer with optimal corrosion resistance, particularly saline corrosion according to ISO 9227. The layer formed also has a suitable surface condition and roughness to ensure optimal adhesion of paint systems and varnishes, which is the main function sought.

[0023] Upon contact with the anti-corrosion substance of the invention, the magnesium of the alloy is attacked by permanganate ions according to the following reversible chemical reaction (1):

[0024] 2MnO4 + 3Mg + 8H+ 2MnO2(s) + 3Mg2+ + 4H2O (1)

[0025] A porous layer of solid manganese oxide (MnO2) forms on the surface of the treated part or portion. Due to the combination of the pH value and the specific concentrations of permanganate [MnO4] and dihydrogen phosphate [H2PO4] ions contained in the corrosion inhibitor, the ions precipitate as magnesium phosphate (Mg(PO4)2) and magnesium permanganate (Mg(MnO4)2), thus sealing the pores of the manganese oxide layer. Furthermore, this layer remains less than 5 µm thick thanks to the aforementioned ionic concentrations and the initial pH of the corrosion inhibitor.

[0026] Within the indicated pH range, phosphate ions are predominant. They can also clog the porous layer of manganese oxide or be transformed within it into phosphoric acid, which will precipitate in situ once the part is no longer in contact with the anti-corrosion substance, thus clogging the pores of the anti-corrosion layer.

[0027] Advantageously, the molar concentration of permanganate ions [MnO4] can be greater than or equal to 0.08 mol / L and less than or equal to 0.18 mol / L and in particular equal to 0.13 mol / L.

[0028] According to an advantageous embodiment of the invention, the predetermined duration may be greater than or equal to 1 min (minute) and less than or equal to 10 min and preferably equal to or greater than 3 min and less than or equal to 7 min.

[0029] The predetermined duration can thus be equal to or greater than 4 min and less than or equal to 6 min. Such values ​​make it possible to obtain a porous layer of manganese oxide less than 5 µm thick, having a microscopic structure exhibiting good adhesion to varnishes and anti-corrosion paint systems, as well as good anti-corrosion resistance to neutral salt spray according to ISO 9227. A chemical affinity also exists between the nature of the porous layer of manganese oxide and the nature of the paints.

[0030] The predetermined time thus reduced for the implementation of the process makes it possible to improve its industrialization.

[0031] In practice, when the portion of the part is brought into contact at least locally with the anti-corrosion substance, the anti-corrosion substance may have a temperature equal to that of the ambient air.

[0032] Furthermore, the solubility limit of permanganate ions can be advantageously increased by raising the temperature of the anti-corrosion substance. This temperature allows for optimization of the permanganate ion concentration in the substance to achieve the concentration levels necessary for obtaining a high-performance anti-corrosion coating that ensures optimal adhesion to paint systems.

[0033] The use of an anti-corrosion substance at ambient temperature makes it possible to simplify the industrialization of such a treatment process.

[0034] Advantageously, the anti-corrosion substance can have a dynamic viscosity between 1 pascal-second (Pa.s) and 1000 Pa.s.

[0035] Such a range of viscosity values ​​of the anti-corrosion substance thus makes it possible to remain in contact with the part or portion being treated regardless of its shape, size or orientation.

[0036] Such a dynamic viscosity of the anti-corrosion substance allows application on a vertically oriented surface of a part without flowing on that surface and at ambient temperature.

[0037] In practice, prior to contacting the anti-corrosion substance with the part, the anti-corrosion substance can be agitated. Such agitation can advantageously be carried out manually using a dedicated device such as a stirrer.

[0038] Advantageously, regardless of the embodiment of the invention, the anti-corrosion substance may also contain ions selected from potassium, ammonium, sodium, calcium ions and mixtures thereof. These ions are obtained from the use of one or more dihydrogen phosphate salts. Advantageously, regardless of the embodiment of the process of the invention, the anti-corrosion substance of the invention, before contact with the part, consists of water present in the epoxy gel, hydronium ions (H3O+), potassium ions (K+), permanganate ions (MnO4), and phosphate ions (PO43).

[0039] According to another advantageous example of the invention, the part can be made of a magnesium alloy chosen from alloys of magnesium and at least one rare earth and possibly containing at least one metal chosen from aluminium, zinc, silicon, manganese, copper, zirconium and mixtures thereof and in particular from magnesium alloys containing aluminium, neodymium, gadolinium, zinc and zirconium.

[0040] In particular, the alloy may contain or be composed of magnesium, aluminum, neodymium, gadolinium, and zirconium. The part may be made of EV31A alloy, for example.

[0041] In practice, before bringing the part or portion to be treated into contact with the anti-corrosion substance, the process may include at least one pre-treatment of the part or portion chosen from the following treatments and their combinations: • alkaline degreasing; • pickling with nitric acid followed by pickling with hydrofluoric acid; • pickling with ammonium bifluoride; • pickling with potassium bifluoride; • pickling with sodium bifluoride; • sandblasting followed by alkaline degreasing; • degreasing using a solvent; • aqueous degreasing; • stripping of residues from magnesium treatments; and • contact simultaneously or successively with at least one compound chosen from among sodium hydroxide, hydrogen peroxide, acetic acid, sulfuric acid, hydrofluoric acid, sodium bifluoride, ammonium bifluoride, phosphoric acid, tartaric acid, and hydrochloric acid.

[0042] Such surface preparation of the part or portion of the part then makes it possible to increase the effectiveness of the anti-corrosion treatment.

[0043] In addition, such stripping can be carried out by a light mechanical process using for example an abrasive pad with a stripping solution described above.

[0044] The present invention also relates to an anti-corrosion substance enabling the implementation of the aforementioned process.

[0045] According to the invention, such an anti-corrosion substance is remarkable in that it has a pH equal to or greater than 3.2 and equal to or less than 4.2 and preferably equal to or greater than 3.4 and equal to or less than 4.0, contains permanganate ions at a molar concentration [MnO4] greater than or equal to 0.06 mol / L and less than or equal to 0.31 mol / L and preferably greater than or equal to 0.08 mol / L and less than or equal to 0.18 mol / L and in particular equal to 0.13 mol / L and dihydrogen phosphate ions.

[0046] The anti-corrosion substance of the invention may also contain ions selected from potassium, ammonium, sodium, calcium ions and mixtures thereof. These ions are obtained from the use of one or more dihydrogen phosphate salts. Advantageously, regardless of the embodiment of the process of the invention, the anti-corrosion substance of the invention may, before contact with the part, consist of water, potassium ions (K+), permanganate ions (MnO4), and phosphate ions (PO43).

[0047] According to a preferred embodiment, the corrosion inhibitor is obtained by mixing a powder containing potassium permanganate KMnO4 and potassium dihydrogen phosphate KH2PO4 in solid form with an epoxy gel. This corrosion inhibitor may then have a mass concentration of potassium dihydrogen phosphate equal to or greater than 55 g / L and equal to or less than 95 g / L, preferably equal to or greater than 60 g / L and equal to or less than 80 g / L, a The mass concentration of potassium permanganate is equal to or greater than 10 g / L and equal to or less than 50 g / L, preferably equal to or greater than 15 g / L and equal to or less than 30 g / L. Advantageously, the anti-corrosion substance may also have a pH equal to or greater than 3.2 and equal to or less than 4.2, and preferably equal to or greater than 3.4 and equal to or less than 4.0.

[0048] The present invention also relates to a magnesium alloy part comprising, at least on one surface of a portion of the part, an anti-corrosion layer obtained with the aforementioned process.

[0049] Advantageously, the anti-corrosion layer may contain manganese oxide and at least one phosphate, in particular magnesium phosphate, and the anti-corrosion layer may have a thickness of less than 5pm.

[0050] The manganese oxide layer is porous and its pores are occupied by phosphate molecules. Other ions from the aqueous anti-corrosion solution may also be present.

[0051] In practice, the part can be formed in a magnesium alloy chosen from among the alloys of magnesium and at least one rare earth and containing at least one metal chosen from aluminium, zinc, silicon, manganese, copper, zirconium and their mixtures and in particular from the magnesium alloys containing aluminium, neodymium, gadolinium, zinc and zirconium.

[0052] In particular, the alloy may contain or be composed of magnesium, aluminum, neodymium, gadolinium, and zirconium. The part may be made of EV31A alloy, for example, or may include a layer or portion made of EV31A alloy.

[0053] According to one embodiment of the invention, the anti-corrosion layer may be electrically conductive and may have a standard potential difference AE relative to the standard potential of the alloy of said part greater than IV and in particular greater than or equal to 1.1V; 1.2V, 1.3V or 1.4V and / or in that said anti-corrosion layer resists corrosion for at least 20 hours and in particular 24 hours caused by accelerated aging and / or the anti-corrosion layer may have a grade 0 or 1 for paint adhesion according to ISO 2409.

[0054] Definitions

[0055] The term “phosphate” refers to a part of the constituents of phosphoric acid, dihydrogen phosphate ions, hydrogen phosphate ions, phosphate ions and mixtures thereof.

[0056] Other features and advantages of the invention will become apparent from the following description, with reference to the accompanying drawings, among which:

[0057] Figures 1 and 2 show two test specimens treated with an anti-corrosion coating and then subjected for 24 hours to an accelerated aging corrosion test. The specimen in [Fig. 1] was treated locally according to the process of the invention and has a layer of treatment having a thickness of less than 5pm and for example between 0.5pm and Ipm, while the test specimen in [Fig.2] was treated with a retouching solution marketed under the name "D0W19" and containing Cr6+ ions.

[0058] Figures 3 and 4 show two test specimens previously coated with a primer, then treated with an anti-corrosion coating, and subjected for 24 hours to an accelerated aging corrosion test. The specimen in [Fig. 3] was treated locally according to the process of the invention and has a treatment layer with a thickness of less than 5 µm, for example between 0.5 µm and 1 µm, while the specimen in [Fig. 4] was treated with a D0W19 retouching solution containing Cr6+ ions.

[0059] EXAMPLES

[0060] Example 1: a particular implementation method of the process of the invention

[0061] Preparation of the anti-corrosion substance

[0062] In practice, for an industrial application of the contact treatment process without immersion, an anti-corrosion substance can be prepared in a container.

[0063] By way of illustrative example, a powder containing potassium permanganate KMnO4 in solid form is dissolved in a few centiliters of epoxy gel, giving a molar concentration of permanganate ions [MnO4] equal to 0.13 mol / L with potassium dihydrogen phosphate KH2PO4 in solid form to obtain a molar concentration of dihydrogen phosphate ions [H2PO4] between 0.47 mol / L and 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L, or even equal to 75 g / L so as to obtain an anti-corrosion substance having a pH greater than or equal to 3.4 and less than or equal to 3.6 and in particular equal to 3.5.

[0064] To obtain such a mixture, the powder comprising potassium permanganate KMnO4 and potassium dihydrogen phosphate KH2PO4 in solid form can be manually mixed into an epoxy gel. Such an epoxy gel is chemically inert with the permanganate ions MnO4 and the dihydrogen phosphate ions H2PO4 to obtain the chemical species to be applied, at least locally, to the part.

[0065] Part pretreatment

[0066] A portion of the part or the entire part may undergo a pretreatment selected from the group comprising alkaline degreasing, nitric acid pickling followed by hydrofluoric acid pickling, ammonium bifluoride pickling, potassium bifluoride pickling, sodium bifluoride pickling, sandblasting followed by alkaline degreasing, solvent degreasing, aqueous degreasing, removal of magnesium treatment residues, and simultaneous or successive contact with at least one compound selected from sodium hydroxide, hydrogen peroxide, acetic acid, sulfuric acid, hydrofluoric acid, and bifluoride. sodium, ammonium bifluoride, phosphoric acid, tartaric acid, and hydrochloric acid.

[0067] Such surface preparation of the part or portion of the part then makes it possible to increase the effectiveness of the anti-corrosion treatment.

[0068] In addition, such pretreatment can be carried out by a light mechanical process using for example an abrasive pad with a pickling solution described previously.

[0069] Part processing

[0070] The temperature of the anti-corrosion substance is that of the ambient air. Using an applicator, such as for example a brush or a roller, the anti-corrosion substance from the container is taken and applied to the part or portion of the part to be treated for a predetermined duration greater than or equal to 3 min and less than or equal to 7 min.

[0071] When the predetermined treatment time has elapsed, the anti-corrosion substance is wiped off and the part is rinsed with deionized water. The treated part can then be blown or dried.

[0072] Study of the properties of the anti-corrosion layer formed

[0073] The anti-corrosion layer formed is visible because it has a brown color (light pinkish beige / gold or matte to dark brown). The thickness of the anti-corrosion layer is less than 5 µm.

[0074] The anti-corrosion layer formed by the process of the invention is electrically conductive.

[0075] The corrosion resistance of the anti-corrosion layer obtained according to the process of the invention to accelerated aging was measured. The part treated according to the process of the invention resists corrosion for 24 hours before the appearance of the first pitting corrosion.

[0076] Figure 2 clearly shows that the specimen treated with a D0W19 retouching solution containing Cr6+ ions and subjected to the neutral salt spray corrosion test for 24 hours exhibits a completely corroded surface. Conversely, the specimen treated according to the method of the invention, illustrated in Figure 1, shows an uncorroded surface after 24 hours of the same test.

[0077] The corrosion resistance of the anti-corrosion layer obtained according to the process of the invention, coated with a varnish, to a neutral salt spray according to ISO 9227 was also measured. The varnish-coated part of the invention resisted for more than 250 hours before the first signs of corrosion appeared.

[0078] The corrosion resistance of the anti-corrosion layer obtained according to the process of the invention, coated with a paint system comprising a primer and a topcoat or varnish, subjected to accelerated aging, was also measured. The part The invention, coated with paint, resists for more than 1000 hours before the first signs of corrosion appear.

[0079] The adhesion of varnishes and paints was tested according to ISO 2409. It was found that the anti-corrosion layer obtained according to the process of the invention exhibits a grade of 0 for paint adhesion according to the aforementioned standard. After humid aging, and again according to the aforementioned standard, the anti-corrosion layer obtained according to the process of the invention exhibits a grade of 1 for paint adhesion (according to ISO 2409). The aging cycle lasts 24 hours and is broken down as follows: 6 hours of exposure in a humid environment: 80% RH, 40°C, then immersion for 15 minutes in a saline solution (5% NaCl), and the remaining time is spent drying in ambient air.

[0080] Accelerated aging tests in a climatic chamber

[0081] Furthermore, aging cycles as indicated in the preceding paragraph on the anti-corrosion treated EV31A alloy reveal the presence of first corrosion pits after 2 cycles of 24h for a part treated by chrome etching whereas after the fifteenth cycle of 24h no pitting is detected in the case of a part having an anti-corrosion layer obtained according to the process of the invention and having a thickness of less than 5pm.

[0082] Naturally, the present invention is subject to numerous variations in its implementation. Although several embodiments have been described, it is understood that it is not conceivable to exhaustively identify all possible embodiments. It is, of course, conceivable to replace a described means with an equivalent means without departing from the scope of the present invention as defined by the claims.

Claims

Demands

1. A method for the anti-corrosion treatment of a part made of a magnesium alloy, said method comprising bringing a portion of said part into contact, at least locally, with an applicator, with an anti-corrosion substance for a predetermined time, characterized in that said anti-corrosion substance contains: - permanganate ions MnO4, - dihydrogen phosphate ions H2PO4, and - an epoxy gel, and in that, after said predetermined time, said method comprises rinsing said portion of said part with water.

2. A process according to claim 1, characterized in that said anti-corrosion substance is obtained by mixing said epoxy gel with a powder containing potassium permanganate KMnO4 and potassium dihydrogen phosphate KH2PO4 in solid form, after mixing said anti-corrosion substance having a molar concentration of permanganate ions [MnO4] greater than or equal to 0.06 mol / L (mol per liter) and less than or equal to 0.31 mol / L, and a molar concentration of dihydrogen phosphate ions [H2PO4] greater than or equal to 0.47 mol / L (mol per liter) and less than or equal to 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L.

3. A process according to claim 2, characterized in that said molar concentration of permanganate ions [MnO4] is greater than or equal to 0.08 mol / L and less than or equal to 0.18 mol / L and in particular equal to 0.13 mol / L.

4. A method according to any one of claims 1 to 3, characterized in that said predetermined duration is greater than or equal to 1 min and less than or equal to 10 min and preferably equal to or greater than 3 min and less than or equal to 7 min.

5. A method according to any one of claims 1 to 4, characterized in that, when said portion of said part is brought into contact at least locally with said anti-corrosion substance, said anti-corrosion substance has a temperature equal to that of the ambient air.

6. A method according to any one of claims 1 to 5, characterized in that said anti-corrosion substance has a dynamic viscosity between 1 pascal-second (Pa.s) and 1000 pascal-seconds (Pa.s).

7. A method according to any one of claims 1 to 6, characterized in that said part is formed in a magnesium alloy selected from alloys of magnesium and at least one rare earth and containing at least one metal selected from aluminium, zinc, silicon, manganese, copper, zirconium and mixtures thereof.

8. A method according to any one of claims 1 to 7, characterized in that, before contacting said part with the anti-corrosion substance, said method comprises at least one pretreatment of said part selected from the following treatments and combinations thereof: - alkaline degreasing; - nitric acid pickling followed by hydrofluoric acid pickling; - ammonium bifluoride pickling; - potassium bifluoride pickling; - sodium bifluoride pickling; - sandblasting followed by alkaline degreasing; - solvent degreasing; - aqueous degreasing; - contact simultaneously or successively with at least one compound chosen from among sodium hydroxide, hydrogen peroxide, acetic acid, sulfuric acid, hydrofluoric acid, sodium bifluoride, ammonium bifluoride, phosphoric acid, tartaric acid, hydrochloric acid.

9. Anti-corrosion substance enabling the implementation of the process according to any one of claims 1 to 8, characterized in that said anti-corrosion substance has a pH equal to or greater than 3.2 and equal to or less than 4.2 and preferably equal to or greater than 3.4 and equal to or less than 4.0, contains permanganate ions in a molar concentration [MnO4] greater than or equal to 0.06 mol / L and less than or equal to 0.31 mol / L and preferably greater than or equal to 0.08 mol / L and less than or equal to 0.18 mol / L and in particular equal to 0.13 mol / L and dihydrogen phosphate ions.

10. A magnesium alloy part comprising, on at least one surface of a portion of said part, an anti-corrosion coating obtained by the process according to any one of claims 1 to R

11. of O. Part according to claim 10, characterized in that said anti-corrosion layer contains manganese oxide and at least one phosphate, in particular magnesium phosphate and in that said anti-corrosion layer has a thickness of less than 5pm.

12. Part according to any one of claims 10 to 11, characterized in that said part is made of magnesium alloy selected from alloys of magnesium and at least one rare earth and optionally containing at least one metal selected from aluminium, zinc, silicon, manganese, copper, zirconium and mixtures thereof.

13. Part according to any one of claims 10 to 12, characterized in that said anti-corrosion layer is electrically conductive and has a standard potential difference AE relative to the standard potential of the alloy of said part greater than IV and in particular greater than or equal to 1.1V; 1.2V, 1.3V or 1.4V and / or in that said anti-corrosion layer resists corrosion for at least 20 hours and in particular 24 hours caused by accelerated aging and / or in that said anti-corrosion layer has a grade 0 or 1 for paint adhesion according to ISO 2409.

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