Method for anticorrosion treatment of a magnesium alloy part, associated anticorrosion aqueous solution and part

Abstract

The invention relates to a method for anticorrosion treatment of a magnesium alloy part, in which the part is immersed for a given duration in an anticorrosion aqueous solution having a given temperature and containing permanganate ions MnO4- and dihydrogen phosphate ions H2PO4-. According to the invention, the anticorrosion aqueous solution contains, prior to immersion of the part, a molar ion concentrate of permanganate ions [MnO4-] of greater than or equal to 0.18 mol / L and less than or equal to 0.32 mol / L, the pH of the anticorrosion aqueous solution is maintained for the given duration at a value of greater than or equal to 3.2 and less than or equal to 4.2, preferably greater than or equal to 3.4 and less than or equal to 4.0, by addition of phosphoric acid, and, after the given duration, the part is removed from the solution and rinsed with water.

Description

[0001] METHOD FOR ANTICORROSION TREATMENT OF A MAGNESIUM ALLOY PART - ANTICORROSION AQUEOUS SOLUTION AND ASSOCIATED PART

[0002] The present invention relates to a method for anti-corrosion treatment of a magnesium alloy part, an aqueous anti-corrosion solution allowing the implementation of the method and the part obtained after treatment according to the method of the invention.

[0003] Magnesium alloys are mixtures of magnesium and other alloying metals, usually at least one metal selected from aluminum, zinc, silicon, manganese, copper, zirconium, neodymium, gadolinium, and other rare earths and mixtures thereof. Some alloys may also include rare earths.

[0004] Furthermore, one of the remarkable characteristics of magnesium is its density, 1.7 g / cm3; magnesium-based alloys are therefore used where weight saving 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. Furthermore, magnesium alloys are generally used as cast alloys.

[0005] Document EP 1277853 A1 describes a method for the anti-corrosion treatment of a cast plate of a magnesium alloy AZ91 D (containing 90% magnesium, 9% aluminum and 1% zinc). The plate is immersed in a solution of degreasing agent at 40°C (degree Celsius) for 10 minutes and 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. It is finally immersed in one liter of an anti-corrosion bath, consisting of an aqueous solution of manganese phosphate containing 100g of ammonium dihydrogen phosphate and 20g (gram) of potassium permanganate, the pH of which is adjusted to 3.5 with orthophosphoric acid. The treatment takes place at 40°C for 15 minutes. The plate is then rinsed with water and dried.

[0006] The process described in the aforementioned document certainly complies with certain regulations, such as the REACH regulation, aimed at improving the protection of human health and the environment against risks linked to chemical substances because the solutions used do not contain hexavalent chromium. However, the contact time with the anti-corrosion solution is relatively long, which makes it difficult to industrialize.

[0007] Furthermore, the aforementioned document does not provide information on the thickness of the anti-corrosion layer formed. However, the thickness of the layer is of great importance when it comes to treating parts which have precise reliefs which must be respected so that the part can be used in a mechanism, for example in an engine or when it is a part subjected to variable forces. The influence of the treatment layer on the fatigue resistance of the treated alloy is then also important.

[0008] Document CN 1294203 discloses a method for surface treatment of magnesium alloys.

[0009] A treatment solution has a NasPCh phosphate concentration of between 0.1 - 0.5 g / L.

[0010] Document CN 1 1 1424271 discloses a treatment solution based on potassium phosphate, with the generally accepted chemical formula K3PO4, and a molar mass of approximately 212 g / mol. Document JP2003277944 describes a treatment method comprising several stages using a solution containing 30 g / L of KMnCh and 100 g / L of NahhPCh at a corrected pH of 2.5.

[0011] An aim of the present invention is therefore to propose an anti-corrosion treatment process which is easily industrializable and which makes it possible to guarantee the fatigue resistance of the parts thus treated.

[0012] Another aim of the present invention is to propose a method which makes it possible to obtain a coating capable of providing anti-corrosion protection of less than 5 pm, the dimensions of which must be respected so that the part can perform its function in a mechanism or form a casing capable of housing and guiding a mechanism, for example in rotation or translation.

[0013] Another object of the invention is to provide an aqueous anti-corrosion solution which does not contain chromium.

[0014] Another object of the present invention is to provide an anti-corrosion treated magnesium alloy part which has an anti-corrosion treatment layer whose thickness is less than 5 pm (micrometer).

[0015] Another object of the present invention is to provide an anti-corrosion treated magnesium alloy part whose anti-corrosion treatment layer has a standard potential difference with respect to the potential of the magnesium alloy greater than 1 V (volt).

[0016] Another aim of the present invention is to provide a magnesium alloy part treated against corrosion, the anti-corrosion treatment layer of which has good adhesion to paints, in particular according to the ISO 2409 standard. According to a first aspect, the present invention relates to a method for the anti-corrosion treatment of a magnesium alloy part, according to which said part is immersed for a given period in an aqueous anti-corrosion solution having a given temperature and containing permanganate ions MnCh' and dihydrogen phosphate ions H2PO4'.Characteristically, according to the invention, said aqueous anti-corrosion solution contains, before immersion of said part, a molar concentration of permanganate ions [MnCh'] greater than or equal to 0.18 mol / L (mole per liter) and less than or equal to 0.32 mol / L, the pH of said aqueous anti-corrosion solution is maintained during said given duration at 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, by adding phosphoric acid and after said given duration, said part is removed from said aqueous solution and rinsed with water.

[0017] In practice, the aqueous anti-corrosion solution may contain, before immersion of the part, a molar concentration of dihydrogen phosphate ions [H2PO4-] greater than or equal to 0.47 mol / L (mole per liter) and less than or equal to 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L.

[0018] The above-mentioned molar concentrations and the pH value make it possible to obtain a particularly thin layer with good resistance to corrosion, particularly saline corrosion according to ISO 9227. The layer formed also has a suitable surface condition and roughness to ensure good adhesion to paint systems and varnishes.

[0019] In contact with the aqueous solution of the invention, the magnesium of the alloy is attacked by permanganate ions according to the following reversible chemical reaction (1): 2MnO4- + 3Mg + 8H + 2MnO2(s) + 3Mg 2++ 4H2O (1 ) A porous layer of manganese oxide forms on the surface of the part. Due to the combination of the pH value and the particular concentrations of permanganate ions [MnCh'] and dihydrogen phosphate [H2PO4-] contained in the aqueous anti-corrosion solution, the latter precipitate into magnesium phosphate Mg(PO4)2 and magnesium permanganate Mg(MnO4)2 and block the pores of the manganese oxide layer. Furthermore, the latter remains thin thanks to the values ​​of the aforementioned ionic concentrations and the pH at which said aqueous solution is maintained.

[0020] Adding phosphoric acid controls the pH by adding H+ ions and also allows the addition of phosphate ions. In the indicated pH range, phosphate ions are predominant. They can also clog the porous manganese oxide layer or transform into phosphoric acid within it, which will precipitate in situ once the part is removed from the anti-corrosion solution, thus clogging the pores of the anti-corrosion layer.

[0021] Preferably, said aqueous anti-corrosion solution may contain, before immersion of said part, a molar concentration of permanganate ions [MnCh'] greater than or equal to 0.23 mol / L and less than or equal to 0.29 mol / L and in particular equal to 0.25 mol / L.

[0022] The given duration is not limited according to the invention. It is preferably greater than or equal to 1 min (minute) and less than or equal to 10 min and preferably equal to or greater than 2 min and less than or equal to 5 min. Such values ​​make it possible to obtain a porous layer of manganese oxide of less than 5 pm having a microscopic structure exhibiting good adhesion for varnishes and anti-corrosion paint systems, in particular in terms of anti-corrosion resistance to neutral salt spray according to standard ISO 9227. A chemical affinity also exists between the nature of the porous layer of manganese oxide and the nature of the paints.

[0023] The reduced implementation time of the process thus makes it possible to improve its industrialization.

[0024] Advantageously, regardless of the method of implementing the method of the invention, said given temperature may be greater than or equal to 55°C and less than or equal to 65°C and preferably equal to 60°C. In addition, the solubility limit of the permanganates may be advantageously increased as a function of the temperature value of the aqueous anti-corrosion solution. Such a temperature range of 55°C to 65°C makes it possible to optimize the concentration of the solution in permanganate ions in order to reach the concentration levels necessary to obtain a high-performance anti-corrosion coating which guarantees adhesion to the paint systems.

[0025] In practice, during said immersion of said part, said aqueous anti-corrosion solution can be agitated. Such agitation can advantageously be carried out by means of a propeller arranged at the bottom of the bath and driven in rotation by a motor.

[0026] Advantageously, whatever the method of implementing the invention, said aqueous anti-corrosion solution also contains ions chosen from potassium, ammonium, sodium, calcium ions and their mixtures. These ions come from the use of one or more dihydrogen phosphate salts. Advantageously, whatever the method of implementing the method of the invention, said aqueous anti-corrosion solution of the invention is, before immersion of said part, made up of water, hydronium ions HsO -, potassium ions K + , phosphoric acid H3PO4, permanganate ions Mn04' and phosphate ions PCh 3 '. Potassium ions have low steric hindrance and therefore do not hinder the penetration and retention of phosphoric acid in the pores of the manganese layer.

[0027] The magnesium alloy constituting said part is not limited according to the invention. Whatever the mode of implementation of the invention, said part may advantageously be made of a magnesium alloy chosen from alloys of magnesium and at least one rare earth and optionally containing at least one metal chosen from aluminum, zinc, silicon, manganese, copper, zirconium and their mixtures and in particular from magnesium alloys containing aluminum, neodymium, gadolinium, zinc and zirconium. In particular, the alloy may contain or consist of magnesium, aluminum, neodymium, gadolinium and zirconium. The part may be made of EV31 A alloy, for example.

[0028] Whatever the embodiment of the invention, it is possible, before immersing said part in said aqueous anti-corrosion solution, to carry out at least one pre-treatment of said part, such as a surface preparation making it possible to increase the effectiveness of the anti-corrosion treatment, chosen from the following pre-treatments and their combinations: alkaline degreasing; nitric acid pickling followed by hydrofluoric acid pickling; ammonium bifluoride pickling; potassium bifluoride pickling; sodium bifluoride pickling; sandblasting followed by alkaline degreasing; pickling of magnesium treatment residues;contacting simultaneously or successively with at least one compound chosen from soda, hydrogen peroxide, acetic acid, sulfuric acid, hydrofluoric acid, sodium bifluoride, ammonium bifluoride, phosphoric acid, tartaric acid, hydrochloric acid, nitric acid, chromic acid and mixtures thereof.;

[0029] According to a second aspect, the present invention also relates to an aqueous anti-corrosion solution allowing the implementation of the method according to the invention. Characteristically, said aqueous anti-corrosion solution 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 [MnCh'] greater than or equal to 0.18 mol / L and less than or equal to 0.32 mol / L and preferably greater than or equal to 0.23 mol / L and less than or equal to 0.29 mol / L and in particular equal to 0.25 mol / L, dihydrogen phosphate ions at a molar concentration [hhPCh'] 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 and phosphoric acid in solution.

[0030] Said aqueous anti-corrosion solution of the invention may also contain ions chosen from potassium, ammonium, sodium, calcium ions and their mixtures. These ions come from the use of one or more dihydrogen phosphate salts. Advantageously, whatever the method of implementing the method of the invention, said aqueous anti-corrosion solution of the invention is, before immersion of said part, made up of water, potassium K ions + , MnCh' permanganate ions and PO4 phosphate ions 3 '.

[0031] Optionally, the aqueous anti-corrosion solution of the invention can be applied locally to parts to be treated. For example, a pad, roller or brush can be used to deposit the solution on the parts by contact. Local application of the aqueous solution thus makes it possible to retouch parts without the need to completely immerse them in a bath.

[0032] Thus, according to a preferred embodiment, said aqueous anti-corrosion solution contains a concentration of potassium dihydrogen phosphate equal to or greater than 55g / L and equal to or less than 95g / L, preferably equal to or greater than 67g / L and equal to or less than 83g / L, a quantity of potassium permanganate equal to or greater than 30g / L and equal to or less than 50g / L, preferably equal to 36g / L and equal to or less than 44g / L and a quantity of phosphoric acid necessary to obtain 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.

[0033] According to a third aspect, the present invention relates to a magnesium alloy part comprising on its surface an anti-corrosion layer and which can be obtained according to the method of the invention. Typically, said anti-corrosion layer contains manganese oxide and at least one phosphate, in particular a magnesium phosphate, and it has a thickness of less than 5 μm.

[0034] 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.

[0035] The alloy of said part is not limited according to the invention. For example, said part may be made of a magnesium alloy chosen from alloys of magnesium and at least one rare earth and optionally containing at least one metal chosen from aluminum, zinc, silicon, manganese, copper, zirconium and mixtures thereof and in particular from magnesium alloys containing aluminum, neodymium, gadolinium, zinc and zirconium. In particular, the alloy may contain or consist of magnesium, aluminum, neodymium, gadolinium and zirconium. The part may be made of EV31 A alloy, for example, or include a layer or a portion of EV31 A alloy.

[0036] The anti-corrosion layer may be electrically conductive and have a standard potential difference AE relative to the potential of a reference electrode greater than 1 V and in particular greater than or equal to 1.1 V; 1.2V 1.3V or 1.4V and / or resist at least 20 hours and in particular 24 hours to corrosion in neutral salt spray according to standard ISO 9227 and / or have a grade 0 or 1 for adhesion to paint according to standard ISO 2409.

[0037] Definitions

[0038] The term "phosphate" refers to a part of the constituents of phosphoric acid, dihydrogen phosphate ions, hydrogen phosphate ions, phosphate ions and their mixtures.

[0039] Other characteristics and advantages of the invention will emerge from reading the description which follows, with reference to the appended drawings, among which:

[0040] Figures 1 and 2 represent two test pieces treated with anti-corrosion treatment and then subjected for 24 hours to the so-called neutral salt spray corrosion test. The test piece in Figure 1 was treated according to the method of the invention and has a treatment layer having a thickness of less than 5 pm and for example between 0.5 pm and 1 pm, while the test piece in Figure 2 was treated by etching with Cr6+ ions.

[0041] EXAMPLES Example 1: particular method of implementing the method of the invention

[0042] Preparation of the aqueous anti-corrosion solution

[0043] In practice, for an industrial application of the treatment process, a solution of several hundred, or even thousands, of liters can be prepared.

[0044] As an illustrative example, a mass of potassium permanganate is dissolved in 1000 L (liters) of water, making it possible to obtain a molar concentration of permanganate ion equal to [MnO4'] = 0.25 mol / l. Potassium dihydrogen phosphate KH2PO4 is then added to obtain a mass concentration of [KH2PO4] of between 65 g / l and 85 g / l, and preferably of between 70 g / l and 80 g / l, or even equal to 75 g / l, and phosphoric acid H3PO4 or an aqueous solution of phosphoric acid so as to obtain an aqueous solution whose pH is greater than or equal to 3.4 and less than or equal to 3.6 and in particular equal to 3.5. This pH value range must be maintained throughout the treatment of the part by adding phosphoric acid H3PO4 or an aqueous solution of phosphoric acid.

[0045] Part treatment

[0046] The temperature of the above-mentioned aqueous anti-corrosion solution is measured. It is brought to 60°C if its temperature is different from this value. The part to be treated is then immersed in the above-mentioned aqueous anti-corrosion solution for a period of time greater than or equal to 2 min and less than or equal to 5 min. Throughout the immersion period, the pH of the anti-corrosion solution is monitored and kept within the above-mentioned range of values ​​by adding phosphoric acid H3PO4 or an aqueous phosphoric acid solution. When the treatment period has elapsed, the part is removed from the aqueous anti-corrosion solution and rinsed twice with deionized water.

[0047] Study of the properties of the anti-corrosion layer formed

[0048] The formed layer is visible as it has a brown color (light / golden or matte to dark brown). The thickness of the layer is less than 5pm.

[0049] An electrochemical study allowed to compare the corrosion potentials (Ecorr) of anticorrosion layers obtained according to different treatments available, both on the market and those obtained for chromium VI etching. The corrosion density (jcorr) and the annual corrosion e (pm / year) were also studied. In addition, the test medium for measuring the corrosion potentials (Ecorr) can be a saline solution with a mass concentration of sodium chloride equal to [NaCI]=50g / l for example.

[0050] The results obtained are grouped in Table 1 below.

[0051] [Table 1] EV31 A alloy is an alloy with the theoretical formula (by mass): (Mg-3Nd-1.5Gd-0.3Zn-0.5Zr).

[0052] As a reminder, the closer the Ecorr value is to zero, the more the material is protected from corrosion.

[0053] It is noted from the results in Table 1 that the anti-corrosion layer formed according to the method of the invention has the best protection against corrosion, even compared to that of the layer obtained by etching with chromate ions or hexavalent chromium. It is also noted that the corrosion rate of the anti-corrosion layer obtained by the method of the invention is the lowest. Finally, it is also noted that the annual corrosion is the lowest for the anti-corrosion layer obtained according to the method of the invention.

[0054] The difference AE between the standard potential of the formed anti-corrosion layer and that of the alloy is 1.3V + - 0.1V.

[0055] The anti-corrosion layer formed by the method of the invention is electrically conductive.

[0056] The corrosion resistance of the anti-corrosion layer obtained according to the process of the invention to a neutral salt spray according to the ISO 9227 standard was measured. The part treated according to the process of the invention resists 24 hours before the appearance of the first corrosion pits.

[0057] Figure 2 clearly shows that the specimen treated by Cr6+ ion etching and subjected to the neutral salt spray corrosion test for 24 hours has a completely corroded surface. On the contrary, the specimen treated according to the method of the invention, illustrated in Figure 1, has a slightly corroded surface after 24 hours of the same test. The corrosion resistance of the anti-corrosion layer obtained according to the method of the invention covered with a varnish to a neutral salt spray according to standard ISO 9227 was also measured. The part of the invention covered with varnish resists more than 250 hours before the appearance of the first corrosion pits.

[0058] The corrosion resistance of the anti-corrosion layer obtained according to the method of the invention, covered with a paint system comprising a primer and a neutral salt spray finish according to ISO 9227 standard, was also measured. The part of the invention covered with paint resists more than 1000 hours before the appearance of the first corrosion pits.

[0059] The adhesion of varnishes and paints was tested according to the ISO 2409 standard. It turns out that the anti-corrosion layer obtained according to the method of the invention has a grade 0 for adhesion to paint according to the aforementioned standard. After humid aging, and still according to the aforementioned standard, the anti-corrosion layer obtained according to the method of the invention has a grade 1 for adhesion to paint (according to the ISO 2409 standard). The aging cycle has a duration of 24 hours which is broken down as follows: 6 hours of exposure in a humid environment: 80% RH, 40°C, then immersion of 15m in a saline solution (5% NaCl) and the rest of the time, drying cycle in ambient air.

[0060] Accelerated aging tests in a climatic chamber

[0061] Furthermore, aging cycles as indicated in the previous paragraph on the EV31 A alloy treated with anti-corrosion treatment reveal the presence of first corrosion pits after 2 cycles of 24 hours for a part treated by chromium etching whereas after the fifteenth cycle of 24 hours no pitting is detected in the case of a part comprising an anti-corrosion layer obtained according to the process of the invention and having a thickness of less than 5 μm.

[0062] 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 possible to replace a means described by an equivalent means without departing from the scope of the present invention.

Claims

CLAIMS 1 . Method for anti-corrosion treatment of a magnesium alloy part according to which said part is immersed for a given period in an aqueous anti-corrosion solution having a given temperature and containing permanganate ions Mn04- and dihydrogen phosphate ions H2PO4-, characterized in that said aqueous anti-corrosion solution contains, before immersion of said part, a molar concentration of permanganate ions [Mn04-] greater than or equal to 0.18 mol / L (mole per liter) and less than or equal to 0.32 mol / L, a molar concentration of dihydrogen phosphate ions [H2PO4-] greater than or equal to 0.47 mol / L (mole per liter) and less than or equal to 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L, in that the pH of said aqueous anti-corrosion solution is maintained during said given period at 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,by adding phosphoric acid and in that after said given time, said part is removed from said aqueous solution and rinsed with water., 2. Method according to claim 1, characterized in that said aqueous anti-corrosion solution contains, before immersion of said part, a molar concentration of permanganate ions [MnO4-] greater than or equal to 0.23 mol / L and less than or equal to 0.29 mol / L and in particular equal to 0.25 mol / L.

3. Method according to any one of the preceding claims, characterized in that said given duration is greater than or equal to 1 min and less than or equal to 10 min and preferably equal to or greater than 2 min and less than or equal to 5 min.

4. Method according to any one of the preceding claims, characterized in that said given temperature is greater than or equal to 55°C and less than or equal to 65°C and preferably equal to 60°C. Method according to any one of the preceding claims, characterized in that, during said immersion of said part, said aqueous anti-corrosion solution is stirred.

5. Method according to any one of the preceding claims, characterized in that said part is made of a magnesium alloy chosen from among alloys of magnesium and at least one rare earth and optionally containing at least one metal chosen from among aluminum, zinc, silicon, manganese, copper, zirconium and their mixtures and in particular from among magnesium alloys containing aluminum, neodymium, gadolinium, zinc and zirconium.

6. Method according to any one of the preceding claims, characterized in that before immersing said part in said aqueous anti-corrosion solution, at least one pre-treatment of said part is carried out, 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; - sodium bifluoride pickling; - sandblasting followed by alkaline degreasing; - stripping of residues from magnesium treatments; - contacting simultaneously or successively with at least one compound chosen from soda, hydrogen peroxide, acetic acid, sulfuric acid, hydrofluoric acid, sodium bifluoride, ammonium bifluoride, phosphoric acid, tartaric acid, hydrochloric acid.

7. Aqueous anti-corrosion solution allowing the implementation of the method according to any one of claims 1 to 8, characterized 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 in a molar concentration [MnO4-] greater than or equal to 0.18 mol / L and less than or equal to 0.32 mol / L and preferably greater than or equal to 0.23 mol / L and less than or equal to 0.29 mol / L and in particular equal to 0.25 mol / L, dihydrogen phosphate ions [H2PO4-] in a molar concentration greater than or equal to 0.47 mol / L (mole per liter) and less than or equal to 0.63 mol / L, and preferably between 0.51 mol / L and 0.59 mol / L and phosphoric acid in solution.

8. Magnesium alloy part comprising on its surface an anti-corrosion layer and obtainable according to the method according to any one of claims 1 to 7, characterized in that said anti-corrosion layer contains manganese oxide and at least one phosphate, in particular a magnesium phosphate and in that it has a thickness of less than 5 pm.

9. Magnesium alloy part according to claim 9, characterized in that said part is made of magnesium alloy chosen from alloys of magnesium and at least one rare earth and optionally containing at least one metal chosen from aluminum, zinc, silicon, manganese, copper, zirconium and their mixtures and in particular from magnesium alloys containing aluminum, neodymium, gadolinium, zinc and zirconium.

10. Part according to claim 9 or 10, 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 1 V and in particular greater than or equal to 1.1 V; 1.2V, 1.3V or 1.4V and / or in that it resists at least 20 hours and in particular 24 hours to corrosion in neutral salt spray according to standard ISO 9227 and / or in that said layer has a grade 0 or 1 for adhesion to paint according to standard ISO 2409.

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