Aluminum alloy part and associated manufacturing process

Abstract

The invention relates to a part (1) comprising: - an aluminum alloy body (10), - a first layer (12) that contains aluminum oxide and is arranged on the body (10), - an intermediate layer (14) containing chromium, and - a second layer (16) containing aluminum oxide, the intermediate layer (14) being arranged between the first layer (12) and the second layer (16).

Description

[0001] DESCRIPTION

[0002] TITLE: ALUMINUM ALLOY PART AND ASSOCIATED MANUFACTURING METHOD

[0003] Technical field of the invention

[0004] The invention relates to the technical field of aluminum alloy parts and in particular, parts having undergone an anodic oxidation step. In particular, the invention relates to parts having undergone anodic oxidation, impregnation and sealing steps.

[0005] The invention also relates to the field of methods for manufacturing aluminum alloy parts, including in particular steps of anodic oxidation, impregnation and sealing.

[0006] Technical background

[0007] In the aeronautics industry, it is common to use aluminum alloy parts. Indeed, aluminum alloys offer an excellent mechanical properties / weight ratio and their manufacturing cost is relatively low. However, these parts are likely, depending on the environment in which they are located, to be affected by several types of localized corrosion, causing the degradation of the part and possibly leading to its shrinkage or failure. In order to improve the corrosion resistance of titanium alloy parts, it is known to carry out a surface treatment of the part.

[0008] The surface treatment typically includes a chromic anodic oxidation step of the part which forms a surface layer of aluminum oxide with a thickness of between 2 μm and 15 μm. Since the aluminum oxide layer is porous, the anodic oxidation step is typically followed by a sealing step to fill the pores of the aluminum oxide layer. This improves the corrosion resistance of the part. The anodic oxidation step is typically carried out by immersing the part in a chromic acid solution and the sealing step of the part is carried out by immersing the part in a solution comprising hexavalent chromium. However, hexavalent chromium is a compound that is prohibited by new health and environmental standards.

[0009] In order to avoid hexavalent chromium, document FR-B1-3 106 838 proposes a surface treatment of an anodized aluminum alloy part comprising the steps of impregnating the part by immersing the part in a solution comprising a hexafluorozirconate salt and a trivalent chromium salt and sealing the part by immersing the part in a solution comprising a silicate. Thus, each of the solutions of the impregnation and sealing steps uses alternative compounds to hexavalent chromium and complying with safety and environmental standards. A part is then formed comprising an aluminum alloy body coated with a first layer comprising aluminum oxide, an intermediate layer comprising chromium and a layer comprising silicate.

[0010] The present invention provides a novel solution to provide a part that is corrosion resistant while complying with safety and environmental standards.

[0011] Summary of the invention

[0012] For this purpose, the invention proposes a part comprising:

[0013] - an aluminum alloy body,

[0014] - a first layer comprising aluminum oxide, arranged on the body,

[0015] - an intermediate layer comprising chromium, and

[0016] - a second layer comprising aluminum oxide, the intermediate layer being arranged between the first layer and the second layer. The part according to the invention is therefore remarkable in that it has a second layer of aluminum oxide. This second layer of aluminum, rich in oxygen and aluminum, gives the part very good corrosion resistance properties. It has also been found that this layer of aluminum could give the part better paint adhesion.

[0017] Therefore, thanks to the invention, the part is corrosion resistant, has good paint adhesion properties and can be manufactured in accordance with health and environmental standards.

[0018] The invention may comprise one or more of the following features, taken in isolation from each other or in combination with each other:

[0019] - the second layer has a thickness between 10 nm and 500 nm,

[0020] - a layer of paint arranged on the second layer,

[0021] - the second layer is obtained by sealing the body in a sealing solution comprising an aluminate.

[0022] The invention also relates to a method of manufacturing a part comprising the following chronological steps:

[0023] (a) provision of an aluminium alloy body,

[0024] (d) anodic oxidation of the body,

[0025] (e) impregnating the body with an impregnating solution comprising a trivalent chromium salt and an oxidizing compound, and

[0026] (f) sealing the body with a sealing solution comprising an aluminate.

[0027] According to the invention, the step of sealing the aluminum alloy part is carried out using a sealing solution comprising an aluminate. Indeed, it has been demonstrated that an anodized aluminum alloy part impregnated with chromium / zirconium oxide (Crlll / Zr) for example and then sealed with an aluminate has good corrosion resistance, which is at least equivalent to a part sealed with silicate. Furthermore, the aluminate sealing improves the adhesion of the part to the paint. It has also been found that the aluminate sealing improves the breakdown voltage of the process. The treatment is more resistant to electrical stresses.

[0028] Also, the process of the invention makes it possible to do without hexavalent chromium and is therefore compatible with safety and environmental standards.

[0029] The invention may comprise one or more of the following features, taken in isolation from each other or in combination with each other:

[0030] - the sealing solution has a mass concentration of aluminate between 0.1 g / L and 10 g / L, preferably between 0.5 g / L and 5 g / L,

[0031] - aluminate is sodium aluminate,

[0032] - in step (d), the body is immersed in a tank comprising a sulfuric anodic oxidation solution comprising sulfuric acid at a mass concentration of between 150 g / L and 250 g / L, preferably 180 g / L,

[0033] - the impregnation solution has a temperature between 30°C and 50°C, preferably 40°C,

[0034] - between steps (e) and (f), a step (e') of post-impregnation of the body with a post-impregnation solution comprising hydrogen peroxide and a lanthanum salt,

[0035] - the oxidizing compound is a hexafluorozirconate salt, *

[0036] - the sealing solution has a pH between 7 and 12, preferably 11, -- the sealing solution has a temperature between 50°C and 150°C, preferably between 80°C and 100°C,

[0037] -- step (f) is carried out for a period of between 5 min and 60 min, preferably between 10 min and 20 min,

[0038] -- step (d) is a sulfuric anodic oxidation,

[0039] -- step (e) is carried out for a period of between 5 min and 60 min, preferably between 10 min and 20 min,

[0040] - in step (f), the sealing solution comprises an aluminate prior to oxidation, impregnation, post-impregnation and sealing of the body,

[0041] - in step (e'), the temperature of the post-impregnation solution is between 15°C and 30°C, preferably 25°C,

[0042] - the duration of step (e') is between 2 min and 30 min, preferably 5 min.

[0043] - in step (e'), the body is immersed in the post-impregnation solution.

[0044] Brief description of the figures

[0045] Other characteristics and advantages will emerge from the following description of a non-limiting embodiment of the invention with reference to the appended drawings in which: Figure 1 is a schematic cross-sectional representation of a part according to the invention; Figure 2 is a schematic representation of a method according to an embodiment of the invention.

[0046] Detailed description of the invention

[0047] A part 1 according to one embodiment of the invention is illustrated in Figure 1. The part 1 according to the invention comprises a body 10 made of aluminum alloy. The aluminum alloy is for example chosen from the 2000 series such as alloy 2014, 2017, 2024, 2214, 2219, 2618, or the 6000 series such as alloy 6061, 6063, or the 7000 series such as alloy 7010, 7020, 7050, 7055, 7068, 7075, 7085, 7175, 7475. According to another example, the aluminum alloy is a casting alloy type AS7G06, AS7G03, AS10G or even AS9U3.

[0048] The part 1 further comprises a first layer 12 arranged on the body 10. The first layer 12 comprises aluminum oxide. The first layer 12 has a thickness of between 2 μm and 30 μm, preferably between 5 μm and 25 μm. The first layer 12 is obtained by anodic oxidation of the part 1.

[0049] The part 1 according to the invention further comprises an intermediate layer 14 comprising chromium. The thickness of the intermediate layer 14 is between 1 μm and 10 μm, for example between 3 μm and 5 μm. The intermediate layer 14 is obtained by impregnating the part 1 with a solution comprising a trivalent chromium salt and an oxidizing compound.

[0050] The part 1 according to the invention further comprises a second layer 16. The intermediate layer 14 is arranged between the first and second layers 12, 16. The second layer 16 comprises aluminum oxide 16. The second layer 16 has a thickness of between 10 nm and 500 nm, preferably 200 nm. The second layer 16 is obtained by sealing the part 14 with an aluminate. The second layer 16 covers the intermediate layer 14 and has its own thickness.

[0051] Preferably, the part 1 further comprises a layer of paint 18. The layer of paint 18 is arranged on the second layer of aluminum oxide 16. The layer of paint 18 has a thickness of between 10 μm and 100 μm, preferably 50 μm.

[0052] The part 1 according to the invention has good corrosion resistance and good adhesion to paint. Indeed, the combination of the intermediate layer 14 and the second layer 16 of aluminum oxide provides good corrosion resistance to the part 1. In addition, the second layer 16 of aluminum oxide promotes the adhesion of the paint layer 18.

[0053] A method of manufacturing the part 1 will now be described. The manufacturing method comprises the following chronological steps: (a) providing the body 10 of aluminum alloy,

[0054] (b) optionally, degreasing the body 10,

[0055] (c) optionally, stripping the body 10,

[0056] (d) anodic oxidation of body 10,

[0057] (e) impregnating the body 10 with an impregnation solution comprising a hexafluorozirconate salt and a trivalent chromium salt,

[0058] (e') optionally, post-impregnation of the body 10,

[0059] (f) sealing the body 10 with a sealing solution comprising an aluminate,

[0060] (g) optionally, application of paint to the body 10. Where applicable, aqueous compositions / solutions are made up to 100% of their volume with water.

[0061] The degreasing step (b) advantageously makes it possible to dissolve any greases that may be present on the surface of the body 10. The degreasing step can be carried out with an aqueous alkaline degreasing solution. The aqueous alkaline degreasing solution comprises, for example, SOSOCLEAN A3432 at a volume concentration of, for example, 11% (volume / volume). The temperature of the aqueous alkaline degreasing solution is, for example, between 30°C and 50°C, typically 45°C. The duration of step (b) is, for example, between 1 min and 30 min, typically 10 min. Step (b) can be carried out by soaking, spraying, or any other technique known to those skilled in the art.

[0062] The pickling step (c) advantageously makes it possible to dissolve any oxides formed on the surface of the body 10. The pickling step can be carried out with an aqueous alkaline pickling solution. The aqueous alkaline pickling solution comprises, for example, a mixture of SOCOSURF A1858 at a volume concentration of, for example, 42% and SOCOSURF A1806 at a volume concentration of, for example, 10% (volume / volume). The temperature of the aqueous alkaline pickling solution is, for example, between 30°C and 100°C, typically 50°C. The duration of step (c) is, for example, between 1 min and 30 min, typically 10 min. Step (c) can be carried out by dipping, spraying, or any other technique known to those skilled in the art.

[0063] Step (d) according to the invention makes it possible to create the first layer 12 of aluminum oxide. At the end of this step, the first layer of aluminum oxide 12 is porous. The anodic oxidation is advantageously a sulfuric anodic oxidation. During this step, the body 10 is immersed in a tank comprising a sulfuric anodic oxidation solution comprising sulfuric acid at a mass concentration of between 150 g / L and 250 g / L, preferably 180 g / L. The solution is in particular an aqueous solution. Preferably, the sulfuric anodic oxidation solution is at a temperature of between 10°C and 20°C, typically 18°C. The tank further comprises an anode connected to the body 10. The anode and the body 10 are typically connected to a current generator. The body 10 is subjected to a current density of between 1 A / dm 2 and 5 A / dm 2 , typically 2 A / dm 2The body 10 is subjected to a direct voltage, for example between 10 V and 30 V, typically between 15 V and 20 V. The duration of step (d) is for example between 10 min and 120 min, for example 60 min.

[0064] The impregnation step (e) is advantageously carried out by immersing the body 10 in the impregnation solution. The impregnation solution is an aqueous solution. According to the invention, the impregnation solution comprises a trivalent chromium salt and an oxidizing compound. In the present invention, trivalent chromium is understood, in a known manner, to mean chromium in the +3 oxidation state.

[0065] The trivalent chromium salt may be in the form of fluoride, chloride, nitrate, acetate, acetate hydroxide, sulfate, potassium sulfate, etc., of trivalent chromium, for example CrF3,xH2O, CrCI3,xH2O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3(OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O. The oxidizing compound according to the invention is preferably a hexafluorozirconate salt. It is for example chosen from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6), potassium hexafluorozirconate (K2ZrF6). Advantageously, the impregnation solution has a mass concentration of trivalent chromium salt of between 0.5 g / L and 50 g / L and a mass concentration of oxidizing compound of between 0.5 g / L and 50 g / L.

[0066] Advantageously, the impregnation solution has a temperature between 30°C and 50°C, preferably 40°C.

[0067] Advantageously, the duration of step (e) is between 5 min and 60 min, preferably between 10 min and 20 min.

[0068] Preferably, the impregnation solution comprises SOCOSURF TCS at a volume concentration of 34% (volume / volume).

[0069] Preferably, step (e') is carried out with a post-impregnation solution. Step (e') is for example carried out by immersing the body 10 in the post-impregnation solution. The post-impregnation solution is an aqueous solution comprising for example hydrogen peroxide and a lanthanum salt. The lanthanum salt is for example a lanthanum carbonate (La2(COs)3).

[0070] Preferably, the impregnation solution comprises SOCOSURF PACS at a volume concentration of 10% (volume / volume). The temperature of the impregnation solution is for example between 15°C and 30°C, typically 25°C. The duration of step (e') is for example between 2 min and 30 min, typically 5 min.

[0071] Steps (e) and optionally (e') make it possible to form the intermediate layer 14 and to fill the pores of the aluminum oxide layer formed in step (d).

[0072] According to the invention, in step (f), the sealing solution is an aqueous solution comprising an aluminate. The aluminate is, for example, a sodium aluminate (NaAIO2), a potassium aluminate (KAIO2), a lithium aluminate (ÜAIO2). Preferably, the aluminate is a sodium aluminate (NaAlC ). The mass concentration of aluminate is between 0.1 g / L and 10 g / L, preferably between 0.5 g / L and 5 g / L. Preferably, the sealing solution has a temperature between 50°C and 150°C, preferably between 80°C and 100°C. Preferably, the duration of this step is between 5 min and 60 min, preferably between 10 min and 20 min. Preferably, the sealing solution has a pH of between 7 and 12, preferably 11. The sealing step makes it possible to form the second layer 16.

[0073] Advantageously, in step (g), the paint is applied mechanically, for example using a spray gun or manually using a brush.

[0074] The impregnation and sealing steps make it possible to fill the pores of the aluminum oxide layer 12 and to reinforce the corrosion resistance of the part 1. The sealing step being carried out with an aluminate, the corrosion resistance of the part 1 is improved and the adhesion of the paint is improved.

[0075] [Examples]

[0076] Example 1: Corrosion tests

[0077] Parts made of 2618 T6 aluminum alloy and parts made of 2024 T3 aluminum alloy were subjected to degreasing, pickling, sulfuric anodizing, impregnation and post-impregnation steps according to the conditions set out in Table 1. Then, different sealing steps were carried out according to the conditions set out in Table 2, in particular sealing 1 with aluminate according to the invention, sealing 2 with silicate according to the prior art and sealing 3 with water according to the prior art.

[0078] [Table 1]

[0079]

[0080] [Table 2]

[0081] The parts were then subjected to a salt spray resistance test in accordance with standard NF EN ISO 9227. The results are shown in Table 3.

[0082] [Table 3]

[0083] It is observed that sealants 1, 2 increase the corrosion resistance of the parts compared to sealant 3. Sealing with silicate and aluminate therefore improves the corrosion resistance of aluminum alloy parts. Sealing with aluminate therefore gives the part good corrosion properties at least equivalent to sealing with silicate.

[0084] Example 2: Extreme surface analysis

[0085] The parts that underwent sealing step 1 and sealing step 2 were subjected to X-ray photoelectron spectrometric (XPS) analyses.

[0086] The results are presented in Table 4. [Table 4]

[0087] Elemental analysis of the extreme surface of the parts over a thickness of 50 nm, demonstrates the presence of aluminum oxide (presence of aluminum and oxygen). This layer of aluminum oxide on the surface promotes the corrosion resistance of the part as demonstrated in example 1 compared to water clogging and adhesion to paint.

Claims

CLAIMS 1. Part (1) comprising: - a body (10) made of aluminum alloy, - a first layer (12) comprising aluminum oxide, arranged on the body (10), - an intermediate layer (14) comprising chromium, and - a second layer (16) comprising aluminum oxide, the intermediate layer (14) being arranged between the first layer (12) and the second layer (16).

2. Part according to the preceding claim, in which the second layer (16) has a thickness of between 10 nm and 500 nm.

3. Part according to any one of the preceding claims, comprising a layer of paint (18) arranged on the second layer (16).

4. Part according to any one of the preceding claims, in which the second layer is obtained by sealing the body (10) in a sealing solution comprising an aluminate.

5. Method of manufacturing a part (1) comprising the following chronological steps: (a) providing a body (10) made of aluminum alloy, (d) anodic oxidation of the body (10), (e) impregnating the body (10) with an impregnating solution comprising a trivalent chromium salt and an oxidizing compound, and (f) sealing the body (10) with a sealing solution comprising an aluminate.

6. Method according to the preceding claim, characterized in that the sealing solution has a mass concentration of aluminate of between 0.1 g / L and 10 g / L, preferably between 0.5 g / L and 5 g / L.

7. A method according to any one of claims 5 to 6, wherein the aluminate is sodium aluminate.

8. Method according to any one of claims 5 to 7, characterized in that in step (d), the body (10) is immersed in a tank comprising a sulfuric anodic oxidation solution comprising sulfuric acid at a mass concentration of between 150 g / L and 250 g / L, preferably 180 g / L.

9. Method according to any one of claims 5 to 8, in which the impregnation solution has a temperature between 30°C and 50°C, preferably 40°C.

10. Method according to any one of claims 5 to 9, further comprising, between steps (e) and (f), a step (e') of post-impregnation of the body (10) with a post-impregnation solution comprising hydrogen peroxide and a lanthanum salt.

11. Method according to any one of claims 5 to 10, characterized in that the oxidizing compound is a hexafluorozirconate salt.

12. Method according to any one of claims 5 to 11, in which the sealing solution has a pH of between 7 and 12, preferably 11.

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