Aluminum Alloy Part and Associated Manufacturing Method

The invention relates to the technical field of aluminum alloy parts and in particular to parts that have undergone an anodic oxidation step. In particular, the invention concerns parts that have undergone anodic oxidation, impregnation and sealing steps.

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

In the aerospace industry, it is common practice to use aluminum alloy parts. The aluminum alloys offer an excellent mechanical properties/weight ratio and are relatively inexpensive to manufacture. However, depending on the environment in which they are found, these parts are likely to be affected by several types of localised corrosion, causing the degradation of the part and leading to its removal or failure. In order to improve the resistance to corrosion of the titanium alloy parts, it is known to carry out a surface treatment of the part.

The surface treatment typically comprises a chromic anodic oxidation step on the part to form a surface layer of aluminum oxide with a thickness of between 2 μm and 15 μm. As the aluminum oxide layer is porous, the anodic oxidation step is typically followed by a sealing step that allows the pores in the aluminum oxide layer to be filled. This allows the part's corrosion resistance to be improved.

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. Yet the hexavalent chromium is a compound that is prohibited by the new health and environmental standards.

In order to be free from the hexavalent chromium, the document FR-B1-3106 838 proposes a surface treatment for an anodised 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. In this way, each of the solutions for the impregnation and sealing steps uses compounds that are alternatives to the hexavalent chromium and comply 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.

The present invention offers a new solution for providing a part that is resistant to corrosion while complying with safety and environmental standards.

To this end, the invention proposes a part comprising:

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 was also found that this aluminum layer could give the part better paint adhesion.

As a result of the invention, the part is corrosion resistant, has good paint adhesion properties and can be manufactured in compliance with health and environmental standards.

The invention may comprise one or more of the following characteristics, taken alone or in combination with each other:

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

According to the invention, the step of sealing the aluminum alloy part is carried out using a sealing solution comprising an aluminate. It has been shown that an anodised aluminum alloy part impregnated with chromium/zirconium oxide (CrIII/Zr), for example, and then sealed with an aluminate, has good corrosion resistance, which is at least equivalent to a part sealed with silicate. In addition, the aluminate sealing allows the part to adhere better to the paint. It was also found that the aluminate sealing improves the breakdown voltage of the method. The treatment is more resistant to electrical stress.

The method also allows you to be hexavalent chromium-free and is therefore compatible with safety and environmental standards.

The invention may comprise one or more of the following characteristics, taken alone or in combination with each other:

A partaccording to one embodiment of the invention is illustrated in. The partaccording to the invention comprises an aluminum alloy body. The aluminum alloy is for example selected from the 2000 series such as alloy 2014, 2017, 2024, 2214, 2219, 2618, or the series 6000 such as the alloy 6061, 6063, or the series 7000 such as the alloy 7010, 7020, 7050, 7055, 7068, 7075, 7085, 7175, 7475. According to another example, the aluminum alloy is a cast alloy of the AS7G06, AS7G03, AS10G or AS9U3 type.

The partalso comprises a first layerarranged on the body. The first layercomprises aluminum oxide. The first layerhas a thickness of between 2 μm and 30 μm, preferably between 5 μm and 25 μm. The first layeris obtained by anodic oxidation of the part.

The partaccording to the invention also comprises an intermediate layercomprising chromium. The thickness of the intermediate layeris between 1 μm and 10 μm, for example between 3 μm and 5 μm. The intermediate layeris obtained by impregnating the partwith a solution comprising a trivalent chromium salt and an oxidising compound.

The partaccording to the invention also comprises a second layer. The intermediate layeris arranged between the first and second layers,. The second layercomprises aluminum oxide. The second layerhas a thickness of between 10 nm and 500 nm, preferably 200 nm. The second layeris obtained by sealing the partwith an aluminate. The second layercovers the intermediate layerand has its own thickness.

Preferably, the partalso comprises a paint layer. The paint layeris arranged on the second aluminum oxide layer. The paint layerhas a thickness of between 10 μm and 100 μm, preferably 50 μm.

The partaccording to the invention has good corrosion resistance and good paint adhesion. The combination of the intermediate layerand the second aluminum oxide layergives the partgood corrosion resistance. In addition, the second aluminum oxide layerpromotes adhesion of the paint layer.

A method for manufacturing the partwill now be described. The manufacturing method comprises the following chronological steps of:

Where applicable, the aqueous compositions/solutions are completed with water up to 100% of their volume.

It is advantageous that the degreasing step (b) dissolves any grease present on the surface of the body. The degreasing step can be carried out using an aqueous alkaline degreasing solution. The aqueous alkaline degreasing solution comprises, for example, SOSOCLEAN A3432 at a concentration by volume 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. The step (b) can be carried out by soaking, spraying or any other technique known to the skilled in the art.

The pickling step (c) advantageously allows any oxides formed on the surface of bodyto be dissolved. The pickling step can be carried out using an aqueous alkaline pickling solution. The aqueous alkaline pickling solution comprises, for example, a mixture of SOCOSURF A1858 at a concentration by volume of, for example, 42% and SOCOSURF A1806 at a concentration by volume of, for example, 10% (volume/volume). The temperature of the alkaline aqueous 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. The step (c) can be carried out by soaking, spraying or any other technique known to the skilled person.

The step (d) according to the invention allows the first layerof aluminum oxide to be created. At the end of this step, the first layerof aluminum oxide is porous. The anodic oxidation is advantageously a sulfuric anodic oxidation. During this step, the bodyis immersed in a tank comprising a sulfuric anodic oxidation solution comprising sulfuric acid in 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 sulphuric anodic oxidation solution is at a temperature of between 10° C. and 20° C., typically 18° C. The tank also comprises an anode connected to the body. The anode and the bodyare typically connected to a current generator. The bodyis subjected to a current density of between 1 A/dmand 5 A/dm, typically 2 A/dm. The bodyis subjected to a DC voltage of between 10 V and 30 V, for example, typically between 15 V and 20 V. The duration of step (d) is between 10 min and 120 min, for example 60 min.

The impregnation step (e) is advantageously carried out by immersing the bodyin the impregnating solution. The impregnating solution is an aqueous solution. According to the invention, the impregnating solution comprises a trivalent chromium salt and an oxidising compound. In the present invention, a trivalent chromium is taken to mean chromium in the +3 oxidation state.

The trivalent chromium salt may be in the form of fluoride, chloride, nitrate, acetate, acetate hydroxide, sulphate, potassium sulphate, etc, of trivalent chromium, for example CrF3,xH2O, CrCl3,xH2O, Cr(NO3)3,xH2O, (CH3CO2)2Cr,xH2O, (CH3CO2)7Cr3(OH)2,xH2O, Cr2(SO4)3,xH2O, CrK(SO4)2,xH2O.

The oxidising compound according to the invention is preferably a hexafluorozirconate salt. It is, for example, selected from the group consisting of ammonium hexafluorozirconate ((NH4)2ZrF6), sodium hexafluorozirconate (Na2ZrF6) and potassium hexafluorozirconate (K2ZrF6).

Advantageously, the impregnating solution has a mass concentration of trivalent chromium salt of between 0.5 g/L and 50 g/L and a mass concentration of oxidising compound of between 0.5 g/L and 50 g/L.

Advantageously, the impregnating solution has a temperature of between 30° C. and 50° C., preferably 40° C.

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

Preferably, the impregnating solution comprises SOCOSURF TCS at a volume concentration of 34% (volume/volume).

Preferably, the step (e′) is carried out with a post-impregnating solution. The step (e′) is carried out, for example, by immersing the bodyin the post-impregnating solution. The post-impregnating solution is an aqueous solution comprising, for example, hydrogen peroxide and a lanthanum salt. The lanthanum salt is, for example, a lanthanum carbonate (La2(CO)).

Preferably, the impregnating solution comprises SOCOSURF PACS at a volume concentration of 10% (volume/volume). The temperature of the impregnating 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.

The steps (e) and optionally (e′) form the intermediate layerand fill the pores in the aluminum oxide layer formed in step (d).

According to the invention, in step (f), the sealing solution is an aqueous solution comprising an aluminate. The aluminate is, for example, a sodium (NaAlO), potassium (KAlO) or lithium (LiAlO) aluminate. Preferably, the aluminate is a sodium aluminate (NaAlO). 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 of 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 forms the second layer.

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

The impregnation and sealing steps fill the pores in the aluminum oxide layerand improve the corrosion resistance of the part. As the sealing step is carried out using an aluminate, the corrosion resistance of the partis improved and paint adhesion is enhanced.

Parts made from 2618 T6 aluminum alloy and parts made from 2024 T3 aluminum alloy were subjected to degreasing, pickling, sulphuric anodising, impregnation and post-impregnation steps under the conditions set out in Table 1. Various sealing steps were then carried out in accordance with the conditions set out in Table 2, in particular an aluminate sealingin accordance with the invention, a silicate sealingin accordance with the prior art and a water sealingin accordance with the prior art.

The parts were then subjected to a salt mist test in accordance with the standard NF EN ISO 9227. The results are shown in Table 3.

It can be seen that the sealingandincreases the corrosion resistance of parts compared with the sealing. The sealing of the silicate and aluminate can therefore improve the corrosion resistance of aluminum alloy parts. The aluminate sealing therefore gives the part good corrosion properties at least equivalent to silicate sealing.

The parts that had undergone sealing stepand sealing stepwere subjected to X-ray photoelectron spectrometry (XPS) analysis.

The results are presented in Table 4.

The elemental analysis of the extreme surface of the parts over a thickness of 50 nm shows the presence of aluminum oxide (presence of aluminum and oxygen). This layer of aluminum oxide on the surface improves the corrosion resistance of the part, as demonstrated in Example 1, compared with water sealing and paint adhesion.