WO2017207909A1

WO2017207909A1 - Chromating method and component obtained by this method

Info

Publication number: WO2017207909A1
Application number: PCT/FR2017/051337
Authority: WO
Inventors: Julien GURT SANTANACH; Alain Viola; Jean-Baptiste CAMBON; Rémy TURBAN; Jérémie Christian André COTINOT
Original assignee: Safran Helicopter Engines; Safran; Safran Transmission Systems; Safran Aircraft Engines
Priority date: 2016-05-30
Filing date: 2017-05-30
Publication date: 2017-12-07
Also published as: EP3464681A1; CN109312468A; FR3051805B1; FR3051805A1; US20200040461A1

Abstract

The invention relates to a method of chromating a component comprising an aluminium-based alloy and comprising the steps of supplying the component comprising, on at least one surface of the component, an aluminium oxide-based layer, immersing the component in a chromating bath comprising chrome III and carrying out a chemical conversion of the component in the chromating bath. The invention also relates to a component obtained by this method.

Description

CHROMATATION PROCESS AND PIECE OBTAINED BY THIS METHOD

Background of the invention

The present disclosure relates to the protection against corrosion of a part comprising an aluminum-based alloy.

The alloys based on aluminum have the advantage of being light. However, they may be susceptible to corrosion. Also, it is known to protect parts made from aluminum alloys against corrosion by performing, for example, a chemical conversion of the surface of the workpiece.

This chemical conversion treatment is generally carried out by putting the piece in contact with a bath containing hexavalent chromium (or chromium VI or Cr VI). The bath can be made from a solution such as the solution commonly known by the trademark Henkel Alodine® 1200S. This chemical conversion treatment is a chromating treatment of the aluminum-based alloy in which the alloy is converted to surface in order to precipitate aluminum oxy-hydroxides and aluminum chromates. This treatment makes it possible to produce on the surface of the part a coating which increases the resistance to corrosion as well as the wear resistance of the part made of aluminum-based alloy. Moreover, this coating makes it possible to maintain electrical conductivity of the coated zone and to allow easy and good quality adhesion of organic paints.

Naturally, the aluminum-based alloys which are exposed to air oxidize and form, on the surface of the room, a passivation layer of aluminum oxide (Al ₂ O ₃ ), also called alumina. This layer of aluminum oxide that naturally forms on the surface of the part is called a native oxide layer.

[0005] Also, before carrying out the chemical conversion treatment, the surface of the part is stripped in order to remove this passivation layer and to expose the aluminum-based alloy. This stripping makes it possible to put in contact the aluminum-based alloy and the solution comprising hexavalent chromium and thus to perform the chromium reaction of the part.

However, in application of the REACH regulation (abbreviation for "Registration, Evaluation, Authorization and Restriction of Chemicals"), the use of hexavalent chromium will be prohibited.

An alternative treatment proposes to use, in place of the solution comprising hexavalent chromium, a solution comprising trivalent chromium (or chromium III or Cr III). However, it has been observed that the corrosion resistance properties of the coating thus obtained are generally worse than the corrosion resistance properties of the coating obtained from the hexavalent chromium solution.

Chromatization process means a chemical conversion process of a metal part by immersion of the workpiece in a chromating bath and performing a chemical conversion of the workpiece in the chromating bath, the chromating bath comprising chromium, in the form of chromium VI or in the form of chromium III, for example.

Obiet and summary of the invention

The present disclosure aims to remedy at least in part these drawbacks.

For this purpose, the present disclosure relates to a method of chromating a workpiece comprising an aluminum-based alloy comprising the following steps:

- Providing the part comprising on at least one surface of the part a layer based on aluminum oxide whose thickness is greater than or equal to 5 nm;

immersion of the part in a chromating bath comprising chromium III;

- Performing a chemical conversion of the workpiece in the chromating bath.

Thanks to the presence of the aluminum oxide-based layer, during the realization of the chemical conversion of the part in the chromating bath, the formed coating has good resistance to corrosion. corrosion, especially salt spray. This layer based on aluminum oxide makes it possible to satisfactorily grow, on the surface of the part, a coating comprising chromium III and having desired anticorrosion properties.

Also, this chromating process, instead of wanting to expose the aluminum-based alloy before immersion in the chromating bath, as taught by all chromating processes, seeks to promote the presence of an oxide layer on the part before it is immersed in the chromating bath. It is therefore clear that the chromating bath does not include products for dissolving the aluminum oxide layer, as can be the case in conventional methods.

By aluminum-based alloy is meant an alloy whose average mass content of aluminum is predominant. It is understood that aluminum is the element whose mass content in the alloy is the highest. The aluminum-based alloy has, for example, a mass content of at least 50% aluminum, preferably at least 70% aluminum, even more preferably at least 80% aluminum.

By layer based on aluminum oxide (Al ₂ O 3) is meant a layer whose average mass content of aluminum oxide is predominant. It is understood that aluminum oxide is the compound whose mass content in the layer is the highest. The aluminum oxide-based layer has, for example, a mass content of at least 30% of aluminum oxide, preferably at least 40% of aluminum oxide, even more preferably at least 50% of aluminum oxide. aluminum oxide.

The chromating bath may comprise zirconium.

The presence of zirconium in the chromating bath, for example in the form of hexafluorozirconate, for example sodium hexafluorozirconate or potassium hexafluorozirconate, used to activate the chemical conversion reaction, forming in particular an Al-F complex with the aluminum at the interface between the part and the coating, the zirconium dissolved in solution then being deposited by reduction together with the chromium and forming a co-deposition of chromium III oxide and zirconium oxide. The aluminum oxide-based layer may be a native oxide layer and the part and the aluminum oxide-based layer may be degreased before immersion in the chromating bath.

The workpiece is generally machined to give it a shape close to the finished form. This machining operation is performed in the presence of a machining fluid, generally an oily fluid. Despite the presence of oily fluid on the part, a layer of native oxide forms on the surface of the piece. Indeed, this fluid allows oxidation of the aluminum-based alloy. The removal of this oily fluid is achieved by degreasing the part.

For example, the part and the aluminum oxide-based layer can be degreased with an aqueous solution and neutral for example.

It is also possible to use a solvent or a mixture of solvents under vacuum, for example perchlorethylene.

One can also consider using a mixture of azeotropic solvents at atmospheric pressure, such as a mixture of fluorinated ether and chlorocarbon.

This degreasing can be assisted by a method for generating ultrasound or cavitation.

This degreasing operation is performed so that the native oxide layer is retained on the workpiece.

The thickness of this native oxide layer may be greater than 5 nm, preferably greater than 10 nm, even more preferably greater than 15 nm, and less than 130 nm, preferably less than 120 nm, even more preferably lower. at 110 nm.

The aluminum oxide-based layer may be formed by a mechanical process.

It can thus accurately control the thickness of the aluminum oxide layer present on the workpiece.

Before applying the mechanical method of forming the aluminum oxide layer, one can degrease the room to clean any elements that pollute the surface piece.

The mechanical process may comprise a step of projecting solid particles onto the surface of the part. By projecting solid particles under pressure on the surface of the workpiece, the native oxide layer is removed and the aluminum-based alloy is exposed. This bare aluminum alloy is very reactive and a new layer of aluminum oxide is formed on all surfaces of the part on which the solid particles have been projected. This process is also known as "sanding".

The solid particles may be glass beads, for example silica beads, or ceramic balls, for example zirconia beads.

The mechanical process may comprise a step of abrasion of the surface of the workpiece with abrasive paper or a liquid containing abrasive particles.

With this method, as for sandblasting, it exposes the aluminum-based alloy and allows the formation of a new layer based on aluminum oxide on all surfaces of the piece that have suffered abrasion.

It is also possible to add an oxidizing chemical to the abrasive particles to promote the formation of the new layer based on aluminum oxide.

The aluminum oxide-based layer is formed by a chemical process comprising a first step of etching the surface of the part followed by a chemical or electrochemical oxidation step of the workpiece surface.

Moreover, thanks to this chemical process, it is possible to treat parts with complex geometry. Indeed, this chemical process uses solutions in liquid form which makes it possible to reach surfaces that could be difficult to reach by projection of solid particles or by abrasion.

For example, the chemical etching step may be carried out by spraying the part and the native oxide layer with an acidic or alkaline solution chemical solution. It is thus possible to dissolve the oxides present on the surface of the part. The piece is then dried. This chemical etching step may also be performed by immersing the part and the native oxide layer in an acidic or alkaline solution chemical solution.

This chemical etching step may be followed by a step of neutralizing the surface of the part before drying.

The chemical etching step is followed by a chemical or electrochemical oxidation step in order to promote the formation of the new layer based on aluminum oxide, for example by immersing the piece in a bath for a period of time. a time interval of 1 to 30 minutes.

The layer based on aluminum oxide may be porous.

Thus, the contact surface between the aluminum oxide layer and the chromating bath is greater than if the aluminum oxide-based layer is dense. This therefore facilitates the chromating process and increases its efficiency.

The aluminum oxide-based layer can be obtained by arranging the part in an atmosphere comprising between 30% to 100% humidity and / or the temperature of which is between 30 ° C. and 200 ° C., example during a time interval ranging from 5 minutes to 8 hours.

Whether for the formation of the native oxide layer or the formation of a new layer based on aluminum oxide, the humidity and / or the temperature can help promote the formation of the layer. based on aluminum oxide.

The present disclosure also relates to a part comprising an aluminum-based alloy having, on at least one surface of the part, a coating comprising chromium III, the coating comprising a free surface and having a given thickness, in which in a layer of the coating between 15 and 30% of the given thickness of the coating measured from the free surface, the intensity in arbitrary units measured by TOF-SIMS in aluminum oxide varies at most by more or less 50%, preferably of plus or minus 40%, even more preferably of plus or minus 30%.

It is understood that the intensity in arbitrary units is proportional to the concentration of alumina (molar or mass). This small variation in the content of aluminum oxide close to the free surface of the coating makes it possible to determine that the aluminum oxide content of the coating very rapidly reaches a high value, close to the maximum content of oxide of aluminum. aluminum in the coating. This high content of aluminum oxide close to the free surface of the coating is a trace of the aluminum oxide layer present on the part before immersion of the part in the chromating bath.

The thickness of the coating can be determined experimentally by determining the metallic aluminum content of the workpiece starting from the free surface of the workpiece. When the metal aluminum content is constant, it is considered that it is no longer in the coating but in the aluminum-based alloy.

TOF-SIMS is a mass spectrometry analysis of secondary ions in flight time, called TOF-SIMS according to the acronym for "Time Of Flight - Seconda ry-Ions Mass Spectrometry".

The coating may comprise chromium III in the form of chromium oxide (Cr ₂ O ₃ ) or chromium hydroxide (Cr (OH) ₃ ), for example.

The coating comprising chromium III may comprise zirconium.

Brief description of the drawings

Other features and advantages of the invention will emerge from the following description of embodiments of the invention, given by way of non-limiting examples, with reference to the appended figures, in which:

- Figure 1 is a partial schematic sectional view of an aluminum alloy part comprising on at least one surface of the part a layer based on aluminum oxide;

- Figure 2 is a schematic sectional view of a chromating device;

FIG. 3 is a partial schematic sectional view of the part of FIG. 1 after completion of the chromating process FIGS. 4 and 5 are graphical representations of the evolution of the chemical composition of a coating as a function of the thickness of the coating;

- Figures 6 to 8 illustrate results of corrosion tests of coatings obtained by chromating.

Detailed description of the invention

[0053] FIG. 1 represents a piece 10 made of alloy based on aluminum. This part 10 comprises on at least one surface of the part 10 a layer based on aluminum oxide 12. The part 10 and the layer based on aluminum oxide 12 form an oxidized part 14.

This aluminum oxide layer 12 may be a native oxide layer, formed naturally on the surface of the aluminum-based alloy part. This aluminum oxide layer 12 may also have been formed by a mechanical process or a chemical process.

FIG. 2 represents the step of immersing the oxidized part 114 in a tank 16 comprising the chromating bath 18 comprising chromium III. This chromating bath 18 may also comprise zirconium.

After completion of the chemical conversion of the part 10 in the chromating bath 18, a part 10 is obtained comprising on at least one surface of the part 10 a coating 20 comprising chromium III, for example in oxide form. chromium (Cr ₂ O ₃ ) and chromium hydroxide (Cr (OH) 3). When the chromating bath 18 also comprises zirconium, the coating 20 also comprises hard zirconium in the form of zirconium oxide (Zr0 ₂ ). The part 10 and the coating 20 form a coated part 22 having a free surface 24 of the coating 20.

The chromating bath 18 may for example be made from a solution commonly referred to by SurTec trademark SurTec® 650 or Lanthanum 613.3 Coventya.

The chromating step itself is well known and is therefore not described in detail.

The layer based on aluminum oxide 12 may have a thickness greater than 5 nm, preferably greater than 10 nm, even more preferably greater than 15 nm, and less than 130 nm, preferably less than 120 nm, still more preferably less than 110 nm. This aluminum oxide-based layer 12 may be the native oxide layer or a new aluminum oxide layer obtained by a mechanical process or a chemical process.

Thanks to the presence of this aluminum oxide-based oxide layer 12 on the surface of the aluminum alloy part 10 before immersion in the chromating bath 18, after chemical conversion, it is obtained. in the chromating bath 18, a coated part 22 whose coating 20 has a good resistance to corrosion, including salt spray.

Example 1

Take an aluminum alloy piece 10, for example the grade 7175. The piece 10 is machined and then, on the piece 10 is formed a layer based on aluminum oxide 12 of which thickness is between 5-15 nm. The oxidized part 14 is degreased, for example using perchlorethylene under vacuum in the vapor phase. Then, the oxidized part 14 and degreased is immersed in a chromating bath 18 comprising chromium III and zirconium. After chemical conversion, a coated part 22 is obtained comprising the aluminum alloy piece 10 and a coating 20.

Example 2

Take an aluminum alloy piece 10, for example the grade 7175. The workpiece 10 is machined and then, on the workpiece 10 is formed a layer based on aluminum oxide 12 of which thickness is between 5-20 nm. The oxidized part 14 is then sandblasted, that is to say that solid particles are sprayed onto the surface of the oxidized part 14 so as to remove the native oxide layer and expose the alloy-based aluminum. This aluminum-based alloy which has just been exposed is very reactive and a new layer of aluminum oxide 14 is formed on all the surfaces of the part 10 on which the solid particles have been projected. . Then, the oxidized part 14 and degreased is immersed in a chromating bath 18 comprising chromium III and zirconium. After chemical conversion, we obtain a coated part 22 comprising the aluminum alloy piece 10 and a coating 20.

Example 3

Take an aluminum alloy piece 10, for example the grade 7175. The piece 10 is machined and then, on the piece 10 is formed a layer based on aluminum oxide 12 of which thickness is between 5-20 nm. The oxidized part 14 is degreased, for example by alkaline degreasing using a solution commonly known under the name Sococlean, for 6 minutes at 45 ° C., and then etched chemically using a solution commonly referred to as the name Socosurf, for 15 minutes at 31 ° C, so that the native oxide layer is removed and the aluminum-based alloy is exposed. The part 10 is then rapidly immersed in chromating bath 18 comprising chromium III and zirconium, in order to limit the formation of a new layer based on aluminum oxide, the layer based on aluminum oxide can be formed is strictly less than 5 nm. After chemical conversion, a coated part 22 is obtained comprising the aluminum alloy piece 10 and a coating 20.

FIG. 4 shows the evolution of the chemical composition of the coated part 22 of Example 1 as a function of the distance with respect to the free surface 24, that is to say as a function of the thickness of the coating.

FIG. 5 is a graphical representation similar to that of FIG. 4 for example 3.

The results of Figures 4 and 5 were obtained by a mass spectrometry analysis of secondary ions in flight time, called TOF-SIMS according to the acronym for "Time Of Flight - Secondary-Ions Mass Spectrometry". The graphs of FIGS. 4 and 5 have the abscissa "T", ie the erosion time in seconds (s), and the ordinate the molecular composition of the workpiece in logarithmic scale, ie a number of measured strokes "C" in units arbitrary. The erosion time is proportional to the distance from the free surface 24 of the coating 20. Also, at a given time, corresponds a distance from the free surface 24 of the coating 20 and a composition of the coating at this distance. In FIG. 4, it can be observed that the presence on the part 10 of an aluminum oxide-based layer 12 before immersion in the chromating bath 18 is reflected in the coating 20 by a content expressed in terms of number of strokes detected for the aluminum oxide (or alumina or Al2O3) raised at a short distance from the free surface 24 of the coating 20. It is found that this content, in logarithmic scale, has a plateau and that this content varies little in the coating 20.

In Figure 5, we can see that the aluminum oxide content of the coating 20 has no plateau and has a maximum at a distance from the larger free surface 24.

It can also be observed that the evolution of the aluminum fluoride content (AIF ₃ ) is very different depending on whether the part 10 comprises a layer based on aluminum oxide (FIG. 4) or not (FIG. 5).

Since the chemical conversion is a process for converting the surface of the part, there is no straightforward interface between the body of the piece 10 made of aluminum-based alloy and the coating 20. from the coating 20 to the aluminum-based alloy when the content of aluminum metal (Al) is relatively constant. Thus, it is determined that the thickness of the coating 20 corresponds to a distance from the free surface 24 of the coating 20 equivalent to about 200 seconds of erosion time and about 300 seconds of erosion time, respectively for Figures 4 and 5. In practice, the thickness of the coating 20 is determined by the crossing curves for alumina oxide (Al ₂ O ₃ ) and aluminum metal (Al).

It can be observed that in a layer of the coating 20 between 15 and 30% of the given thickness of the coating 24 of Example 1, measured from the free surface 24, the intensity in arbitrary units measured by TOF-SIMS aluminum oxide varies by up to about 15%.

Indeed, the coating 20 having a thickness corresponding to about 200 seconds (see Figure 4), the layer between 15% and 30% of the coating, measured from the free surface 24 of the coating is therefore the numbers of shots measured between 30 and 60 seconds. Between 30 and 60 seconds, the number of strokes measured for aluminum oxide increases by about 15% (multiplying factor of about 1.15).

In the layer of the coating 20 between 15% and 30% of the given thickness of the coating 24 of Example 3, measured from the free surface 24, the aluminum oxide content varies by more than 300%. In Figure 5, the coating 20 has a thickness corresponding to about 300 seconds. The layer between 15% and 30% of the coating, measured from the free surface 24 of the coating therefore corresponds to the numbers of shots measured between 45 and 90 seconds. Between 45 and 90 seconds, the number of strokes measured for aluminum oxide increases by more than 300% (multiplicative factor of about 3.4).

It is understood that the evolution of the aluminum oxide content in a layer of the coating close to the free surface of the coating, that is to say between 15% and 30% of the thickness of the coating measured from the free surface 24 of the coating 20 makes it possible to determine whether an aluminum oxide-based layer having a thickness greater than 5 nm was present or not on the aluminum alloy part 10 when the immersion of the piece in the chromating bath.

Because we compare the coatings in% of their thickness, we can overcome the erosion parameters used to achieve the measurement by TOF-SIMS.

Figures 6 to 7 show photos of the results after a salt spray corrosion test for 168 hours according to ISO 9227, respectively for Examples 1, 2 and 3.

It is found that the coated parts of Examples 1 and 2 do not exhibit pitting corrosion while the coated part of Example 3 has about ten pitting corrosion.

Thus, thanks to the presence of the aluminum oxide-based layer present on the aluminum-based alloy part during the immersion of the piece in the chromium III chromating bath, one obtains a coating having a very good resistance to corrosion.

Although the present description has been described with reference to a specific embodiment, it is obvious that different modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. In addition, individual features of the various embodiments mentioned can be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

1. Process for chromating a part (10) comprising an aluminum-based alloy comprising the following steps:

- supply of the part (10) comprising on at least one surface of the part a layer based on aluminum oxide (12) whose thickness is greater than or equal to 5 nm;

- immersion of the part (10) in a chromating bath (18) comprising chromium III;

- carrying out a chemical conversion of the part (10) in the chromating bath (18).

2. Chromating process according to claim 1, wherein the chromating bath (18) comprises zirconium.

3. Chromating method according to claim 1 or 2, in which the aluminum oxide-based layer (12) is a native oxide layer and in which the part (10) and the oxide-based layer aluminum (14) are degreased before immersion in the chromating bath (18).

4. Chromating process according to claim 1 or 2, wherein the layer based on aluminum oxide (14) is formed by a mechanical process.

5. Chromating process according to claim 4, wherein the mechanical process comprises a step of projecting solid particles onto the surface of the part (10).

6. Chromating process according to claim 4, wherein the mechanical process comprises a step of abrading the surface of the part (10) with abrasive paper or a liquid containing abrasive particles.

7. Chromating process according to claim 1 or 2, in which the layer based on aluminum oxide (14) is formed by a chemical process comprising a first step of chemical stripping of the surface of the part (10) followed by a step of chemical or electrochemical oxidation of the surface of the part (10).

8. Chromating process according to any one of claims 1 to 7, in which the layer based on aluminum oxide (14) is obtained by placing the part (10) in an atmosphere comprising between 30 to 100% humidity and/or whose temperature is between 30°C and 200°C.

9. Part (10) comprising an aluminum-based alloy having, on at least one surface of the part (10), a coating (20) comprising chromium III, the coating (20) comprising a free surface (24) and having a given thickness, in which, in a layer of the coating between 15 and 30% of the given thickness of the coating measured from the free surface (24), the intensity in arbitrary units measured by TOF-SIMS in aluminum oxide varies at most by plus or minus 50%, preferably by plus or minus 40%, even more preferably by plus or minus 30%.

10. Part according to claim 9, in which the coating (20) comprising chromium III comprises zirconium.