EP3999672B1 - Composition, its use for acid pickling of magnesium alloys and method for acid pickling of magnesium alloys - Google Patents

Description

Technical field of the invention

The invention relates to a composition, its use for de-etching magnesium alloys, and a process for de-etching magnesium alloys.

Technical background

Many devices used in aeronautics, such as turboprops, comprise parts made of a magnesium alloy, and more particularly of a magnesium alloy containing silver.

The repair (reconditioning) of these silver-containing magnesium alloy parts involves the step of removing/stripping the initial chemical conversion coating present on these parts in a bath called a "de-etching" bath, etching being the step of creating this chemical conversion coating on the magnesium alloy parts.

The de-etching bath is also used for new production in the case of the presence of a defect on the part during its initial etching: the defect is removed (de-etching) and the coating is repaired (re-etching). etching).

In some de-etching baths currently in use, chromic acid (H ₂ CrO ₄ , CAS#7738-94-5 ) and, optionally, barium chromate (BaCrO ₄ , CAS# 10294-40-3 ).

However, very soon, European REACh regulations will prohibit the use, in particular of these two substances, which are also used in a large number of surface treatment processes.

To replace this soon to be banned de-etching bath, various baths have already been proposed, such as a bath containing hydrofluoric acid (HF), nitric acid (NHOs) and acid hexafluorozirconic (H ₂ ZrF ₆ ).

However, this bath does not allow the pickling of magnesium alloys containing silver as an addition element. Indeed, due to the presence of nitric acid, the silver contained in the alloy is dissolved in the bath and is redeposited during treatment on the part, thus forming a black colored deposit (silver nitrate), thus preventing any subsequent surface treatment operation. In other words, this chemistry is not suitable for the treatment of certain specific alloys currently used.

A second technical reason prevents the use of this bath on mature military equipment.

Indeed, this bath has a high rate of dissolution of the treated alloy, and therefore slightly reduces the dimensions of the treated parts (significant variation for a standard treatment time). This reduction in ratings (or discount) is not acceptable for the division of mature military engines, for which the preservation of the ratings of treated parts is imperative in order to limit scrap requiring replenishment of old parts, generating significant costs.

Examples of chemical pickling baths exempt from hexavalent chromium for magnesium alloy parts are disclosed in each of documents XP001553794, CN 103 898 585 A And JP 2003 277944 A .

• ne pas mettre en oeuvre de substances impactées par REACh (pas de chrome hexavalent) ;
• ne pas abattre les dimensions des matériaux traités (< 5 µm) ;
• permettre l'enlèvement des couches de conversions chimiques de quelques microns sur les alliages de magnésium contenant de l'argent (ex : Nuance MSR-B) ainsi que les autres nuances, éventuellement après un léger frottage manuel au tampon abrasif (on frotte la pièce avec un scotch Brite^® peu abrasif pour retirer les résidus de revêtements)
• laisser la surface dans un état apte au retraitement (absence de poudrage, retrait homogène...).

In this context, the invention aims to provide a composition for a de-etching bath which makes it possible to:

• do not use substances impacted by REACh (no hexavalent chromium);
• do not reduce the dimensions of the treated materials (< 5 µm);
• allow the removal of chemical conversion layers of a few microns on magnesium alloys containing silver (e.g. Grade MSR-B) as well as other grades, possibly after light manual rubbing with an abrasive pad (the part is rubbed with low-abrasive Scotch Brite ^® to remove coating residues)
• leave the surface in a state suitable for reprocessing (no dusting, homogeneous shrinkage, etc.).

Summary of the invention

• elle comprend, en solution dans l'eau :
  • entre 10 et 80 g/l de composition, d'acide phosphorique H₃PO₄, et
  • entre 2 et 15 g/l de composition, de permanganate de potassium KMnO₄,
• le rapport massique H₃PO_4/ KMnO₄ est compris entre 1,5 et 10, de préférence entre 1,8 et 5, le plus préférablement est égal à 2,8,
• ladite composition a un pH compris entre 2,4 et 3, de préférence de 2,5.

To this end, the invention proposes a composition, characterized in that:

• it comprises, in solution in water:
  • between 10 and 80 g/l of composition, of phosphoric acid H ₃ PO ₄ , and
  • between 2 and 15 g/l of composition, potassium permanganate KMnO ₄ ,
• the H ₃ PO _{4 /} KMnO ₄ mass ratio is between 1.5 and 10, preferably between 1.8 and 5, most preferably is equal to 2.8,
• said composition has a pH of between 2.4 and 3, preferably 2.5.

• entre 10 et 30 g/l, de préférence entre 15 et 20 g/l, de composition, d'acide phosphorique H₃PO₄, et
• entre 4 et 8 g/l de composition, de permanganate de potassium KMnO₄.

In one embodiment of the invention, the composition comprises:

• between 10 and 30 g/l, preferably between 15 and 20 g/l, of composition, of phosphoric acid H ₃ PO ₄ , and
• between 4 and 8 g/l of composition, of potassium permanganate KMnO ₄ .

• 17 g/l de composition, d'acide phosphorique H₃PO₄, et
• 6 g/l de composition, de permanganate de potassium KMnO₄,
• cette composition ayant un pH de 2,5.

This composition may include:

• 17 g/l of composition, of phosphoric acid H ₃ PO ₄ , and
• 6 g/l of composition, potassium permanganate KMnO ₄ ,
• this composition having a pH of 2.5.

• entre 20 et 80 g/l de composition, d'acide phosphorique H₃PO₄, et
• le rapport massique H₃PO_4/ KMnO₄ est compris entre 2,5 et 10, de préférence entre 2,5 et 5, le plus préférablement est égal à 2,8.

In another embodiment of the invention, the composition comprises:

• between 20 and 80 g/l of composition, of phosphoric acid H ₃ PO ₄ , and
• the H ₃ PO _{4 /} KMnO ₄ mass ratio is between 2.5 and 10, preferably between 2.5 and 5, most preferably is equal to 2.8.

The composition of the invention may also comprise an agent for adjusting the pH different from nitric acid HNOs, preferably chosen from acetic acid CH ₃ COOH, sulfuric acid, H ₂ SO ₄ phosphoric acid H ₃ PO ₄ sodium hydroxide NaOH and potassium hydroxide KOH

The invention also proposes the use of the composition according to the invention for the de-etching of a part made of a magnesium alloy, in particular containing silver.

The invention also proposes a process for de-etching a part made of a magnesium alloy, characterized in that it comprises a step a) of immersing said part in a bath comprising the composition according to the invention.

Preferably, this step a) is carried out at a temperature of between 10 and 35° C., preferably between 15 and 35° C., for 5 to 20 min, preferably for 10 min.

Still preferably, the method of the invention further comprises, before step a), a step a1) of adjusting the pH of the bath to a pH of between 2.4 and 3, preferably to a pH of 2.5.

• soit par ajout d'un acide différent de l'acide nitrique, de préférence par ajout d'un acide choisi parmi l'acide acétique, l'acide sulfurique, l'acide phosphorique, pour diminuer le pH,
• soit par ajout d'hydroxyde de sodium et/ou d'hydroxyde de potassium, de préférence d'hydroxyde de potassium, pour augmenter le pH.

In this case, step a1) is preferably implemented:

• either by adding an acid different from nitric acid, preferably by adding an acid chosen from acetic acid, sulfuric acid, phosphoric acid, to reduce the pH,
• or by adding sodium hydroxide and/or potassium hydroxide, preferably potassium hydroxide, to increase the pH.

A characteristic of the method of the invention resides in the fact that said part is made of a magnesium alloy containing silver.

Brief description of figures

The invention will be better understood, and other advantages and characteristics of the invention will appear more clearly on reading the explanatory description which follows and which is made with reference to the figure 1 annexed.
[ Fig. 1 ] there figure 1 shows the variation in mass loss of an MSR-B T6 grade magnesium alloy comprising 2 to 3% silver 2 to 3%, rare earths and 0 to 4% zirconium immersed for 8 minutes in a bath (a composition) according to the invention, at 25° C., depending on the pH;

Detailed description of the invention

The composition of the invention makes it possible to dissolve the layer to be removed on a magnesium alloy part, such as a rare-earth magnesium alloy (grades WE43, E121, for example), a magnesium-aluminum alloy (for example grade AZ91), a magnesium-zinc alloy (for example of grade ZRE1) and above all a magnesium alloy containing silver, without attacking the magnesium, that is to say without significant loss of mass.

It comprises an acid which makes it possible to pickle the surface of the magnesium alloy.

Due to European REACh regulations, the acid cannot be the chromic acid currently used and which will be used in the near future.

However, for decades chromic acid was used because it made it possible to guarantee a negligible reduction in the geometric dimensions of the treated parts because with it the phenomenon of dissolution of the surface layer was limited: chromic acid allows the formation of a film of Cr ₂ O ₃ and Cr(OH) ₃ . Chromic acid acted as both an inhibitor and an acid.

The acids used in the field of pickling magnesium alloys are hydrofluoric, nitric, sulfuric and acetic acids.

However, these acids lead to too high dissolution rates of the surface layer, that is to say to an unacceptable reduction in the geometric dimensions of the treated parts.

To overcome this drawback, the invention proposes to use, in combination with the acid (different from chromic acid), a second component which would not have the aim of reducing the rate of dissolution of the acid but of protect the surface exposed by the acid.

This second component is an inhibitor which acts by forming a thin protective layer.

This thin layer of protection must then be able to be removed by simply rinsing with distilled water.

As such an inhibitor, the inventors tested cerium nitrate, Ce(NO ₃ ) ₃ which forms a layer of cerium oxide and potassium permanganate, KMnO ₄ which forms a layer of manganese oxide.

However, there again, the speeds of dissolution of the surface layer are too high, leading to an unacceptable reduction in the geometric dimensions of the treated parts.

The inventors then discovered that a composition comprising defined amounts of phosphoric acid and potassium permanganate, in precise proportions and in defined mass ratios (concentration ratio), and at a defined pH, could be used for the de-etching of magnesium alloy parts of all grades, including those containing silver.

The two species phosphoric acid and potassium permanganate are not species commonly used by those skilled in the art of surface treatment and have never been used before, to the knowledge of the inventors, for the specific operation of de-etching magnesium alloys.

Thus, the inventors have discovered that a composition comprising a solution in water of between 10 and 80 g/l of composition, of phosphoric acid H ₃ PO ₄ , and between 2 and 15 g/l of composition, of permanganate potassium KMnO ₄ at an H ₃ PO _{4 /} KMnO ₄ mass ratio of between 1.5 and 10, preferably between 1.8 and 10, even between 1.8 and 5 or between 2.5 and 5, most preferably equal to 2.8, said composition having a pH of between 2.4 and 3, could be used for the de-etching of magnesium alloy parts of all shades, including containing silver.

Below the low limits indicated above, the respective actions of stripping the phosphoric acid and protecting the bare surface of the part from potassium permanganate are not sufficient.

Using values above the upper limits shown above does not achieve any additional effect.

A preferred composition comprises between 10 and 30 g, or even between 15 and 20 g, per liter of composition, of phosphoric acid, and between 2 and 15 g, or even between 4 and 8 g, per liter of composition, of potassium permanganate.

The most preferred composition comprises 17 g, per liter of composition, of phosphoric acid and 6 g, per liter of composition, of potassium permanganate KMnO ₄ .

Another preferred composition comprises between 20 and 80 g, per liter of composition, of phosphoric acid, and between 2 and 15 g, per liter of composition, of potassium permanganate. The H ₃ PO _{4 /} KMnO ₄ mass ratio is between 2.5 and 10, preferably between 2.5 and 5, most preferably is equal to 2.8.

The H ₃ PO ₄ / KMnO ₄ mass ratio is preferably 2.8

The composition has a pH between 2.4 and 3.0. Most preferably the composition of the invention has a pH of 2.5.

To obtain the desired pH, the composition of the invention may additionally contain a pH adjusting agent.

When the pH needs to be lowered, such an agent may be an acid.

However, of course, chromic acid cannot be used. Nor can nitric acid be used because otherwise silver nitrate will form on the surface of the part and the surface treatment cannot continue.

The preferred acids are acetic acid CH ₃ COOH, sulfuric acid, H ₂ SO ₄ phosphoric acid (H ₃ PO ₄ ) and mixtures thereof.

When the pH needs to be increased, such an agent may be a base.

Preferred bases are sodium hydroxide NaOH and potassium hydroxide KOH.

Potassium hydroxide is particularly preferred because it has the same spectator ion as KMnO4, which limits interactions between ions. The invention also provides a process for de-etching magnesium alloys, including those containing silver.

The de-etching process for a magnesium alloy part of the invention comprises a step a) of immersing said part in a bath comprising the composition according to the invention.

The bath used during the implementation of the process of the invention may contain, in addition to the composition of the invention, other agents such as, for example, other magnesium inhibitors, that is to say other chemical species capable of forming precipitates on the surface of the part in order to form a thin film stopping the attack of the acid. Mention may be made, by way of examples, of cerium nitrate or H ₂ ZrF ₆ .

For optimum effect, step a) is carried out at a temperature between 10 and 35° C., preferably between 15 and 35° C., for 5 to 20 min, preferably for 10 min. It is preferable to maintain agitation of the bath during step a).

In order to carry out step a) at the correct pH, it may be necessary to implement, before step a), a step a1) of adjusting the pH of the bath to a pH of between 2.4 and 3, preferably 2.5.

• soit par ajout d'un acide différent de l'acide nitrique, de préférence par ajout d'un acide choisi parmi l'acide acétique, l'acide sulfurique, l'acide phosphorique, pour diminuer le pH,
• soit par ajout d'hydroxyde de sodium et/ou d'hydroxyde de potassium, de préférence d'hydroxyde de potassium, pour augmenter le pH.

In this case, step a1) is implemented:

• either by adding an acid different from nitric acid, preferably by adding an acid chosen from acetic acid, sulfuric acid, phosphoric acid, to reduce the pH,
• or by adding sodium hydroxide and/or potassium hydroxide, preferably potassium hydroxide, to increase the pH.

Before carrying out step a), it is preferable to check that the loss of mass of the parts to be treated obtained by immersion in the bath in step a) at the pH, at the temperature and for the time planned, is much lower or equal to the tolerated mass loss.

This mass loss, for an application for the division of mature military engines, must be less than 90 g/dm ² , preferably less than or equal to 85 g/dm ² .

For this purpose, a test step a2) is implemented before step a) and after step a1).

This step a2) consists in immersing a follow-up specimen, made of the same magnesium alloy as that of the part to be treated and of known surface in the bath of step a), for the time and at the temperature which will be applied to the part to be treated itself. Then, the specimen is weighed and it is verified that it has undergone a loss in mass of less than 90 mg/dm ² , preferably less than or equal to 85 gd/m ² .

The method of the invention then preferably comprises two successive steps b1) of rinsing the parts obtained after step a) and b2).

These steps b1) and b2) are carried out in water, preferably demineralised, for at least 1 minute.

Finally, the method of the invention comprises a step c) called cleaning of the treated parts.

This step c) consists in eliminating, by any method appearing to those skilled in the art, the layer of manganese oxide formed on the surface of the part treated. This layer is usually a few microns thick and has a darker color than the untreated part. Thus, rubbing the surface of the part with an abrasive pad until the dark layer disappears is sufficient to eliminate it.

In order to make the invention better understood, several embodiments will now be described in the examples which follow.

EXAMPLES Example 1

6 test specimens with a surface area of 0.5 dm ² were cut from a magnesium alloy containing silver of grade MSR-B

Example 2

6 de-etching baths were prepared which contained 17g/l of phosphoric acid, 6g/l of potassium permanganate, the balance being water.

The first bath, denoted B1, was adjusted to a pH of 1.99. Bath B1 is not in accordance with the invention.

The second bath, denoted B2, was adjusted to a pH of 2.18. Bath B2 is not in accordance with the invention.

The third bath, denoted B3, was adjusted to a pH of 2.44.

The fourth bath, denoted B4, was adjusted to a pH of 2.5.

The fifth bath, denoted B5, was adjusted to a pH of 2.72.

The sixth bath, denoted B6, was adjusted to a pH of 3.0.

Example 3

The 6 specimens prepared in Example 1 were each immersed respectively in one of the baths B1 to B6 for 8 minutes at a temperature of 25°C. Stirring was maintained in the bath using a propeller placed in the bath.

After 8 minutes, the specimens are removed from the baths, rinsed by two successive soakings in a water bath for 1 minute.

They are then dried and the black manganese oxide layer formed on their surface is removed with an abrasive pad.

They are then weighed and their mass losses in mg/dm ² are measured.

There figure 1 appended shows the results obtained in terms of mass loss as a function of the pH of the bath.

As seen in figure 1 , when the pH of the bath is less than 2.3, the mass losses are too high: they are greater than or equal to 90 mg/dm ² .

It can also be seen that the higher the pH of the bath, the lower the mass losses.

But, above pH 3, it is increasingly difficult to pickle (eliminate the layer of manganese oxide) the parts.

The pH of the de-etching bath of the invention must therefore be maintained between 2.4 and 3.

Most preferably the pH is 2.5.

Claims (11)

1. A composition, characterised in that:

   - it comprises, in a solution of water:

   between 10 and 80 g/l of a composition of phosphoric acid H₃PO₄, and

   between 2 and 15 g/l of a composition of potassium permanganate KMnO₄,

   - the weight ratio H₃PO₄/ KMnO₄ is between 1.5 and 10, preferably between 1.8 and 5, most preferably equal to 2.8,

   - said composition has a pH between 2.4 and 3, preferably 2.5.
2. The composition according to claim 1, characterised in that:

   - it comprises:

   between 10 and 30 g/I, preferably between 15 and 20 g/l of a composition of phosphoric acid H₃PO₄, and

   between 4 and 8 g/l of a composition of potassium permanganate KMnO₄.
3. The composition according to claim 1 or 2, characterised in that

   - it comprises:

   17 g/l of a composition of phosphoric acid H₃PO₄, and

   6 g/l of a composition of potassium permanganate KMnO₄,

   - said composition has a pH of 2.5.
4. The composition according to claim 1, characterised in that:

   - it comprises:

   between 20 and 80 g/l of a composition of phosphoric acid H₃PO₄, and

   the weight ratio H₃PO₄/ KMnO₄ is between 2.5 and 10, preferably between 2.5 and 5, most preferably equal to 2.8.
5. The composition according to one of the preceding claims, characterised in that it further comprises an agent for adjusting the pH different from nitric acid HNOs, preferably selected from acetic acid CH₃COOH, sulfuric acid H₂SO₄, phosphoric acid H₃PO₄, sodium hydroxide NaOH and potassium hydroxide KOH.
6. Use of the composition according to one of the preceding claims for the de-etching of a part made of a magnesium alloy, in particular containing silver.
7. A method for de-etching a part made of a magnesium alloy, characterised in that it comprises a step a) of immersing said part in a bath comprising the composition according to one of claims 1 to 5.
8. The method according to claim 7, characterised in that the step a) is implemented at a temperature of between 10 and 35°C, preferably between 15 and 35°C, for 5 to 20 min, preferably for 10 minutes.
9. The method according to claim 7 or 8, characterised in that it further comprises, prior to the step a), a step a1) of adjusting the pH of the bath to a pH between 2.4 and 3, and preferably 2.5.
10. The method according to claim 9, characterised in that the step a1) is implemented:

    either by adding an acid other than nitric acid, preferably by adding an acid selected from acetic acid, sulfuric acid and phosphoric acid, to reduce the pH,

    or by adding sodium hydroxide and/or potassium hydroxide, preferably potassium hydroxide, to increase the pH.
11. The method according to one of claims 7 to 10, characterised in that said part is made of a magnesium alloy containing silver.

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