METHOD FOR CONTROLLING A METAL PARTS TREATMENT BATH

Abstract

The invention relates to a method for controlling a treatment bath for metal parts comprising titanium or a titanium alloy, the bath comprising iron. The method comprises the following steps: - taking a sample from the treatment bath, - adding a predetermined quantity of chromium(II) chloride to the sample based on the concentration of titanium or titanium alloy and iron, - continuously adding potassium permanganate, - determining the concentration of titanium and iron by potentiometry. Figure for the abstract: Fig. 6

Description

Title of the invention: METHOD FOR CONTROLLING A TREATMENT BATH FOR METAL PARTS Technical field of the invention

[0001] The present invention relates to a bath for preparing or treating aeronautical metal parts, and in particular metal parts comprising titanium. It is specifically aimed at controlling the surface treatment bath. Technological background

[0002] Most parts used in the aeronautical field are generally made of a composite material that is lightweight and robust. However, some parts, such as aircraft turbomachine blades, are equipped with reinforcing metal parts, for example at their leading and / or trailing edges, so that they resist, among other things, impacts from particles or foreign bodies known by the English acronym "FOD" for "Foreign Object Debris". The reinforcing metal parts are typically bonded to the leading and / or trailing edge.

[0003] These reinforcing metal parts generally comprise titanium or a titanium alloy, titanium having good thermomechanical properties and very good corrosion resistance. However, these reinforcing metal parts for leading and / or trailing edges, comprising titanium or titanium alloys, require surface preparation. The surface treatment can be carried out using a chemical treatment bath which typically allows for the chemical dissolution of at least a few microns of the surface layers to pickle the metal part, clean it of impurities (oxides, particles, etc.), or even chemically machine certain parts of the metal parts. In the case of turbomachine blades, the chemical treatment bath of the metal part to be bonded also allows for increasing its adhesion surface (specific surface area) through preferential chemical machining of certain phases of the microstructure.This increase in specific surface area allows for improved mechanical performance at the interface between the metal part and the turbomachine blade.

[0004] The chemical treatment bath generally includes acids such as hydrofluoric acid and possibly nitric acid.

[0005] It is strongly recommended to monitor the chemical treatment bath in order, firstly, to determine the concentration of dissolved metallic elements, for example titanium, which are found in the chemical treatment bath after chemical dissolution, and secondly, to ensure that the chemical treatment bath treats the parts effectively. This control can be carried out daily or weekly to guarantee the quality of the chemical dissolution and the conformity of the treated parts.

[0006] One solution for monitoring a chemical treatment bath is inductively coupled plasma spectrometry (ICP), which allows for the determination or quantification of the desired chemical elements through chemical analysis. This method is very expensive and not 100% reliable for determining the actual titanium concentration, which can pose a risk to the quality of the parts.

[0007] Moreover, since not all workshops are equipped with a dedicated installation for this method, the control of the chemical treatment bath can then be subcontracted, which can lengthen the time to obtain the results (on the order of a week) and present a risk of manufacturing non-conforming metal parts while waiting for the results.

[0008] The control method is also longer in itself because each chemical compound is titrated separately.

[0009] Another control method is the potentiometric titration method using a potassium dichromate (K2Cr2O7) titrant solution. However, this method, which uses a potassium dichromate (K2Cr2O7) titrant solution, is prohibited in Europe because potassium dichromate is known to be harmful to the environment and to human health. Indeed, potassium dichromate is classified as a CMR IB substance, the acronym for which stands for carcinogenic and / or mutagenic and / or reprotoxic chemicals.

[0010] There is therefore a need to resolve all or part of the aforementioned drawbacks. Summary of the invention

[0011] The objective of the present invention is to provide a solution enabling the rapid and reliable determination of the concentration of chemical compounds, particularly metallic ones, in a treatment bath while being respectful of the environment and the health of operators.

[0012] We achieve this objective in accordance with the invention by means of a method for controlling a treatment bath for metal parts comprising titanium or an alloy thereof. The bath comprises metal elements containing at least iron, the method comprising the following steps: - taking a sample from a treatment bath, - Addition of a predetermined quantity of chromium(II) chloride to the sample, depending on the concentration of titanium or titanium alloy, and iron; - Continuous addition of potassium permanganate to the sample. - determination of the concentration of titanium and iron in the sample by potentiometry.

[0013] Thus, this solution makes it possible to achieve the aforementioned objective. The use of potassium permanganate as a titrant solution is safer for the environment and presents less risk to operators than potassium dichromate K2Cr2O7.

[0014] Moreover, this process is faster than the so-called ICP method because the two chemical compounds (titanium and iron) are titrated within the same analytical test thanks to potassium permanganate which makes it possible to distinguish distinctly each jump of potential relating to the oxidation of titanium ions and then of iron ions.

[0015] In this solution, the potassium permanganate used as a titrant decomposes into K+ and MN04 ions. The strong oxidizing power of the MN04 ion will react with the ionic species present. Each reaction results in a potential jump in the sample solution, and the acquisition of a potential U curve as a function of the volume of titrant solution makes it possible to distinguish each reaction equilibrium related to the reduction of chromium, titanium, and then iron ions (inflections respectively represented by points EP1, EP2, and EP3 in [Fig. 6]).

[0016] Finally, knowing the volume and concentration of the titrant species used for each of the chemical equilibria, we can deduce the dissolved iron and titanium contents.

[0017] The process also includes one or more of the following features, taken alone or in combination:

[0018] - adding at least one acid to the sample of the treatment bath so as to obtain an acidic solution with a predetermined pH.

[0019] - the treatment bath is acidic so that the sample from the treatment bath The sample taken is acidic. - the alkaline treatment bath sample includes at least one sodium hydroxide.

[0020] - hydrochloric acid and sulfuric acid are added to the sample of treatment bath, said hydrochloric and sulfuric acids being concentrated and diluted respectively between 5 to 10% and between 10 to 20% relative to the total volume of the sample of the treatment bath.

[0021] - the predetermined pH of the acidic solution is between 0 and 6, and is preferably less than 1 in order to ensure that the metallic elements in the bath are in the form of metallic ions (and not metallic oxides) for a given potential.

[0022] - titanium has a threshold value between 3000 ppm and 5000 ppm and iron presents a threshold value between 100 ppm and 200 ppm.

[0023] - the minimum quantity of chromium II chloride is 0.2 g (± 0.01).

[0024] - the minimum quantity of chromium(II) chloride is greater than the threshold values ​​of quantity of titanium or titanium alloy, and iron.

[0025] - the concentration of potassium permanganate is at least 0.1 mol / L.

[0026] - the sample of the treatment bath comprises a volume between 5 mL and 20 mL.

[0027] — the treatment bath has a temperature between 80°C and 94°C.

[0028] - - the composition of the bath is as follows: —sodium hydroxide between 10% and 30% by total volume, —triethanolamine between 5% and 10% by total volume, —diethanolamine between 1% and 5% by total volume, —water as a supplement. Brief description of the figures

[0029] The invention will be better understood, and other objects, details, features and advantages thereof will become more apparent upon reading the following detailed explanatory description, of embodiments of the invention given by way of purely illustrative and non-limiting examples, with reference to the accompanying schematic drawings in which: - Fig. 1 is a cross-sectional view of a tank containing a treatment bath and a metal part immersed in this bath according to the invention; - Fig. 2 is a cross-sectional view of a container holding a sample of treatment bath to be analyzed according to the invention; - Fig. 3 represents a Pourbaix diagram of the metallic elements iron and titanium according to the invention; - Fig. 4 represents the different chemical reactions in a sample for a control process when adding potassium permanganate according to the invention; - Figure 5 represents a detection device for performing a potentiometric measurement step of metallic compounds contained in a treatment bath according to the invention; and Figure 6 is an example of a potentiometric titration curve for the metallic elements contained in a sample from a treatment bath according to the invention. Detailed description of the invention

[0030] Figure 1 represents a tank 1 comprising a chemical treatment bath 2 for metal parts. These metal parts advantageously comprise titanium and / or its alloys, and are generally intended for the aeronautical field. but not exhaustively. Of course, other materials can be used for the manufacture of aeronautical parts.

[0031] The metal parts may be a leading edge or trailing edge reinforcement of a turbomachine blade. The turbomachine blades are preferably movable fan or propeller blades.

[0032] The chemical treatment bath 2 allows for the treatment of metal parts by pickling, machining, or cleaning. The composition and / or concentration of the chemical compounds in the treatment bath may differ depending on the desired treatment.

[0033] Preferably, treatment bath 2 is used for pickling. Pickling is used to remove the layers of metal oxides that form on the surface of metal parts that have undergone forging and / or heat treatment. In the case of a titanium or titanium alloy metal part, titanium oxides may form on its surface.

[0034] According to one embodiment, the treatment bath 2 is advantageously alkaline. In this description, the term "alkaline" means that the pH of the treatment bath is greater than 7.

[0035] For this purpose, the treatment bath 2 comprises, for example, at least one sodium hydroxide (NaOH). Advantageously, at least one of the compounds in the alkaline bath is selected from sodium hydroxide, triethanolamine (C6Hi5NO3), or diethanolamine (C4HiNO2).

[0036] Advantageously, the treatment bath 2 comprises a volume percentage of sodium hydroxide (NaOH) between 10 and 30% relative to the total volume of the sample in the treatment bath, a volume percentage of triethanolamine between 5 and 10% relative to the total volume of the sample in the treatment bath, and a volume percentage of diethanolamine between 1 and 5% relative to the total volume of the sample in the treatment bath.

[0037] According to one embodiment, the treatment bath 2 can be made from the commercially known product BONDERITE C-AK 5578-GL Aero manufactured by HENKEL or from the commercially known product TURCO 4008-3, also manufactured by HENKEL. Advantageously, but not exclusively, between 75% and 100% of the treatment bath is made with BONDERITE C-AK 5578-GL Aero. The treatment bath is advantageously topped up with water.

[0038] According to another embodiment, the treatment bath 2 is advantageously acidic. In this description, we mean by the term "acidic" that the pH of the treatment bath is less than 7. Preferably, the pH is between 6 and 0, and even more preferably is less than 1. In this embodiment of In this embodiment, the treatment bath 2 comprises at least one of the following acids: hydrofluoric acid (HF), nitric acid (HNO3), hydrochloric acid (HCl), and / or sulfuric acid (H2SO4). These acids advantageously allow for pickling of metal parts containing titanium. Following a non-limiting example, the treatment bath comprises all three acids: hydrofluoric acid, nitric acid, and sulfuric acid.

[0039] In [Fig. 1], a finished metal part 3 is immersed in the chemical treatment bath 2 for a predetermined period of time so that it may be treated, and in this case pickled. The predetermined time can be between 5 min and 1h.

[0040] The treatment bath 2 advantageously, but not exclusively, has a temperature between 80°C and 94°C.

[0041] The treatment bath 2 advantageously, but not exclusively, comprises at least one metallic element, which is preferably iron. The iron is added, for example, to regulate the pickling rate of titanium and / or its alloys by sodium hydroxide. The iron is added, for example, in the form of iron sulfate and preferably at a concentration between 40 ppm and 60 ppm. Even more preferably, the concentration of iron sulfate is approximately 50 ppm.

[0042] During the immersion of the metal part 3, chemical reactions occur which involve, on the one hand, the enrichment of the treatment bath 2 with titanium, and possibly, on the other hand, the consumption of alkali species. The treatment bath 2 can also advantageously, but not exclusively, be enriched with iron by dissolution during the chemical reactions, as some metal parts may contain iron. The chemical reactions will continue to occur at the expected rate as long as the quantity of titanium does not exceed a certain threshold value (during production).

[0043] Advantageously, but not limitingly, the threshold value of the quantity of metallic compounds not to be exceeded in the treatment bath 2 is 3000 ppm (mg / L) for titanium and 100 ppm (mg / L) for iron.

[0044] The amount of titanium and iron in the treatment bath 2 is controlled by a treatment bath control method. This control is performed daily or weekly. It allows the treatment bath 2 to be changed or the amount of other chemical compounds (for example, sodium hydroxide) in the treatment bath to be modified to obtain an acceptable amount of titanium and iron for treating the metal parts.

[0045] The process advantageously includes a step of taking a sample 4 of said chemical treatment bath 2 (alkaline or acidic). The quantity of treatment bath 2 taken from the tank 1 is placed, for example, in a beaker 5. The latter A sample of the treatment bath 2 is shown in [Fig. 2]. The quantity taken is, for example, between 5 ml and 20 ml. The quantity taken will, of course, depend on the container in which the sample 4 is contained.

[0046] In the case of an alkaline treatment bath, the process advantageously includes a step of adding at least one acid to the sample taken so as to obtain an acidic solution with a predetermined pH. The sample 4 from treatment bath 2 is acidified so that the chemical compounds to be monitored (namely titanium and iron) are converted into ions. Only ions can be measured in the present monitoring process.

[0047] In an alkaline sample 4, with a high pH (greater than 7), titanium and iron are in the form of oxides or hydroxides.

[0048] The sample 4, before being acidified after the step of adding at least one acid, may include the following oxides or hydroxides: Ti(OH)3, HTiO4, HTiO3, TiO, TiO3, TiO2, Fe3O4, FeO42, Fe2O3, Fe(OH)2. The addition of acid(s) allows the oxides or hydroxides to be transformed into ions, for example.

[0049] Advantageously, but not limitingly, the predetermined pH of the acidic solution of sample 4 obtained after acidification is less than 7. Preferably, the predetermined pH is between 6 and 0, and more preferably is less than 1. Such a predetermined pH makes it possible to guarantee that the metallic elements in sample 4 are in the form of metal ions (and not metal oxides) for a given potential.

[0050] When the pH of the sample (or of the treatment bath 2) is less than 1, titanium and iron are predominantly found in the form of ions and also in the form of oxides such as: TiO22+, TiO2+, Ti3+, Ti2+, Fe2+, Fe3+, FeO42. More precisely, to obtain titanium ions, the pH must be less than 2 (for a potential less than -0.2V) and to obtain iron ions, the pH must be less than 1 (for a potential less than 1.6V).

[0051] Figure 3 shows a Pourbaix diagram of metallic elements such as iron and titanium. In this figure, we see that the pH and potential (in mV) conditions are necessary to obtain only metallic ions in a given form (i.e., without oxides). In the case of iron, for example, at a pH less than 1 and a potential of 0.4 V, iron is in the form of the Fe²⁺ ion. In the case of titanium, at a pH less than 1 and a potential less than -0.4 V, titanium is in the form of the Ti²⁺ ion. Thus, with a pH advantageously equal to 1 and a potential between -0.6 V and 0 V, both titanium and iron will be simultaneously in their ionic forms.

[0052] Advantageously, the acid added to sample 4 of treatment bath 2 can be hydrochloric acid (HCl). Alternatively, the acid can be sulfuric acid (H2SO4).

[0053] According to another embodiment, a mixture of hydrochloric acid (HCl) and sulfuric acid (H2SO4) is added to the sample of the treatment bath so as to obtain an acidic solution.

[0054] In the embodiment with an acidic treatment bath (having a pH less than 1), the process for checking the treatment bath does not include the step of adding at least one acid. Since the treatment bath is already acidic, the sample 4 taken is an acidic solution containing titanium and iron, mainly in the form of ions and also as oxides.

[0055] Generally, the parts are produced in an alkaline / basic environment. Analysis of the sample in an acidic environment is appropriate and preferred.

[0056] In this example of a possible step involving the addition of at least one acid, the concentrated hydrochloric acid is advantageously, but not limited to, a dilution of 5 to 10% relative to the total volume of the sample in the treatment bath. For example, the hydrochloric acid is concentrated to a maximum of approximately 37% of the aqueous medium. The concentrated sulfuric acid is advantageously, but not limited to, a dilution of 10 to 20% relative to the total volume of the sample in the treatment bath.

[0057] Optionally, the process includes a step of stirring or mixing the sample in the treatment bath when adding the acid or acids. This helps to eliminate any bubbles that may form.

[0058] The control process includes a step of adding a predetermined amount of chromium(II) chloride (CrCl2) to the acidic solution (following acidification or to a solution that is already acidic). The predetermined amount is sufficient to initiate a redox reaction. The minimum amount of CrCl2 can be 0.2 g (± 0.01). Advantageously, the minimum amount of chromium(II) chloride to be added depends on the amount of titanium and iron that must not be exceeded (i.e., the threshold value) and is preferably greater than the amount of titanium and iron (and possibly another chemical compound in the treatment bath).

[0059] Advantageously, but not limitingly, chromium II chloride is (always) added in excess compared to the other chemical compounds to be dosed from the treatment bath (titanium and iron).

[0060] The control process includes a step of continuously adding potassium permanganate (KMnO4) to the sample (comprising an acidic solution). Advantageously, but not limitingly, the molar concentration of permanganate The potassium concentration is 0.1 mol / L. This molar concentration allows for the observation of equivalence points and enables precise visualization of chemical reactions. A higher molar concentration would lead to a very rapid reaction and would be less easily visualized on the potentiometry curve.

[0061] Of course, in the case, for example, of titanium with a threshold value of 5000 ppm and iron with a threshold value of 200 ppm, a larger volume of titrant solution will be required to determine the concentration of these compounds. The quantity of titrant solution can, for example, be multiplied by at least two (with the quantity of chromium(II) chloride always exceeding the threshold values ​​for the quantity of titanium or titanium alloy and iron, and the concentration of potassium permanganate exceeding 0.1 mol / L, for example).

[0062] Advantageously, but not limitingly, the evolution of the potential U is continuously monitored as a function of the amount of permanganate added, which allows the potentiometric curve to be plotted: U = f(V permanganate). For a given concentration, the flow rate of the titrant is controlled so that the potentiometer has sufficient time to efficiently measure the potential over time.

[0063] Once potassium permanganate is added to the treatment bath sample, three reactions advantageously occur and are shown schematically in [Fig.4]. The latter represents in particular the successive redox reactions initiated by MnO4- ions as a function of the standard potentials (ordinate axis in V) of the ionic couples present.

[0064] Potassium permanganate dissociates advantageously, but not exclusively, in aqueous solution into K+ and MnO4 ions. The permanganate ions (MnO4) will then successively oxidize the reducing agents of the ionic couples present, starting with the couple with the lowest standard potential (Cr37Cr2+), and ending with the couple with the highest potential (Fe37Fe2+) as shown schematically in [Fig.4].

[0065] Advantageously, but not limitingly, the K+ ion resulting from the dissociation does not participate in the redox reactions.

[0066] The permanganate ions (MnO4) will, according to the first chemical reaction 1, therefore first react with the chromium(II) chloride ions (2Cr2+) by oxidizing the Cr2+ ions to Cr3+ ions, such that: MnO4 + 4H+ + 3Cr2+ MnO2 + 3Cr3+ + 2H2O.

[0067] Permanganate ions (MnO4) will be reduced to MnO2 ions. Ti3+ + Fe2+ ions do not take part in the first reaction.

[0068] Next, the second chemical reaction 2 and the third chemical reaction 3 of oxidation and reduction which take place in the treatment bath sample following the addition of potassium permanganate (KMnO4), the following can be expressed successively: MnO4 + 4H+ + 3Ti3+ MnO2 + 2H2O + 3Ti4+ MnO4 + 4H+ + 3Fe2+ MnO2 + 2H2O + 3Fe3+.

[0069] The process includes a step of determining the quantity of titanium and iron by potentiometric titration. Potentiometric titration is a simple and well-known method that allows the measurement of a potential difference between two electrodes immersed in an electrolytic solution to be analyzed. Chemical reactions then initiate depending on the oxidizing / reducing couples present, inducing a potential difference between the electrodes.

[0070] Plotting the rising potential curve (in mV) as a function of the volume of permanganate titrant solution (in mL) indicates, for example, the number of reactions occurring and the equilibrium potentials of each reaction. After precise identification of the reactions, and knowing the volume and concentration of the titrant, it is therefore possible to determine the concentration of the metallic elements dissolved in the solution.

[0071] With reference to [Fig. 5], the measurement is advantageously carried out using a detection device 10 which includes, for example, an electrode 11. The electrode 11 is connected to a processing unit 13 which is capable of measuring the potential difference between the electrode and the sample solution. In the present example, only one electrode 11 is immersed in the sample 4 to perform the measurement.

[0072] Advantageously, but not exclusively, the electrode 11 is a platinum electrode. The detection device 10 is simple and can be used in all types of workshops or laboratories without additional costs. The detection device 10 can be, for example, the titrator commercially known as the Metrohm Titrando 905. The electrode 11 used can be the commercially known Pt Ring Titrode 6.0451.100. The potential difference is measured by the electrode 11, whose potential remains constant.

[0073] Figure 6 shows a potentiometric curve of the different chemical compounds / metallic elements using potassium permanganate. In particular, the potentiometric curve represents the evolution of the potential U, expressed in mV (on the y-axis), as a function of the volume V of potassium permanganate, expressed in ml (on the x-axis). Three equivalence points are shown on the curve, for example: - EPI represents the potential at equivalence during the reaction involving MnO4 with Cr2+, - EP2 represents the potential at equivalence during the reaction involving MnO4 with Ti3+; - EP3 represents the potential at equivalence during the reaction involving MnO4 with Fe2+.

[0074] We see from this curve that the concentration of EPI is -140 mV for 2.3 ml, that the concentration of EP2 is approximately 330 mV for 6.5 ml, and that the concentration of EP3 is approximately 640 mV for approximately 7 ml. Optionally, there is an established acceptance range for each chemical compound; for example, EPI can be between -160 mV and -120 mV, EP2 can be between 130 mV and 340 mV, and EP3 can be between 500 mV and 700 mV.

[0075] When the chromium(II) chloride is insufficient to allow the subsequent chemical reactions to proceed, i.e., when the titanium concentration is above the threshold value for titanium concentration, the sample must be diluted. The dilution factor is, for example, 2. In this case, chromium(II) chloride is added in a predetermined quantity (at least 0.2 g), and potassium permanganate is also added so that the potential of the sample solution increases sufficiently to allow the reactions with the titanium and iron metal ions to take place.

[0076] Such a process makes it possible to simultaneously determine the chemical compounds iron and titanium using potassium permanganate (KMnO4) (which presents less risk to health and the environment than potassium dichromate) by distinctly identifying the potential jumps related to the oxidation of iron and titanium ions and by excluding jumps related to parasitic reactions for example from impurities present in the treatment bath.

Claims

Demands

1. Method for controlling a treatment bath for metal parts comprising titanium or an alloy thereof, the bath comprising metallic elements containing at least iron, the method comprising the following steps of: - taking a sample from a treatment bath, adding a predetermined amount of chromium II chloride to the sample based on a threshold value of titanium or titanium alloy, and iron, - continuously adding potassium permanganate to the sample, - determining the concentration of titanium and iron in the sample by potentiometry.

2. A method according to claim 1, characterized in that it comprises a step of adding at least one acid to the sample of the alkaline treatment bath so as to obtain an acidic solution with a predetermined pH.

3. A method according to claim 1 or 2, characterized in that the alkaline treatment bath sample comprises at least one sodium hydroxide.

4. A process according to any one of claims 1 to 3, characterized in that hydrochloric acid and sulfuric acid are added to the treatment bath sample, said hydrochloric and sulfuric acids are concentrated and diluted respectively between 5 to 10% and between 10 to 20% relative to the total volume of the treatment bath sample.

5. A method according to any one of claims 2 to 4, characterized in that the predetermined pH of the acidic solution is between 0 and 6 and is preferably less than 1 in order to ensure that metallic elements in the bath are in the form of metallic ions for a given potential.

6. A process according to any one of claims 1 to 5, characterized in that titanium has a threshold value between 3000 ppm and 5000 ppm; and iron has a threshold value between 100 ppm and 200 ppm.

7. A process according to any one of claims 1 to 6, characterized in that the minimum quantity of chromium II chloride is 0.2 g (± 0.01).

8. A process according to claim 6, characterized in that the minimum quantity of chromium II chloride is greater than the threshold values ​​of quantity of titanium or titanium alloy, and iron.

9. A process according to any one of claims 1 to 8, characterized in that the molar concentration of potassium permanganate is at least 0.1 mol / L.

10. A method according to any one of claims 1 to 9, characterized in that the sample of the treatment bath comprises a volume between 5 mL and 20 mL.

11. Use of the process for controlling a treatment bath according to any one of claims 1 to 10, for pickling, machining or cleaning.

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