Chemical polishing bath for iron or nickel alloy, and process using such a bath

Abstract

A chemical polishing bath and process for polishing an iron or nickel alloy part, or a portion thereof, is presented. The process comprises: nitric acid as an oxidizing agent capable of attacking iron and / or nickel, at a nitrate concentration between 0.3 and 1.1 mol / L; a chloride and / or fluoride complexing agent capable of forming a complex with oxidized iron and / or nickel, at a concentration between 2.8 and 5.8 mol / L; phosphoric acid at a phosphate concentration between 4.1 and 6.3 mol / L; and sulfuric acid at a sulfate concentration between 0.3 and 0.8 mol / L. The bath is particularly suitable for polishing parts produced by additive manufacturing (3D printing).

Description

Title of the invention: Chemical polishing bath for iron or nickel alloy, and method using such a bath. Technical field

[0001] The present invention relates generally to the field of surface treatment of iron or nickel alloy parts, and in particular to the field of polishing iron or nickel alloy parts. The invention is especially suited to polishing parts made with these materials by additive manufacturing. State of the art

[0002] Additive manufacturing, or 3D printing, is a major focus of R&D in France and worldwide, enabling the production of three-dimensional polymer or metal parts from powders. Due to the manufacturing process involving successive powder melting, additive manufacturing processes allow for the production of parts with complex geometries and also make it possible to consider producing new alloy grades inaccessible through traditional metallurgy, while simultaneously reducing the amount of raw material required. Additive manufacturing is a particularly attractive technology for the aerospace, automotive, and naval industries, with the aim of improving the performance and weight reduction of structures.However, additive manufacturing processes known as powder bed fusion inherently lead to the creation of parts whose final surface finish bears the hallmarks of powder-based manufacturing, particularly surface roughness (expressed by the parameter Ra) ranging from 5 to 25 µm depending on the process, which is significantly higher than the roughness of a machined part (on the order of a few microns). Furthermore, the parts produced are generally characterized by the presence of partially melted particles on the extreme surface, which can pose a risk of detachment during operation and thus degrade the performance of these parts (clogging of pipes, entrainment of particles in hydraulic systems, reduction of static and dynamic mechanical properties, etc.).

[0003] It is therefore necessary to reduce the roughness of additively manufactured parts, but also to eliminate surface irregularities and thus prevent the risk of particle / foreign body detachment from the surface during operation, which could generate functional problems related to the use of these parts. Improving the surface finish of the produced parts can also improve certain performance characteristics in terms of fatigue and corrosion resistance, or even flow on surfaces for hydraulic applications.

[0004] Today, such a post-processing step represents a major technological barrier for the development of functional metal parts from additive manufacturing with controlled and reproducible characteristics.

[0005] Various polishing processes exist, such as electropolishing, plasma polishing, or laser polishing, but these processes require the use of electricity or even lasers, resulting in a high associated cost. Furthermore, the use of lasers or working with high electrical currents necessitates special protection for operators. Finally, these complex processes require precise control of their parameters. Electropolishing baths are also difficult to manage because they are highly hygroscopic, and dehydration steps are required periodically.

[0006] Chemical polishing processes consisting of immersing the part to be treated in a bath, called a chemical polishing bath, containing acidic or basic compounds as well as oxidizing agents, without the application of an electric current or a laser, are simpler to implement.

[0007] Four examples of polishing baths for nickel alloy parts are described in the article “Surface finish control of additively manufactured Iconel 625 components using combined chemical-abrasive flow polishing” by N. Mohammadian, S. Turenne, and V. Brailovski, in the Journal of Materials Processing Technology (2018), vol. 252, pp. 728–738. These baths comprise nitric acid and at least one of phosphoric acid, sulfuric acid, acetic acid, hydrochloric acid, and hydrofluoric acid. These baths exhibit an effect more akin to chemical machining than polishing, and they have only a slight surface smoothing effect.

[0008] US Patent 3,709,824 describes a so-called polishing solution for stainless steel parts, but which is closer to 'shining', that is to say, whose objective is to smooth out dents and obtain a shiny finish. The bath comprises phosphoric acid, nitric acid, hydrochloric acid and sulfosalicylic acid, as well as possibly other additives, acidic or non-acidic.

[0009] The objective of the invention is to provide a polishing bath for iron or nickel alloy parts that makes it possible to obtain a surface with an average roughness (Ra) of less than 10 pm and a limited material removal, typically less than 200 pm. General description of the invention

[0010] With this objective in mind, the present invention relates to a chemical polishing bath for iron or nickel alloy parts, and a method using such a bath.

[0011] Chemical polishing aims to reduce the surface roughness of a metal part by means of a selective dissolution operation of a layer of material of variable thickness, ranging from tens of nanometers to several tens of micrometers. This material removal is achieved chemically, using reagents that can be acidic or basic, but also possess oxidizing properties. It should be noted that the dissolution caused by the chemical polishing process has a direct impact on the dimensional appearance of the part, even if this is not the intended goal. This distinguishes it from chemical machining, whose sole objective is to reduce the dimensions of the part (by several tens or hundreds of microns) to achieve a target size.

[0012] The principle of chemical polishing is also different from that of chemical pickling, which is more akin to the chemical machining process. Indeed, the objective of chemical pickling is to remove an oxide layer, the thickness of which is on the order of a few microns, without impacting the dimensional characteristics of the part and its surface roughness, but allowing for better adhesion of the surface treatment applied subsequently.

[0013] A difference can also be noted compared to the chemical degreasing / decontamination steps, the purpose of which is to solubilize chemical species adsorbed on the surface of the material to be treated. No impact on the dimensional characteristics of the part is observed in this latter case.

[0014] Thus, in the present text, polishing means reducing the surface roughness of a part, while changing its dimensions as little as possible.

[0015] According to the invention, a chemical polishing bath for polishing a part made of iron or nickel alloy, or a part thereof, comprises: - nitric acid as an oxidizing agent capable of attacking iron and / or nickel, at a nitrate concentration between 0.3 and 1.1 mol / L, i.e. between 20 and 70 g / L; - a chloride and / or fluoride complexing agent capable of forming a complex with oxidized iron and / or oxidized nickel, at a concentration between 2.8 and 5.8 mol / L; - phosphoric acid at a phosphate concentration between 4.1 and 6.3 mol / L, or between approximately 400 and approximately 600 g / L; and - sulfuric acid at a sulfate concentration between 0.3 and 0.8 mol / L, i.e. between 30 and 80 g / L.

[0016] One of the merits of the invention is to have identified compounds, and their respective concentrations for the production of a polishing bath allowing a significant reduction of roughness, without significantly increasing the discount, whereas these two effects generally require antagonistic parameters.

[0017] The solution according to the invention comprises an oxidizing agent, which attacks the surface of the iron or nickel alloy part by forming an iron or nickel oxide, and a complexing agent for the iron or nickel oxide formed. The density of the bath is controlled by the mixture of sulfuric acid and phosphoric acid. In order to achieve a polishing effect, this bath generally has a higher viscosity, or density, than other chemical etching solutions, such as those used in chemical machining. Therefore, the aggressiveness of this solution differs from conventional pickling or chemical machining solutions, even when using similar reagents. The two acids used (sulfuric acid and phosphoric acid) also have an etching function on the surface of the iron or nickel alloy. The arrival of H+ ions from these two acids near the surface to be polished is also limited by their diffusion, which allows for control of their etching action.

[0018] The higher viscosity of the polishing solution slows down the diffusion of chemical species from the solution to the surface of the workpiece, where they are consumed. The aim is to make diffusion slower than the reaction rate. The surface thus becomes depleted of etching agents (oxidizing agent, sulfuric acid, or phosphoric acid) and complexing agents, because they are consumed faster than they reach the surface. Their arrival therefore occurs first near the crests of the raised areas of the surfaces to be polished, where their action is prioritized. Conversely, the recesses will not receive as many reactants (etching agents and complexing agents), as these have already been consumed by a reaction on the most prominent points of the workpiece. This phenomenon is thus responsible for a surface polishing effect through the selective dissolution of surface asperities.

[0019] In the context of the invention, talking about an increase in the viscosity of the bath is equivalent to talking about an increase in its density, and vice versa.

[0020] According to the current understanding of the invention, the effectiveness of the present chemical polishing bath relies on controlling the diffusion kinetics of the oxidizing and complexing agents within the bath towards the surface of the part to be polished, as well as the reaction kinetics of the oxidizing agents with the surface of the part to be polished. The operating principle of the chemical polishing bath according to the invention is as follows. An appropriate viscosity (higher than that of conventional chemical etching baths) of the chemical polishing bath slows down the diffusion of the active chemical species (here, the oxidizing and complexing agents) from the solution towards the surface of the part. In the vicinity of the surface, the active species are consumed, which induces a local depletion of the bath.To restore the balance of concentrations, the active species diffuse from the bath to the surface, reacting as soon as they come into contact with the surface to be polished. Since the most accessible areas are the peaks of the surface roughness, the attack on these peaks is favored, thus reducing the surface relief and therefore the overall roughness of the part.

[0021] Initial tests were carried out with the present polishing bath, for bath volumes between 100 mL and 400 L, and proved its effectiveness, both on a laboratory and industrial scale. The chemical polishing bath according to the invention was specifically developed for polishing parts produced using additive manufacturing techniques, but can be applied to all kinds of iron or nickel alloy parts, regardless of the manufacturing process. Furthermore, the dimensions and / or geometries can be highly varied. Moreover, the iron or nickel alloy part to be treated can be part of a component. That is to say, the iron or nickel alloy part to be treated can be combined with another material, to which it is juxtaposed or superimposed.

[0022] In the present text, the expression "iron or nickel alloy part" therefore refers to any part comprising at least one portion of iron or nickel alloy, and the invention is not limited to objects made solely of iron or nickel alloy. In particular, the expression "chemical polishing bath / process for polishing an iron or nickel alloy part" refers both to a polishing bath / process for the chemical polishing of a part made of an iron or nickel alloy and to a bath / process for the chemical polishing of a part comprising a portion made of an iron or nickel alloy and a portion made of another material.

[0023] In this text, the expression "iron or nickel alloy" means an alloy among any iron-based alloy (including nickel-free iron alloys), any nickel-based alloy (including iron-free nickel alloys) and any iron- and nickel-based alloy.

[0024] According to some embodiments, the complexing agent may be present at mass concentrations between 100 and 200 g / L in total chlorides and / or between 53 and 106 g / L in total fluorides.

[0025] During preliminary tests, the inventors observed that if the complexing agent is present at concentrations above 5.8 mol / L, the rate of etching and therefore the material removal is too high, whereas if the concentration is below 2.8 mol / L, the reduction in roughness is insufficient. Furthermore, the inventors also observed that if the nitric acid concentration is above 1.1 mol / L, the material loss (i.e., the reduction in surface finish) is too high without any change in roughness (i.e., no reduction in roughness), and if the nitric acid concentration is below 0.3 mol / L, there is no significant etching of the workpiece (no reduction in roughness and no reduction in surface finish). Thus, only a bath containing the identified compounds within the claimed concentration ranges makes it possible to obtain the desired effect.

[0026] Similarly, the inventors discovered that the present polishing bath exhibited optimal chemical polishing properties for sulfuric acid concentrations between 0.3 and 0.8 mol / L and phosphoric acid concentrations between 4.1 and 6.3 mol / L. If the bath contains more phosphoric and / or sulfuric acid, its density increases, which slows the diffusion kinetics of the etching agents to the surface of the workpiece. Conversely, if the bath contains less phosphoric and / or sulfuric acid, its density decreases, which promotes the diffusion kinetics of the species and results in a greater reduction in polishing.

[0027] According to preferred embodiments of the invention, the polishing bath is substantially water-free, in that no water is added during the preparation of the polishing bath. However, it is not impossible for the bath to contain water, particularly if the acids used during the preparation of the bath are not pure acids but acids in aqueous solution.

[0028] Preferably, a polishing bath according to the invention is constituted, directly after its assembly (i.e., its preparation) and before any use thereof:

[0029] - of nitric acid as an oxidizing agent capable of attacking iron and / or nickel, at a nitrate concentration between 0.3 and 1.1 mol / L, i.e. between 20 and 70 g / L;

[0030] - of a chloride and / or fluoride complexing agent capable of forming a complex with the oxidized iron and / or oxidized nickel, at a concentration between 2.8 and 5.8 mol / L;

[0031] - of phosphoric acid at a phosphate concentration between 4.1 and 6.3 mol / L, i.e. between approximately 400 and approximately 600 g / L;

[0032] - of sulfuric acid at a sulfate concentration between 0.3 and 0.8 mol / L, i.e. between 30 and 80 g / L;

[0033] - of water; and

[0034] - of the counter-ion(s) of the complexing agent.

[0035] According to some embodiments, the polishing bath comprises nitric acid as the sole oxidizing agent. According to particularly preferred embodiments, the bath is free of permanganate, the associated reduced form of which is a manganese oxide that would precipitate on the surface of the workpiece (or part thereof) to be polished, thus interfering with its polishing. According to these same or other embodiments, the polishing bath is free of hydrogen peroxide, i.e., hydrogen peroxide is not added to the polishing bath, as it is not stable over time in an aqueous environment, and degradation of hydrogen peroxide leads to a degradation of the polishing performance of the present bath.

[0036] The role of the chloride and / or fluoride complexing agent in the invention is to combine with the oxidized iron and / or oxidized nickel (in the form of oxide, hydroxide, and / or oxyhydroxide) that forms on the surface of the part under the attack of the oxidizing agent, or that may be in solution. Advantageously, the chloride ions (Cl⁻), and Fluorides (F₂) are capable of complexing a variety of metals, which is advantageous for iron or nickel alloy parts. Furthermore, chlorides, and fluorides in particular, are extremely stable complexing agents in solution, and the risk of their degradation over time and the formation of uncontrolled byproducts is minimal.

[0037] Advantageously, the chloride complexing agent added to the chemical polishing bath is added to the bath in the form of hydrochloric acid and / or ferric chloride FeCl3 and / or ferrous chloride FeCl2, preferably in the form of ferric chloride FeCl3. Thus, according to certain particularly preferred embodiments, the source of the chloride complexing agent is ferric chloride, which makes it possible to supply a large quantity of chloride to the bath without significantly increasing the bath volume.

[0038] Preferably, the chloride complexing agent is not added to the polishing bath in the form of a monovalent chlorine salt. Indeed, to achieve the desired concentrations, such large quantities of monovalent salts would be required that they would not be soluble in the polishing bath, and the monovalent counter-ions of these salts (such as, for example, but not limited to, Na+ or K+) would disrupt the action of the etching agents (oxidant and complexing agent).

[0039] Preferably, the complexing agent comprises, relative to the total amount of complexing agent, more than 50 mol% chloride, preferably more than 75 mol%, and more preferably more than 90 mol% chloride. The fluoride content in the chemical polishing bath is preferably limited to reduce the risk of formation and release of difluorine gas, and to reduce the risks associated with handling a concentrated acid bath containing fluorides.

[0040] Advantageously, the fluoride complexing agent added to the chemical polishing bath is added to the bath in the form of NH4F.HF, NaF.HF, KF.HF, HF, NH4F, NaF, KF or mixtures thereof.

[0041] Depending on the variant, the chloride and / or fluoride complexing agent is present at a concentration between 2.9 and 5.6 mol / L, preferably between 3.0 and 5.4 mol / L, more preferably between 3.5 and 5.0 mol / L, and more preferably between 4.2 ± 0.5 mol / L. When the complexing agent comprises only chlorides, this refers to the chloride concentration. When the complexing agent comprises only fluorides, this refers to the fluoride concentration. When the complexing agent comprises both chlorides and fluorides, this refers to the sum of the respective molar concentrations of chlorides and fluorides.

[0042] According to variants, the oxidizing agent is present at a nitrate concentration between 0.5 and 1.0 mol / L.

[0043] Depending on the variants, phosphoric acid is present at a phosphate concentration between 4.3 and 6.3 mol / L, preferably at a concentration of 5.30+0.25 mol / L, and / or sulfuric acid is present at a sulfate concentration between 0.5 and 0.7 mol / L.

[0044] During use, the polishing bath becomes saturated with dissolved iron and / or nickel due to the chemical attack on surface roughness. It is possible to regenerate such a bath after use by simply adding back the compounds consumed during polishing, particularly oxidizing and complexing agents. In this case, the regenerated bath still contains iron and / or nickel, as is also the case with traditional chemical polishing baths. However, traditional baths are destroyed when the iron and / or nickel concentration becomes too high. The presence of iron and / or nickel can indeed disrupt the bath's operation and impair polishing performance, notably by slowing the reaction kinetics, which could negatively impact the polishing process and compete with the diffusion of reagents (oxidizing and complexing agents).It is therefore preferable to define the permissible concentration limits for iron and / or nickel that do not degrade the chemical polishing mechanism. Advantageously, iron can be present at a concentration between 1.4 and 2.3 mol / L, or between 80 and 130 g / L, preferably between 80 and 100 g / L, and nickel can be present up to a maximum concentration of 0.51 mol / L, or up to a maximum concentration of 30 g / L, preferably between 0 and 15 g / L. Since these iron and / or nickel concentrations are lower than those traditionally observed for a chemical pickling bath, for example, the bath used here is indeed a polishing bath.

[0045] Depending on the variants, the bath has a density ranging from 1.60 to 1.80 g / cm3, preferably from 1.60 to 1.70 g / cm3, more preferably from 1.60 to 1.65 g / cm3.

[0046] In the context of the invention, the concentration ranges mentioned should be understood in the broadest possible sense, i.e., including the upper and lower limit values ​​of said concentration ranges. As used in this text, the term "approximately" means a range of values ​​within ±10% of the stated value.

[0047] In another aspect, the invention proposes a method for chemically polishing a part made of iron or nickel alloy, or a part thereof, using the chemical polishing bath as described above.

[0048] The method according to the invention comprises at least the following steps: - provide a chemical polishing bath as described above; - immerse a part to be polished, made of iron or nickel alloy, or a part thereof, in the chemical polishing bath; - remove the part from the bath after a predetermined immersion time.

[0049] In order to control the concentrations of reactants, particularly oxidizing and complexing agents, in the polishing bath, these concentrations can be determined by titration. The determination of the different chemical species can be carried out by acid-base titration in a non-aqueous medium, by thermo-titration, or by spectrophotometry. Alternatively, it is also possible to determine the different reactants by ion chromatography or by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0050] The temperature of the polishing bath during immersion of the workpiece is between 35°C and 60°C, preferably between 40°C and 45°C. The immersion time of the iron or nickel alloy workpiece in the polishing bath depends on the initial condition of the workpieces to be treated, the desired roughness, and the permissible material removal rate; it can preferably be between 20 and 400 minutes, preferably between 10 and 120 minutes, or preferably between 100 and 200 minutes. The bath temperature influences the reaction kinetics of the polishing process, and in particular the rate of etching of the iron or nickel. The higher the temperature, the faster the etching agents (oxidizing agent, sulfuric acid, or phosphoric acid) and complexing agents act.The difference between the reaction rate of the reactants and their diffusion is accentuated, which improves the polishing process and the results obtained, i.e. a reduction in surface roughness while limiting the depreciation.

[0051] Thanks to the process according to the invention, which uses a polishing bath at a certain temperature and immerses the part or the portion thereof to be polished for a specific time, it is possible to obtain a significant reduction in the surface roughness of the part without significantly increasing the reduction in surface finish. These two effects generally require opposing parameters: immersing an iron or nickel alloy part for a longer time in the polishing bath promotes a reduction in roughness but increases the reduction in surface finish. Surprisingly, applying an immersion time of up to 3 hours makes it possible to limit the reduction in surface finish to a few hundred micrometers. In other words, in the present context, the reduction in surface finish observed for a part polished using a chemical polishing bath or a chemical polishing process according to the invention is less than 300 µm.The parameter Sa can be considered more relevant than Ra (roughness, or height, arithmetic mean along a line) when it comes to quantifying the roughness, or a decrease in roughness, of a surface because it is an arithmetic mean roughness measured over a surface and no longer solely along a line like Ra. Iron or nickel alloy parts polished using a chemical polishing bath according to the invention advantageously exhibit an Sa of the order of 1 pm.

[0052] In the present text, the expression "in the order of 1 pm" means less than 10 pm, preferably between 1 and 5 pm.

[0053] Alternatively, the chemical polishing process for polishing an iron or nickel alloy part, or a portion thereof, may include the following steps: - to provide a chemical polishing bath according to the present invention; - circulate, preferably in a forced manner, the chemical polishing bath inside cavities and / or porosities of the iron or nickel alloy part, or part thereof, to be polished, the chemical polishing bath being preferably maintained at a temperature between 35°C and 60°C, in particular between 40°C and 45°C.

[0054] Advantageously, the chemical polishing bath can be circulated within cavities and / or pores of the part to be treated using suitable equipment such as pumps, fittings, and flanges. This equipment, which is known to those skilled in the art, will not be described in detail here.

[0055] Advantageously, an activation step, or chemical pretreatment, can be carried out on the surface of the part to be polished before the part, or the portion thereof, is brought into contact with the chemical polishing bath. Those skilled in the art will note that the activation step is more akin to a deoxidation step without material removal than to chemical pickling. In other words, according to certain embodiments, the iron or nickel alloy part, or the portion thereof, to be polished is subjected to an activation step before being immersed in the chemical polishing bath or before the chemical polishing bath is circulated within the part to be polished.Such an activation step is a surface preparation step that allows the surface to be cleaned and, in particular, facilitates access to the oxidizing and / or complexing agents that react with the surface during the chemical polishing process, increasing its effectiveness.

[0056] According to variants, the activation step is carried out by immersion in a bath comprising hydrochloric acid at a chloride concentration between 200 and 330 g / L for a period of between 5 and 10 min.

[0057] Advantageously, the removal of the workpiece from the polishing bath, or the end of the polishing bath's circulation within the workpiece, can be followed by rinsing and then a bleaching treatment applied to the workpiece or the part thereof that was in contact with the polishing bath. Indeed, the substrate etching / oxidation step is responsible for the formation of a black layer of iron or nickel hydroxide and possibly hydroxide of alloying elements. This black layer is not removed during rinsing (because it is typically insoluble). Bleaching being a process involving an acidic solution, such as a nitric acid solution and / or hydrofluoric acid, well known to those skilled in the art, will not be detailed here. However, according to certain embodiments, the bleaching step may consist of immersion in an HF / HNO3 solution with an HF concentration between 0.9 and 1.7 mol / L and an HNO3 concentration between 2.5 and 3.5 mol / L at room temperature for 5 to 15 min, preferably 10 min.

[0058] The present chemical polishing bath and polishing process have been developed in particular for iron alloys such as stainless steels, for example Inox 316, Inox 316L, Inox 326, Inox 326L, Inox 304, Inox 304L, Inox 15-5PH, Inox 174-PH, maraging steel, and for nickel alloys such as Inconel 718, Inconel 625, Inconel 690, Inconel 939, Hastelloy X, René 77, René 220, ABD900. As is well known in the field, these alloys are widely used, particularly in the automotive industry, which increases the interest in developing such a chemical polishing bath.

[0059] Advantageously, the part to be polished in whole or in part using a chemical polishing bath according to the invention or the process using such a bath is a part made of iron or nickel alloy obtained by an additive manufacturing process, or three-dimensional printing.

[0060] Thus, the present chemical polishing bath or chemical polishing process is particularly applicable for the treatment of parts produced for the aerospace, aeronautical, and automotive industries, to eliminate surface irregularities and thus prevent the risk of particle detachment from the surface of said parts in service, which could generate functional problems related to their use such as the clogging of pipes, or the reduction of their static and / or dynamic mechanical properties. Brief description of the figures

[0061] Other features and characteristics of the invention will become apparent from the detailed description of at least one advantageous embodiment shown below by way of illustration, with reference to the accompanying drawings. These show:

[0062] [Fig.1] of the representative diagrams (points 1 to 6) of the operating principle of the present process;

[0063] [Fig.2] scanning electron microscope images allowing comparison of the surface condition of a part made of Inconel 718 nickel alloy after immersion in a chemical polishing bath according to a first embodiment of the present invention (bath 1 - part A) for 0 min (a), 10 min (b), 20 min (c), 30 min (d), 40 min (e), 50 min (f), 60 min (g), 70 min (h), 80 min (i), 90 min (j);

[0064] [Fig. 3] a graph representing the evolution of the roughness Sa (roughness - immersion time) for a sample of a part made of Inconel 718 nickel alloy immersed in a chemical polishing bath according to a first embodiment of the present invention (bath 1 - part A) for different durations;

[0065] [Fig.4] a graph representing the evolution of the discount (discount - immersion time) for a sample of a part made of Inconel 718 nickel alloy immersed in a chemical polishing bath according to a first embodiment of the present invention (bath 1 - part A) for different durations;

[0066] [Fig.5] scanning electron microscope images allowing comparison of the surface condition of a 316L stainless steel iron alloy part after immersion in a chemical polishing bath according to a first embodiment of the present invention (bath 1 - part G) for 0 min (a), 10 min (b), 15 min (c), 20 min (d), 25 min (e), 30 min (f), 35 min (g), and 40 min (h);

[0067] [Fig.6] a graph representing the evolution of the roughness Sa (roughness - immersion time) and the discount (discount - immersion time) for a sample of a part made of 316L stainless steel iron alloy immersed in a chemical polishing bath according to a first embodiment of the present invention (bath 1 - part G) for different durations;

[0068] [Fig.7] confocal microscope images allowing comparison of the surface condition of a part made of Inconel 718 nickel alloy before (a) and after (b) chemical polishing using a second preferred embodiment of a chemical polishing bath according to the invention (bath 2 - part B);

[0069] [Fig. 8] confocal microscope images allowing comparison of the surface condition of an Inconel 718 nickel alloy part before (a) and after (b) chemical polishing using a preferred third embodiment of a chemical polishing bath according to the invention (bath 3 - part C); and

[0070] [Fig.9] confocal microscope images allowing comparison of the surface condition of a part made of Inconel 718 nickel alloy before (a) and after (b) chemical polishing using a fourth preferred embodiment of a chemical polishing bath according to the invention (bath 2 - part D). Detailed description with examples

[0071] The operating principle of the polishing bath according to the invention will first be explained with reference to [Fig. 1]. As explained previously, the present invention surprisingly proposes a polishing bath that allows for a significant reduction in roughness without increasing the discount, whereas these two effects generally require opposing parameters.

[0072] The increased viscosity / density of the bath, controlled by sulfuric and phosphoric acids, slows down the diffusion of chemical species from the solution to the surface of the part. This depletion of species on the surface active substances, due to their consumption in the vicinity of the surface, are responsible for a surface polishing effect by selectively dissolving the surface relief.

[0073] The principle is shown in [Fig. 1] for polishing a nickel-based part. It is similar for an iron-based part.

[0074] 1): attack and oxidation of the material by the oxidant under acidic conditions and formation of a first passivation layer (metallic oxides from nickel and, where applicable, alloying elements);

[0075] 2, 3): complexation of nickel oxides and, where applicable, alloy oxides by complexing agents to solubilize oxides;

[0076] 4, 5): slow diffusion of oxidizing compounds from the solution to the surface of the part to be polished, sulfuric acid and phosphoric acid allowing to slow down the arrival of oxidizing compounds;

[0077] 6): attack of the upper parts of the relief, resulting in a reduction of roughness. Examples#:

[0078] Four examples of polishing nickel alloy parts in four different baths, baths 1 to 4, according to the invention, will be described below, as well as two examples of treating nickel alloy parts in two different baths, baths 5 and 6, not according to the invention (comparative examples). An example of polishing an iron alloy part in one bath, bath 1, according to the invention, will also be described.

[0079] Six Inconel 718 nickel alloy parts, referenced A to F, produced by the same additive manufacturing process, and one 316L stainless steel iron alloy part, referenced G, also produced by an additive manufacturing process, are each in the form of a 20 x 40 mm plate. The plates first underwent an activation step using hydrochloric acid to prepare their surface, according to conventional methods well known to those skilled in the art. The seven parts A to G are then each partially covered with self-adhesive masking tape.

[0080] The partially masked nickel alloy parts A to D thus obtained were then subjected to chemical polishing processes using polishing baths of different compositions, referenced 1 to 4, according to four preferred but non-limiting embodiments of the invention. The partially masked nickel alloy parts E and F thus obtained were subjected to chemical polishing processes using comparative polishing baths of different compositions, not in accordance with the present invention, referenced 5 and 6. In addition, the partially masked iron alloy part G thus obtained was then subjected to a chemical polishing process using polishing bath 1 according to the invention. The six different baths 1 to 6 were carried out in similar tanks, with solution volumes of 2 L; their compositions are detailed in Table 1. [Tables 1] Bath PO^ g / L SO% ao; g / L Cl Fe g / L Ni g / LT °C Bath 1,517 71 68 165 86 0 40 Bath 2,530 70 41 170 88 7.2 55-60 Bath 3,493 67 21 152 83 7.5 40 Bath 4,540 70 135 200 80 0 40-45 Bath 5 (comparative) 525 68 163 123 110 0.4 40 Bath 6 (comparative) 590 78 41 90 101 2.7 40-45

[0081] For each of the baths, the chlorides are added to the bath in the form of FeCl3.

[0082] Parts A and G were immersed in bath 1, part B was immersed in bath 2, part C was immersed in bath 3, part D was immersed in bath 4, part E was immersed in bath 5, part F was immersed in bath 6. For each of the parts, the area covered by the masking tape is not in direct contact with the bath in which the part is immersed and does not undergo any change in its surface condition.

[0083] In the context of the present invention, the high viscosity of the polishing bath (typically a density of 1.6 to 1.8 g / cm³) and the local inhomogeneities in concentrations (due to the thickness of the diffusion layer) of oxidizing and complexing agents are crucial for controlling the polishing and promoting the chemical attack of the peaks. Therefore, the agitation of the bath when the workpiece is immersed is strictly controlled. Preferably, the bath is agitated at a lower speed when the workpiece is immersed (treatment phase) than during the preparation of the polishing bath (homogenization phase).In particular, the agitation speed influences the renewal of chemical species present in the liquid layer, called the diffusion layer, in the vicinity of the part being treated; a lower agitation speed allows the formation of a thicker diffusion layer, which slows down the diffusion of reagents and promotes polishing.

[0084] After immersion in the polishing bath, the treated part is rinsed and then observed under a scanning electron microscope and a confocal microscope to determine the reduction in its roughness and its discount. In particular, for each part, the masking tape is removed and an area comprising a treated surface and an untreated surface (because previously masked by the masking tape) is examined. The surface was observed using an Olympus DSX510 microscope to determine its roughness. The topography of each surface was recorded and analyzed to determine the discount due to material removal and the variations in roughness parameters caused by the polishing effect of the different baths. Scanning electron microscopy observations were performed using a Zeiss commercially available scanning electron microscope.

[0085] The surface roughness obtained after manufacturing (untreated surface) and after chemical polishing (treated surface) for parts A to G in iron or nickel alloy, as well as the reduction of parts which took place during this polishing are reported in Table 2. In this, the expression before polishing refers to the part of the surface which was masked by the self-adhesive masking tape during immersion in the bath, while the expression after polishing refers to the part of the surface not masked by the tape and subjected to the chemical polishing treatment.

[0086] As can be seen in Table 2 and in [Fig. 2], [Fig. 5] and [Fig. 7]-9, parts A to D and G obtained by the additive manufacturing process have a similar surface finish before polishing. Parts E and F also have a similar surface finish.

[0087] After polishing, parts A to G all underwent a reduction in surface finish (Table 2). Figures 3-4 and 6, as well as Table 2, indicate a significant decrease in the surface relief of the parts treated by the four baths according to the invention, and therefore a polishing of parts A to D and G. It should be noted that among the parts treated with a bath according to the invention, part C exhibits a greater reduction in surface finish and roughness than part B after a similar immersion time in the chemical polishing bath (Table 2). This can be attributed to the slightly lower density of bath 3 compared to bath 2 (lower concentrations of phosphoric and sulfuric acids - Table 1). A lower density does indeed promote the diffusion of the etching agents towards the surface of the part being treated. [Tables 2] Part (Polishing Time) Sa before polishing pm Sa after polishing pm Ra before polishing pm Ra after polishing pm Discount pm A (after 90 min) 12.1 2.5 / / 280 B (after 80 min) 12.4 3.5 12.8 3.5 120 C (after 60 min) 13.4 3.05 14.1 2.67 200 D (30 min) 11.3 6.3 11.3 6.2 160 F (40 min) 13.1 3.7 / / 243 G (30 mins) 12.4 6.8 13.1 7.1 60

[0088] The results presented in Table 2 are results of measurements carried out on the respective upper face of the different parts.

[0089] By comparing the results obtained using bath 2 (according to the invention) - part B and bath 6 (comparative example) - part F, it is noted that increasing the concentration of chloride complexing agent does not appear to significantly influence the reduction of roughness, however it allows the discount to be limited to acceptable values ​​(typically less than 200 pm).

[0090] By comparing the results obtained using bath 2 (according to the invention) and bath 5 (comparative example), the inventors observed that increasing the concentration of nitric acid while decreasing the concentration of the complexing agent does not produce a chemical polishing effect. Indeed, the inventors found that the roughness after treatment is greater than that measured in the initial state. Bath 5 is therefore more akin to a chemical machining bath and not a polishing bath.

Claims

Demands

1. Chemical polishing bath for polishing an iron or nickel alloy part, or part thereof, said chemical polishing bath comprising: - nitric acid as an oxidizing agent capable of attacking iron and / or nickel, at a nitrate concentration between 0.3 and 1.1 mol / L; - a chloride and / or fluoride complexing agent capable of forming a complex with oxidized iron and / or oxidized nickel, at a concentration between 2.8 and 5.8 mol / L; - phosphoric acid at a phosphate concentration between 4.1 and 6.3 mol / L; and - sulfuric acid at a sulfate concentration between 0.3 and 0.8 mol / L.

2. Chemical polishing bath according to claim 1, wherein the chloride complexing agent is added to the bath in the form of hydrochloric acid and / or ferric chloride FeCl3, preferably in the form of ferric chloride FeCl3.

3. Chemical polishing bath according to claim 1 or 2, wherein the fluoride complexing agent is added to the bath in the form of NH4F.HF, NaF.HF, KF.HF, HF, NH4F, NaF, KF or mixtures thereof.

4. Chemical polishing bath according to any one of the preceding claims, wherein the complexing agent comprises, relative to the total amount of complexing agent, more than 50 mol% of chloride, preferably more than 75 mol%, more preferably more than 90 mol% of chloride.

5. Chemical polishing bath according to claim 1 or 2, wherein the complexing agent is present at a concentration between 2.9 and 5.6 mol / L, preferably between 3.0 and 5.4 mol / L, preferably also 4.2+0.5 mol / L.

6. Chemical polishing bath according to any one of the preceding claims, wherein the oxidizing agent is present at a nitrate concentration between 0.5 and 1.0 mol / L.

7. A chemical polishing bath according to any one of the preceding claims, wherein phosphoric acid is present at a phosphate concentration between 4.3 and 6.3 mol / L, preferably at a concentration of 5.30 ± 0.25 mol / L, and / or acid sulfuric acid is present at a sulfate concentration between 0.5 and 0.7 mol / L.

8. Chemical polishing bath according to any one of the preceding claims, comprising iron at a concentration between 1.4 and 2.3 mol / L and / or comprising nickel at a concentration less than 0.51 mol / L.

9. Chemical polishing bath according to any one of the preceding claims, said bath having a density of 1.60 to 1.80 g / cm3, preferably 1.60 to 1.70 g / cm3.

10. A chemical polishing method for polishing an iron or nickel alloy part, or a part thereof, comprising the following steps: i. providing a chemical polishing bath according to any one of the preceding claims; ii. immersing an iron or nickel alloy part to be polished, or a part thereof, in the chemical polishing bath, which is preferably maintained at a temperature between 35°C and 60°C, in particular between 40°C and 45°C; iii. removing the part after a predetermined immersion time.

11. Chemical polishing method according to claim 10, wherein the immersion time is between 20 and 400 minutes.

12. A chemical polishing method for polishing an iron or nickel alloy part, or part thereof, comprising the following steps: i. providing a chemical polishing bath according to any one of claims 1 to 9; ii. circulating, preferably in a forced manner, the chemical polishing bath inside cavities and / or porosities of the iron or nickel alloy part, or part thereof, to be polished, the chemical polishing bath being preferably maintained at a temperature between 35°C and 60°C, in particular between 40°C and 45°C.

13. A chemical polishing method according to claim 10, 11 or 12, wherein the iron or nickel alloy part, or the part thereof, to be polished is subjected to an activation step before being

14. contact with the chemical polishing bath by immersion in it or circulation of it, preferably the activation step is carried out by immersion in a bath comprising hydrochloric acid at a chloride concentration between 200 and 330 g / L for a duration of between 5 and 10 min. Chemical polishing bath according to any one of claims 1 to 9, or chemical polishing process according to any one of claims 10 to 13, wherein the iron or nickel alloy part to be polished is obtained by an additive manufacturing process.

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