Technical field of the Invention

The invention relates to a composition for plasma electropolishing of metal surfaces and to the related electropolishing process using said composition. In particular, said composition consists of an aqueous solution of soluble salts of hydrofluoric, boron hydrofluoric, fluorosilicic, or hexafluorotitanic acids, and to the related plasma electropolishing process.

Known art

The present invention relates to the plasma electropolishing process of metal surfaces. In particular, the present invention relates to a plasma electropolishing process using an aqueous solution of soluble salts of hydrofluoric, Boron hydrofluoric, fluorosilicic, or hexafluorotitanic acids. More particularly, the present invention refers to the composition of the polishing solution and to the optimal process parameters to be applied to the plasma electropolishing process for metal surfaces such as Titanium (Ti), Zirconium (Zr), Hafnium (Hf), Vanadium (Va), Niobium (Nb), Tantalum (Ta), or from alloys of said metals.

Typically, metal surface polishing is carried out with processes of chemical or electrochemical nature.

In chemical polishing, the piece to be treated is immersed in a concentrated aqueous solution of plural acids whose action allows to remove a superficial layer of metal, preferably acting on asperities present and decreasing their roughness.

In the electrochemical process, called electropolishing or electrolytic polishing, the piece to be treated is immersed in a concentrated aqueous solution of several acids, and connected to the positive pole of an electric circuit, thereby acting as the anode of an electrolytic cell. A second metallic electrode, immersed in the same solution, faced to the piece to be treated and at a suitable distance from the latter (usually of the order of tens of cm), acts as the cathode. Applying a potential difference between anode and cathode triggers an oxidation reaction on the metal of the piece to be treated, partially dissolving it into the solution. Said oxidation reaction occurs preferentially at asperities of the metal surface, which are then dissolved to a greater extent and smoothed. The overall effect is a reduction of piece roughness. With reference to Nb or Nb alloy surfaces, the solutions used for chemical polishing consist of 46% hydrofluoric acid (HF), 85% phosphoric acid (H3PO4) and 56% nitric acid (HNO3) mixed together i n a 1 : 2 : 1 vo l u m e rat i o, re s p ect ive l y . The solutions for electropolishing instead consist of 46% hydrofluoric acid (HF) and 98% sulfuric acid (H2SO4) mixtures in a 1:9 volume ratio. The typical time required to remove 200 pm of thickness of the material is between 6 and 10 hours, the voltage applied generally is from 5 V to 20 V.

An alternative approach is the so-called plasma electropolishing, or plasma electrolytic polishing (PEP), which is different from traditional electropolishing owing to the regime of higher voltages, from 200 V to 400 V, enabling faster processing times and the use of diluted solutions. PEP, in fact, uses diluted aqueous solutions of salts enabling to abate the generation of hazardous gases. The process is industrially known and applied for various metals and alloys, such as Chrome-Cobalt-Molybdenum, stainless steel, and brass, for which polishing formulas are available; whereas it is not used for treatment on Niobium and its alloys. In the literature, only one formula for Niobium plasma electropolishing is known [Y. Aliakseyeu, A. Bubulis, V. Minchenya, A. Korolyov, V. Niss, and R. Kandrotaite Janutiene, 'Plasma Electrolyte Polishing of Titanium and Niobium Alloys in Low Concentrated Salt Solution Based Electrolyte', mech, vol. 27 , no. 1, pp. 88-93, Feb. 2021, doi: 10.5755/j02.mech.25044] using a solution containing exclusively NH4F.

Despite the advantages of plasma electropolishing compared with the described polishing methods, singling out parameters of electrolyte composition and ideal conditions of time and current to be applied to the treatment still remains challenging.

Object of the present invention is to provide a composition of the polishing solution and optimal process parameters to be applied to the plasma electropolishing process plasma for the treatment of metal surfaces of Titanium (Ti), Zirconium (Zr), Hafnium (Hf), Vanadium (Va), Niobium (Nb), Tantalum (Ta), or from alloys of said metals, such as to obtain better results compared to the traditional electropolishing techniques.

Summary of the Invention

The present invention generally refers to the composition of a solution to be used in the plasma electropolishing process of metal surfaces. More particularly, the invention relates to an aqueous solution of soluble salts of hydrofluoric, boron hydrofluoric, fluorosilicic, or hexafluorotitanic acids, and the use of said solution in the related plasma electropolishing process of metal surfaces.

Object of the invention is the composition of an aqueous solution of soluble salts to be used in a plasma electropolishing process of metal surfaces; said solution comprising at least one salt selected from the following two groups, respectively:

A. Ammonium fluoride, NH4F; ammonium hydrogen difluoride, (NH^HFz; ammonium tetrafluoroborate, NH4BF4; ammonium hexafluorosilicate, (NF hSiFg; ammonium hexafluorotitanate, (NF hTiFg;

B. Sodium fluoride, NaF; potassium fluoride, KF; lithium fluoride, LiF; sodium hexafluorosilicate, NajSiFs; lithium hexafluorosilicate, LizSiFs; potassium hexafluorosilicate, KaSiFs; sodium hexafluorotitanate, NajTiFs; potassium hexafluorotitanate, KjTiFs, lithium hexafluorotitanate, LiaTiFs; moreover, object of the present invention is the plasma electropolishing process using said composition. In particular, said plasma electropolishing process relates to metal surfaces consisting of at least one of the following metals: Titanium (Ti), Zirconium (Zr), Hafnium (Hf), Vanadium (V), Niobium (Nb), Tantalum (Ta), or alloys of said metals.

Brief description of the figures

Figure 1: schematic description of the plasma electropolishing process: 1 - surface to be treated, 2 -composition for plasma electropolishing, 3 -second metallic member, 4- positive pole, 5 - negative pole, 6 - voltage generator.

Detailed description of the Invention

In the description of the present invention, by "surface to be polished" it is meant any one item exhibiting an exposed metal surface whose roughness has to be reduced; by "anode" it is meant the electrode of an electrochemical cell which undergoes an oxidation reaction, and by "cathode" the electrode in which a reduction reaction takes place.

According to the present invention, a composition of salts soluble in aqueous solution is made, to be used in a plasma electropolishing process of metal surfaces as defined in claim 1.

For a better understanding of the present invention a preferred embodiment is presently described, purely by way of a non-limiting example. Composition of salts in aqueous solution comprising at least one salt selected from the following two groups, respectively:

A. Ammonium fluoride, NH4F; ammonium hydrogen difluoride, (NH^HFz; ammonium tetrafluoroborate, NH4BF4; ammonium hexafluorosilicate, (NF hSiFg; ammonium hexafluorotitanate, (NF hTiFg;

B. Sodium fluoride, NaF; potassium fluoride, KF; lithium fluoride, LiF; sodium hexafluorosilicate, NajSiFs; lithium hexafluorosilicate, LizSiFs; potassium hexafluorosilicate, KaSiFs; sodium hexafluorotitanate, NajTiFs; potassium hexafluorotitanate, KjTiFs, lithium hexafluorotitanate, LiaTiFs-

According to an aspect of the invention the mass concentration of the salts selected in group A is from 1.0% to 5.0%, and the mass concentration of the salts selected in group B is from 0.5% to 3.0%.

In a preferred embodiment of the invention, the salt selected in group A is ammonium fluoride (NH4F) with a mass concentration of 3.0% and the salt selected in group B is sodium fluoride (NaF) with a mass concentration of 1.0%

In a second preferred embodiment of the invention, the salt selected in group A is ammonium hydrogen difluoride ((NF jHFz) with a mass concentration of 2.5% and the salt selected in group B is potassium fluoride (KF) with a mass concentration of 1.0%

In a third preferred embodiment of the invention, the salt selected in group A is ammonium tetrafluoroborate, NH4BF4 with a mass concentration of 3.0% and the salt selected in group B is lithium fluoride (LiF) with a mass concentration of 0.5%.

Advantageously, according to the invention, the composition of the described solution is used in the plasma electropolishing process of metal surfaces consisting of at least one of the following metals: Titanium (Ti), Zirconium (Zr), Hafnium (Hf), Vanadium (Va), Niobium (Nb), Tantalum (Ta), or alloys of said metals.

With reference to Figure 1, according to an aspect of the invention, said plasma electropolishing process comprises the steps of: a) Placing the surface to be polished 1 in contact with the composition 2; b) Placing at least one further metal element 3 in contact with the same composition, so that electrical contact between all metal surfaces through the same composition is ensured; c) Connecting the surface to be polished 1 to the positive pole 4 and the other elements 3 to the negative pole 5 of a voltage generator 6, so that the surface to be polished acts as the anode of an electrochemical cell; d) Setting the temperature of the composition so that it is between 60°C and 95°C; e) Applying a continuous potential difference between the two electrodes between 200 V and 400 V. Advantageously, according to the invention, the potential difference applied between the two electrodes is 300 V while the temperature of the solution is maintained at 85°C.

Object of the present invention is to provide a composition for plasma electropolishing of metal surfaces, and the related electropolishing process using said composition, such as to overcome the limitations of the known art.

An advantage of the present invention compared to the known art is the rate at which the desired treatment is carried out; in fact, the process according to the invention proves to be 10 times faster than the conventional electropolishing process and about 3.5 times faster than chemical polishing.

A further advantage of the present invention is the greater efficiency compared to the known art; in fact, the polishing effect of the treated surface is obtained with the removal of lesser amounts of material compared to the known processes.

Another advantage relates to the compositions subject-matter of the present invention, which by consisting of solutions at lower concentrations prove to be less hazardous for the environment and the operators' health compared to those used in the known art.

A further advantage of the invention is the ability to remove contaminating elements from the surface to be treated by virtue of the higher power employed for area unit, thereby avoiding the need to apply pre-treatments commonly used in the conventional electropolishing and chemical polishing processes: (degreasing, ultrasound cleaning, activation, polishing).