The present invention relates to a metal alloy and to a method for manufacturing products suitable for use in environments exposed to hydrogen.

As is known, hydrogen, indicated by the symbol H in its atomic form or H ₂ in its molecular form, is considered by many as the fuel of the future, given its extreme abundance in nature and most of all the lack of emission of pollutant gases when it is burned.

The only byproduct of hydrogen combustion is in fact water (H ₂ O).

Currently, hydrogen can be produced in gas form by means of various chemical or electrochemical processes that define its quality and characteristics.

Among the known processes, it is important to mention the electrolysis process, by means of which it is possible to obtain "clean" hydrogen (so-called green hydrogen), requiring water as the only source substance.

By using this process it is therefore possible to establish a perfect virtuous cycle, in which hydrogen is electrolytically extracted from water and is subsequently used to generate energy, again producing only water.

For this reason, hydrogen is rightfully considered an energy vector as well as an energy source.

Even considering the many advantages, first of all of environmental nature, linked to the use of hydrogen as fuel, the diffusion of these technologies is still very limited.

There are various reasons that have a negative impact in this regard.

Among those reasons, there is a significant economic and technological cost of the design and installation of dedicated infrastructures and systems.

Hydrogen in fact entails various difficulties in terms of management, particularly for storage, transport and use.

The high pressures involved, which reach hundreds of bars, mean that polymeric materials cannot be considered for many applications.

On the other hand, various metallic materials are characterized by significant embrittlement when they are exposed to hydrogen, especially in conditions of high pressure and/or temperature of the gas.

Among the high-performance alloys that are most affected by hydrogen embrittlement, there are alloys based on titanium (Ti), such as for example Ti Gr.2 and Ti Gr.5, and alloys based on nickel (Ni), such as the alloys Inconel® and Hastelloy®, among others.

While regarding ferrous alloys, carbon steels and stainless steels with a ferritic or martensitic structure have a better, but still rather limited, compatibility with respect to the preceding ones.

The only family of steels having a good resistance to hydrogen degradation is the Series 300 austenitic steels, including AISI 304L (UNS S30404) and AISI 316L (UNS S31603).

In particular, it has been demonstrated that the high content in nickel (Ni) and molybdenum (Mo) coupled with the austenitic matrix allows AISI 316L to excel in this regard, even with respect to the "poorer" AISI 304L.

However, despite having a good chemical-physical compatibility with hydrogen, those steels have low mechanical properties, which make them difficult to use for very high gas pressures.

The aim of the present invention is to obviate the drawbacks of the cited prior art.

Within the scope of this aim, a particular object of the invention is to provide a metal alloy and a method for manufacturing products made of the alloy that are suitable for use in environments exposed to hydrogen and at the same time have improved performance in terms of mechanical properties, being usable even at high pressures.

A further object of the invention is to provide a metal alloy, which is adapted for use with additive manufacturing methods.

A further object of the invention is to provide a metal alloy that is obtained by using fully recycled material.

Not least object of the invention is to provide a metal alloy and a method based mainly on the use of exhausted material and therefore advantageous also from an economic standpoint as well as from an environmental standpoint.

This aim and these objects, as well as others which will become better apparent hereinafter, are achieved by a metal alloy as claimed in the appended claims.

The aim and objects of the invention are also achieved by method as claimed in the appended claims.

Further characteristics and advantages will become better apparent from the description of preferred but not exclusive embodiments of a metal alloy and of a method according to the invention, particularly suitable for additive manufacturing methods for making products suitable for use in environments exposed to hydrogen.

The metal alloy according to the invention is an alloy with a ferrous base which includes at least chromium (Cr), nickel (Ni) and molybdenum (Mo).

In greater detail, this is a metal alloy which has a chemical composition that is consistent with the reference specifications provided by ASTM F3184-16, i.e., by the international standards for AISI 316L austenitic steel alloys used in additive manufacturing, but which is abundantly enriched in some specific alloy elements among the main ones.

In this regard, it should be noted in fact that among the austenitic steels of greatest interest the AISI 316L alloy has at the same time a good compatibility with the hydrogen environment and a wide diffusion in the field of 3D printing.

In the metal alloy according to the invention, the content of chromium (Cr) is preferably comprised between approximately 17% and approximately 18% by weight, the content of nickel (Ni) is comprised between approximately 12% and approximately 14% by weight, and the content of molybdenum (Mo) is comprised between approximately 2.2% and approximately 3% by weight.

Even more preferably, in the metal alloy according to the invention the content of nickel (Ni) is comprised between approximately 13% and approximately 14%, and the content of molybdenum (Mo) is comprised between approximately 2.4% and approximately 3% by weight.

In addition to the above cited elements, the metal alloy furthermore includes carbon (C), manganese (Mn), phosphorus (P), sulfur (S) and silicon (Si).

Preferably, the content of carbon (C) is lower than or equal to approximately 0.03% by weight, the content of manganese (Mn) is lower than or equal to approximately 2% by weight, the content of phosphorus (P) is lower than or equal to approximately 0.045% by weight, the content of sulfur (S) is lower than or equal to approximately 0.03% by weight, and the content of silicon (Si) is lower than or equal to approximately 1% by weight.

The metal alloy can be in powder form or in wire form, depending on the specific requirements of use.

Advantageously, the metal alloy according to the invention is obtained starting from fully recycled material, preferably using a method that is substantially equivalent to the one described in Italian patent application No. 102021000006527, in the name of this same Applicant, the content of which is to be understood as incorporated herein by explicit reference.

Substantially, the method comprises finding materials to be recycled, which essentially consist of processing byproducts, rejects or exhausted components, which are sorted and separated on the basis of their chemical composition.

If necessary, after separation, the materials to be recycled are subjected to cutting, shredding and/or chemical washing.

In order to provide the feedstock in the desired metal alloy, two or more materials to be recycled with different chemical compositions are selected and introduced in a crucible, where they are melted, so as to form a mixture.

Once the material has melted, the chemical composition of the mixture contained in the crucible is checked to determine whether it is substantially identical to that of the desired feedstock.

The expression "substantially identical chemical composition" indicates that the two materials have the same alloy elements with the same concentrations by weight, on the total weight of the alloy.

If the check yields a negative result, the method resumes substantially from the preceding selection step.

In practice, in order to correct the chemical composition of the mixture contained in the crucible, additional materials to be recycled, appropriately selected among those available, are introduced in the crucible. If instead the check yields a positive result, the mixture is extracted from the crucible to then produce the desired feedstock, which can be produced both in powder form and in wire form.

By means of the process described above, the chemistry of an appropriate base material is substantially enriched, aiming to maximize its response in environments exposed to hydrogen until the alloy according to the invention is obtained.

Table 1 lists the chemical composition of 316L austenitic steel for 3D printing according to the ASTM reference specification (ASTM F3184), and the chemical composition of the metal alloy according to the invention, in which the content of some of the key elements is maximized by successive steps, by means of the method described above, while complying with the acceptability intervals of the source steel.

Table 1 As listed in Table 1 , it is possible to choose a different enrichment level depending on the properties required for the product to be provided. The levels referenced herein, for the sake of simplicity, as Level 2 (Lv. 2) and Level 3 (Lv. 3), imply a growing precision in the enrichment process in order to produce a material that is increasingly alloyed but in any case in compliance with the initial ASTM specifications.

In this manner, by refining by successive steps the chemical composition of the base material (for example ASTM F3184 -> Lv. 2 -> Lv. 3), it is possible to obtain the most desirable conditions to have a product that has a high resistance in environments exposed to hydrogen.

Once the metal alloy with the desired chemical composition has been provided, the feedstock is used to 3D print the products of interest.

If the feedstock is in powder form, the powder is converted, by means of one of the most technologically relevant processes, according to the particle size of the powder.

Advantageous processes include, for example, binder jetting (BJ) or a similar process, laser beam powder bed fusion (LB-PBF) or a similar process such as selective laser melting, electron beam melting (EBM) or a similar process such as electron beam powder bed fusion (EB-PBF), laser-based direct energy deposition (L-DED) or a similar process.

If the feedstock is in the form of metal wire, it can be converted for example by means of the printing technology that provides for wire arc additive manufacturing (WAAM) or a similar process.

By means of one of those technologies, a product suitable for use in environments exposed to hydrogen is thus provided.

For example, the metal alloy and the method according to the invention allow to provide, among the various possible products, engineered components such as steel plates for fuel cells, steel plates for electrolyzers, and parts containing pressure, to be used for valves, pipes or systems for the storage and transport of hydrogen.

Experimental tests performed by the Applicant, and a careful analysis of the results, have allowed to demonstrate that the material printed starting from a feedstock made of the metal alloy according to the invention has mechanical properties that are vastly superior to the traditional counterpart, obtained for example from a bar or by forging, while maintaining excellent resistance to hydrogen embrittlement.

These two characteristics allow the products thus obtained to be used effectively for everything that relates to the use, transport and storage of hydrogen under pressure, from adjustment valves to the liners of pressure vessels.

Likewise, the response of the printed material, following electrochemical tests, has found that the metal alloy according to the invention is a valid solution for the provision of metal plates for fuel cells or electrolyzers.

Currently, in fact, various technologies of fuel cells and electrolyzers are affected by considerable problems of corrosion of the metal plates, especially on the cathode side.

These problems usually lead to the use of very expensive alloys such as titanium (Ti) alloys and/or to the application of protective coatings made of precious materials such as for example gold (Au).

Conversely, the metal alloy according to the present invention represents a more economical alternative in this regard, reducing the manufacturing costs of these apparatuses.

Not least advantage of the present invention is that it ensures the recycling of metallic material that has a high added value for the provision of components for the hydrogen production chain, with great benefits from the economic standpoint and from the environmental standpoint.

Other characteristics and advantages of the metal alloy and of the method according to the invention will become evident from the example that follows, given by way of non-limiting illustration.

EXAMPLE

In order to produce a feedstock in powder form, constituted by metal alloy according to the invention, 100% recycled material was used as a start.

In particular, waste material of the AISI 316 and 316L type, originally having the chemical composition in accordance with ASTM A479 - 20, was enriched with some specific alloy elements among the fundamental ones, i.e., chromium (Cr), nickel (Ni), and molybdenum (Mo). This was done to obtain a metal alloy that had a greater compatibility with hydrogen, according to the principles described earlier.

Table 2 shows the original chemical composition of the waste used (AISI 316L), compared with the reference ASTM standards (ASTM A479), and the final composition of the enriched powder, together with the reference specifications used (ASTM F3184).

The initial chemical composition was enriched within the atomization process, by adding to the initial AISI 316L austenitic steel some enrichment material, so as to bring the values of nickel (Ni) and chromium (Cr) to the limit of what is allowed and also raise the value of molybdenum (Mo). In particular, in order to perform the enrichment, waste materials were added which had a chemical composition according to the F55 super duplex steel reference alloys (high Cr content) and Inconel® 625 (high content of Ni and Mo).

The enrichment process can be advantageously provided according to what is described in Italian patent application No. 102021000006527 in the name of this same Applicant.

Table 2

Analysis of the results shows that the atomized material was found to be perfectly in line with the ASTM specification for AISI 316L steel for 3D printing.

In particular, with reference to what is described in Table 1 , a considerably enriched alloy which could be classified as Level 3 (Lv. 3) was chosen.

The powder thus produced was then used to print sample products by laser beam powder bed fusion (LB-PBF) technology.

The samples were characterized from a mechanical point of view, both in pristine conditions and after aging in a hydrogen-rich atmosphere; the tests performed showed that the printed material has a far superior response than the traditional counterpart from a bar or forging.

The yield strength and the breaking load obtained by means of the traction tests according to ASTM E8 are shown in the following Table 3.

The results obtained were compared with the acceptability intervals of the reference specifications, which were found to coincide for the bar/forging and the printed part.

Table 3

Direct comparison allows to appreciate the superiority of the material obtained by using the metal alloy according to the invention and by utilizing the LB-PBF 3D printing method with respect to the traditional counterpart. The use of waste material did not at all negatively condition the mechanical properties of the printed material, as confirmed in Table 3.

Indeed, the specimens have breaking strains and yield strengths that are far higher than the minimum requirements required by the reference specification.

Similar samples were subjected to a pre-exposure in a hydrogen environment, so as to evaluate the effect of this gas on the tensile properties of the printed steel.

The specimens were exposed to an atmosphere containing pure hydrogen (>99.9%), at a temperature of 300°C and a pressure equal to 100 bar, for a total time comprised between 5 and 15 days.

The mechanical tests were repeated at the end of this period. Table 4 shows a comparison between the tensile properties before and after exposure in hydrogen:

Table 4

The tests performed on the printed samples demonstrate the soundness of the material for applications in the hydrogen field.

The yield and breaking loads remain substantially constant and so does the useful area reduction.

The breaking elongation decreases by a few percentage points (approximately 10%) following the tests. However, this decrease, if compared with the literature and with the reference specifications, is not per se a negative result.

In this regard, for example, the values quoted by D.-H. Lee et al., highlighted in figure 1 , allow to deduce that the printed material exhibits a reduction in breaking elongation of 7% and 19% after 1 and 5 days respectively, values far higher than what has been observed here.

A variation of the breaking elongation from 44% ± 1% to 39% ± 3% is considered acceptable both with respect to the ISO 6892-1 reference specification (maximum variation allowed for 316L equal to 15%) and with respect to ASTM F3184 (minimum elongation value set at 30%).

Accordingly, therefore, the above cited alloy obtained with powder from recycling is an ideal candidate for producing components for the hydrogen production chain.

The detailed description and the example given above clearly show the advantages achieved by means of the metal alloy and the method according to the present invention.

The metal alloy and the method, particularly for providing products suitable for use in environments exposed to hydrogen, according to the invention, are susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with technically equivalent

SUBSTITUTE SHEET (RULE 26) elements.

The materials used, as well as the dimensions and shapes, may of course be any according to the requirements and the state of the art.

This application claims the priority of Italian Patent Application No. 102021000024502, filed on September 23, 2021 the subject matter of which is incorporated herein by reference.

SUBSTITUTE SHEET (RULE 26)