CORROSION-RESISTANT ALLOY COMPOSITION

Abstract

An alloy composition includes 40-60 wt. % of a nickel-based superalloy and 40-60 wt. % of a cobalt-based superalloy based on the total weight of the alloy composition. The nickel-based superalloy includes 40-60 wt. % of nickel and 15-25 wt. % of chromium based on the total weight of the nickel-based superalloy. The cobalt-based superalloy includes 50-70 wt. % of cobalt and 25-35 wt. % of chromium based on the total weight of the cobalt-based superalloy. The nickel-based superalloy and the cobalt-based superalloy are homogeneously distributed in the alloy composition. Further, the alloy composition is a spark plasma product of spherical particles having an average particle size of 10 micrometers (m) to 45 m of the nickel-based superalloy and particles having an average particle size of 5-40 m of the cobalt-based superalloy. The alloy composition is more corrosion-resistant than a pure nickel-based superalloy and a pure copper-based superalloy.

Claims

1. An alloy composition, comprising: 40-60 wt. % a nickel-based superalloy; and 40-60 wt. % a cobalt-based superalloy based on a total weight of the alloy composition, wherein the nickel-based superalloy comprises 40-60 wt. % of nickel and 15-25 wt. % of chromium based on a total weight of the nickel-based superalloy, the cobalt-based superalloy comprises 50-70 wt. % of cobalt and 25-35 wt. % of chromium based on a total weight of the cobalt-based superalloy, the nickel-based superalloy and the cobalt-based superalloy are homogeneously distributed in the alloy composition, and the alloy composition is a spark plasma product of spherical particles having an average particle size of 15 to 45 m of the nickel-based superalloy and particles having an average particle size of 5 to 40 m of the cobalt-based superalloy.

2. The alloy composition of claim 1, wherein: the nickel-based superalloy comprises Inconel 718, and the cobalt-based superalloy comprises Co 212.

3. The alloy composition of claim 2, wherein the Inconel 718 comprises: 52.4 wt. % of Ni; 18.85 wt. % of Cr; 4.96 wt. % of Nb and Ta in total, 3.09 wt. % of Mo; 0.95 wt. % of Ti; 0.48 wt. % of Al; 0.05 wt. % of Co; 0.04 wt. % of Si; 0.04 wt. % of C; 0.02 wt. % of Mn; 0.02 wt. % of Cu; and Fe, based on the total weight of the nickel-based superalloy.

4. The alloy composition of claim 3, wherein the Co 212 comprises: 28.5 wt. % of Cr; 6 wt. % of Mo; 0.75 wt. % of Fe; 0.35 wt. % of C; 1.0 wt. % or less of Ni; 1.0 wt. % or less of Si; 1.0 wt. % or less of Mn; and Co, based on the total weight of the cobalt-based superalloy.

5. The alloy composition of claim 4, wherein: the Inconel 718 and the Co 212 has a weight ratio of 1:1 in the alloy composition.

6. The alloy composition of claim 5, wherein: the alloy composition is more corrosion-resistant than a pure Inconel 718 alloy and a pure Co 212 alloy.

7. The alloy composition of claim 6, wherein: the alloy composition has a corrosion rate (CR) of about 6.0410.sup.3 mils per year (mpy) in a solution containing 3.5 wt. % of NaCl for an open circuit potential of 10 mV, and the CR of the alloy composition is 55%-75% lower than those of the pure Inconel 718 alloy and the pure Co 212 alloy.

8. The alloy composition of claim 6, wherein: the alloy composition has a corrosion current density (I.sub.corr) of about 41.5 nA in a solution containing 3.5 wt. % of NaCl for an open circuit potential of 10 mV, and the Icorr of the alloy composition is 55%-75% lower than those of the pure Inconel 718 alloy and the pure Co 212 alloy.

9. The alloy composition of claim 6, wherein: the alloy composition has a polarization resistance (Rp) of about 628.1 k in a solution containing 3.5 wt. % of NaCl for an open circuit potential of 10 mV, and the R.sub.p of the alloy composition is 150%-270% higher than those of the pure Inconel 718 alloy and the pure Co 212 alloy.

10. The alloy composition of claim 6, wherein: corrosion protectiveness of the alloy composition is 150%-200% higher than those of the pure Inconel 718 alloy and the pure Co 212 alloy.

11. The alloy composition of claim 1, wherein: the nickel-based superalloy and the cobalt-based superalloy form separate phases that are homogeneously distributed in the alloy composition.

12. The alloy composition of claim 11, wherein: an interface between the separate phases has no secondary phases, reaction products, de-bonded areas or voids.

13. The alloy composition of claim 11, wherein: the separate phases have an average dimension of 20 to 60 m.

14. The alloy composition of claim 1, wherein the alloy composition is obtained by: obtaining powders of Inconel 718 and powders of Co 212 by gas atomization; forming a mixture of the powders of Inconel 718, the powders of Co 212 and ethanol in an ultrasonic probe sonicator; removing the ethanol from the mixture by heating the mixture; and densifying the mixture by spark plasma sintering.

15. The alloy composition of claim 14, wherein: the spark plasma sintering is executed at a pressure of 50 MPa and a temperature of 1100 C. for 15 minutes, with a heating rate of 100 C./min.

16. The alloy composition of claim 14, wherein obtaining the alloy composition further comprises: grinding the mixture on a diamond disk to remove a graphite film used in the spark plasma sintering; grinding the mixture with a SiC sandpaper of 120 to 180 grit; polishing the mixture on a polishing cloth with a diamond paste solution; and rinse the mixture with ethanol.

17. The alloy composition of claim 14, wherein: the powders of Inconel 718 have an average particle size of 15 to 45 m as a result of the gas atomization, and the powders of Co 212 have an average particle size of 10 to 20 m as a result of the gas atomization.

18. The alloy composition of claim 14, wherein: the gas atomization is executed with argon.

19. The alloy composition of claim 14, wherein: the mixture is heated at 70 C. for 24 hours to remove the ethanol from the mixture.

20. The alloy composition of claim 1, wherein: the nickel-based superalloy is Inconel 718, the cobalt-based superalloy is Co 212, and the alloy composition consists of about 50 wt. % of Inconel 718 and about 50 wt. % of Co 212 based on the total weight of the alloy composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] A more complete appreciation of this disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

[0029] FIG. 1 is a schematic flowchart depicting a method of making an alloy composition according to certain embodiments.

[0030] FIG. 2 is a schematic illustration of a die setup using materials made of graphite to conduct spark plasma sintering (SPS), according to certain embodiments.

[0031] FIG. 3 is a schematic block diagram depicting an exemplary electrochemical test setup, according to certain embodiments.

[0032] FIG. 4A is a scanning electron microscopy (SEM) image of an alloy composition (including 50% Inconel 718 and 50% Co 212) showing a distribution pattern, according to certain embodiments.

[0033] FIG. 4B is a SEM image of the alloy composition depicting interfacial characteristics between the Co 212 alloy and the INC 718 alloy, according to certain embodiments.

[0034] FIG. 5A is an optical image depicting energy dispersive X-ray spectroscopy (EDX) results for the alloy composition, according to certain embodiments.

[0035] FIG. 5B is an optical image showing the presence of chromium (Cr) in EDX mapping of the alloy composition, according to certain embodiments.

[0036] FIG. 5C is an optical image showing the presence of iron (Fe) in EDX mapping of the alloy composition, according to certain embodiments.

[0037] FIG. 5D is an optical image showing the presence of nickel (Ni) in EDX mapping of the alloy composition, according to certain embodiments.

[0038] FIG. 5E is an optical image showing the presence of cobalt (Co) in EDX mapping of the alloy composition, according to certain embodiments.

[0039] FIG. 6 is a graph depicting X-ray diffraction spectra for 100% INC 718 alloy, 100% Co 212 alloy, and the alloy composition, according to certain embodiments.

[0040] FIG. 7 is a graph depicting open circuit potential curves for 100% INC 718 alloy, 100% Co 212 alloy, and the alloy composition, according to certain embodiments.

[0041] FIG. 8A is an electrochemical impedance spectroscopy (EIS) curve showing Nyquist plots for 100% INC 718 alloy, 100% Co 212 alloy, and the alloy composition in a 3.5% NaCl solution at 25 C., according to certain embodiments.

[0042] FIG. 8B is a graph depicting Bode plots for 100% INC 718 alloy, 100% Co 212 alloy, and the alloy composition in a 3.5% NaCl solution at 25 C., according to certain embodiments.

[0043] FIG. 8C is a schematic electrical circuit used to fit the EIS data, according to certain embodiments.

[0044] FIG. 9 shows linear polarization resistance (LPR) plots and the variation of corrosion rate for 100% INC 718 alloy, 100% Co 212 alloy, and the alloy composition, according to certain embodiments.

[0045] FIG. 10A is a cyclic potentiodynamic polarization (CPDP) plot of the 100% INC 718 alloy in the 3.5% NaCl solution at 25 C., according to certain embodiments.

[0046] FIG. 10B is a CPDP plot of the 100% Co 212 alloy in the 3.5% NaCl solution at 25 C., according to certain embodiments.

[0047] FIG. 10C is a CPDP plot of the alloy composition in the 3.5% NaCl solution at 25 C., according to certain embodiments.

DETAILED DESCRIPTION

[0048] In the drawings, like reference numerals designate identical or corresponding parts throughout the several views. Further, as used herein, the words a, an and the like generally carry a meaning of one or more, unless stated otherwise.

[0049] Furthermore, the terms approximately, approximate, about, and similar terms generally refer to ranges that include the identified value within a margin of 20%, 10%, or preferably 5%, and any values therebetween.

[0050] Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the disclosure are shown.

[0051] As used herein, the words about, approximately, or substantially similar may be used when describing magnitude and or position to indicate that the value and/or position described is within a reasonable expected range of values and/or positions. For example, a numeric value may have a value that is +/0.1% of the stated value (or range of values), +/1% of the slated value (or range of values), +/2% of the stated value (or range of values), +/5% of the slated value (or range of values), +/10% of the staled value (or range of values), +/15% of the stated value (or range of values), or +/20% of the stated value (or range of values). Within the description of this disclosure, where a numerical limit or range is stated, the endpoints are included unless stated otherwise. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

[0052] The present disclosure is intended to include all hydration states of a given compound or formula, unless otherwise noted or when heating a material. For example, Ni(NO.sub.3).sub.2 includes anhydrous Ni(NO.sub.3).sub.2, Ni(NO.sub.3).sub.2.Math.6H.sub.2O, and any other hydrated forms or mixtures.

[0053] In addition, the present disclosure is intended to include all isotopes of atoms occurring in the present compounds and complexes. Isotopes include those atoms having the same atomic number but different mass numbers. By way of general example, and without limitation, isotopes of hydrogen include deuterium and tritium.

[0054] Aspects of the present disclosure are directed to an alloy composition fabricated through a spark plasma sintering (SPS) technique. The alloy composition, having a blend of 50% nickel-based superalloy (IN718) and 50% copper-based superalloy (Co212), results in superior corrosion performance compared with the individual In718 and Co212 alloys, making it an ideal candidate for a wide range of engineering applications.

[0055] An alloy composition is described. The alloy composition includes 40-60 wt. %, preferably 45-55 wt. %, preferably 47-52 wt. %, of a nickel-based superalloy and 40-60 wt. %, preferably 45-55 wt. %, preferably 47-52 wt. %, of a cobalt-based superalloy, based on the total weight of the alloy composition. The nickel-based alloy includes nickel in an amount of 40-60 wt. %, preferably 45-55 wt. %, preferably 50-55 wt. %, and more preferably 52-53 wt. %. The nickel-based superalloy also includes chromium. Chromium is generally added to improve anti-corrosion properties and is particularly effective when used in an environment rich in chloride ions. However, it is not very effective if its content in the nickel-based superalloy composition is less than 15 wt. %. Also, if present in higher amounts beyond 25 wt. %, it can impact the dissolution of other elements, such as molybdenum and tantalum, in the matrix of the nickel-based superalloy-thereby deteriorating the anti-corrosion properties of the alloy. Therefore, it is desirable to have chromium in an amount of 15-25 wt. %, preferably 17-23 wt. %, preferably 19-21 wt. %, based on the total weight of the nickel-based superalloy. The nickel-based superalloy also includes other elements such as, but not limited to, niobium, tantalum, molybdenum, titanium, aluminum, cobalt, silicon, carbon, manganese, copper, and iron.

[0056] In some embodiments, the nickel-based superalloy includes 45-60 wt. % of nickel, preferably 50-55 wt. % of nickel, preferably about 52-53 wt. % of nickel; 10-25 wt. % of chromium, preferably 15-25 wt. % of chromium, preferably 18-19 wt. %, of chromium; 1-10 wt. %, of a mixture of niobium and tantalum, preferably 2-9 wt. % of a mixture of niobium and tantalum, preferably 3-6 wt. % of a mixture of niobium and tantalum, preferably between 4-5 wt. % of a mixture of niobium and tantalum; 1-5 wt. % of molybdenum, preferably 2-4 wt. % of molybdenum, preferably 2.5-3.5 wt. % of molybdenum; 0.1-2 wt. % of titanium, preferably 0.5-1.5 wt. % of titanium, preferably 0.75-1 wt. % of titanium; 0.1-1 wt. % of aluminum, preferably 0.2-0.8 wt. % of aluminum, preferably 0.3-0.5 wt. % of aluminum; 0.01-0.1 wt. % of cobalt, preferably 0.03-0.07 wt. % of cobalt, preferably 0.04-0.06 wt. % of cobalt; 0.01-0.1 wt. % of silicon, preferably 0.03-0.07 wt. % of silicon, preferably 0.04-0.05 wt. % of silicon; 0.01-0.1 wt. % of carbon, preferably 0.03-0.07 wt. % of carbon, preferably 0.04-0.05 wt. % of carbon; 0.01-0.1 wt. % of manganese, preferably 0.01-0.05 wt. % of manganese, preferably 0.02-0.03 wt. % of manganese; 0.01-0.1 wt. % of copper, preferably 0.01-0.05 wt. % of copper, preferably 0.02-0.03 wt. % of copper; and the balance being iron and other impurities (about 19-20 wt. %), each based on the total weight of the nickel-based superalloy.

[0057] In a specific embodiment, the nickel-based superalloy includes Inconel 718. The Inconel 718 includes 52.4 wt. % of nickel, 18.84 wt. % of chromium, 4.96 wt. % of a mixture of niobium and tantalum, and 3.09 wt. % of molybdenum, 0.95 wt. % of titanium, 0.48 wt. % of aluminum, 0.05 wt. % of cobalt, 0.04 wt. % of silicon, 0.04 wt. % of carbon, 0.02 wt. % of manganese, 0.02 wt. % of copper, and about 19.1 wt. % of iron and optionally impurities, each based on the total weight of the nickel-based superalloy.

[0058] The alloy composition further includes the cobalt-based superalloy. The cobalt-based superalloy includes cobalt in an amount of 50-70 wt. % of cobalt, preferably 55-65 wt. %, and more preferably of about preferably 60-65 wt. % of cobalt based on the total weight of the cobalt-based superalloy. The cobalt-based superalloy further includes about 25-35 wt. %, preferably 26-30 wt. %, preferably 27-29 wt. % of chromium based on the total weight of the cobalt-based superalloy. The cobalt-based superalloy also includes other elements such as molybdenum and trace amounts of iron, carbon, nickel, silicon, and manganese. In an embodiment, the cobalt-based superalloy includes molybdenum in an amount of 1-10 wt. %, preferably 2-9 wt. %, preferably 3-7 wt. %, preferably 4-6.5 wt. %, preferably about 5.5-6.5 wt. %; iron in an amount of 0.1-1 wt. %, preferably 0.2-0.9 wt. %, preferably 0.3-0.8 wt. %, preferably 0.4-0.75 wt. %, preferably 0.6-0.75 wt. %; carbon in an amount of 0.1-1 wt. %, preferably 0.1-0.5 wt. %, preferably 0.2-0.4 wt. %; nickel in an amount of 0-1.0 wt. %, preferably 0.1-0.6 wt. %, preferably 0.2-0.3 wt. %; silicon in an amount of 0-1.0 wt. %, preferably 0.1-0.6 wt. %, preferably 0.2-0.3 wt. %; and manganese in an amount of 0-1.0 wt. %, preferably 0.1-0.6 wt. %, preferably 0.2-0.3 wt. %; each based on the total weight of the cobalt-based superalloy.

[0059] In a specific embodiment, the cobalt-based superalloy includes Co 212. The Co 212 includes 28.5 wt. % of Cr, 6 wt. % of Mo, 0.75 wt. % of Fe, 0.35 wt. % of C, 1.0 wt. % or less of Ni, 1.0 wt. % or less of Si, and 1.0 wt. % or less of Mn, the balance being Co and optionally impurities, based on the total weight of the cobalt-based superalloy.

[0060] The alloy composition includes nickel-based superalloy and copper-based superalloy in a weight ratio range of 1:5 to 5:1, preferably 1:4 to 4:1, preferably 1:3 to 3:1, preferably 1:2 to 2:1, preferably 1:1. In a specific embodiment, the alloy composition includes Inconel 718 and Co 212 in a weight ratio range of 1:5 to 5:1, preferably 1:4 to 4:1, preferably 1:3 to 3:1, preferably 1:2 to 2:1, preferably 1:1. In other words, the alloy composition can include 50 wt. % of the nickel-based superalloy, particularly, Inconel 718, and 50 wt. % of the copper-based superalloy, particularly, Co 212. In said ranges, the nickel-based superalloy and copper-based superalloy are homogeneously distributed in the alloy composition.

[0061] In some embodiments, the alloy composition consists of 50 wt. % of Inconel 718 and 50 wt. % of Co 212 based on the total weight of the alloy composition. Referring to FIG. 1, a method 50 of making the alloy composition is described. The order in which the method 50 is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method 50. Additionally, individual steps may be removed or skipped from the method 50 without departing from the spirit and scope of the present disclosure.

[0062] At step 52, the method 50 includes obtaining powders of Inconel 718 and powders of Co 212 by gas atomization. Gas atomization is a process in which the liquid metals of Inconel 718 and/or Co 212 are dispersed by a high-velocity jet of inert gas, typically argon or nitrogen, preferably argon. The collision breaks the liquid metals into fine spherical powder particles solidifying in flight. The average particle size of the Inconel 718 powder produced by the gas atomization process is about 15 to 45 m, preferably 20-40 m, preferably 25-35 m, preferably 28-32 m while the average particle size of the Co 212 powder produced by the gas atomization process is about 10-20 m, preferably 12-18 m, preferably 14-16 m, preferably 16 m. In some embodiments, the powder of Inconel 718 has a particle size of about 5, 7, 8, 10, 12, 15, 16, 17, 18, 19, 20, 22, 24, 26, 28, 30, 32, 34, 46, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60 m, and the Co 212 powder has particle sizes of about 1, 2, 3, 4, 5, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30 m, with the average particle size being 16 m. In some embodiments, the particle sizes of the powders beyond these ranges are also possible. Several factors determine the particle size of powders produced by the gas atomization methodfor example, the liquid-to-inert gas ratio, the velocity of the inert gas, etc., and the parameters can be adjusted depending on the desired particle size. The particles of the Inconel 718 are preferably spherical.

[0063] At step 54, the method 50 includes forming a mixture of the powders of Inconel 718, the powders of Co 212, and ethanol in an ultrasonic probe sonicator. The mixture, including the powders of Inconel 718 and Co 212, is dissolved in ethanol to homogenize the mixture. Although ethanol is preferred, other alcohols, such as methanol, butanol, isopropanol, or a mixture thereof, may also be used. In an embodiment, the ethanol is neat ethanol. The homogenization of the mixture is done by sonicating the mixture at room temperature for about 10-60 minutes, preferably 12, 14, 16, 18, 20, 22, 24, 26, 28, and 30 minutes. Other techniques to homogenize the powders, such as stirring, agitation, swirling, and the like, may also be used.

[0064] At step 56, the method 50 includes removing the ethanol from the mixture by heating the mixture. The ethanol/or any other solvent used to homogenize the mixture can be evaporated by heating the mixture to a temperature of about 60-80 C., preferably 65-75 C., preferably to about 70 C. for 15-30 hours, preferably 18-28 hours, preferably 20-24 hours, preferably 24 hours. Heating appliances such as hot plates, muffle furnaces, tube furnaces, heating mantles ovens, microwaves, autoclaves and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, or hot-air guns can be used to remove the ethanol from the mixture.

[0065] At step 58, the method 50 includes densifying the mixture by spark plasma sintering (SPS) to obtain the alloy composition. The mixture is densified by SPS. As used herein, spark plasma sintering (SPS), which is also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), is a sintering technique, in which the pulsed DC current directly passes through a graphite die, as well as the mixture, in the case of conductive samples. Joule heating has been found to play a role in the densification of powder compacts, achieving near theoretical density at lower sintering temperatures than conventional sintering techniques. The heat generation is internal, in contrast to the conventional hot pressing, where the heat is provided by external heating elements. This facilitates a very high heating or cooling rate. Hence, the sintering process generally is very fast. The general speed of the process ensures it has the potential to densify powders/mixtures with nanosize or nanostructure while avoiding coarsening, which accompanies standard densification routes.

[0066] The mixture can be fed directly into a graphite die without a pre-compaction step (e.g., by vibration or applying suitable pressure). The graphite die has a thickness of about 10-30 mm, preferably 15-25 mm, preferably about 20 mm. The die containing the mixture can be placed directly in an SPS chamber or furnace, and spacers can be used if necessary. In some embodiments, a thin graphite foil, preferably a graphite film/sheet, was used as a spacer between the mixture and the die to facilitate sample ejection after sintering, to reduce the friction between the die walls and the mixture, and to prevent punch wear. The graphite sheet has a thickness of about 0.2-0.4 mm, preferably 0.22 mm, 0.24 mm, 0.26 mm, 0.28 mm, 0.3 mm, 0.32 mm, 0.34 mm, 0.36 mm, and 0.38 mm. In a preferred embodiment, the graphite sheet has a thickness of about 0.35 mm. The graphite die was further covered with a graphite blanket during sintering to reduce heat loss. The SPS chamber is closed, and the sintering is carried out under an argon atmosphere with a partial vacuum at a pressure of no higher than 100 MPa being applied in the chamber, preferably 30-100 MPa, such as 32 MPa, 34 MPa, 36 MPa, 38 MPa, 40 MPa, 45 MPa, 50 MPa, 55 MPa, 60 MPa, 65 MPa, 70 MPa, 75 MPa, 80 MPa, 85 MPa, 90 MPa, 95 MPa, and 100 MPa. In a preferred embodiment, the sintering is carried out under an argon atmosphere with a partial vacuum at a pressure of about 50 MPa. The SPS heating rate is 50-300 C./min, preferably 50-400 C./min, preferably 150-200 C./min. The SPS heating time or holding period is 2-20 min, preferably 5-15 min or 15 min. The SPS temperature is no higher than 1200 C., for example, 900 C. to 1200 C., preferably 1050 C. to 1150 C., more preferably 1100 C. to 1150 C., and even more preferably 1100 C. to obtain a mixture. The mixture includes the alloy composition.

[0067] The mixture is further ground on a diamond disk to remove a graphite film used in the SPS. The mixture may be contaminated with traces of graphite. This may be removed using SiC abrasives, preferably SiC papers of varying grit sizes from 120 to 800 grit, preferably 300-600 grit, preferably 400-500 grit. The mixture is further polished using a polishing cloth with a diamond paste solution and then rinsed with ethanol to remove any impurities to obtain the alloy composition of the present disclosure. It is a spark plasma product of spherical particles with an average particle size of 15 to 45 m of the nickel-based superalloy and particles with an average particle size of 10 to 20 m of the cobalt-based superalloy.

[0068] The microstructures of the alloy composition, as revealed by scanning electron microscopy, transmission electron microscopy, or other equivalent microscopy techniques, reveal that the nickel-based superalloy and the cobalt-based superalloy form separate phases and are homogeneously distributed in the alloy composition. No secondary phases, reaction products, de-bonded areas, or voids are observed at the interface between the two separate phases. The separate phases have an average dimension of 20 to 60 m.

[0069] The alloy composition, including Inconel 718 and Co 212 in a weight ratio of 1:1, is more corrosion-resistant than a pure Inconel 718 alloy and a pure Co 212 alloy. The corrosion resistance can be measured with various parameters, such as corrosion rate (CR), corrosion current density (I.sub.corr), polarization resistance (Rp), corrosion protectiveness, and the like. CR is the speed at which any given metal deteriorates in a specific environment. A low CR indicates a higher lifespan and vice versa. In some embodiments, the alloy composition has a corrosion rate (CR) of about 6.0410.sup.3 mils per year (mpy) in a solution containing 3.5 wt. % of NaCl, which is at least 55%-75% lower than those of the pure Inconel 718 alloy (about 15.9210.sup.3 mpy) and the pure Co 212 alloy (about 21.6310.sup.3 mpy).

[0070] In some embodiments, the alloy composition, including Inconel 718 and Co 212 in a weight ratio 1:1, has a corrosion current density (I.sub.corr) of about 41.5 nA in a solution containing 3.5 wt. % of NaCl. The I.sub.corr of the alloy composition is 55%-75% lower than that of the pure Inconel 718 alloy (109.04 nA) and the pure Co 212 alloy (148.6 nA). In some embodiments, the alloy composition, including Inconel 718 and Co 212 in a weight ratio of 1:1, has a polarization resistance (Rp) of about 628.1 k in a solution containing 3.5 wt. % of NaCl. In such embodiments, the Rp of the alloy composition is 150%-270% higher than those of the pure Inconel 718 alloy (238.1 cm.sup.2) and the pure Co 212 alloy (175.3 (2 cm.sup.2).

[0071] In some embodiments, the alloy composition, including Inconel 718 and Co 212 in a weight ratio 1:1, has a corrosion protectiveness 150%-200% higher than that of the pure Inconel 718 and Co 212 alloys. The increased corrosive protectiveness can be attributed to the synergistic effect of the Co 212 and Inconel 718 in the alloy composition, where the individual elements of the alloy composition, when used in defined weight percentages as will be provided in Table 1, work together to influence the ability to quickly form a passive film over the surface of the alloy composition, compared to the individual pure alloys.

EXAMPLES

[0072] The following examples describe an alloy composition for improved corrosion resistance. The examples are provided solely for illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.

Example 1: Materials

[0073] The starting materials were nickel alloy 718 powder of grade: LPW-718API-A APE, Carpenter Additive, UK and cobalt superalloy 212. Inconel 718 and INC 718 are used interchangeably in the present disclosure. Both superalloy powders were produced by an argon gas atomized method. The Inconel 718 powder had a spherical shape with an average particle size of about 15 micrometers (m) to 45 m. Further, the Co 212 powder had an average particle size of about 16 m. An elemental composition of a precursor of the powders is listed in Table 1 as a weight percentage (wt. %). An ultrasonic probe sonicator (Model VC 750, Sonics, USA) was used in conjunction with ethanol as a mixing liquid to homogenize the samples. After 30 minutes of probe sonication, the samples were heated to about 70 C. for 24 hours to evaporate the ethanol. Referring to FIG. 2, a schematic diagram of Spark Plasma Sintering (SPS) equipment (FCT System, HP D5, Germany) is illustrated. The SPS equipment was used to consolidate the powder mixtures. The powder combination was pressed at 50 mega Pascals (MPa) and heated at a rate of 100 degrees Celsius per minute ( C./min), while being placed in a 20 mm graphite die. The holding period was 15 minutes, and all the samples were sintered at 1100 C.

[0074] A graphite sheet of a thickness of about 0.35 mm was placed between the graphite die and the powders to make it easier to remove the sample from the die and prevent punch wear. The die was further covered with a graphite blanket during sintering to reduce heat loss. The actual temperature during sintering was measured using a thermocouple and a pyrometer. The thermocouple was placed in the graphite die beside the sample to measure the temperature; the thermocouple can measure temperatures up to 400 C. In some cases, a pyrometer was used for monitoring higher temperatures. The sintered samples were initially ground on a diamond disk to remove the graphite film on the surface. Furthermore, the surfaces of the samples were ground with silicon carbide (SiC) papers of varying grit sizes ranging from 120 grits to 800 grits. Mirror-like surfaces were produced by polishing the samples on a polishing cloth with a diamond paste solution and subsequently the samples were rinsed in ethanol. The density of the samples was measured using Archimedes method with a density measurement kit (Mettler Toledo) in an immersion medium of deionized water. The microstructure was examined using an optical microscope (DSX510, Olympus, Japan) and a field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) was employed for the phase analysis. A study on X-ray diffraction was conducted utilizing X-rays with wavelengths of 0.154 nm, 10 mA current, and 30 kV accelerating voltage, respectively.

TABLE-US-00001 TABLE 1 Elemental composition of powders of the precursor Element Ni Cr Nb + Ta Mo Ti Al Co Si C Mn Cu Fe INC Wt. % 52.4 18.85 4.96 3.09 0.95 0.48 0.05 0.04 0.04 0.02 0.02 Bal. 718 Element Co Cr Mo Fe C Ni Si Mn Co Wt. % Bal 28.5 6 0.75 0.35 1.0 1.0 1.0 212

Example 2: Electrochemical Measurement

[0075] The samples were rinsed with distilled water, followed by sonication in acetone, and further air-dried before commencement of electrochemical test. The electrochemical test solution was aerated with 3.5% sodium chloride (NaCl) solution. An experimental setup for the electrochemical corrosion measurement is shown in FIG. 3. The experimental setup includes a graphite counter electrode, a silver/silver chloride (Ag/AgCl) reference electrode, and the specimen to be tested as the working electrode. Readings were acquired using the Gamry potentiostat/Galvanostat (Reference 600) instrument. The working electrode was immersed in a 3.5% NaCl test solution for a period of about 1 hour where the open circuit potential (OCP) was measured, before performing electrochemical impedance spectroscopy (EIS), linear polarization resistance (LPR), or cyclic potentiodynamic polarization (CPDP) measurements.

[0076] For all the testing, the OCP measurement was monitored for 1 hour to ensure it maintains a steady state condition, after which the EIS was initiated in the frequency range from 10 MHz to 100 kHz with an AC amplitude peak-to-peak value of +10 mV (vs OCP). LPR measurements were then carried out by a potential dynamic scanning from +0.025 V (vs OCP) at a scanning rate of 0.125 mV/s. In addition, the CPDP test was conducted by polarizing the system with a forward scan rate of 1 mV/s to a potential of 0.25 V cathodically and 0.25 V anodically against the OCP. The reverse scan rate was 2 mV/s. Data analysis and curve fittings were performed utilizing Gamry EChem Analyst 5.5 software.

Example 3: Microstructural Characteristics

[0077] The scanning electron micrographs of the alloy composition including 50% Inconel 718 and 50% Co 212 is shown in FIGS. 4A-4B. The microstructural characterization, as shown in FIG. 4A, revealed a homogeneous distribution of INC 718 and Co 212 alloys within each other, displaying excellent interfacial integrity. The interface between INC 718 particles and Co 212, as shown in FIG. 4B, exhibited no discernible secondary phases, reaction products, de-bonded areas, or voids. Additionally, sporadic occurrences of individual INC 718 particles were observed without clustering in the Co 212 alloy. The relative uniform distribution pattern of INC 718 alloy in the Co 212 alloy, and vice versa may be attributed to the use of suitable blending parameters and the adoption of appropriate sintering parameters.

[0078] The EDX mapping of the etched alloy composition is shown in FIGS. 5A-5E. The elemental mapping confirmed the presence of Cr, Fe, Ni, and Co metals in the alloy composition. The distribution of the elements confirmed that the unetched darker part of the image represents the Co 212 alloy while the etched lighter areas represent the INC 718 alloy. Further, the XRD results corresponding to pure INC 718, pure Co 212, and the alloy composition were analyzed, as shown in FIG. 6. The pattern of the alloy composition was compared with that of pure INC 718 and Co 212 alloys and other related phases. However, only peaks corresponding to the pure INC 718 and pure Co 212 alloys were identified in the alloy composition.

Example 4: Electrochemical Corrosion Test

[0079] The open circuit potential (OCP) measurement served to observe the unaltered corroding potential of the specimen in the absence of any applied external current. It is anticipated that the system may reach a stable state when the potential is independent of the immersion duration. Consequently, the OCP serves as an indicator of the stability of an electrochemical environment before system polarization, offering insights into the thermodynamic inclination towards corrosion of the samples. Referring to FIG. 7, a graph showing the OCP of the samples is illustrated. FIG. 7 reveals that the specimens achieved equilibrium after the 1 hour monitoring period. Both the INC 718 and the alloy composition reached a stable state, displaying a nobler potential in contrast to the Co 212 alloy. In particular, the Co 212 alloy exhibited a potential of approximately 243.7 mV, whereas the INC 718 and the alloy composition maintained stable potentials of 139.3 mV and 146.1 mV, respectively, after the 1 hour monitoring period. Furthermore, the observed trend in the OCP may be attributed to the development of the passive film. The consistently lower and increasing OCP over time for the Co 212 alloy suggests a gradual film formation, while the IN718 and the alloy composition mostly maintained a stable OCP over an extended duration, signifying the early establishment of the film and its sustained stability and protectiveness. Notably, the alloy composition, despite indicating an early formation of a protective film, demonstrated a continual increase in OCP over time, with the potential to surpass that of the INC 718 alloy. In general, the degree of nobility of the OCP after reaching equilibrium reflects the inertness of the sample, indicating a lower propensity for corrosion in the test environment.

Example 5: Electrochemical Impedance Spectroscopy (EIS)

[0080] FIGS. 8A-8B illustrates Nyquist plots and corresponding Bode plots, respectively, for the EIS spectra obtained in aerated 3.5% NaCl for the three alloys at 25 C. The Nyquist plots, as shown in FIG. 8A, exhibited depressed capacitive loops of different sizes such that the diameter of the loops is increasing with increasing corrosion resistance in the NaCl solution. The solution resistance is denoted at the high-frequency intercept on the real axis of the Nyquist plot, while the low-frequency intercept denotes the charge transfer resistance. Therefore, the size of the Nyquist capacitive loop is directly linked to the resistance to charge transfer of the metal, controlled reactions at the metal/solution interface. This result reveals that the film resistance on the alloy composition was higher compared to pure INC 718 and Co 212 alloys. As a result, the surface film served to isolate the surface of the alloy from the corrosion media, thereby impeding the dissolution rate of the metal. The frequency dependent depictions of the impedance measurement are illustrated as phase angle and impedance plots in the corresponding Bode plots, as shown in FIG. 8B. The three alloys exhibited a single peak in the phase angle plots. Furthermore, the impedance behavior of the alloys was interpreted utilizing the equivalent circuits (EC), as shown in FIG. 8C. The EC includes a constant phase element (CPE.sub.dl) representing the non-ideal capacitive behavior of the electrical double-layer connected in parallel with the surface resistance to charge transfer (R.sub.ct). Both CPE.sub.dl and R.sub.ct were then connected in series with the resistance of the oxide films (R.sub.f) and a constant phase element (CPE.sub.f), accounting for the capacitive properties of the oxide film. The solution resistance (R.sub.s) was connected in series with the R.sub.f and CPE.sub.f elements. The chi-square (.sup.2) statistic is commonly employed to assess the concordance between experimental and simulated data, with a lower .sup.2 indicating better agreement and increased reliability of the obtained fitting parameters. In general, a .sup.2 value of 10.sup.3 or lower is considered indicative of high-quality EIS fitting. The impedance of a CPE is described by equation 1, as follows:

[00001]$\begin{array}{cc}{Z}_{\mathrm{CPE}}={Y_o^{-1}\left(j\right)}^{-n}& \left(1\right)\end{array}$ [0081] where n (with value 1

[00002]$\begin{array}{cc}{C}_{\mathrm{dl}}={Y_o^{\frac{1}{n}}\left[\frac{1}{R_s}+\frac{1}{R_{\mathrm{ct}}}\right]}^{\frac{n-1}{n}}& \left(2\right)\end{array}$

[0082] As documented in Table 2, both R.sub.ct and R.sub.f increased for the 50% IN-50% Co alloy when compared to the INC 718 and Co 212 alloys. This analysis shows that the increased resistance of the alloy composition is due to both the impediment of further charge, mass transfer, and resistance of the surface oxide coating. In addition, the higher the R.sub.ct values, the lower the C.sub.dl values, shows that the passive layer created on the surface of the metal improves the hydrophobicity of the surface. The total resistance (R.sub.t), which is the sum of the R.sub.ct and R.sub.f, was utilized to evaluate the protective performance. The alloy composition has enhanced protectiveness by approximately 170% and 171% when compared to INC 718 and Co 212, respectively.

TABLE-US-00002 TABLE 2 Fitted EIS parameters R.sub.f CPE.sub.fdl CPE.sub.dl R.sub.t Sample ID Rs() R.sub.ct(10.sup.6 ) (10.sup.6 ) (mF) (F) (10.sup.6 ) .sup.2 INC 718 25.36 0.093 0.149 4.707 83.630 0.242 0.0091 Co 212 25.67 0.198 0.043 0.162 71.473 0.241 0.0038 Alloy 26.25 0.446 0.209 0.132 28.837 0.655 0.0002 composition

Example 6: Linear Polarization Resistance (LPR)

[0083] LPR measurement was utilized to investigate the dissolution rate of the superalloys in 3.5% NaCl solution. In general, LPR is a non-destructive technology which measures the corrosion rate in real time by picking a modest voltage value of 10 mV around the OCP. This prevents permanent disruption of the corrosion process and ensures accurate results. The linear relationship between E/I and I.sub.corr is valid at this low voltage setting. R.sub.p, I.sub.corr, and CR values obtained are documented as a graphical representation in FIG. 9. The values presented in FIG. 9 indicate an increase in the polarization resistance when the two alloys were combined in 50-50 ratio. Co 212 exhibited the least R.sub.p value of 175.3 cm.sup.2 compared to INC 718 which exhibited Rp value of 238.1 cm.sup.2. EIS and LPR values are in agreement. Both show similar trends in decreased corrosion rates and higher R.sub.ct and R.sub.p values in the alloy composition, respectively. FIG. 9 illustrates LPR plots and the variation in the corrosion rate of the alloys. The enhanced corrosion resistance of the alloy composition may be associated with an increased rate of passivation of the alloy in the NaCl solution compared to the pure individual components. The passive film may subsequently lead to a significant reduction in the number of active sites for corrosion to occur. Consequently, the surface of the metal covered with the passive film becomes protected resulting in a decreased corrosion rate. It can be inferred that the alloy composition enhances microstructure characteristics of the resulting alloy. In general, the occurrence of localized corrosion decreases when common structural defects, such as grain boundaries, dislocations, and segregations, are absent in FIG. 4B. Therefore, the alloy composition exhibited excellent corrosion resistance as a result of their denser structure and effective dissolving of precipitates. The low corrosion resistance of the alloy composition may be attributed to the synergistic effect of cobalt and nickel that significantly influenced the ability to create and sustain passive film compared to the individual components alone.

Example 7: Cyclic Potentiodynamic Polarization Measurements (CPDP)

[0084] CPDP curves for the three alloys in the 3.5% NaCl solution are presented in FIGS. 10A-10C. CPDP is used to apply a complete hysteretic voltage loop on the surface of the sample. The process starts at a possible cathodic potential relative to the original OCP, passes through the primary corrosion potential, breaks down the oxide layer in an anodic manner, re-establishes passivity, recovers, and finally drops to a new stable cathodic position. The anodic arm of the polarization curves exhibited a typical activating to cap M, associated with a dissolution:

[00003]$M.fwdarw.M^{x+}+\mathrm{xe}$

[0085] As can be seen from FIG. 7, the alloy composition exhibited a shift of both cathodic and anodic arms to a region of lower current density. Tafel extrapolation was employed to acquire the respective electrochemical parameters. These parameters are listed in Table 3. The data in Table 2 showed that the I.sub.corr is significantly reduced in the alloy composition compared to either the pure INC 718 or Co 212. Furthermore, the corrosion potential (E.sub.corr) was shifted significantly to a more noble value compared to the pure alloys. The E.sub.corr is an important indicator of the thermodynamic stability of the metal in aerated 3.5% NaCl solution. FIGS. 10A-10C shows the CPDP curves for the three alloys investigated. The anodic branch corresponds to the dissolution of the alloy, whereas the cathodic branch indicates the evolution of oxygen. The anodic dissolution kinetics of the metal ions in NaCl is controlled by a uniform dissolution process via the reaction:

[00004]$M.fwdarw.M^{x+}+\mathrm{xe}.$

[0086] The reduction process occurs at the cathode, where oxygen ions (O.sup.2-) are converted into oxygen gas (O.sub.2) by reduction. The anodic domain may be categorized into three clearly defined regions: active dissolution, passivation, and an area where the current density rises with increasing voltage. The active-to-passive transition behavior of INC 718 and Co 212 indicated a sharp rise in potential, as shown in FIGS. 10A-10B. Pitting is detected above the passive zone of both INC 718 and Co 212. Pitting is likely caused by the chloride anions breaking down the passivation layer at specific surface regions, such as pores. The primary factor behind this phenomenon is the minor changes in the structure and thickness of the passive film. The passivation potential of both INC 718 and Co 212 is significantly higher compared to the alloy composition, as illustrated in FIG. 7.

[0087] The passive portions of the alloy composition, as shown in FIG. 7 exhibit a greater potential range, indicating that the surface of the alloy is more effectively passivated. Hysteresis in the CPDP curve refers to the situation when the forward curve does not align with the reverse scanning curve. The disparity in current density between forward and reverse directions at identical potential levels illustrates the magnitude of hysteresis. The greater disparity in current densities is caused by the disturbance of surface passivity at high potentials. Therefore, a larger hysteresis loop indicates a greater disruption of the passive film, resulting in more difficulties in repairing the broken passive film.

TABLE-US-00003 TABLE 3 CPDP fitted parameter E.sub.corr I.sub.corr E.sub.p CR Sample ID (mV) (A/cm.sup.2) (mV) (mpy) IN 718 348 0.32 1131.0 0.146 Co 212 458 0.39 746.4 0.155 50% INC 718-50% Co212 274 0.09 248.0 0.041

[0088] To summarize, the present disclosure provides an alloy composition including 50% Inconel 718 and 50% Co-212. The alloy composition is synthesized using spark plasma sintering (SPS) by blending 50% INC 718 and 50% Co 212 alloys. Further, microstructural analysis revealed a uniform dispersion of INC 718 and Co 212 powders, with good interfacial integrity and the absence of voids or reaction products, thus further securing the structural integrity and overall performance of the alloy. In particular, electrochemical corrosion investigations unveiled exceptional resistance to corrosion of the alloy composition, surpassing the capabilities of its individual constituents. This enhanced corrosion resistance positions the alloy composition as a versatile candidate for a myriad of engineering applications, ranging from aerospace components to marine equipment.

[0089] Numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.