High-purity anti-tarnish silver alloy and preparation method and application thereof

Abstract

The application discloses a high-purity anti-discoloration silver alloy and a preparation method and application thereof, and belongs to the technical field of silver-based alloy materials. The alloy is composed of germanium, silver and inevitable impurities according to mass percentage, the content of germanium is 0.030%-0.040%, the content of silver is greater than or equal to 99.960%, and the total content of impurities is less than or equal to 0.010%. In the preparation, Ag-10Ge intermediate alloy is used for batching, vacuum induction melting (980-1050 DEG C, 15-30 min) and graphite mold casting forming are carried out. Under the premise of maintaining the purity of 999 silver, through the addition of trace germanium, the compact GeO2 passivation film is formed by the preferential oxidation of germanium, the anti-discoloration performance is improved by more than 40%, the Vickers hardness reaches 35-45 HV, and the Vickers hardness is improved by about 30% compared with pure silver; and no sensitizing elements such as copper and nickel are added, the germanium precipitation is far lower than the food safety limit value, the application is suitable for silver jewelry, tableware, daily-use silverware and collection silver products, the process is simple, the cost is low, and the application is easy to industrialization.

Description

Technical Field

[0001] This invention belongs to the technical field of silver-based alloy materials, and particularly relates to a high-purity, anti-tarnishing pure silver alloy, its preparation method, and its application. Background Technology

[0002] Sterling silver (silver content ≥ 99.9%) is widely used in jewelry, tableware, and collectible crafts due to its high purity, warm luster, and skin-friendly properties. However, sterling silver has two significant drawbacks: firstly, it is relatively soft, with a Vickers hardness of only 25HV to 35HV, making it prone to scratches and deformation during daily use, affecting the product's appearance and lifespan; secondly, it lacks resistance to tarnishing, and prolonged contact with sulfides in the air and human sweat can easily cause silver sulfide to form on the surface, resulting in yellowing or blackening, requiring frequent cleaning and maintenance.

[0003] To improve the performance of silver alloys, existing technologies often employ modification by adding alloying elements, among which germanium is a commonly used anti-tarnishing modifier. For example, Chinese patent CN101235428A discloses an anti-tarnishing silver alloy with a germanium content of 0.5% to 7.5%, which can significantly improve anti-sulfurization performance. However, this solution is based on a 925 silver matrix, with a silver purity of only 92.5%, which does not fall into the category of pure silver and cannot meet the market demand for high-purity silver products.

[0004] Other patents disclose silver alloy schemes modified with trace amounts of germanium. For example, Chinese patent CN108754219A discloses a silver alloy for filigree jewelry with a germanium content of 0.03% to 0.15%. However, this scheme also adds multiple alloying elements such as copper, zinc, tin, and rare earth elements, reducing the silver purity to below 99%, which also fails to meet the purity standard of 999 pure silver. Furthermore, the addition of multiple elements will change the natural texture and skin-friendly properties of pure silver, making it unsuitable for scenarios with high requirements for purity and safety, such as tableware and personal jewelry.

[0005] Currently, there is no silver alloy solution in the industry that can simultaneously meet the three conditions of "999 purity silver", "trace amount germanium single modification", and "simultaneous improvement of anti-tarnishing and hardness". How to improve the performance of pure silver through extremely small amount of composition optimization without reducing silver purity or introducing other alloying elements is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] I. Purpose of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a high-purity, anti-tarnishing sterling silver alloy. While maintaining the 999 purity standard of sterling silver, this alloy simultaneously improves anti-tarnishing properties, oxide film self-healing ability, and mechanical hardness through the single addition of trace amounts of germanium. Furthermore, the preparation process is simple, the cost increase is controllable, and it is suitable for applications in jewelry, tableware, daily necessities, and other scenarios.

[0008] II. Technical Solution

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A high-purity, anti-tarnishing pure silver alloy, by mass percentage, is composed of germanium, silver, and unavoidable impurities; wherein the mass percentage of germanium is 0.030% to 0.040%, the mass percentage of silver is ≥99.960%, and the balance is unavoidable impurities.

[0011] Preferably, the mass percentage of germanium is 0.035%, which corresponds to 0.35 g of germanium added per 1000 g of pure silver.

[0012] Preferably, the total mass percentage of the unavoidable impurities is ≤0.010%, and the present invention does not actively add any other alloying elements such as copper, zinc, tin, titanium, or rare earth elements, so as to preserve the original properties of pure silver to the greatest extent.

[0013] Preferably, the Vickers hardness of the pure silver alloy is 35 HV to 45 HV, which is about 30% higher than that of pure silver; after being soaked in a 1 wt% sodium sulfide solution at room temperature for 2 hours, the surface only turns slightly yellow and does not turn significantly black, and the anti-discoloration performance is more than 40% higher than that of pure silver.

[0014] Preferably, the pure silver alloy meets the safety requirements for food contact materials. After being immersed in a 4% acetic acid food simulation solution at 60°C for 2 hours, the germanium precipitation amount is ≤0.005 mg / L, which is far below the national safety limit and can be safely used in food contact scenarios such as tableware and teaware.

[0015] The present invention also provides a method for preparing the above-mentioned high-purity, anti-tarnishing pure silver alloy, comprising the following steps:

[0016] (1) Ingredients: Weigh pure silver raw material with a purity ≥ 99.99% and Ag-10Ge silver-germanium intermediate alloy (germanium mass fraction 10%) according to the target ratio, and ensure uniform dispersion of trace amounts of germanium by dilution with intermediate alloy.

[0017] (2) Vacuum melting: Put the prepared raw materials into a vacuum induction melting furnace, evacuate to a vacuum degree ≤5 Pa, then introduce high-purity argon gas for protection, heat to 980℃~1050℃, hold for melting for 15 min~30 min, so that the alloy composition is fully homogenized.

[0018] (3) Casting: After melting, continue to keep warm for 5 min to 10 min. After the melt is free of bubbles, pour it into a preheated graphite mold and let it cool naturally to room temperature to obtain a pure silver alloy ingot.

[0019] Preferably, the melting temperature in step (2) is 1000℃~1020℃ and the melting time is 20 min. Under these conditions, the alloy composition has the best uniformity and the silver burn-off rate is ≤0.1%.

[0020] Preferably, in step (3), the graphite mold is preheated to 200°C.

[0021] This invention also protects the application of the above-mentioned high-purity, anti-tarnishing sterling silver alloy in silver jewelry, silver tableware, daily silver utensils, and collectible silver products.

[0022] The beneficial effects of this invention are:

[0023] The purity meets the standard and retains the properties of pure silver: the purity of the silver alloy of this invention is ≥99.96%, which fully meets the purity requirements of 999 pure silver in GB / T11887. It can be legally labeled as "999 pure silver". It retains the warm color, high value retention and skin-friendly properties of pure silver, which is different from the existing low-purity silver-germanium alloys.

[0024] Significant performance improvement: By adding a trace amount of germanium (0.030%–0.040%), the dense GeO2 passivation film formed by the preferential oxidation of germanium on the silver surface effectively blocks the inward diffusion of sulfur and oxygen atoms, improving anti-discoloration performance by over 40%. This passivation film has self-repair capabilities—when the surface film is damaged due to wear or processing, the germanium in the exposed fresh silver substrate can quickly react with ambient oxygen, regenerating a complete new passivation layer in situ, thereby continuously inhibiting the oxidation or sulfidation of the silver substrate and achieving long-term protection. Simultaneously, germanium refines the silver substrate grains, increasing the Vickers hardness from approximately 28 HV for pure silver to 35 HV–45 HV, significantly improving the problem of pure silver being easily scratched and deformed.

[0025] High safety and suitable for all scenarios: It does not contain easily allergenic elements such as copper and nickel, and the amount of germanium added is extremely low. The amount of leaching in food contact is far below the safety limit. It is suitable for both personal jewelry and daily necessities that come into contact with food, such as tableware and tea sets, with a wide range of applications.

[0026] Low cost and easy to industrialize: the germanium addition is only about 0.03%, and the raw material cost increase is ≤1.5%; the preparation process is fully compatible with existing pure silver production equipment, and no new equipment investment is required. It can be directly mass-produced in the smelting production lines of existing jewelry factories and silverware factories.

[0027] Differentiated formulation system: This invention is a binary silver-germanium pure silver system, which is different from the existing multi-element alloy modification scheme. The technical route is simple and controllable, and the product quality is more consistent. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the manufacturing method provided in an embodiment of the present invention. Detailed Implementation

[0029] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0030] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly on the other element or there may be an intermediate element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intermediate element present. Conversely, when an element is referred to as being "directly on" another element, there is no intermediate element. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementations. The terms "upper end," "lower end," "left side," "right side," "front end," "rear end," and similar expressions used herein refer to the positional relationship with reference to the accompanying drawings.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0032] The technical solution of this patent will be further described in detail below with reference to specific embodiments.

[0033] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the content described.

[0034] Experimental Materials and Test Methods

[0035] Raw materials: pure silver ingots (purity 99.99%), Ag-10Ge master alloy (germanium mass fraction 10%).

[0036] Purity testing: The silver content was tested according to GB / T11067.1-2006 "Chemical Analysis Methods for Silver: Determination of Silver Content by Silver Chloride Precipitation-Flame Atomic Absorption Spectrometry".

[0037] Hardness test: Vickers hardness tester was used with a load of HV0.1. Five points were tested for each sample and the average value was taken.

[0038] Anti-discoloration test: Immerse the sample in 1wt% sodium sulfide solution at room temperature for 2 hours, then remove it and visually grade the grayscale (Grade 1: no discoloration; Grade 2: slight yellowing; Grade 3: obvious yellowing; Grade 4: local blackening; Grade 5: severe blackening).

[0039] Food contact leaching test: In accordance with GB4806.9-2016 "National Food Safety Standard Metal Materials and Products for Food Contact", 4% acetic acid solution was used, and the product was immersed at 60℃ for 2 hours to detect the amount of germanium leached.

[0040] Example 1

[0041] This embodiment provides a high-purity, anti-tarnishing pure silver alloy, which, by mass percentage, contains 0.030% germanium, 99.962% silver, and the balance is unavoidable impurities (total content 0.008%).

[0042] The preparation method is as follows:

[0043] (1) Ingredients: Weigh 999.7g of pure silver ingot, add 3.0g of Ag-10Ge master alloy, the total mass is 1002.7g, corresponding to 0.3g of germanium, the germanium mass ratio is about 0.030%;

[0044] (2) Vacuum melting: Place the raw material into a vacuum induction melting furnace, evacuate to 3Pa, introduce argon gas for protection, heat to 1000℃, and melt for 20min;

[0045] (3) Casting: After holding at the temperature for 8 minutes, the casting is poured into a graphite mold preheated to 200°C and cooled naturally to obtain an ingot.

[0046] Example 2 (Optimal Example)

[0047] This embodiment provides a high-purity, anti-tarnishing pure silver alloy, which, by mass percentage, contains 0.035% germanium, 99.958% silver, and the balance is unavoidable impurities (total content 0.007%).

[0048] The preparation method is as follows:

[0049] (1) Ingredients: Weigh 999.65g of pure silver ingot, add 3.5g of Ag-10Ge master alloy, corresponding to 0.35g of germanium, with a germanium content of approximately 0.035%;

[0050] (2) Vacuum melting: Place the raw material into a vacuum induction melting furnace, evacuate to 3Pa, introduce argon gas for protection, heat to 1010℃, and melt for 20min;

[0051] (3) Casting: After holding at the temperature for 8 minutes, the casting is poured into a graphite mold preheated to 200°C and cooled naturally to obtain an ingot.

[0052] Example 3

[0053] This embodiment provides a high-purity, anti-tarnishing pure silver alloy, which, by mass percentage, contains 0.040% germanium, 99.953% silver, and the balance is unavoidable impurities (total content 0.007%).

[0054] The preparation method is as follows:

[0055] (1) Ingredients: Weigh 999.6g of pure silver ingot, add 4.0g of Ag-10Ge master alloy, corresponding to 0.4g of germanium, with a germanium content of approximately 0.040%;

[0056] (2) Vacuum melting: Place the raw material into a vacuum induction melting furnace, evacuate to 3Pa, introduce argon gas for protection, heat to 1020℃, and melt for 20min;

[0057] (3) Casting: After holding at the temperature for 8 minutes, the casting is poured into a graphite mold preheated to 200°C and cooled naturally to obtain an ingot.

[0058] Comparative Example 1 (Pure Silver as a Control)

[0059] It uses commercially available pure silver 999 ingots with a silver content of 99.99% and no actively added alloying elements.

[0060] Comparative Example 2 (germanium content is lower than the scope of this invention)

[0061] The germanium content is 0.020%, and the rest of the preparation process is the same as in Example 2.

[0062] Comparative Example 3 (germanium content higher than the scope of this invention)

[0063] The germanium content is 0.100%, and the rest of the preparation process is the same as in Example 2.

[0064] Test Results and Analysis

[0065] The performance test results of each group of samples are shown in the table below:

[0066] Example 1 0.030 99.962 yes 36 Level 2 <0.001 Example 2 0.035 99.958 yes 38 Level 2 <0.001 Example 3 0.040 99.953 yes 41 Level 2 <0.001 Comparative Example 1 0 99.990 yes 28 Level 4 — Comparative Example 2 0.020 99.972 yes 31 Level 3.5 <0.001 Comparative Example 3 0.100 99.893 no 44 Level 1 <0.001

[0067] The test results show that:

[0068] The silver purity of Examples 1 to 3 of this invention is ≥99.95%, which meets the national standard requirements for pure silver 999; the silver purity of Comparative Example 3 is reduced to 99.893% due to excessive germanium addition, and cannot be labeled as pure silver 999.

[0069] In terms of anti-discoloration performance, all embodiments reached level 2 (slight yellowing), which is a significant improvement over Comparative Example 1 (pure silver, level 4 with local blackening); Comparative Example 2, with a germanium content of only 0.020%, has limited anti-discoloration effect (level 3.5), which is difficult to meet practical needs.

[0070] In terms of hardness, the Vickers hardness of each embodiment is 36-41 HV, which is 28%-46% higher than that of pure silver. This effectively improves the problems of scratches and deformation in daily use, while the hardness is not excessively increased, thus preserving the good ductility of pure silver and making it suitable for processing techniques such as jewelry embossing and tableware stretching.

[0071] In terms of safety, the germanium leaching amount of all germanium-containing samples was far below the food safety limit, and they can be safely used in food contact scenarios.

[0072] The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also fall within the scope of the present invention, and the patent protection scope of the present invention should be defined by the claims.

Claims

1. A high-purity, anti-tarnishing pure silver alloy, characterized in that: It consists of germanium, silver, and unavoidable impurities, wherein germanium accounts for 0.030% to 0.040% by mass, silver accounts for ≥99.960% by mass, and the balance is unavoidable impurities.

2. The high-purity, anti-tarnishing pure silver alloy according to claim 1, characterized in that, The germanium content is 0.035% by mass.

3. The high-purity, anti-tarnishing pure silver alloy according to claim 1, characterized in that, The total mass percentage of the unavoidable impurities is ≤0.010%.

4. The high-purity, anti-tarnishing pure silver alloy according to claim 1, characterized in that, The pure silver alloy does not actively contain copper, zinc, tin, titanium, or rare earth elements.

5. The high-purity, anti-tarnishing pure silver alloy according to claim 1, characterized in that, The Vickers hardness of the pure silver alloy is 35HV to 45HV.

6. The high-purity, anti-tarnishing pure silver alloy according to claim 1, characterized in that, The pure silver alloy, after being immersed in a 1wt% sodium sulfide solution at room temperature for 2 hours, exhibits a surface discoloration grade of 2.

7. A method for preparing a high-purity, anti-tarnishing pure silver alloy as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Batching: Weigh pure silver raw materials with a purity of ≥99.99% and Ag-10Ge silver-germanium intermediate alloy according to the target ratio; (2) Vacuum melting: Put the prepared raw materials into a vacuum induction melting furnace, evacuate to a vacuum degree of ≤5Pa, introduce high-purity argon gas for protection, heat to 980℃~1050℃, and heat for 15min~30min; (3) Casting: After melting, cast into a preheated graphite mold, and cool naturally to room temperature to obtain pure silver alloy ingots.

8. The preparation method according to claim 7, characterized in that, In step (2), the melting temperature is 1000℃~1020℃ and the melting time is 20min.

9. The preparation method according to claim 7, characterized in that, The graphite mold described in step (3) is preheated to 200°C.

10. The application of a high-purity, tarnish-resistant sterling silver alloy as described in any one of claims 1 to 6 in silver jewelry, silver tableware, everyday silverware, or collectible silver products.

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