Causes of silver darkening and cleaning methods. A Great Method to Remove Tarnish from Silver How to Avoid Tarnishing in Silver

Why does silver darken? Why do some silver jewelry retain its soft shine for years, while others turn black literally in hours? Is the darkening of silver associated with damage to its owner? Or did the silver darken because of her illness? The last statement is partly true, but not always.

Unlike gold, silver actively reacts with sulfur, forming sulfides. This is why pure silver can actually darken - as a result of interaction with sulfur. But jewelry silver, from which earrings, chains, bracelets and rings are made, contains copper in addition to pure silver. It is copper, interacting with sweat (containing sulfur), that oxidizes in the jewelry alloy, thereby causing the darkening of the silver. Therefore, the higher the standard of your silver jewelry (the less copper it contains), the slower it oxidizes.

Silver of standard 999 is least susceptible to oxidation. Silver of standard 875 is a little more susceptible to oxidation. True, sulfur, which is part of sweat, can cause darkening of the silver itself. But pure silver in the alloy is the last to oxidize.

Consequently, the more sweat is released, the faster the silver turns black. For example, silver darkens faster if you play sports without removing your jewelry. Or if you are experiencing stress - a person always sweats more if he is nervous.

Also, excessive sebum production can be caused by hormonal changes in the body. The largest number of sebaceous glands are located on the chest. If only the chains turn black, this may be due to “hormonal storms”, which are observed, for example, in pregnant women.

They say that silver darkens if its owner has problems with the liver and kidneys. Or if the owner of the silver was damaged. But there is no evidence of such phenomena. There are only unnecessary worries and damaged nerves due to unfounded superstitions.

Jewelry silver turns black when interacting with sulfur. Moreover, it can be sulfur, which is part of the air, or part of water, cosmetics, secreted along with sweat. That is why, in order for silver not to darken longer, it is necessary to follow certain rules for wearing it: remove it when applying cosmetics, in the shower, when swimming in the sea. Remove any jewelry before doing homework. You should also not wear silver to the gym (it also looks ridiculous).

It also happens that silver begins to darken after taking some kind of medicine. This is due to the fact that medications have different effects on the composition of sweat secreted. Most likely, silver turns black due to a change in the composition of sweat and an increase in the proportion of sulfur in it.

Not the entire silver jewelry may turn black, but only one side of it. For example, a silver cross will turn black only on the outside. As a rule, the inside of the crosses is smooth, which ensures maximum tightness to clothing and limits the access of air and sulfur. And the more open and prominent side will oxidize more strongly. There may also be no blackening where the jewelry rubs against clothing.

It happens that silver jewelry turns black immediately after cleaning. This is due to the fact that immediately after cleaning, the surface of silver easily enters into all sorts of reactions, and therefore will produce a strong oxide from interaction with sweat. Therefore, immediately after cleaning, it is better not to wear silver for a couple of days, so that a thin protective layer of oxides can form on its surface. After such “exposure”, the silver darkens more slowly.

But not everything is so sad. It happens that when worn, silver, on the contrary, brightens. Some associate this with a light aura, others - again with impaired kidney function. In reality, everything is again simple: silver is brightened by nitrogen-containing substances in human sweat, which react with it and return its shine.

No matter how much we write about the properties of silver, the need to return to this topic again and again arises constantly. Not every jewelry store salesperson, even in metropolitan regions, can professionally and competently explain to the buyer “why does silverware quickly and unevenly darken, turn black, turn yellow?..”. Zhanna Perevalova, General Director of the ArgentA silver factory, answers questions from retail representatives regularly received by the Russian Jewelry Trade Club.

It is well known that silver items develop a patina over time. First, a thin yellow film forms on the metal, then a dark brown, almost black coating appears.

Some objects created by Russian craftsmen of the pre-revolutionary era develop a velvety golden-brown film that does not develop into a shiny black film. Sometimes the shade of darkened silver is so beautiful that they prefer to preserve it, despite the fact that the original appearance of the item was undoubtedly different. Silver actively reacts with sulfur, which is present everywhere in our lives (from surrounding household objects and the composition of the atmosphere, to food and waste products of the person himself). Interaction with sulfur is the main reason why silver inevitably darkens, although there are other reagents that are dangerous to it - chlorine, various salts.

The alloy of 925 silver, from which cutlery and dishes are made, contains copper in its composition - in that optimal proportion with the noble metal, which was determined several centuries ago by jewelry craftsmen. Copper is necessary to give the alloy the required rigidity, because pure silver is a rather soft metal and is not very suitable for the production of functional items. On the other hand, copper helps accelerate oxidation processes in the alloy. Therefore, the higher the standard of a silver product (in other words, the lower its copper content), the slower it oxidizes. The maximum sample is 999.

What other factors accelerate the process of patina formation on silver?

First of all, environmental pollution. In a metropolis, where the air is filled with exhaust gases, combustion products, and emissions from industrial facilities, this, of course, will happen faster. The patination process will be more noticeable near the sea than on the continental plain.

Sea water itself is a very aggressive environment for various metals, but the air of these places, actively saturated with hydrogen sulfide, is also dangerous for silver. Elevated temperature and humidity are also factors that accelerate the formation of sulfide film.
Then why has silver tableware been so popular at all times, if it is clear in advance that it will darken and lose its “marketable appearance”? It's all about benefit!

Since ancient times, humanity has used the disinfecting, antimicrobial, and healing properties of silver, which can sometimes rival antibiotics in effectiveness. Scientists continue to discover new properties of this metal that have a beneficial effect on the health of living organisms to this day. It happens that traces of touch remain on a silver product, even if it was touched with gloves. Thin fabric does not protect the metal surface from the local effects of microelements containing sulfur. Sometimes silver turns black immediately after cleaning. This is due to the fact that it is after deep cleansing that the metal surface easily enters into all sorts of reactions, which means it easily oxidizes. Therefore, it is better to wait a while and not use the product immediately after the procedure, so that a thin protective layer of oxides has time to form on its surface. Then the silver will darken more slowly.

Silverware should not be allowed to come into contact with rubber, as it also contains sulfur, which catalyzes the auto-oxidation of the metal. Let us remind you once again that silver is a soft metal and therefore scratches easily. Cutlery made from this precious material must be handled with care. Such items should be kept in specially designed cases in a cool, dark place. The mirror surface of polished silver and the original color of the product are preserved for a long time if, after each washing or rinsing, it is thoroughly wiped or dried in the open air.

There is an opinion that products from past centuries are of higher quality, darken more slowly and are easier to clean. There is some truth in this.

The level of science and technology today is incomparably higher than, say, at the end of the 19th century, and modern alloys (silver alloy compositions) are more diverse. These alloys may include, in addition to silver and copper, also impurities of iron, lead, antimony, bismuth, etc. (the average content of which, by the way, is regulated by GO ST 6839-80 for the SrM92.5 grade). Another fair question arises: why deliberately “contaminate” silver, because it is clear that the more foreign impurities there are in the alloy, the more susceptible it is to oxidation when exposed to external factors. The answer is simple: silver has poor fluidity when melted. Therefore, in order to get a budget lightweight product, it needs to be made thin, and without auxiliary metals that optimize processing, this is extremely difficult to do.

But not all modern silver products contain impurities. There are many high-quality collections on the market that are not inferior in their characteristics to “grandmother’s silver.” "ArgentA" reveals the secret of its proprietary alloy: only silver and oxygen-free copper. Moreover, the alloy is not purchased, but is manufactured directly at the enterprise in order to strictly control the composition of the alloy.

Yes, due to the lack of excipients we cannot make very light objects. Accordingly, the price of such products is a little higher, but we are sure that our collections are for centuries!

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Introduction……………………………………………………3

1. Research part…………………………………. 4

1.1. Reasons for oxidation of silver products………………4

1.3.Physical and chemical properties of silver……………5

2. Practical part…………………………………………… 6

2.1. Survey method…………………………………………… 6

2.2.Method of scientific experiment………………………… 7

2.3.Experiment results………………………………8

Conclusions……………………………………………………… 10

Conclusion……………………………………………………11

Bibliography……………………………………………………………12

Applications…………………………………………………………… 13

1. Reminder when cleaning silver items……………… 13

2.1-2.5Photographs of the research carried out…… 14

Introduction

Silver is rightfully considered one of the most amazing metals. Many centuries ago, man learned to make not only dishes from it, but also jewelry. Due to its antiseptic properties, silver is used in the treatment of various diseases. Many centuries have passed, but even today silver is popular in various fields of human activity: medicine, technology, science, culture.

 M. Maksimov “Essay on Silver”

But, unfortunately, over time, silver products lose their original shine, become dull, and become covered with a black coating. Every person who wears silver jewelry or uses cutlery made from this metal has encountered such a problem.

So my favorite silver ring has lost its original appearance. I asked my chemistry teacher for clarification on this issue. And he, in turn, suggested that I study this problem from a chemical point of view. This is how the idea for this work was born.

We have set ourselves target: investigate the reasons for the darkening of silver, select affordable cleaning methods that do not require a lot of time and money.

To achieve this goal, several tasks:

Study the scientific literature on this issue.

Find out the reasons for the darkening of the objects under study.

Identify the most accessible cleaning methods.

Conduct experiments in the school laboratory.

Summarize and analyze the data obtained.

Practical significance: The results of the study will help everyone who wants to keep their silver items in their original form.

G hypothesis:

1) We believe that the blackening of silver products is associated with a chemical process occurring between the metal and the air.

2) Darkening and lack of shine can be eliminated at home using available methods.

RESEARCH PART

The antiseptic properties of silver have been known since ancient times. So, even in Ancient Egypt - 4500 years ago, before a military campaign, soldiers were given silver plates, which, if necessary, were applied to wounds, which helped to quickly cope with the disease and avoid infection. Our ancestors could not explain these phenomena and attributed them to the action of higher forces.

Rene Marcard “A Brief History of Chemistry and Alchemy”

Reasons for oxidation of silver products

Why does silver turn black? This question has worried people since ancient times. With the development of science, the reasons leading to this result became clear. It turns out that copper, which is part of silver, interacts with sulfur. As a result, oxidation of the metal occurs, and as a result, darkening. The amount of copper in silver depends on the sample. The lower the sample, the more copper the alloy contains. Where does sulfur come from? Science has proven that sulfur-containing substances are released by human sweat. Therefore, it is recommended to remove jewelry when playing sports. The human sebaceous glands begin to work intensively not only during physical activity, but also during stressful situations, as well as during various types of diseases. In addition, sulfur can contain cosmetics, medications, air and water. http://www.stramam.ru

There is a version that darkening of silver indicates improper functioning of the kidneys or liver. A change in the color of silver may indicate problems with the nervous system. And the darkening of silver items on certain parts of the body can indicate local disruptions in the functioning of the endocrine system.

Physical and chemical properties of silver

Silver is a soft metal with a white color.

Its density is 10.5 g/cm 3 - it is considered a heavy metal.

Silver has, under normal conditions, best electrical conductivity from all metals.

Silver is capable reflect 95% of the visible spectrum. This is the best indicator among metals. This property determines the unique shine of products made from it.

In silver there is highest thermal conductivity among metals.

Silver is not as soft as gold, but in terms of ductility, i.e. the ability to change shape under the influence of external forces surpasses it. Thanks to all these qualities and properties, silver is widely used in jewelry. Chemical information. Directory

Chemical properties of silver

Silver is chemically inactive, so it belongs to the family of noble metals.

Silver does not interact with oxygen, water, alkali solutions, hydrochloric and dilute sulfuric acids.

But silver dissolves in nitric and concentrated sulfuric acids, for example:

Ag + 2HNO 3 (conc.) = AgNO 3 + NO 2 + H 2 O

It dissolves in ferric chloride, which is used in iron etching.

Ag+FeCl 3 →AgCl + FeCl 2

Oxygen in the air, even at high temperatures, does not oxidize silver.

But in the presence of traces of divalent sulfur (hydrogen sulfide) in moist air, silver sulfide is formed - a slightly soluble substance, which causes the darkening of silver items:

4Ag+2H 2 S+O 2 →2Ag 2 S+2H 2 O

When heated with sulfur, silver forms sulfide:

Due to the formation of a chloride film on the surface, silver does not dissolve in aqua regia (a mixture of concentrated hydrochloric and nitric acids in a ratio of 1:3). This property distinguishes it from gold.

I.G. Khomchenko “General Chemistry”

PRACTICAL PART

Survey method

Before starting the practical part of the research work, we conducted a survey among students in our class regarding silver products.

27 people were interviewed. The following results were obtained during the survey:

74.0% (20 people) have silver items;

90.0% (18 people) faced the problem of blackening of silver products;

10.0% (2 people) know how to clean themselves;

0% cleaned in a jewelry workshop;

75% wear a darkened product;

5 people do not wear darkened jewelry because of this flaw;

100% (27 people) want to learn how to clean their jewelry themselves.

Scientific experiment method

Having studied the literature on this topic and identified the causes of oxidation of silver products, we have selected six available methods for cleaning them.

My and my friends' silver items were used as research objects.

Experimental techniques:

Place the products that need to be cleaned into a small container and fill with a 10% solution of ammonia (can be bought at a pharmacy).

After 20-30 minutes, the products can be taken out, rinsed with water and wiped with a napkin to remove drops of water and turbidity.

Ag 2 S + NH 3 + H 2 O  2Ag(NH 3)2 OH

During the reaction, easily soluble silver ammonia is formed.

http://www.mycharm.ru

Prepare a soda solution at the rate of 0.5 liters of water with two tablespoons of soda. Mix thoroughly and put on fire. After the solution boils, immerse aluminum foil in it, and then the product that needs to be cleaned. Even the dirtiest item can be taken out after 15 minutes and washed thoroughly with water.

http://www znajko.ru

3Ag 2 S+2Al+5NaOH+3H 2 O →6Ag↓+2Na+3NaHS

The equation shows that during the reaction, silver is reduced by aluminum to pure metal in an alkaline medium, which is formed by dissolving soda in water.

Cleaning silver items with sulfuric acid.

We prepare a solution of sulfuric acid of 10% concentration, observing safety precautions. Dip the silver into it, put it on the fire and let it simmer for 1-2 minutes. After the solution has cooled, rinse thoroughly with water and wipe.

You must be careful not to allow the acid to come into contact with your skin or clothing or to inhale its fumes.

Ag 2 S + H 2 SO 4  Ag 2 SO 4 + SO 2 + H 2 O

Ag 2 S + Ag 2 SO 4  4Ag +2SO 2 

Cleaning silver with salt.

Dissolve 2 teaspoons of table salt in a glass of water and leave the silver in the solution overnight. For greater effectiveness, you can boil it in a soda solution for 10 minutes in the morning.

After completing the procedure, rinse with water and wipe with a soft cloth.

Ag 2 S + 2NaCl  2AgCl +Na 2 S

2AgCl + Na 2 CO 3 → 2Ag + 2NaCl + CO 2 ￪ + O 2 ￪

Cleaning silver items with toothpaste.

For such cleaning you need a toothbrush and toothpaste.

Apply toothpaste to the product and rub thoroughly. Then rinse with water and dry. www helprf.com/Uvlikbez/Cerebro

Research results

During the experiments, the advantages and disadvantages of each method were identified.

Cleaning silver items with ammonia solution.

Advantages of this method:

Accessible;

Easy to organize;

Effective;

Flaws:

strong smell of ammonia;

people with upper respiratory tract disease and allergies should not use this method;

Cleaning silver items with aluminum (food) foil in a soda solution.

Advantages of this method:

efficiency;

speed in execution;

no pungent odors;

pristine shine;

Advantages of this method:

rapidity;

efficiency;

Flaws:

sulfuric acid is an aggressive chemical that can be harmful to health;

it is inappropriate to use a strong acid, as it has a negative effect on the metal surface;

Cleaning silver with salt:

Advantages of this method:

ease of execution;

Flaw:

the silver item has not been completely cleaned;

Advantages of this method:

ease of execution;

Flaw:

the process is labor-intensive;

there are scratches on the surface of the product;

conclusions

The hypothesis of the research work was confirmed. We have solved all the tasks set for ourselves. Our goal has been achieved - the reasons for the darkening have been clarified, available cleaning methods have been selected, and recommendations have been formulated that will allow you to clean silver items at home without much effort and time.

Based on the results obtained, the following conclusions are formulated:

The darkening of silver products is caused by the chemical process of interaction of the metal with sulfur compounds contained in the air, as well as in the soil or the human body.

Some possible and accessible cleaning methods have been studied and the simplest and most effective ones have been identified.

In our opinion, the most effective method is using aluminum foil in a soda solution. It is safe for human health, available reagents are used, and does not take much effort and time. The products acquire their original appearance.

Based on the results of the research, the following recommendations can be offered:

It is necessary to remove jewelry before visiting a bathhouse or sauna.

Do not allow the product to come into contact with chemically aggressive substances.

Store silver items separately in a tightly closed box.

Conclusion

In conclusion, I would like to say that the loss of shine and blackening of silver products is associated with many factors. This may be the presence of sulfur-containing compounds in the air and increased air humidity and hormonal changes occurring in the human body. But it is possible to restore the former shine and radiance on your own, at home. And we believe that this work will help everyone who wants to solve this problem.

The research results were presented to my classmates in chemistry class, who immediately became interested in the issue of purification. I hope that our recommendations will help them keep their favorite jewelry in its original form.

One cannot help but admire silver: at all times it has been associated with abundance and dignity, it has calmed and given mysterious beauty. And with proper care, silver items will delight us and our loved ones for many years.

Bibliography

I.G. Khomchenko “General Chemistry” // New Wave, 2001

Rene Marcard “A Brief History of Chemistry and Alchemy” // Enigma, 2014

Chemical information. Directory. Chemistry, 1988

M. Maksimov “Essay on silver”, Nedra, 1981

V. Stanzo, M. Chernenko “Popular library of chemical elements” Book 2, Science 1983

I.V. Pyatnitsky “Analytical chemistry of silver” // Science, 1975

Informational resources:

http://www.mycharm.ru

http://www.stramam.ru

http://www znajko.ru

http://www helprf.com/Uvlikbez/Cerebro

Annex 1

REMINDER WHEN CLEANING SILVER PRODUCTS

If the jewelry has darkened, you should wash it in a 10% solution of ammonia, then rinse in clean water and dry (never leave the jewelry wet).

Pour 0.5 liters of water into a container, add 1-2 tablespoons of baking soda, mix and put on fire. After the soda solution boils, lower the aluminum foil and the silver item into the solution. After 10-15 minutes, the product can be taken out and rinsed with water.

If it is lightly soiled, it is enough to wipe the item with a cloth soaked in the solution, and if the jewelry is very dark, you can simply dip it in the solution and wait a little.

Products with precious and semi-precious stones should be cleaned very carefully using a soft flannel cloth.

When cleaning, do not use toothbrushes or other hard materials that may have a harmful effect on the product.

Do not use harsh chemicals for cleaning. This will harm your health.

Appendix 2.1

Photo materials of the conducted research

Cleaning with ammonia solution

The process of cleaning a silver spoon with ammonia solution (10%)

Appendix 2.2

Cleaning silver items with aluminum (food) foil in a soda solution.

Product before cleaning Product after cleaning

The process of cleaning a silver product with aluminum (food) foil in a soda solution.

Appendix 2.3

Cleaning silver items with sulfuric acid:

Product before cleaning Product after cleaning

The process of cleaning silver products with sulfuric acid:

Appendix 2.4

Cleaning silver items with salt:

Product before cleaning Product after cleaning

The process of cleaning a silver item with salt:

Appendix 2.5

Cleaning silver items with toothpaste:

Cleaning material:

Product after cleaning.

Source: SCIFUN.ORG

If you own anything silver or plated with silver, then you know that the bright, shiny surface of the metal gradually darkens and loses its shine. This is because silver reacts chemically with sulfur-containing substances in the air. With the help of chemicals, you can reverse the tarnishing and make your silver shiny again.

For this you will need:

Tarnished silver

A pan that can completely immerse your silver,

Aluminum foil to cover the bottom of the pan,

Water to fill the pan,

Kitchen mitts,

200 g of baking soda per 4 liters of water.

Cover the bottom of the pan with aluminum foil. Place your silver on the foil - it should touch the aluminum.

Boil water, remove it from the stove and place it in the sink. Add 200 g of soda per 4 liters of water to boiling water. The mixture will foam a little, so we put the pan in the sink.

Pour the mixture into the silver pan until it completely covers the silver.

The tarnish will begin to fade almost immediately. If the silver is only slightly tarnished, the shine will return within a few minutes. If the silver is heavily stained, you may need to reheat the mixture and repeat the procedure several times to remove all the plaque.

When silver tarnishes, it combines with sulfur to form silver sulfide. Silver sulfide - black. When a thin layer of silver sulfide forms on the surface of the silver, it darkens. Silver can be restored to its former shine by removing silver sulfide from its surface.

There are two ways to remove silver sulfide. One of them is to remove it from the surface. The second reverses the chemical reaction and turns silver sulfide back into silver. With the first method, some of the silver is removed during the polishing process. The second method allows you to keep all your silver. Polishes containing abrasives during the polishing process erase silver sulfide and part of the silver itself along with it. Another plaque solvent dissolves silver sulfide in the liquid. These polishes use immersing the silver in liquid, or rubbing a liquid into the silver using a cloth, then rinsing the silver. They also remove some of the metal.

The plaque removal method described here uses a chemical reaction to convert silver sulfide back into silver. Many other metals besides silver form compounds with sulfur. Some of them attract sulfur more strongly than silver. Aluminum is one such metal. In this experiment, silver sulfide reacts with aluminum. During this process, sulfur atoms are transferred from the silver to the aluminum, releasing the silver and forming aluminum sulfide.

The reaction between silver sulfide and aluminum occurs when the two metals are immersed in a soda solution and come into contact. The reaction occurs faster when the solution is warm. The solution transfers sulfur from the silver to the aluminum. Aluminum sulfide may adhere to the aluminum foil, or form tiny, pale yellow flakes at the bottom of the pan. Silver and aluminum must be in contact with each other because the reaction between them produces a small electric current. This type of reaction is used in batteries to produce electricity.

When processing silver alloys from an ingot to a finished product, one of the most important operations is recrystallization annealing, which at industry enterprises is in most cases carried out in air and less often in a protective atmosphere or vacuum. If heating is carried out in air, then the surface of the product oxidizes and after etching it becomes discolored and the mechanical properties of the alloy deteriorate. The reason for these phenomena lies in the properties of silver itself and in the content of alloying additives, which form oxides during annealing. Defects caused by oxidation, especially with frequent and prolonged annealing, can greatly complicate further processing, and their elimination requires lengthy etching or grinding, and sometimes the alloy is completely unsuitable for processing. The quality alloy supplied by the foundry can be completely ruined by improper heat treatment.

Eliminating these shortcomings is of significant economic interest, as this will lead to a reduction in irretrievable losses of expensive alloys, a reduction in the percentage of defects and the elimination of difficulties encountered when processing silver alloys. However, before eliminating these shortcomings, it is necessary to know the oxidation processes that take place during annealing, to properly develop and follow the heat treatment process.

It is known that silver is a good conductor of oxygen and forms a number of chemical compounds with it that are unstable at high temperatures.

When silver is annealed in an oxygen-containing atmosphere, a decrease in weight and the appearance of roughness on the surface of the product are observed. This is explained by the formation of volatile silver oxide at high temperatures. In this case, the silver seems to evaporate from the surface. Leirox and Raub, when studying the volatility of silver oxides, found that about 3 grams are lost from 1 m 2 of the surface of a silver sheet during ten-hour annealing in air at 750 o C, and about 8 grams at 850 o C in oxygen.

Base additives have a much greater tendency to oxidize than silver and form persistent oxides with oxygen, which can be volatile, such as zinc oxide or cadmium oxide. The most important filler metal for silver, copper, forms two types of oxides Cu2O and CuO with oxygen.

Silver-copper alloys form with cuprous oxide at a temperature of 776 o a ternary eutectic Ag-Cu-Cu 2 O composition: 66.5% Ag; 32.8% Cu; 0.7% Cu 2 O, close to the binary eutectic Ag - Cu.

Oxidation of copper during the annealing process of silver-copper alloys is the cause of most defects in forming.

Along with the appearance of an oxide layer on the surface, an internal oxide zone may appear inside the sample.

While external oxidation causes a change in surface quality and increases deadweight loss, the process of internal oxidation in silver and its alloys changes the chemical, physical and mechanical properties of the material, including corrosion resistance, electrical conductivity, tensile strength, yield strength, etc. d.

Unlike the outer oxide layer, the inner oxide zone is heterogeneous and consists of a metal matrix in which oxide particles of the base component are embedded.

Silver and its alloys with base metals, due to the significant difference in oxygen affinity between silver and base metals, have a tendency to internal oxidation. At high temperatures, due to the high dissociation pressure of silver oxide, only oxides of the base components of the alloy are formed. In addition, internal oxidation is facilitated by the high solubility and significant rate of diffusion of oxygen into silver.

In technically pure silver (purity level 99.9 - 99.99%), the main impurity is copper, the content of which ranges from 0.1-0.01%.

Oxidative annealing causes a rapid transformation of copper, which forms a solid solution with silver, into cuprous oxide, the crystals of which are located predominantly along the boundaries of silver grains. This leads to a significant change in the properties of the metal.

The processes of internal oxidation of commercially pure silver and silver alloys can be considered as oxide formation processes occurring in the alloy-gas system, with silver playing the role of an oxygen carrier. In this regard, the rate of the process is determined by the rate of oxygen diffusion into silver, which, in turn, depends on temperature.

The rate of oxidation, or rate of growth of the oxide layer during internal oxidation of silver and its alloys, can be expressed as the increase in oxygen content in milligrams per unit surface area or per gram of alloy.

Spengler, studying the internal oxidation of silver and its alloys, determined that the process of internal oxidation of chemically pure silver (purity 99.999%, the rest is copper) obeys a linear law.

Technically pure silver containing up to 0.1% copper forms a homogeneous solid solution of copper and silver. When annealing at temperatures above 300 o C, the process of internal oxidation obeys the parabolic law. Dissolved oxygen in the air combines with copper, which forms a solid solution with silver, causing the formation of cuprous oxide. Cuprous oxide particles then coagulate, located predominantly along the boundaries of the silver grains. This leads to an increase in electrical conductivity and hardness, and the hardness increases the more, the lower the oxidation temperature, i.e., the more dispersed the released cuprous oxide particles are. Electrical conductivity, on the contrary, increases with increasing annealing temperature, since the size of cuprous oxide crystals increases.

Internal oxidation during annealing of silver-copper alloys depends to a greater extent than in chemically and technically pure silver, on factors such as temperature, annealing duration, grain size, partial pressure of the oxidizing agent in the surrounding atmosphere, etc.

The parabolic law is usually used to describe the internal oxidation of silver-copper alloys. However, a number of researchers have come to the conclusion that at an annealing temperature of about 500 o C there is a cubic dependence, and at lower temperatures there is a logarithmic or inverse logarithmic dependence.

The amount of oxygen absorbed by the alloy, and therefore the degree of oxidation, depends on the annealing time. During short-term annealing, the maximum oxygen absorption occurs in an alloy with 90% silver.

With prolonged annealing, the maximum shifts to the alloy containing 80% silver. The minimum oxygen absorption is in the region of alloys with a eutectic structure. According to Leroix and Raub, the total amount of oxygen adsorbed by silver-copper alloys depending on the annealing time can be calculated using the formula:

x 2 =k. t

Where X- amount of adsorbed oxygen, g;

t- annealing time, sec;

k- constant oxidation.

The rate of internal oxidation is greatly influenced by grain size.

Large grains, regardless of the formation conditions, favor internal oxidation, while a fine-grained structure prevents the penetration of oxygen into the alloy. As the copper content in the alloy increases, large silver crystals that conduct oxygen well decrease and the amount of eutectic increases.

The passage of oxygen through numerous grain boundaries and eutectic plates is hampered, and oxidation of the alloy occurs mainly at the surface. The finely dispersed eutectic structure at 72% Ag therefore determines a minimum of oxidation.

According to Raub and Plata, with long-term annealing at a temperature of 700 o C, the internal oxidation zone is twice as large as with the same annealing time at 600 o C.

The high partial pressure of oxygen in the annealing atmosphere favors the diffusion of oxygen into the silver and promotes internal oxidation.

At a low partial pressure of the oxidizer, its diffusion into the alloy decreases, and in this case, external oxidation predominates, i.e., an oxide layer is formed on the surface of the alloy with an underlying thin zone of internal oxidation.

The processes of internal oxidation of silver and its alloys can be traced in photographs of microsections given in Schlegel's work.

In Fig. Figure 1 shows the structure of the polished surface of a plate made of commercially pure silver. After 4 hours of annealing in an oxygen environment, cuprous oxide particles formed along the boundaries of the silver grains.

In the 960 silver alloy, after annealing for an hour in air at a temperature of 700 o C, an internal heterogeneous oxide zone with a thickness of 96 microns was formed under the outer oxide layer (Fig. 2). With 6-hour annealing, this zone increased to 214 μm (Fig. 3). Along the boundaries of the metal grains in the oxide zone, copper oxide particles begin to separate.

Brittle particles of copper oxide and oxide formed during the oxidation of copper destroy the structure of the metal. In addition, copper oxide Cu 2 O is also harmful because during annealing it tends to form large fractions that accumulate in the form of plates or stripes under the surface layer. This greatly impairs the machinability of the alloys.

In the processing technology of silver-copper alloys, the outer oxide layer is removed by etching in a hot sulfuric acid solution. When annealed again in air, the copper again diffuses to the surface and oxidizes again. After several annealings and etchings, a zone enriched with silver appears on the surface, through which oxygen easily penetrates. Further oxidation of copper no longer occurs on the surface, but under this enriched layer of silver. In Fig. Figure 4 shows a section of a plate made of 800 silver alloy subjected to repeated annealing at a temperature of 700 o C and etching. An oxide layer consisting of CuO formed under the surface of the plate. Under this layer there is a heterogeneous zone of Cu 2 O, followed by unoxidized metal. The oxide layers formed make further processing difficult. When rolling, stamping, drawing, these oxide layers can cause delamination of the metal, the formation of cracks, tears, etc. on the surface. When grinding or polishing, the outer layer enriched with silver is removed, and the inner oxidized layer appears on the surface in the form of gray-blue spots .

The process of oxidation of products coated with silver, or bimetals, one of the layers of which is silver, occurs in the same way as the oxidation of silver alloys during repeated annealing and etching. Oxygen passes through the silver layer and oxidizes the base metal. An oxide zone is formed at the boundary of the metal connection, which weakens the adhesion of the metals, or even leads to delamination. In Fig. Figure 5 shows the adhesion zone in a bimetallic plate made of iron and rib after 6 hours of annealing in air at a temperature of 700 o C. Iron particles diffuse into silver and are oxidized there by oxygen. An oxide zone is formed at the adhesion interface between metals. In this case, the strength of the metal connection decreases, and pressure processing is difficult.

If the bimetal uses not pure silver, but a silver alloy, for example 960 standard, then the diffusion of oxygen through this layer slows down due to its interaction with the copper of the alloy and the formation of an internal oxidation zone.

When oxidized silver alloys or commercially pure silver are annealed in a hydrogen-containing atmosphere, hydrogen diffuses into the metal and reduces copper oxides to copper, producing water vapor.

The decrease in the deformability of alloys in this case becomes especially noticeable. In Fig. Figure 6 shows a section of a plate made of 960 silver alloy after oxidative annealing in air at a temperature of 700 o C for 5 hours and then, after slight deformation, subjected to annealing in a hydrogen environment. There are many pores in the metal structure. Annealing of silver and its alloys in a hydrogen environment is only possible if the metal was melted in a vacuum or in an inert gas environment.

Copper oxide and oxide formed during internal oxidation have a larger specific volume than the metal, and this leads to the formation of internal stresses, which, in turn, lead to the appearance of cracks with minor pressure treatment and to an increase in the hardness of the alloy. Cracks that appear on the surface of workpieces during rolling, rolling or drawing lead not only to stress concentration in the tears, but also to even deeper oxidation during intermediate annealing. Such workpieces are difficult to process by pressure. It is impossible to obtain thin sheets or wire from them.

The tensile strength, elongation, and transverse contraction of high-grade silver alloys initially decrease sharply with an increase in the degree of oxidation; however, further, with an increase in the duration of annealing and an increase in the internal oxide zone, the dependence of mechanical properties on the degree of oxidation decreases.

To eliminate defects arising from oxidation of copper in silver-copper alloys during annealing and to successfully perform further processing operations, the following annealing conditions must be observed:

1. To reduce the oxidation of copper, it is necessary to reduce the number of intermediate annealings to a minimum, i.e., during pressure treatment, give the maximum permissible hardening. Thus, when processing the most commonly used silver-copper alloys with a silver content of 80 to 90%, hardening up to 80% should be given. For example, rolling an ingot from a thickness of 10 to 2 mm or drawing wire from 3 to 1.4 mm should be done without intermediate annealing. Heavily deformed alloys recrystallize faster and at lower temperatures. This produces a fine-grained strugura. Large ingots of alloys with a silver content of more than 92% should be water quenched before pressure treatment;

2. The duration of annealing depends on the size of the products and on the type of heat exchange (heating in electric muffle furnaces, salt baths, open gas flame, etc.) / This should be taken into account and avoid too high and prolonged heating, as it leads to the formation of coarse grains. structure, which worsens the mechanical properties of the alloy, and in addition, large grains contribute to the oxidation of the alloy;

3. Small and thin parts made of high-grade silver alloys, which often have to be annealed due to complex processing, are especially susceptible to oxidation. To prevent it, it is necessary to anneal under a layer of calcined charcoal or coat it with brown or boric acid before annealing. Good results are obtained by annealing silver alloys in salt baths.

Recently, annealing of noble metal alloys in furnaces with a protective atmosphere has found widespread use. As a protective atmosphere when annealing silver-copper alloys, the most favorable is a weakly reducing exogas atmosphere, obtained by burning natural gas with an air flow coefficient α = 97-99.

From the above it follows that oxidation of silver and its alloys during annealing is an undesirable phenomenon and should be avoided. However, in some cases, internal oxidation can be used to improve the mechanical properties of silver and its alloys. Properties such as fatigue strength, tensile strength, and creep depend on the conditions of formation of the internal oxidation layer and, in particular, on the size and distribution of oxide particles, which in turn depend on the concentration of the alloying metal and the oxidation temperature.

Spengler discovered that adding 1% nickel to homogeneous silver-copper alloys reduced the size of cuprous oxide precipitates at grain boundaries during internal oxidation. At the same time, due to the release of fine particles of copper oxide, the mechanical properties of the alloys after oxidation are higher than those of alloys that do not contain nickel.

Meijerling and Drunvestein (9) studied the hardening of a large number of binary alloys based on silver and copper. They found that silver-copper alloys could have much higher hardness as a result of internal oxidation. Thus, after 2 hours of heating in air to 800 o C, the Vickers hardness of a silver alloy containing 1.2% magnesium increases from 40 to 170 kg/mm 2. When replacing magnesium with 1.6% aluminum, 2.4% beryllium or manganese, the hardness of the alloy is respectively 160, 135 and 140 kg/mm 2.

Addition 1.3% Zn; 1.4 Sn or 1% Cd either does not increase hardness at all or increases it very little (60, 40 kg/mm 2, respectively). From this we can conclude that in order to obtain certain mechanical properties of silver-copper alloys, in some cases internal oxidation should be used rather than developing new alloys.

LITERATURE

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2. Leroux A. und Raub E. “Untersuchungen fiber das Verhalten von Silber-Kupfer-Legierungen beim Cliihcn in Sauerstoff und Luft.”Z. Anorg, Allg. Chem. 188, 1930.

3. Raub E. und Plate W. “Einflu8 der inneren Oxydation auf die iechnishen Eigenschaften von Silber-Legierungen.” Z, Metall, 10, 1955.

4. Raub E. “Die Edelmetalle und ihre Legierungen.” Berlin, 1940.

5. Sch1ege1 H. “Die Oxydation beim Gliihen als Fehlerursache bei der Verarbeitung der Silber-Kupfer-Legierungen.” Feinmechanik und Optik, 75, 1958, No. 7, 8.

6. Brepohl E. “Theorie und Praxis des Goldschmieds.” VEB, Leipzig, 1962.

7. Raub E. und Plate W. “Uber das Verhalten der Edelmetalle und ihrer Legierungen zu Sauerstoff bei hoher Temperatur irn festen Zustand.” Z. Metallkunde, 48, 1957.

8. Speng1er H. “Die innere Oxydation von Silber und Silberlegierungen.” Z. Metall, 1970, 24, !No 7.

9. Meijering J. L. et Druyvesteyn M. J. Philips Res Rep. 1947, v. 2, p. 81, 260.

10. Ghaston J. C. J Inst Metals, 1945, vol. 71, p. 23.

11. I. Bern R. Oxidation of metals. M. Metallurg, vol. 2, 1969.

12. Fratsevich I. M. Votkovich R. F., Lavrenko V. A. High-temperature oxidation of metals and alloys. Kyiv, 1963.

13. Frohlich K “Das System Kupfer-Silber-Sauerstoff”. Mitteilun-aus dem Forschungsinstitut und Probieramt fiir Edelmetalle, Ichwabisch Gmiind, Nr 10, 11, 1932, S. 100.

14. SpenglerH. “Die Zunderung technischer Goldlegierungen und ihre Vermeidung bei Wahrmebehandlung” Z. Metal], 10, 1956, S. 617-620.