What is metal oxidation

105 anodic oxidation

Russian-English dictionary of normative and technical terminology > anodic oxidation

106 oxidation in neutral environment

Russian-English dictionary of normative and technical terminology > oxidation in a neutral environment

107 internal oxidation

internal oxidation Formation of dispersed oxides in steel or alloy as a result of diffuse penetration of oxygen through the surface layer.

Occurs during the selective interaction of alloy components with oxygen, when one of the alloy components has a greater affinity for oxygen, and the base component has a much lower affinity or does not oxidize at all.

Internal oxidation is used as a method of hardening, in particular increasing the hardness of steel and alloys.

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EN

Russian-English dictionary of normative and technical terminology > internal oxidation

108 secondary oxidation

secondary oxidation Oxidation of liquid metal during its release from the furnace, casting and crystallization, as well as in a steel-smelting furnace after deoxidation due to contact of the metal with oxidized liquid slag. As a result of secondary oxidation, the content of oxygen and oxide non-metallic inclusions in the metal (steel) begins to increase after an initial decrease.

The main process of secondary oxidation when releasing metal from the furnace occurs on the chute as a result of direct oxidation of the jet with air, when casting steel from a ladle into molds, on a continuous casting machine, when the melt flows from the main ladle into a tundish and from it into the mold.

The most radical method of combating secondary oxidation is smelting and casting metal in a vacuum, in vacuum chambers at reduced pressure or in a protective, neutral gas.

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What means and methods exist for bluing metal yourself?

To ensure that the product can serve for a long time without painting, metal bluing is used at home to protect against corrosion. This processing method creates a thin oxide film on the surface of the parts. It does not allow moisture (H₂O) and gases (CO and O₂) to pass through to the open metal (in real conditions, a chemical reaction occurs to reduce the carbon content with a decrease in hardness).

Oxidation proceeds in such a way that iron exhibits divalent properties. Oxides form on the surface or a layer of insoluble salts forms. Only in the presence of strong acids can the resulting film be dissolved.

Causes of metal corrosion in the open air

The combined action of water and oxygen occurs according to the scheme where both aggressive substances act on pure iron: 4Fe + 6H₂O + 4O₂ = 4Fe(OH)₃.

As a result, iron hydroxide Fe(OH)₃ is formed; it has a red (orange) color and has a loose structure.

Carbon steel, when periodically heated in the environment, loses carbon, and the iron it contains is more susceptible to corrosion. Decarbonization of steel products occurs through the following chemical reactions:

• Fe₃C + CO₂ = 3Fe + 2CO;
• Fe₃C + H₂O = 3Fe + H₂ + CO;
• Fe₃C + 2H₂ = 3Fe + CH₄,

where Fe₃C is cementite, one of the important components of steel. It is this connection that determines the strength and hardness of steel products.

The surface layer contains more pure iron. It is susceptible to water and surrounding gases. The process is growing. Loose rust quickly spreads inside parts.

To prevent corrosion and decarburization, products are coated with paints or primers. Chemical exposure is reduced hundreds of times. But it is not always possible to use liquid or powder paints. A whole group of steel parts is operated without painting:

1. Cold steel and firearms.
2. Products obtained as a result of cold forging.
3. Claiming couples working with gases and liquids.

For them you need to use a different method of protection.

What does steel blueing do?

Some inexperienced craftsmen often do not know what metal bluing is. This method of processing is used quite rarely, believing that it is a rather complicated procedure. In fact, any manufacturer can achieve positive results with his own hands with minimal costs of materials and labor.

During chemical or thermal bluing, an oxide film with a thickness of 13 to 1015 microns is formed on the surface of the steel. The thickness of the resulting layer largely depends on the technology used. When performing this operation, the main goals are achieved:

• the steel product's corrosion resistance increases;
• parts that have been processed are able to remain resistant to aggressive environmental influences.

If there is action of alkalis, acids and special heat treatment, then the surface is covered with oxide,

2Fe + O₂ = 2FeO.

Oxidation, when iron exhibits diatomic properties, forms black films. Depending on their thickness, the color of the product also changes. If there is a sufficiently thick film (up to 1215 microns), then the surface is black. To damage such a coating, you need to apply quite a significant amount of force.

How to oxidize metals at home?

In practice, bluing is performed according to various schemes:

1. Heat treatment of parts followed by oil coating.
2. Heating to boiling temperatures in chemical solutions.
3. Painting with special compounds that penetrate the surface layer. Diffusion of the coating layer with the base occurs.

At home, you can implement any of the methods, you just need to familiarize yourself with the features of the technology, as well as prepare the necessary equipment.

Selection of technology for bluing parts

To choose the right bluing method, it is advisable to decide on a number of specific parameters and recommendations:

• In the case of restoration of damaged coating, the extent of existing damage should be determined. If you need to revive limited areas where there are scratches or small abrasions, the choice is made on the alkaline method of producing an oxide film on the metal surface.
• If the surface of the part is significantly damaged and there is corrosion on it, the existing coating will have to be completely removed.
• Each product has a certain value. Based on this, it is worth estimating how long subsequent operation is expected. If the item will spend most of its time on the wall as an exhibit, simple chemical processing methods are possible.
• For products actively used on hikes or in the open air (knives, utensils or firearms), thermal methods using oil are used.
• When performing work to order, much depends on the price that the customer can pay. Cheap orders are carried out using simple methods, expensive ones involve the use of more expensive bluing techniques. When performing work for third-party customers, the question is: “How much does it cost?” – will be very relevant.
• At the training stage you will have to pay for everything yourself. Any professional training is expensive, but self-learning the basics of the profession has the advantage: knowledge is acquired without additional payment to the mentor.

All that remains is to study the basic methods of applying an oxide film, and then practice the techniques through real experience and gain the necessary skills.

When carrying out bluing work using chemicals, you will have to use tools to clean the surface from dirt and various types of grease. You can do the work manually or use a power tool. The work must be done on a desktop (workbench or cover the table in the apartment with a removable tabletop). In the room itself you need to have equipment and certain protective items:

1. You need a working container for bluing. It should ensure that the part is completely immersed in the solution.
2. Create good exhaust ventilation and fresh air flow. It is advisable to install a probe above the place where processes are carried out, then harmful fumes will not affect humans.
3. When working, you need a container made of material that is neutral to alkalis and acids. It should contain water or a solution that can quickly neutralize the drugs used. The container is made of plastic, glass, porcelain or earthenware.
4. The work must be done with gloves. Here you will need rubber and other types of gloves to prevent solutions from getting on your hands. They must also protect against possible thermal effects.
5. Respiratory protection is achieved by using respirators. When using strong acids, activated carbon filters will be needed.
6. Vision protection is achieved by using safety glasses. There are special glasses for working with chemical reagents; they are sold in work clothing stores.
7. Aprons and jackets made of tarpaulin will protect the body from drops of solutions.

Alkaline steel bluing

The simplest way to apply an oxide film to products is alkaline bluing. The operating principle is based on the use of caustic soda and sodium nitrate (used as a fertilizer). Both components have a pronounced alkaline reaction.

To process a part weighing about 900-1000 g you need to have:

• 100 g caustic soda (NaOH);
• 30 g of sodium nitrate (sodium nitrate NaNO₃).

Process:

1. Dissolve the components in 100 ml of water. If this amount of working solution is not enough, then increase the amount of liquid, as well as caustic soda and nitrate in proportion.
2. The solution is heated to 135145 ⁰С (salt solutions boil at a temperature higher than pure water boils).
3. The part is washed with soap or weak alkali to remove possible greasy stains.
4. Place in the working solution for 30-35 minutes and maintain a high temperature. In a hot state, the rate of film formation on the surface of a steel object will be high.
5. After completing the procedure, it is necessary to wash off the remaining working solution.
6. Wipe the product with vegetable or technical oil.
7. Then thoroughly wipe the item with a rag; only a thin layer of oil film will remain, which will further protect the finished product.

As a result, the metal surface will receive a black coating with a bluish tint. It is quite durable and can withstand minor mechanical stress.

Other alkaline treatments are possible in NaOH and KOH solutions. Prepare a solution in which 300-400 g of each component is dissolved. The minimum concentration of alkalis is 700 g per 1 liter of solution.

The processing procedure takes place according to the technology described above. In this case, the result is a film with slightly more blue.

Attention! During alkaline processing, the liquid level should be maintained so that the part is always completely in the solution. Then the entire coating will be uniform.

Before starting acid coating, the workpiece is cleaned and washed. You can get by with pure alcohol or a 40% strength solution (regular vodka). Some people use white spirit. Using a swab, clean the surface from greasy stains.

Recipe No. 1

Working solution for bluing:

• 2 g citric acid;
• 2 g oxalic acid;
• 1 liter of water.

Process:

1. The solution is heated to a temperature of 120-125 ⁰C.
2. Place the product in it for 20 minutes.
3. Once completed, the part is removed and washed with a slightly alkaline solution. You can use a sponge soaked in shampoo.
4. The treatment is completed by rubbing with machine oil.

The product takes on a black color with some brown tint.

Recipe No. 2

Vegetable tannins (tannins) are used. They are obtained from oak or willow branches.

Preparation of concentrate:

• willow or oak branches (about 3 kg) are boiled in a 10 liter container;
• 3 hours after cooking, a black solution forms;
• the branches are removed from the solution;
• evaporate the solution to 3 liters. The result is concentrated tannic acid. Only part of the solution is used for bluing. The remainder of the concentrate is poured into a glass container and closed with a tight lid (can be stored for up to 34 years).

Blued parts:

1. 2030 g of tanning concentrate is used per 1 liter of working solution.
2. To activate the process, bluing is carried out with citric acid, which is added to the working solution (23 g per 1 liter of water).
3. The part that needs to be blued is placed in the prepared solution.
4. The duration of the process is 24..30 hours.
5. After processing, the part is removed and washed.
6. The finished part is wiped with machine oil.
7. Remove any remaining oil from the part using a rag.

Using "Rusty Varnish"

“Rusty varnish” is on sale. It is used for surface coating of parts:

• the part is wiped clean from residual grease;
• varnished;
• use an acetone-based solvent, for example, No. 646, to wash off excess varnish from the surface;
• This is one of the fastest bluing methods using ready-made solutions.

bluing at home.

Thermal bluing of parts

In industrial conditions, they prefer to perform bluing using a thermal method. Usually this operation is combined with low tempering, which relieves internal stresses inside steel products:

1. The part is heated in a muffle furnace for 20-30 minutes to a temperature of 180-220 ⁰C.
2. The removed part is wiped with machine oil using a swab. It is advisable to treat all surfaces.
3. If necessary, the treatments are repeated.
4. Usually, a double heat treatment is sufficient to obtain a high-quality blued surface.

Information: weapons factories producing firearms and bladed weapons use thermal bluing. The coating on many products lasts more than 100 years. Example, Mosin rifles manufactured in 1891 and Nagan revolvers released at the end of 1888.

Source: https://metmastanki.ru/voronenie-metalla-v-domashnih-usloviyah-sposoby

Oxidation of metals at home

This article will focus on analyzing the phenomenon of metal oxidation. Here we will look at a general idea of ​​this phenomenon, get acquainted with some varieties and study them using the example of steel. The reader will also learn how to perform a similar process on their own.

Definition of oxidation

First, we will focus on the concept of oxidation itself. This is a process that creates an oxide film on the surface area of ​​the product as well as on the workpiece.

It becomes possible due to redox reactions. Most often, such measures are used for the oxidation of metals, decorative elements and to form a dielectric layer.

Among the main varieties, the following are distinguished: thermal, plasma, chemical and electrochemical form.

Species diversity

Dwelling on the description of the above listed types, about each of them we can say that:

• The thermal form of oxidation can be carried out by heating a specific product or tool in an atmosphere of water vapor or oxygen. If metals, such as iron and low-alloy steel, are oxidized, the process is called bluing.
• The chemical form of oxidation characterizes itself as a processing process through the use of melts or solutions of oxidizing agents. These can be representatives of chromates, nitrates, etc. Most often, this is done with the aim of giving the product protection against corrosion processes.
• Oxidation of the electrochemical type is characterized by the fact that it occurs inside electrolytes. It is also called micro-arc oxidation.
• The plasma form of oxidation can only be carried out in the presence of low-temperature plasma. It must contain O2. The second condition is the presence of a direct current discharge, as well as RF and/or microwave.

General concept of oxidation

To better understand what this is the oxidation of metals, it would also be advisable to familiarize yourself with the general, brief characteristics of oxidation.

Oxidation is a process of a chemical nature, which is accompanied by an increase in the degree of atomic oxidation of a substance that is subject to this phenomenon. This occurs through the transfer of negatively charged particles - electrons, from the atom, which is the reducing agent. It may also be called a donor. The transfer of electrons occurs in relation to the oxidizing atom, the electron acceptor.

Sometimes, during oxidation, the molecules of the original compounds can become unstable and break down into smaller constituent fragments. In this case, some part of the atoms of the resulting molecular particles will have a higher degree of oxidation than the same types of atoms, but remaining in the original, original state.

Using the example of steel oxidation

What is metal oxidation? The answer to this question will be better considered using an example, for which we will use this process with steel.

Chemical oxidation of a metal - steel - is understood as the process of performing work during which the metal surface is covered with an oxide film. This operation is carried out, most often, to form a protective coating or give a new feature to a decorative element; This is also done to create a dielectric layer on steel products.

Speaking about chemical oxidation, it is important to know: first, the product is treated with some alloy or solution of chromate, nitrate or some other oxidizing agent. This will give the metal protection against corrosion. The procedure can also be carried out using compositions of an alkaline or acidic nature.

The chemical form of oxidation, carried out through the use of alkalis, must be carried out at a temperature between 30 and 180 °C. For such procedures it is necessary to use alkalis mixed with a small amount of oxidizing agents. After the part has been treated with an alkaline compound, it must be rinsed very thoroughly and then dried. Sometimes a workpiece that has already gone through the oxidation procedure can be additionally oiled.

More details about the acid method

To apply the acid operation method, it is necessary to use several acids, most often two or three. The main substances of this type are hydrochloric, phosphoric and nitric acids. A small amount of manganese compounds and others are added to them. The variation in temperature parameters at which oxidation of the metal - steel, can occur, using the acid method, ranges from 30 to 100 ° C.

Chemical oxidation, described for two methods, gives a person the opportunity to obtain, both in production and at home, a film that provides sufficiently strong protection for the product. However, it will be important to know that the protection of steel and other metals will be more reliable if an electrochemical procedure is used. It is precisely because of the advantages of electrochemical. method over chemical oxidation, the latter is used less frequently in relation to steel objects.

Anodic form of oxidation

The oxidation of metals can take place using the anodic process. Most often, the electrochemical oxidation process is called anodic. It is carried out in the thickness of electrolytes in a solid or liquid state of aggregation. Also, the use of this method will allow you to apply a high-quality film to the object:

• The thickness of the thin-layer coating ranges from 0.1 to 0.4 micrometers.
• Ensuring electrical insulating and wear-resistant properties is possible if the thickness ranges from two to three to three hundred microns.
• Protective coating = 0.3 – 15 µm.
• Layers with properties similar to enamel can be applied. Experts most often call this film an enamel coating.

A characteristic of a product that has been anodized is the presence of positive potential. This procedure is recommended for the purpose of imparting protection to the elements of integrated circuits, as well as when creating a dielectric coating on the surface of semiconductors, alloys and steels.

The process of oxidizing anodized metals can, if desired, be performed by anyone in a domestic environment, at home. However, it will be very important to comply with all safety conditions, and this must be done unconditionally. This is due to the use of very aggressive compounds in this method.

One of the special cases of anodization is considered to be the method of microarc oxidation. It allows a person to obtain a number of unique coatings with high decorative, heat-resistant, protective, insulating and anti-corrosion parameters.

The microarc form of the process can only be carried out under the influence of alternating or pulsed current in the thickness of electrolytes that are slightly alkaline in nature. The method under consideration makes it possible to obtain a coating thickness from two hundred to two hundred and fifty microns.

After the operation, the surface will look like ceramics.

Bluening process

Oxidation of ferrous metals in professional terminology is called bluing.

When we talk about steel blueing, for example, oxidation, blackening or blueing, we can say that this is a process during which a layer of iron oxide is formed on cast iron or low-alloy steel. As a rule, the thickness of such a film ranges from one to ten microns. The thickness of the layer also determines the presence of a certain tarnish color. Depending on the increasing thickness of the film layer, the colors can be: yellow, brown, cherry, purple, blue and gray.

Currently, there are several types of bluing:

• The alkaline type is characterized by the use of appropriate solutions, with the addition of oxidizing agents, at temperatures ranging from 135 to 150 degrees Celsius.
• The acid type of bluing uses acidic solutions and chemical or electrochemical methods.
• The thermal form of processing is characterized by the use of fairly high temperatures (from 200 to 400 °C). The process takes place in the atmosphere of superheated water vapor. If an ammonia-alcohol mixture is used, then the temperature requirements increase to 880 °C, and in molten salts - from 400 to 600 °C. The use of an air atmosphere requires preliminary coating of the surface of the spare part with a thin layer of varnish, which must be asphalt or oil-based.

Thermal oxidation of metals is a technique in which an oxide film is applied to steel in an atmosphere of water vapor. Other oxygen-containing environments with sufficiently high temperatures can also be used. It is quite difficult to carry out heat treatment at home, and therefore, as a rule, it is not performed. When mentioning the plasma type of oxidation, it is important to know that it is almost impossible to do this at home.

Performing the operation yourself

You can do metal oxidation at home yourself. It is easiest to subject steel products to this type of treatment. To do this, you first need to polish or clean the part on which the oxidation work will be carried out. Next, oxides should be removed from the surface using solutions of five percent H2SO4 (sulfuric acid). The product must be kept in the liquid for sixty seconds.

Further actions

After the stage of placing the part in a bath of acid has passed, it should be rinsed under warm water and passivation work should be carried out or, in other words, boil the object for five minutes. To do this, use a solution of water from the tap with fifty grams of simple laundry soap. Here the calculation is for 1 liter of liquid. Having carried out all these steps, we have come to the completion of oxidation. To implement the procedure, you must:

• Use containers that are enameled and have no chips or scratches on the inner surface.
• Fill the container with water and dilute it with the appropriate amount of grams of caustic soda (per 1 liter = 50 grams).
• Transfer the vessel with water to the stove and place the product on top.
• Heat the mixture to approximately 135-150 °C.

After 90 minutes, you can pull out the part and contemplate your own work.

Some data

The reader will know that if there is a need for such an operation, but in the absence of the skill or desire, such a request can be addressed to various specialists. Oxidation of metals in Moscow, for example, can be performed both by specialists in various service sectors, and at home, by people.

Some types of such means of imparting protection to the part can be quite expensive. In the capital of the Russian Federation, the anodized type of oxidation will be quite expensive, but will give a high reliability indicator to the object.

To find specialists in such a matter, just type in a Google search query, for example: “performing chemical oxidation in (a certain city or region),” or something similar.

Source: https://FB.ru/article/369828/oksidirovanie-metallov-v-domashnih-usloviyah

Metal oxidation methods

[Metal oxidation] at home allows you to solve two problems at the same time: renew the metal surface of any product and additionally protect it from corrosion.

Previously, it was believed that oxidation processing could only be performed in a production environment using industrial equipment, but human intelligence has proven that this is not the case.

The differences between processing metal products at home and in production lie in the difference in the technologies used, but they pursue the same goal.

As a result of the industrial oxidation process, a change in structure occurs in the upper layer of the metal.

At home, the steel surface is coated with a special substance that helps change the shade and protect it.

Features of the chemical process

Chemical treatment of a metal surface involves the use of solutions and melts of various oxidizing agents, for example, salts of chromic or nitric acid.

Their use makes it possible to provide anti-corrosion protection to the metal. In this case, processing can be carried out using both alkaline and acidic compounds.

The process of chemical oxidation by the alkaline method occurs at a temperature of 30-1800, which is determined by the type of metal.

For example, chemical oxidation of aluminum and its alloys is performed at a temperature of 80-1000, the processing time is 10-20 minutes.

The shade of the film formed on the surface of non-ferrous metal depends on the thickness and structure of the alloys.

If the chemical oxidation of aluminum is performed in an alkaline solution of low concentration and at low temperature, a thin protective film with a tarnished color can be obtained.

Conversely, if you make a too concentrated alkali solution for aluminum and its alloys and use a high processing temperature, the protective coating will be loose.

A long period of oxidation can result in etching of the metal.

Processing of complex alloyed stainless steel (steel oxidation) occurs through the use of a concentrated solution of nitric acid.

At a temperature of 18-550 with a duration of 15-60 minutes.

Features of anodic oxidation of metal

Anodic oxidation of metal products at home is performed using electrolyte compositions under the influence of direct current.

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In this case, the vessel in which anodic oxidation will be carried out should not be conductive.

Sulfuric acid (H2SO4) diluted with water can act as an electrolyte at the rate of 20% per 800 ml of water.

In this case, the acid is not diluted with water, but the water is diluted with acid. You can replace H2SO4 with baking soda and salt.

Using an aluminum hanger, the workpiece to be processed is attached to the anode, and a lead plate is attached to the cathode.

If the metal product has a complex shape, then more lead elements are used.

The distance between the plates and the product should not be more than 90 mm. The processing temperature should be 200, with a current density of 2-3 Amps per square dm.

The voltage at which anodizing will be carried out is 12-15V for 60 minutes.

Micro-arc oxidation is considered one of the anodizing technologies; the technical result of its use is to obtain a coating with pronounced decorative characteristics and higher protective ability.

Microarc oxidation gives the surface of non-ferrous metal uniformity, anti-corrosion resistance and microhardness.

The components of the composition are:

• water;
• H3BO3 (20-30 g/l);
• potassium alkali (4-6 g/l);
• starch (6-12 g/l).

Using the above list, you can make an electrolyte at home by regular mixing.

Next, microarc oxidation of aluminum alloys is performed in anode-cathode mode at a temperature of 25-300.

At a current density of 15-20 Amps per square dm, with a duration of 90-120 minutes.

Thermal oxidation of metals

Thermal oxidation of iron, alloys and stainless steel is a process that results in the formation of an oxide film layer on the surface of metal products.

Thermal oxidation is performed under high temperature conditions using steam or oxygen.

The equipment used to carry out thermal oxidation is a special furnace.

Therefore, it will not be possible to carry out heat treatment using this method at home.

The use of furnaces in oxidation technology eliminates the use of chemicals, etching, washing and a number of other processes.

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The processing temperature of metal products in thermal furnaces can range from 3500 to 7000, depending on the type of steel.

Technology of oxidation of copper and its alloys

Oxidation of copper is not difficult to perform using chemical and electrochemical methods, as a result of which the copper surface can acquire a variety of colored coatings.

To obtain copper film, cyanide or acidic liquid is used. Good performance is achieved by oxidizing copper with cyanide electrolyte.

At the same time, copper alloys, which contain alloying metals in their structure, are more difficult to process.

An example is bronze, which contains a certain percentage of tin, which helps protect copper from oxides.

Or an alloy of bronze with nickel and chromium additives, such a metal is even more difficult to process.

Bronze with a minimum presence of zinc, not exceeding 20%, lends itself well to processing, while a large amount of it complicates the process.

With the help of sulfur compounds, cold processing of copper sculptures is most often performed. As a rule, these are liver sulfur, ammonium sulphide and sodium.

Ammonium sulfide allows you to make a cold black with a blue tint oxide coating. You can give a decorative look to a product made of bronze and tin using sulfur liver.

But if you use it to color pure copper or bronze and tombac, you can achieve a red tint to the film layer.

Silver oxidation technology

Oxidizing silver allows the white metal to obtain a blue, black or purple tint, while the structure of the processed product is not subject to deformation or destruction.

You can treat silver items at home using liver sulfur.

To prepare the composition at home, you need to take potassium alkali and sulfur (you can buy it where fertilizers are sold).

Then you need to combine the substances in an iron container: 1 part alkali and 2 parts sulfur, and keep the composition on fire until completely melted.

The mixture must be stirred periodically. Next, the finished sulfur liver is removed from the heat and allowed to cool.

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When the alloy has cooled, it is broken into pieces and transferred to a container with a tight lid.

Now that you have a sulfur liver at home, you can start processing silver. You need to take a piece of alloy, about the size of a pea, put it in a container and fill it with hot water.

After the lump is dissolved by stirring, the silver item is placed in sulfuric water.

After half an hour, the silver will begin to change its color, as mentioned above, the white metal can take on a purple, black or blue tint.

When the product acquires the desired color, it is removed from the liquid and rinsed with hot, warm and, finally, cold water.

Titanium oxidation technology

Oxidation of titanium is mandatory due to the low wear resistance of this type of metal.

Obtaining an oxide film allows titanium products to acquire chemical strength, increase the frictional characteristics of the material and change the color of the surface coating.

To carry out the oxidation of titanium, anodic treatment is most often used, since titanium does not withstand the effects of chemical solutions during the chemical oxidation process.

Anodic oxidation of titanium involves the use of oxalic, chromic and other acids or mixtures thereof, as well as other additives.

The black oxide film helps to strengthen the surface structure of titanium products; it is the result of using anodizing technology with an 18% sulfuric acid solution.

Depending on the processing mode, the protective film acquires a certain thickness.

For example, if the process is performed at a temperature of 800C, the anode current density is 0.5 Amperes with a processing time of 8 hours, the film layer will be about 2.5 microns.

When anodizing in the mode: 100ºС, duration – 2 hours, current density – 1 Ampere – the film thickness will be 1 micron.

Source: https://rezhemmetall.ru/oksidirovanie-metalla.html

Rust removal

According to statistics, losses from rust (corrosion) amount to up to 12% of all metal produced. Operating conditions for metal products and structures are constantly becoming more stringent, including due to air pollution. Thus, emissions of sulfur or nitrogen oxides lead to the formation of microdroplets of sulfuric or nitric acids, and the rate of rust formation increases several times.

Rust - the chemical basis of the process

Iron is a reactive metal that, in the presence of water and oxygen, easily oxidizes, forming several compounds - oxides, hydroxides and their hydrates. Oddly enough, there is no exact formula for rust: depending on environmental conditions, the iron oxidation product has a variable composition: nFe(OH)3*mFe(OH)2*pH2O.

Rust occurs over the entire surface of the metal, but the most vulnerable places are welds, internal corners of structures, and holes for threaded connections. The structure of rust is very loose; there is practically no adhesion to metal.

Due to its high porosity, the rust layer easily retains atmospheric moisture, creating favorable conditions for further destruction of the metal.

The danger of the process is that it is not possible to visually assess the degree of damage to a metal structure: under the red-brown layer of rust, the metal can be completely destroyed. If measures are not taken in a timely manner, the result may be disastrous, including complete destruction of the product. It’s one thing if it’s a rusty nail in the wall of a country house, and quite another thing if the rust has affected a power line support or the hull of a marine vessel.

Methods for removing rust

Popular wisdom says that it is easier to prevent any problem than to later make heroic efforts to eliminate its consequences. Rust is no exception. Over the past 20-30 years, chemists and physicists have proposed many ways to prevent corrosion - from protective coatings to complex engineering structures - electrochemical protection stations.

If rust does appear, this is not a reason to give up: there are many effective ways to remove it, and the sooner active measures are taken, the greater the effect of their use will be. So, first things first.

Mechanical rust removal

Corrosion products have low adhesion and are therefore easily removed from the metal surface by mechanical action - for example, when treated with a metal brush.

You can remove rusty deposits from large products using a grinding machine, while following a simple rule: you need to start with coarse grains, and use the finest grains for finishing. Areas of metal from which rust has been removed are completely defenseless against atmospheric influences.

If they are not treated with anti-corrosion compounds that prevent contact with water and oxygen, the rusting process will only accelerate.  

Chemical methods for removing rust

Knowing the nature and chemical composition of rust, it is logical to assume that it can be removed with the help of acids. It is known from a school chemistry course that metal oxides and hydroxides easily interact with acids, resulting in the formation of iron salts and the corresponding acid, and water.

For example, when exposed to hydrochloric acid, the following reactions occur:

• 2Fe(OH)3 + 6HCL → 2FeCl3 + 6H2O
• Fe (OH)2 + 2HCL → FeCl2 +2H2O

The resulting ferric chloride is a water-soluble salt that must be removed from the surface of the workpiece by simply rinsing in water and then wiping the surface dry. There is no need to wait until rust begins to form again; the cleaned areas should be treated with protective compounds.

When treating with acids, there is a danger of dissolving the metal, since iron in the electrochemical voltage series comes before hydrogen, it reacts actively with many dilute acids:

For this reason, before doing chemical experiments at home, you need to read the relevant literature. A corrosion inhibitor, methenamine, will help eliminate the side effect; when adding only 1-2 g per liter of hydrochloric acid solution, the reaction with iron does not occur.

Rust converters

Liquid compounds based on phosphoric acid are an excellent way to prevent further rust formation on the surface of steel and iron products. With this processing method, only those areas of rust that are weakly adhered to the base are first removed.

The iron orthophosphate formed during the reaction creates a durable protective film through which moisture and oxygen do not penetrate, thereby preventing further corrosion of the metal.

To speed up the drying process, you can add 30-40 ml of isobutyl alcohol or 15 grams of tartaric acid per liter of 25% phosphoric acid solution.

Modern equipment for rust removal

Mechanical methods of cleaning surfaces from rust using improvised means are not always applicable; if the product has a complex shape, then it is not possible to treat all areas. Chemical methods also have their drawbacks; if basic safety rules are not followed, you can get a chemical burn or poisoning.

Disposal of waste solutions also poses a certain problem.

The optimal way to remove rust, especially from the surface of products with complex geometric shapes, is soft blasting. The essence of the method is as follows: a stream of compressed air containing special abrasive granules is directed at the metal product.

By changing the pressure, you can adjust the depth of the layer removed from the surface - this way only the layer of rust or scale is removed, and the metal remains untouched. ARMEX granules used in the Nordblast NB 28-2 soft blasting machine consist of tiny particles of soda and chalk.

By applying high pressure to the surface, they easily remove not only rust, but also paint and varnish materials.

A distinctive feature of the method is its absolute environmental safety: the components used are chemically inert. Numerous studies have proven that there are practically no scratches or other microscopic defects on the metal surface, which can subsequently become centers for the re-formation of rust. The alkaline nature of the granules promotes the formation of a passive film on iron or steel products, protecting the bulk of the metal from corrosive destruction.

The best results from using the Nordblast NB 28-2 soft blasting machine were obtained when processing machine or yacht parts. The rust removal process depends on the degree of corrosion, usually it takes 1 day to completely treat a car, 2 days for a yacht.

The earlier the problem is detected, the easier it is to fight rust. Which method is most preferable - everyone decides for themselves, but you should not use old-fashioned methods if there is equipment that removes rust with 100% efficiency!

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Source: https://blastingservice.ru/services/udalenie-kraski/udalenie-rzhavchiny/

Chemical corrosion

Chemical corrosion is a type of corrosion destruction of metal associated with the interaction of the metal and the corrosive environment, in which the metal is simultaneously oxidized and the corrosive environment is restored. Chemical corrosion is not associated with the formation or exposure to electric current.

The driving force (root cause) of chemical corrosion is the thermodynamic instability of metals. They can spontaneously transition to a more stable state as a result of the process:

Metal + Oxidizing component of the medium = Reaction product

In this case, the thermodynamic potential of the system decreases.

By the sign of the change in the thermodynamic potential, one can determine the possibility of spontaneous occurrence of chemical corrosion. The criterion is usually the isobaric-isothermal potential G. When a chemical process occurs spontaneously, a decrease in the isobaric-isothermal potential is observed. Therefore, if:

Δ GT < 0, then the process of chemical corrosion is possible;

Δ GT > 0, then the process of chemical corrosion is impossible;

Δ GT = 0, then the system is in equilibrium.

Chemical corrosion includes:

— gas corrosion — corrosion destruction under the influence of gases at high temperatures;

— corrosion in non-electrolyte liquids.

Gas corrosion

Gas corrosion is the most common type of chemical corrosion. At high temperatures, the metal surface is destroyed under the influence of gases. This phenomenon is observed mainly in metallurgy (equipment for hot rolling, forging, stamping, parts of internal combustion engines, etc.)

The most common case of chemical corrosion is the interaction of metal with oxygen. The process proceeds according to the reaction:

Me + 1/2O2 - MeO

The direction of this reaction (oxidation) is determined by the partial pressure of oxygen in the gas mixture (pO2) and the dissociation pressure of oxide vapor at a certain temperature (pMeO).

This chemical reaction can occur in three ways:

1) pO2 = pMeO, equilibrium reaction;

2) pO2 > pMeO, the reaction is shifted towards the formation of oxide;

3) pO2 < pMeO, the oxide dissociates into pure metal and oxide, the reaction proceeds in the opposite direction.

Knowing the partial pressure of oxygen in the gas mixture and the dissociation pressure of the oxide, it is possible to determine the temperature range at which this reaction is thermodynamically possible.

The rate of gas corrosion is determined by several factors: ambient temperature, the nature of the metal or alloy composition, the nature of the gaseous environment, the time of contact with the gaseous environment, and the properties of the corrosion products.

The process of chemical corrosion largely depends on the nature and properties of the oxide film formed on the surface.

The process of the appearance of an oxide film on the surface can be divided into two stages:

— oxygen molecules are adsorbed on the surface of the metal, which is in direct contact with the atmosphere;

- metal reacts with gas to form a chemical compound.

At the first stage, an ionic bond occurs between surface atoms and oxygen: the oxygen atom takes two electrons from the metal. This creates a very strong bond, much stronger than the bond between oxygen and the metal in the oxide.

Perhaps this phenomenon is observed due to the effect on oxygen of the field created by metal atoms.

After complete saturation of the surface with the oxidizer, which occurs almost instantly, at low temperatures, due to Van der Waltz forces, physical adsorption of oxidant molecules can be observed.

As a result, a very thin monomolecular protective film is formed, which thickens over time, making it difficult for oxygen to reach.

At the second stage, due to chemical interaction, the oxidizing component of the medium takes away valence electrons from the metal and reacts with it, forming a corrosion product.

If the resulting oxide film has good protective properties, it will inhibit the further development of the chemical corrosion process. In addition, the oxide film greatly affects the heat resistance of the metal.

There are three types of films that can form:

- thin (invisible to the naked eye);

- medium (gives tarnished colors);

- thick (clearly visible).

In order for the oxide film to be protective, it must meet certain requirements: have no pores, be continuous, adhere well to the surface, be chemically inert in relation to its environment, have high hardness, and be wear-resistant.

If the film is loose and porous, and also has poor adhesion to the surface, it will not have protective properties.

There is a continuity condition, which is formulated as follows: the molecular volume of the oxide film must be greater than the atomic volume of the metal .

Continuity is the ability of the oxide to cover the entire surface of the metal with a continuous layer.

If this condition is met, then the film is continuous and, accordingly, protective.

But there are metals for which the continuity condition is not an indicator. These include all alkaline, alkaline earth (except beryllium), even magnesium, which is important technically.

To determine the thickness of the oxide film formed on the surface and study its protective properties, many methods are used. The protective ability of a film can be determined during its formation, by the rate of metal oxidation and the nature of the rate change over time.

If the oxide has already formed, it is advisable to study the thickness and its protective properties by applying some reagent suitable for this case to the surface (for example, a solution of Cu(NO3)2, which is used for iron).

By the time of penetration of the reagent to the surface, the thickness of the film can be determined.

Even the already formed continuous film does not stop its interaction with the metal and the oxidizing environment.

The influence of external and internal factors on the rate of chemical corrosion.

Temperature has a very strong influence on the rate of chemical corrosion. When it increases, oxidation processes occur much faster. In this case, the decrease in the thermodynamic possibility of the reaction occurring has no significance.

Variable heating and cooling have a particularly strong effect. Cracks form in the protective film due to the appearance of thermal stresses. Through cracks, the oxidizing component of the medium has direct access to the surface. A new oxide film is formed, and the old one gradually peels off.

The composition of the gas environment plays a major role in the corrosion process. But this is individual for each metal and changes with temperature fluctuations. For example, copper corrodes very quickly in an oxygen atmosphere, but is stable in an environment containing SO2. Nickel, on the contrary, corrodes intensively upon contact with an SO2 atmosphere, but is stable in O2, CO2 and H2O environments. Chromium is relatively stable in all four environments.

If the oxide dissociation pressure is higher than the pressure of the oxidizing component, the oxidation of the metal stops and it becomes thermodynamically stable.

The rate of oxidation depends on the composition of the alloy. Let's take iron, for example. Additions of sulfur, manganese, phosphorus and nickel do not affect its oxidation. Silicon, chromium, aluminum slow down the process. And beryllium, cobalt, titanium and copper greatly inhibit oxidation. At high temperatures, tungsten, molybdenum, and vanadium can intensify the process. This is explained by the volatility or fusibility of their oxides.

Observing the rate of iron oxidation at different temperatures, we note that with increasing temperature, the slowest oxidation is observed with an austenitic structure. It is the most heat-resistant compared to others.

The rate of chemical corrosion is also influenced by the nature of surface treatment. If the surface is smooth, then it oxidizes a little slower than a bumpy surface with defects.  

Chemical corrosion in non-electrolyte liquids

Non-electrolyte liquids are liquid media that are not conductors of electricity. These include: organic (benzene, phenol, chloroform, alcohols, kerosene, oil, gasoline); inorganic origin (liquid bromine, molten sulfur, etc.).

Pure nonelectrolytes do not react with metals, but with the addition of even a small amount of impurities, the interaction process accelerates sharply. For example, if the oil contains sulfur or sulfur-containing compounds (hydrogen sulfide, mercaptans), the process of chemical corrosion accelerates.

If in addition the temperature increases, there will be dissolved oxygen in the liquid - chemical corrosion will intensify.

The presence of moisture in non-electrolyte liquids ensures intensive corrosion through the electrochemical mechanism.

Chemical corrosion in non-electrolyte liquids is divided into several stages:

— approach of the oxidizing agent to the metal surface;

— chemisorption of the reagent on the surface;

— reaction of the oxidizing agent with the metal (formation of an oxide film);

— desorption of oxides with metal (may be absent);

— diffusion of oxides into a non-electrolyte (may be absent).

To protect structures from chemical corrosion in non-electrolyte liquids, coatings that are stable in this environment are applied to its surface.

Source: https://www.okorrozii.com/ximichiskakorrozia.html