The presence of scale, oxides, welds and other defects on the surface of stainless steel products impairs their performance properties, spoils the appearance of objects. In the presence of such defects, additional processing is required. The most popular type of processing is stainless steel etching at home.
Stainless steel processing methods
For the processing of stainless steel, a number of methods have been developed to give products the required parameters and aesthetic appearance. At home, you can use the following methods:
- etching of stainless steel;
- painting;
- satin finish (grinding and polishing);
- bluing;
- chrome plating.
Stainless steel pickling procedure
The main way to clean the surface of stainless steel is pickling (chemical and electrochemical). This method is used after heat treatment of the product, cold and hot plastic deformation, as well as to remove traces of welding. In addition to cleaning the surface, pickling stainless steel restores the passive layer that protects the stainless alloy from the damaging effects of high temperatures.
The method is based on the chemical interaction of the metal surface with acids of various concentrations, mainly sulfuric or hydrochloric, molten alkaline compositions. The use of acids involves a two-stage process: first treatment with a sulfuric acid solution, then placement in a nitric acid environment. When using the alkaline method, a caustic soda solution is used.
When processing stainless steel, careful control over compliance with technological conditions is necessary. The aggressive environment in which the metal is placed should only affect its surface, destroying defects without changing the structure of the object itself. Special additives are used to prevent over-etching.
Electrochemical (galvanic) etching
To do this, you need to follow a number of procedures.
Solution preparation. Prepare an aqueous acid solution chosen for etching. It is necessary to carefully calculate its percentage.
Surface preparation. It is necessary to perform degreasing by any method so that the protective varnish is qualitatively fixed on the stainless steel. After processing, it is not recommended to touch the workpiece, so as not to cause peeling of the varnish and, as a result, non-uniform surface treatment.
Creating a protective layer. It is necessary to apply a protective layer to areas that do not need etching. In industry, special compounds are used for this, which can also be used for home processing (subject to safety precautions). Also at home, you can cook a protective varnish, consisting of tar and rosin dissolved in turpentine.
Etching. During the electrochemical pickling of stainless steel, the workpiece is lowered into a previously prepared solution, after which voltage is applied to the created electrical circuit. In this circuit, the workpiece with the positive electrode connected is the anode, and any steel plate is used as the cathode.
The duration is determined by the type of part, carefully observing the state of the workpiece, but not more than a few minutes. After that, the voltage is turned off, the etched part is removed from the solution and the acid is carefully neutralized with reagents.
Stainless steel cleaning. They clean the stainless steel from the remnants of solutions with special means that do not affect the characteristics of the metal.
chemical etching
It is based only on chemical reactions, without additional exposure to electric current. For pickling stainless steel at home, specially designed compounds are used - pickling pastes. They are safer than reagents used in industrial environments. Hydrochloric acid is not included in the composition of such pastes (due to an increased risk to health).
From industrial substances, home formulations differ in a reduced content of harmful chlorides. Pastes are made on the basis of nitric and hydrofluoric (hydrofluoric) acids. It is a liquid jelly-like transparent substance that requires caution when using.
Before using the paste, the stainless alloy workpiece must be thoroughly cleaned and degreased. To apply the paste on stainless steel, special tools are used: plastic spatulas and brushes made of acid-resistant materials.
The exposure time of the paste depends on the brand used, indicated in the instructions for the paste. Possible time interval - from 10 minutes. up to an hour. The paste is washed off with a large amount of running water (subject to safety precautions).
When working with large products, the large area of \u200b\u200bwhich makes the application of pastes difficult, use the jet spraying method using pickling sprays for processing stainless steel.
Pickling spray for stainless steel
Etching agents
Among the large number of manufactured pickling pastes, the most popular brands can be distinguished.
Avesta Blue One (Sweden). Reliable etching paste made in Sweden by Avesta BlueOne. Removes corrosion manifestations, minor welding defects, gives shine to products. Processing time depends on the ambient temperature: from 90 min. at 100 ° C up to 20 min. at 300°C. Stainless steel pickling time depends on the surface condition.
Cleans stainless steel from scale and other defects, providing a beautiful view of the weld. Due to its high density, the gel-like paste is effective even on vertical surfaces. The processing time is 10 minutes.
Stain Clean byESAB (Sweden) . A quality brand that restores the anti-corrosion properties of stainless alloys. Does not need mixing. Can be applied on vertical surfaces. It has a low content of harmful nitrogen gases. Not applicable at t below +5 o C.
Other processing methods
Alternative methods can be used for finishing stainless steel products.
Painting
The most affordable and easy to perform method that allows you to quickly create a protective film.
Satin finish (polishing and polishing)
This method of processing stainless steel combines practicality and efficiency. The surface of stainless steel after processing acquires the smoothness of satin. At home, small-sized tools are used for grinding stainless steel: a grinder, an air file, etc. Small areas are satined manually, with sanding sheets or a manual machine. To obtain a high-quality result, grinding begins with the use of sheets with a grit of 180 grit, then they switch to a grain of 320, 600 grit. In conclusion - polished with felt.
Blueing (blackening)
The surface of stainless steel is covered with a blue-black protective oxide film. There are three ways to create a film: acid, alkaline and thermal. The latter method is available only in industrial conditions, when stainless steel at a temperature of 250 to 850 0 C is treated in one of three ways:
- in melted salts;
- in a vaporous solution of alcohol and ammonia;
- in superheated steam.
In everyday life, stainless steel is blackened in solutions of acids or alkalis by an electrochemical or chemical method.
Chrome plating
This method of processing stainless steel reliably protects the surface of products from mechanical and other damage. But its implementation requires knowledge of technology and work experience. Therefore, chrome plating is practically not used at home.
The information presented in this article will be useful for materials scientists, technologists and engineers involved in the field of processing and studying the properties of metals. Obtaining decorative coatings is increasingly being used in various branches of mechanical engineering, automotive industry, and in the production of household items. Consider one of the most requested ways to create a film - the oxidation of stainless steel.
Terms, definitions, types of stainless steel oxidation
By definition, this term refers to the creation of a film of oxides on the surface of stainless steel due to redox reactions. In addition to the protective function, decorative finish, the use of this process is involved in the need to create dielectric layers and change the surface physical processes occurring in an environment of high magnetic and electric fields. Depending on the method by which oxidation was obtained, there are:
- thermal process
- Chemical exposure
- Electrochemical process
- Plasma oxidation
Let's consider these processes in more detail. 
thermal process
The thermal oxidation process consists in the treatment of metal at certain temperatures in an environment of oxygen or water vapor. Low-alloy steels and iron, under this effect, receive a film called bluing. The temperature for bluing is 300-350 degrees Celsius. For high-alloy and high-chromium steels, this figure rises to 700 degrees.
Interesting information: bluing stainless steel at home is quite within the power of everyone. There are several ways to do this. Any method must be preceded by metal grinding and degreasing. We dip the part or product in oil. It can be olive, machine, and best of all weapons. After making sure that the application is uniform and that the oil gets into hard-to-reach places, we take out the workpiece and let the oil drain. After that, we start heating with a blowtorch. The lower the heating rate, the higher the probability of complete removal of light fractions, obtaining a uniform bluing layer. After overcoming the threshold of 400 degrees, a characteristic black color appears on the surface. At the end, it is necessary to polish with soft felts and paste.
Chemical oxidation
Chemical oxidation - obtaining a protective film by the interaction of metal and melts, more often solutions, oxidizing substances. The advantages of this method include:
- relative simplicity
- no high temperature sources
- ease of equipment
- low labor costs
The only and most significant drawback of this method is the low protective characteristics of such a film and low resistance to mechanical stress. The predominant use of chemical oxidation is to apply a touch-up layer, as well as to preserve mechanisms and parts in storage conditions in production workshops and heated warehouses.
The need to use a large amount of water, its subsequent purification, the high cost of water and cleaning reagents leads to a gradual displacement from production, in the face of tightening environmental standards. Cold chemical oxidation of stainless steels has become available due to the introduction of two-component chemical reagents on the market.
Please note: The higher the concentration of active chemical elements, the faster the reaction rate, but the penetration depth is less. Accordingly, the film thickness will be less.
Electrochemical oxidation
Electrochemical oxidation of stainless steels is a method that is widely used in industry. It consists in the fact that the parts are hung on special holders. On this device, they are lowered into a solution with alkali, after which the bath in which it is located is attached to the negative cathode. The parts are connected to the positive anode. When direct current is passed, according to the course of physics, electrolysis processes occur, accompanied by an increase in temperature. The rate of application and the thickness of the resulting film depends on many factors. Main influencing factors:
- The density of the flowing current.
- The electrical conductivity of the solution in which the parts are placed
- Electrolyte temperature
- Part geometry and configuration
An interesting fact: at the end of the previous decade, one of the giants of the Japanese automaker used electroplating blackening of stainless steel on the body of their cars. Studies have shown that in the area of the roof bends in the pillars, the oxidized layer had insufficient thickness. Long-term research on changing the place of attachment of the electrodes to the body, changing the current density, the exposure time did not lead to the expected result. Only by changing the electrical conductivity of the alkaline solution, making special additives, the layer became uniform and sufficient for the quality level of the enterprise.
Complex geometry, sharp corners, curved shapes in the contours of the part lead to a difference in potentials that occur on the surface of stainless steel and, accordingly, lead to a difference in film thickness. For such parts, it is advisable to use the previous oxidation method.
Plasma oxidation
Plasma oxidation occurs under conditions similar to electroplating. When a certain critical value of polarization is reached, a plasma microdischarge occurs on the surface of the anodized part. In contrast to electrochemical nitriding, not only the alkali solution but also the cathode material is involved in the formation of the resulting film. A characteristic feature of the presented method is deep penetration into the stainless metal layer and the possibility of obtaining a uniform coating on objects of complex geometric configuration.
An interesting fact: at the point of spark breakdown during plasma oxidation, the temperature is about 10000K, and the pressure is comparable to 10 2 MPa. After the termination of the spark, a sharp cooling of the surface occurs, which leads to the appearance of new physical properties and their study as elements of nanotechnology.
Coatings that are formed using this method are characterized by increased adhesion to the substrate and properties close to ceramics. Taking into account the price of the equipment and the lack of research in this area, its labor intensity and the need for highly qualified personnel do not allow this process to be widely used in industry, being limited to expensive industries and piece products. For aluminium, titanium and magnesium alloy, plasma oxidation finds niches and distribution in the industry.
Please note: the terminology of this process has not settled down to this day. Therefore, having met in the literature oxidation in the sparking mode, anodic deposition, maximum voltage mode, plasma-electrolytic oxidation, one must understand that this is one and the same process - plasma oxidation.
Blackening challenges associated with stainless steel
All the methods described above are ideal for ferrous alloys and light alloy steels. A special approach is required, a set of measures for blackening stainless steel, as a conditionally inert alloy. The scattered data in the literature on the direct blackening of stainless steel are contradictory and do not always work in practice. On a production scale, it is customary to address this issue with a two-stage approach. The first step is anodizing stainless steel with another metal that is more prone to oxidation. It is mostly nickel, less often copper. The second stage is the oxidation of the resulting surface. Chemists in many countries are developing special passivating pastes, compounds for blackening stainless steels that can incline them to oxidation.
To apply a decorative film that does not work with temperature changes on a surface that does not experience large mechanical loads, the following oxidation method can be used:
- Etching in 10% oxalic acid solution
- Washing and processing in 1% sodium sulfide solution to the required degree of blackening
- Oiling a stainless steel sample.
Based on the information presented, it can be concluded that the use of blackening for stainless steel is in the nature of a commercial decorative coating. The use of oxidation to achieve higher metal characteristics is not justified and cannot be guaranteed. To obtain protective films that expand the scope of stainless steels, it is worth considering other methods and methods.
Steel bluing is increasingly used for painting metal products. It provides reliable surface protection against rust and a decorative finish. With the chemical version of staining, a protective shell appears on the surface. In the process, oils and acids are used. Depending on the type of material used and the degree of heating, the metal joints acquire a natural shade.
Many people think that this processing option is used only in the factory. But it can be organized independently if you follow the sequence of actions and use the right tools.
As a result of the work done, a film of iron oxide will appear on the surface. The dimensions of its thickness are completely dependent on the technology used. It can be 1-10 micrometers. Using the metal burnishing method, you can solve the following problems:
- protect the coating from corrosion, as rust does not appear on blued surfaces;
- give the product a more presentable appearance.
You can get the desired coverage in many ways. But the most popular of them are alkaline, thermal or acid bluing. Alkaline processing is ideal for independent work. To do this, you will need water, a digital scale, sodium nitrate and sodium hydroxide. You will also need a protective mask and gloves for personal safety.
Use only stainless utensils for work. Blueing of steel is carried out under certain temperature conditions. The solution is maintained in a boiling state, which is why it must be handled with extreme care.
To create a durable steel coating at home, the process lasts at least 1.5 hours. Before proceeding with blackening, the surface must be degreased with a solvent. The color of the protective film is determined by the warm-up time, as a result of which it will constantly change. To avoid a strong difference in shades, the surface is completely immersed in the solution.
Important! After processing, metal products must be thoroughly washed in soapy water.
Popular bluing methods
Burning technology includes several methods, the main of which are the following:
- Alkaline method. In this case, you will have to work with oxidizing agents in compliance with the temperature regime of 130-150 degrees. Due to alkaline oxidation, iron is oxidized. But with this method you should not work in a house or apartment, as an unpleasant smell is released during the work.
- acid method. In this case, the work is carried out in acidic solutions using a chemical or electrochemical method. It is important to place the object in the solution for the set time and act in accordance with the instructions.
- Thermal way. Such bluing is considered the oldest and simplest of the existing ones. This technology consists in the need to heat the steel in the open air. The procedure continues until the top layer of metal enters into a chemical reaction with oxygen. The stronger the heat, the darker the part will become.
There are other ways to obtain blued iron. You can choose the most suitable one and do all the work yourself. The main thing is to strictly follow the sequence of actions and use the correct compositions for bluing.
Alkaline
To burnish metal in this way, it is necessary to perform a clear sequence of actions. First of all, you will have to degrease the surface with a solvent. Then do the following:
- Take a bowl and pour 100 ml of water into it.
- It is necessary to dissolve 120 grams of caustic soda and 30 grams of sodium nitrate in it, stirring them until a homogeneous mass.
- Heat the composition to a temperature of 130-150 degrees.
- Place the part in the boiling mixture so that it does not come into contact with the walls.
- The part will turn black after 20 minutes, after which it is washed in distilled water.
When the product is completely dry, it must be lubricated with machine oil and wiped dry. As a result, the coating becomes smooth and wear-resistant. You do not have to additionally polish the product. These rules allow you to give the surface the desired effect without resorting to complex technology. Remember to use a protective mask and thick gloves.
It is important to carry out work only in a stainless container that is of sufficient size. This can be a problem if you plan to burn a large knife. To avoid color differences, the entire surface of the product must be in solution.
Acid
This option is performed in acidic solutions. In this case, electrochemical or chemical processing technology is used. It is important to clean the rust. For this, ordinary sandpaper is suitable. To increase the cleaning speed in advanced cases, you can use a grinder equipped with a metal brush.
Then use a cleaning compound, which is used as sodium triphosphate, ethyl alcohol with kerosene or plain kerosene. The part must be immersed in the solution for 15 minutes. In the end, the product should be washed in running water, wiped well and dried.
While the part dries, it is necessary to prepare a solution for acid bluing. Mix the following ingredients:
- 1 liter of water;
- 2 grams of tannic acid;
- 2 grams of tartaric acid.
The next step is the direct burnishing of the knife. Heat the mixture to 150 degrees. Place the item in the container for 15 minutes. Make sure that the metal surface is completely immersed in the solution. Then rinse it in running water and dip it in boiling water. This allows you to completely clean the surface of the solution.
At the final stage, the process of bluing steel in oil is carried out. In this case, the product is immersed in machine oil for 60 minutes, after which it can be used for its intended purpose.
Important! When using this method, cold bluing is also possible. To do this, use a tool such as rusty varnish.
Thermal
Everything is very simple here. Such bluing is considered the oldest. This technology consists in the need to heat the steel in the open air. The procedure continues until the top layer of metal enters into a chemical reaction with oxygen. The stronger the heat, the darker the part will become.
Applying rusty varnish
This bluing agent is often used to obtain a smooth and even surface. It includes steel shavings, but it can be replaced with cast iron shavings. In this case, financial investments will be minimal. In this case, the workpiece must be placed in a chemical agent. As a result, red or black iron oxide appears. This instruction, as a means for cold bluing, gives a long-lasting and effective result.
To get rid of reddish plaque, use a brush with fairly stiff metal bristles. As a result, only black oxide remains.
To begin with, the metal is machined and degreased. Then the product is placed in a special composition. The bluing agent is prepared in a porcelain container. It is important to work with gloves to protect the skin. The composition is made using hydrochloric acid. It is poured into a container in a small amount. It mixes with metal filings, nitric acid and iron rust.
The mixture is stirred well until the chemical reaction stops, which is accompanied by the evolution of gas. Water and plain vodka are added to the mixture in equal proportions. At the end, it is left for a certain time until it settles. Then it is drained to remove scales and salts that have precipitated.
When the mixture is ready, the processed metal product is placed in it. It is held until it turns black. After that, it is washed under strong pressure of running water.
On the video: burnishing with rusty varnish.
Use of machine oil
Burning a knife can be done at home with oil. You will need the following:
- 500 ml of machine oil;
- equipment for holding the part, such as special hooks, pliers or pliers;
- metal container for bluing in oil;
- rags or paper napkins;
- gas burner, building hair dryer or gas stove.
How to do the procedure if you have a building hair dryer? To do this, the part is laid out on a non-combustible surface, such as a brick. The hair dryer is then set to high heat. The part is heated to the maximum. Then, with the help of pliers or pliers, it is placed in the bath, completely covering the surface with oil. This will ensure an even color at the end of the procedure. Then the part is placed on a paper towel to absorb the oil.
When processing metal in this way, you can use linseed oil. The procedure stops as soon as the desired color is obtained. The process must be repeated 3 or 4 times. At the end of each cycle, the product becomes darker.
If you decide how to burnish a steel product, this method will not make it black. And the surface will not turn out too strong. This bluing agent is suitable if it is necessary to protect a steel product from rust.
Are you thinking how to bury metal objects yourself? To do this, you can use one of the methods suitable for you. Using professional advice, you will get the perfect metal color, and the process itself will take place in a way that is safe for you.
Cold bluing involves the use of a bath or a special container. But it cannot be made of any material. A wooden bath or a galvanized container is optimal.
It is good if the container is additionally equipped with hooks for placing the part in the solution. These elements can be made in the form of curved racks or mounted on the wall. Everything is determined by the height of the sides of the container and the dimensions of the workpiece.
For bluing steel, the container must be properly degreased. An organic-based solvent is best suited for this. It can be acetone, perchlorethylene or B-70 gasoline. The base remains inert with respect to these compositions. But for each type of metal, it is necessary to select the optimal solution. Only with its help you can get the optimal color and maximize the performance of the product.
Even if you are cold bluing, work should be done outdoors or in a well ventilated area. This is due to harmful fumes and a strong chemical smell.
If you are unable to clean the surface well with a brush or abrasive, use etching. For each type of metal there is a tool. If you have a ferrous metal product, the bluing liquid includes 100 grams of nitric, 50 grams of sulfuric and 150 grams of hydrochloric acid per 1 liter of distilled water. The surface is treated for at least 2.5 hours. To give a smooth surface, chemical polishing is used.
Thus, the burnishing of a steel object can be done independently. To do this, it is important to choose the most appropriate processing method and adhere to a clear sequence of actions. As a result, you get a corrosion-resistant surface in a suitable shade.
Do-it-yourself blackening of steel (3 videos)
Metal products after bluing (26 photos)
In order to protect iron from corrosion, bluing is actively used today, both factory and artisanal, which is divided into different types, including blackening of carbon and low-alloy steel at home.
1 What does metal bluing give you?
Most grades of steel tend to rust, corrosion centers occur at the slightest contact with water, if the surface is not then wiped with an oiled rag. Even with high humidity, steel can rust very quickly. For the most part, the metal is coated by spraying or brushing. But this is not suitable for threaded connections, as well as for moving parts. Therefore, burnishing is often used to protect against corrosion, which is also called blackening or blueing of steel and, if we turn to technical terms, oxidation.
In other words, conditions are created for the formation of an iron oxide film on the metal surface, the thickness of which can vary from 1 to 10 micrometers, depending on the processing method. Blueing according to the type of effect on the metal is divided into thermal, acidic and alkaline, that is, in the last two versions, the metal is immersed in the appropriate solution. When heated, the so-called tint colors change on the surface of the steel, approximately the same happens during galvanic treatment in an acid or alkaline bath. You just need to select the desired oxidation color and stop the effect on the steel surface on it.
The change in tint colors is associated with an increase in the thickness of the oxidized metal layer. The thinnest film is formed at the stage of the appearance of yellow, as its thickness grows, brown, cherry, purple, and then blue and gray will replace each other. But the latter does not mean at all that you have come close to what is called blackening. After all, bluing covers almost all colors of tarnish, starting with brown.
2 Steel blueing - recipes of varying degrees of complexity
With prolonged heating, after the glow of the metal passes the white stage, and then yellow, brown and purple, the surface of the steel will turn into a beautiful light blue color, gradually changing to dark. It is on heat treatment that the simplest bluening method is based, and it is with it that we will begin the list of existing oxidation methods. To obtain high-quality bluing at home, you will need a forge, preferably with automatic blowing, in extreme cases, you can use an ordinary metal barrel filled with half chopped brushwood.
We fill a metal box to the size of the part with fine birch charcoal and place it in a well-heated forge or in a barrel with burning chips. When the coal heats up and begins to smolder, we put the steel part wiped with vinegar and dried into it, after which we follow the stages of its heating. At the very beginning of the appearance of blue, we take out the workpiece and cool it slightly in air, while cleaning it with soft charcoal. Then put it back in the box. By repeating these manipulations several times, and at the end bringing the heat to a clear blue, you will get a strong oxide film.
Another way is with the use of a special solution and without heat treatment. For it, you need to get only 2 reagents: 2.5 grams of potassium hexacyanoferrate, popularly referred to as red blood salt and being a powerful oxidizing agent, as well as one and a half chlorine iron. Both ingredients dissolve perfectly, so feel free to fill each in a separate container with 0.5 liters of water, after which we combine the resulting liquids into a single mixture. It is in it that we place the steel part, which must be given corrosion resistance. When the metal surface reaches the desired color, we remove the part from the bath and dry it.
There are more complex methods of blueing, but we will turn to those that are simpler. Oxidation can be carried out not only with solutions, but also with melts of various substances. In particular, a dark blue color can be imparted to a steel product by immersing it in molten sulfur, into which a small amount of soot is previously kneaded after the transition to a liquid state. The second option is a melt of saltpeter, obtained by heating it to a temperature of over 320 degrees. If a steel product is kept in such a bath, it will be covered with a uniform dark blue film.
3 Steel burnishing in the brown spectrum
In some cases, it is quite enough to achieve the formation of a brown oxide film on the surface of the steel part, the shades of which can be different, depending on the substances used. The simplest recipe is based on the same ingredient that we considered earlier, namely, we need 1.5 chlorine iron in the amount of 100 grams, as well as a kilogram of olive oil. We mix these components and get a paste, which should evenly cover the steel product. Then, after a few hours, the surface is polished with a metal brush-brush. The cycle is repeated several times.
Another option is somewhat similar to the previous one. But for the oxidation process at home, in addition to olive oil in the amount of 0.5 kilograms, you will need the same amount of antimony trichloride. The second ingredient, in fact, also refers to oils, therefore, by mixing the components, you will get a kind of ointment. We cover it with a steel product and leave it for a day. After the specified period, we wipe the metal surface with a woolen patch and again apply the ointment for a day. The final step will again be rubbing with wool and polishing with a waxed brush.
You can further simplify the composition, with which a brown anti-corrosion film is applied to steel. To do this, it is enough to make a solution for immersing a metal product in it for a certain time. You only need to stock up on one and a half chlorine iron, which is added in an amount of 150 grams per liter of water, or 0.2 kilograms is measured and mixed with a liter of 90% alcohol. In such a bath, it is enough to dip a steel product several times, while immersing, watching the change in the color of the surface.
4 Recipes for oxidizing steel in noble gray and black colors
Steel that acquires various shades of gray as a result of bluing looks beautiful, while a sufficiently strong film of oxidized metal is obtained that protects against rust. You can get a color in this spectrum in a simple way, which, however, is available only for small products. You will need 70 grams of copper nitrate and 30 grams of denatured alcohol, the first reagent refers to salts, the second is alcohol.
It is better to make the solution by heating the salt until it melts with the addition of denatured alcohol after removing the container from the fire (it is desirable that it be a chemical porcelain cup). We coat the steel product with the composition and heat it over the fire, putting it on a sheet of tin. The following complex recipe is suitable for oxidizing metal to gray. This process, with a considerable duration, can turn into cold blackening of the usual and which is easiest to do at home.
The components needed are as follows: 24% hydrochloric acid - 120 grams, 90% alcohol and water - 100 grams each, sublimate - 40 grams and 2 times less bismuth chloride with copper chloride. We mix acid and water, add alcohol and divide the resulting liquid into 3 equal parts. Pour the last 3 components related to salts separately into each part of the solution and then mix everything in one container. A steel product is immersed in the resulting bath for half an hour, after which it is removed and boiled in clean water. If necessary, repeat the process.
Attention, for your own safety, pour acid into water, but not vice versa, in order to avoid a violent reaction, accompanied by splashing out of the reagent.
And finally, the recipe directly for blackening. In fact, an excellent result can be achieved only by smearing a steel product with linseed oil, wax or animal fat and calcining it well in a fire. As a result of firing, a stable black film is formed on the metal surface. However, it is much more efficient to use the preliminary immersion of steel in a bath with the following solution: 100 grams of copper sulfate are placed per liter of water and 10 grams of ammonia are added. Having coated the cleaned and degreased metal with this composition, and calcined in the fire, you will first get a black-brown film, and after repeated dipping and heating - black.
Technical process "Ultra-Black 407"
BLACK STAINLESS STEEL
Process used for chemical alkaline oxidation of stainless steels with black coloration
Oxidation is carried out in a hot alkaline solution of the salt composition " Ultra Black 407", which representsan invented powdery mixture of salts and caustic soda - the company does not need to purchase any additional chemicals
In the same bath can oxidize not only stainless steel, but also malleable steels, cast irons and mild steels with the formation of black coatings on them
processing stages. Parts to be blackened can be processed in baskets, tumbling drums, or on mild steel hooks or hangers.
1. CLEANING AND DEGREASING: Parts must be thoroughly cleaned and degreased. For chemical degreasing, it is recommended to use branded technical detergents "E-Clean 173" (this TMS is also suitable for electric degreasing), "E-Clean 163" (degreases at low temperatures) and others. Any other chemical degreasing solutions capable of providing adequate surface preparation prior to oxidation may also be used.
2. RINSING: in a cascade type bath with bottom water supply.
3. ACTIVATION: all stainless steels, due to the presence of chromium oxide on their surface, are in a passive state, therefore, before blackening, this oxide must be removed by activation in a solution of the proprietary preparation "E-Peak 211" 120-240 g / l, 2–5 min, temperature from workshop to 65–85°C (selected).
If the passive state is characterized by high stability, then activation may require additional treatment in hydrochloric acid solution (180–200 g/l), 5 min, shop temperature; followed by rinsing in cold water, and then treatment in a dilute solution of the branded preparation "E-Peak 211" (7.5-15 g / l), 30 s - 3 min, 65–85 °С and more.
4. RINSING: in a cascade type bath with bottom water supply.
5. BLACKING (OXIDATION): immerse the parts in a boiling solution of "Ultra-Black 407" ( temperature 124 ± 3 °C) until their surface turns a rich black color. Required dive time may vary from 2 to 15 min depending on the weight of the parts and the type and condition of the stainless steel. Excessive immersion time can lead to poor adhesion of the forming layer. However, most blackening problems can be the result of improper surface preparation or a deviation in the boiling point of the oxidation solution from the recommended values.
