Passivation of metals – ASGARD-Service
Passivation of metals is the transition of the metal surface into an inactive, passive state associated with the formation of thin surface layers of compounds that prevent corrosion.
In engineering, passivation is the technological process of protecting metals from corrosion using special solutions or processes that lead to the creation of an oxide film.
Passivation mechanism
When metals interact with certain components of solutions (melts) in a certain potential range, adsorption or phase layers (films) are formed on the metal surface.
These layers form a dense, almost impenetrable barrier, due to which corrosion is greatly inhibited or completely stopped. Passivation is carried out chemically or electrochemically.
In the latter case, conditions are created when ions of the protected metal, under the influence of current, pass into a solution containing ions capable of forming very poorly soluble compounds.
Passivation of metals in technology
Passivation is one of the methods of protecting metals from corrosion. Often, protective layers are used on the surface of metals - films of oxides under the action of oxidizing agents.
One of the technological options for passivation is bluing. To passivate many metals, solutions based on oxidizing agents capable of forming sparingly soluble compounds (chromates, molybdates, nitrates in an alkaline medium, etc.) are used.
Galvanized parts are often passivated by chromating.
Passivation is used to protect pipelines, boiler and heat exchange equipment from internal corrosion. To do this, by applying an electric field directed radially (that is, across the axis of the pipe) to the pipeline, it is possible to electrically pull free electrons of the metal located on the inner surface of the pipe towards the outer surface. As a result, the metal on the inner surface of the pipeline cannot enter into a chemical reaction.
Passivation of stainless steel
Despite the high resistance of stainless steel to corrosion, it also requires additional protection, which can be obtained through a technological process such as passivation.
In cases where even stainless steel products are at great risk, the need for a passivation procedure is obvious.
Determination of the high corrosion resistance of stainless steels
The essence of such a phenomenon as corrosion is that the surface of the metal begins to deteriorate under the influence of negative external factors and the environment. Typically, corrosion due to constant oxidation affects the metal layer by layer, gradually destroying the internal structure of the steel. In many cases, it no longer makes sense to localize the affected areas of the internal structure of the metal, so steel products have to be replaced with new ones.
Passivation as a technology that allows for reliable protection of steel from corrosion underlies the creation of such a unique metal as stainless steel. The chemical composition of the vast majority of steels belonging to the stainless category may contain various elements:
- Nickel;
- Molybdenum;
- Cobalt;
- Niobium;
- Manganese.
The main alloying element of such steels, the amount of which in their composition can vary between 12–20%, is chromium. The addition of various alloying elements to the composition of stainless steels makes it possible to give them the required physical and chemical characteristics, but it is chromium that is responsible for the corrosion resistance of the steel alloy.
To make the metal resistant to even more aggressive environments, which include hydrochloric, sulfuric and other acids, the quantitative content of chromium in it is not only increased, but also elements such as copper, molybdenum, nickel, etc. are added to its composition. In other words , they passivate the metal, that is, increase its passivity to corrosion processes.
Passivation, in which appropriate alloying elements are added to the chemical composition of stainless steel, is not the only condition for high corrosion resistance of the metal. In order for the protective properties of stainless steel to remain at a high level, the oxide film on its surface, consisting mainly of chromium oxide, must be intact, have a uniform chemical composition and thickness.
Causes of corrosion
Despite the fact that the chemical composition of stainless steel must contain passivators that significantly increase its corrosion resistance, its surface and internal structure can be subject to corrosion.
The main reason why stainless steel begins to deteriorate is insufficient or uneven chromium content in its chemical composition. Contact with metal, which is significantly less resistant to oxidation, can also cause corrosion. Stainless steel products that were connected to each other using welding technology are often subject to destruction.
Even if stainless steel is of very high quality, after welding it can become covered with a layer of rust. To avoid such negative phenomena, the welds used to connect stainless steel products must be thoroughly cleaned and polished.
This procedure allows you to remove from the surface of the weld and the stainless steel products themselves the remnants of less corrosion-resistant metal that was used to perform welding work.
Very often, particles of a metal less resistant to corrosion get onto the surface of a stainless steel in cases where its processing is carried out in close proximity to steel products.
If ordinary metal is being sawed, polished or other types of processing are performed nearby, then its particles, once on the stainless steel, will certainly become sources of its corrosion. They can also appear on stainless steel if you decide to process it with a tool that previously interacted with ordinary metal.
That is why it is advisable to use tools, especially those of the cutting type, to perform processing only of the same type of materials.
The most critical place on the surface of stainless steel products, from the point of view of the occurrence and development of corrosion processes, is the weld. That is why it is important not only to thoroughly clean, grind and polish the site of the formed welded joint, but also to passivate it, for which various acid solutions are used.
Processing of welded joints on stainless steel, after which chemical passivation is performed, is carried out using a metal brush and a grinding machine. At the same time, it is important to ensure that the tools used during passivation do not previously react with ordinary metal, particles of which can become a source of development of corrosion processes.
To check whether there are any inclusions of ordinary metal on the surface of a stainless steel, you can use two methods:
- Treatment with an aqueous solution of nitric acid and potassium ferrocyanide.
Places on the surface of the product that contain inclusions of free iron will immediately turn blue after this treatment. It should be noted that this testing method is used mainly in production laboratories.
- Wetting with ordinary water.
The product is kept in this condition for several hours. If there are inclusions of free iron on the stainless steel, then the areas with such inclusions will begin to become covered with rust.
It is almost impossible to ensure such operating conditions for stainless steel products that they do not come into contact with other metals and aggressive environments, and are not subject to mechanical damage. That is why the above-mentioned technological operation – passivation – is necessary.
The additional degree of protection provided by passivation is often sought to be provided by:
- Stainless steel pipe structures;
- Fastening elements;
- Hull elements of structures and mechanisms operated in sea water.
Meanwhile, passivation is not always advisable even for products of similar purposes.
In many cases, passivation is impractical and may even degrade the protective properties of the steel. Before performing passivation, you should analyze the operating conditions of the product in order to finally decide whether its surface needs additional protection.
Passivation, if the decision to carry it out is made, must ensure the production of a protective layer that is integral and uniform in thickness, which is achieved by strict adherence to the technological process. As a rule, passivation is performed in cases where additional protection is needed on the external rather than internal surface of a stainless steel product.
The essence of a process such as passivation is that the surface of a stainless steel product is treated with a special solution based on nitric and, in some cases, citric acid. Sometimes such a solution can be supplemented with a small amount (2-6%) of sodium bichromate. The chemical composition of such a solution, as well as parameters such as heating temperature and holding time, depend on the grade of stainless steel being processed.
Source: https://asgard-service.com/terms/passivaciya-metallov/
Products – Tekhmashholding – group of companies, official website
- A galvanized bucket is a steel product protected from corrosion by a zinc coating. Zinc acts as a protector, and since under normal conditions it is covered with a thin passive layer, the coating does not dissolve too quickly. Passivation of metals is the transition of the metal surface into an inactive, passive state associated with the formation of thin surface layers of compounds that prevent corrosion.
- In engineering, passivation is the technological process of protecting metals from corrosion using special solutions or processes that lead to the creation of an oxide film.
- One of the technological options for passivation is bluing.
- To passivate many metals, solutions based on oxidizing agents capable of forming sparingly soluble compounds (chromates, molybdates, nitrates in an alkaline medium, etc.) are used.
- Passivation is used to protect pipelines, boiler and heat exchange equipment from internal corrosion. To do this, by applying an electric field directed radially (that is, across the axis of the pipe) to the pipeline, it is possible to electrically pull free electrons of the metal located on the inner surface of the pipe towards the outer surface. As a result, the metal on the inner surface of the pipeline cannot enter into a chemical reaction.
- Tomashov N. D., Chernova G. P., Passivity and protection of metals from corrosion, M., 1965;
- Scorcelletti V.V., Theoretical foundations of metal corrosion, L., 1973;
- Novakovsky V.M., Justification and initial elements of the electrochemical theory of dissolution of oxides and passive metals, in the collection: Corrosion and corrosion protection, vol. 2, M., 1973.
- Overview and main differences
- What is etching
- What is passivation
- What is the difference between etching and passivation
- Conclusion
- Chemical. In this case, the metal product is dipped into a special chemical solution (or the latter is applied to the surface of the part using the appropriate tool). In this case, no electric current is used. In some cases, chemical passivation of metals can be carried out at room temperature. In other cases, heating of the passivator is required.
- Electrochemical. This type of passivation involves the use of oxidizing chemical solutions and electric current. When exposed to the latter, a protective layer is formed from microscopic particles of the substance deposited on the surface of the metal product. A special feature of this technology is that it ultimately produces a film that is more uniform in structure and highly resistant to dissolution.
- Improved corrosion resistance
- Uniform smoothing
- Deburring
- Purity
- Long product life
- Citric acid does NOT remove other elements in the alloy, effectively limiting the depth of the final chromium oxide layer
- Citric Acid DOES NOT introduce heavy metals (hazardous waste) into the bath
- Citric acid only removes iron
- Citric acid is much safer and environmentally friendly
- nickel;
- molybdenum;
- cobalt;
- niobium;
- manganese.
- stainless steel pipe structures;
- fasteners;
- hull elements of structures and mechanisms operated in sea water.
- Electrolyte preparation
- Conducting electric current
- Metal processing after passivation procedure
- 1.Contact of the material with the passivator is carried out by dipping into the liquid, coating or spraying.
- 2.When the metal comes into contact with the passivator, an electric current is passed through it, this makes it possible to reproduce uniform and lasting protection.
- composition of the solution;
- alloy composition;
- condition of the surface of the workpiece.
- Ferritic alloys - potassium dichromate, nitric acid.
- Carbon steels - potassium dichromate, chromic anhydride, phosphoric acid, sodium hydroxide.
- Medium alloy steels - chromic anhydride, phosphoric acid.
- Passivation is used for metal parts for painting. It not only protects against corrosion, but also degreases products. Used in the field of mechanical engineering.
- Passivation of steam turbines.
But why do you need passivation of stainless steel, since it won’t rust anyway? It turns out that if the alloy is in continuous contact with an aggressive environment, it can collapse. An example is a weld. Sometimes there are iron particles on it. And then even stainless steel corrodes.
- Dental field.
The lower part of the implants is processed - the screws that are mounted into the jaw. Passivation is used to prevent destruction of the implant in the jawbone.
- Chemical passivation is often carried out for decorative purposes. With short-term treatment, a rainbow film appears on the surface. Bright objects of use - taps, door handles.
- Passivation of costume jewelry is used to avoid allergic reactions.
Passivation mechanism
When metals interact with certain components of solutions (melts) in a certain potential range, adsorption or phase layers (films) are formed on the metal surface.
These layers form a dense, almost impenetrable barrier, due to which corrosion is greatly inhibited or completely stopped.
Passivation is carried out chemically or electrochemically. In the latter case, conditions are created when ions of the protected metal, under the influence of current, pass into a solution containing ions capable of forming very poorly soluble compounds.
Passivation of metals in technology
Passivation is one of the methods of protecting metals from corrosion. The formation of protective layers on the surface of metal (metal products) - oxide films under the action of oxidizing agents - is often used.
Passivation
Passivation is the phenomenon of a decrease in the rate of gas corrosion with a decrease in partial pressure. Passivation occurs due to the formation of a film on the surface of the metal (Cu, Ti, Zr, Cr, Al, etc.).
Overpassivation is a violation of the passive state. Occurs when partial pressure rises above critical.
Repassivation occurs in such steel grades as: 08Х18Н10Т, 20Х13, 30Х13, 40Х13, 15Х17.
Additional operations
After passivation or filling of a passivating coating, the metal surface is often subjected to additional processing - inhibitors, painting or varnishing, etc.
Literature
In English
Source: https://pellete.ru/stal/passivirovanie-stali.html
Difference between etching and passivation
The main difference between etching and passivation is that etching is a process that is used to remove contaminants as well as create a pattern on a metal surface, whereas passivation is the protection of a metal surface from corrosion.
Both etching and passivation are processes that are used to protect a metal surface. Etching is a form of metal surface treatment. Etching chemically treats the metal surface. Passivation, on the other hand, makes the material “passive” to corrosion. Unlike etching, this protects the metal surface before any impurities reach it.
What is etching?
Etching is the process of treating metal surfaces to remove any contaminants from the surface, as well as to apply a pattern. Impurities may include stains, rust, scale, inorganic contaminants, etc. Metals such as iron and its alloys are subject to etching; precious metals: silver; non-ferrous metals: copper, aluminum alloys, etc.
Knife etching
The chemical agent that is used in this process contains sulfuric acid; hydrochloric acid or phosphoric acid can also be used. In addition, this agent also contains some other ingredients. For example, wetting agents, corrosion inhibitors, etc. The pickling process is common when cleaning steel surfaces in steel smelting processes.
The need for this process is to remove substances on the metal surface that may affect further metal processing such as plating and painting. Descaling is an important step in this process. Many hot working processes leave a discolored oxide layer (scale) on the surface of the metal.
To remove this layer of scale, the part is dipped in a solution of pickling acids.
However, this technique has some disadvantages. Among other things, the process is difficult to handle because the pickling solution is corrosive (it contains strong acids). Additionally, hydrogen embrittlement is another problem for some alloys. Another disadvantage is that it produces sludge as a waste product from the process. Example: Spent liquor is a hazardous waste.
What is passivation?
Passivation is the process of making a material “passive” to corrosion. In other words, it protects the metal surface from corrosion. After passivation of the metal, this metal becomes less susceptible to environmental influences.
During passivation, an outer protective layer is formed. It is used as a micro-coating. This coating is applied using a chemical or electrochemical reaction.
This method of anti-corrosion protection is also used to preserve the appearance of the metal.
Passivation of zinc coated parts
The following metals can be passivated: aluminum, ferrous metals, stainless steel and nickel.
Most metals form a protective oxide layer when they are exposed to air, creating tarnished surfaces on metals. But in some metals, such as iron, no oxide layer appears and rust formation occurs in the open air. In such metals, the presence of a corrosion coating is very important as it reduces further corrosion.
What is the difference between etching and passivation?
Etching is the process of treating metal surfaces to remove any contaminants from the surface or to apply a pattern to the metal. It is made using acid solutions. Thus, they protect the metal surface from contaminants on the metal surface or apply a pattern to this surface.
Passivation is the process of making a material “passive” to corrosion. This is the key difference between etching and passivation. Moreover, passivation is sometimes natural (for example, the formation of an oxide layer in the open air), or through a chemical reaction.
In addition, passivation protects the metal surface before it is even exposed to normal air.
Conclusion – Etching vs Passivation
Metals are generally very reactive when exposed to air. Etching and passivation are two technologies that are used to protect the metal surface. The difference between etching and passivation is that etching is a process used to remove contaminants or create a pattern on a metal surface, whereas passivation is the protection of a metal surface from corrosion.
Source: https://raznisa.ru/raznica-mezhdu-travleniem-i-passivaciej/
Metal passivation: features and methods of implementation
Today, technologies have become widespread to protect various types of metal products from rust.
One of them is metal passivation, which is a process of processing metal material in special chemical solutions called passivators.
Most often these are inorganic substances with high oxidizing properties. Upon contact with metal, the passivator transfers its surface to the so-called passive state.
As a result, a protective ultra-thin film appears on the metal product, preventing the process of oxidation. Its thickness can vary depending on the type of metal, as well as what kind of passivator is used.
After its appearance, the protective film (phase or adsorption) becomes a kind of impenetrable barrier to the spread of rust.
This makes it possible not only to effectively slow down corrosion processes, but also in some cases to completely stop their progress.
What types of passivation are there?
Passivation of metals can be carried out using various principles, namely:
How is passivation of different types of metal carried out?
First of all, we should highlight the process of passivation of a steel alloy , since it is widely used in various industries. First, the surface is degreased and then coated with a special chemical composition consisting of active substances. As a result, this makes the metal passive, increasing its durability and resistance to weathering. Often, a 70-80% solution of sodium nitrite .
If we are talking about the process of copper , then for this purpose, as a rule, solutions are used, the main component of which is chromium . This chemical promotes the formation of a strong, long-lasting film. In this case, the material will be highly resistant to rust when exposed to an environment containing a significant amount of sulfur oxides.
When it is necessary to passivate zinc , it should be noted that such a procedure requires maximum concentration of attention, because it is very important that the protective film is as thin as possible. This is due to the fact that zinc products, as a rule, have a very small thickness of pure metal. Therefore, with a significant film thickness, the layer of pure zinc will be further reduced.
In addition, in some cases, an iron . This metal is particularly susceptible to rust. To create a protective film on its surface, various types of chemicals are used, including a solution of sulfuric acid.
An excellent Soviet video about the features of the process:
Source: http://xn--80affkvlgiu5a.xn--p1ai/slovar/passivaciya-metalla-osobennosti/
Pickling and passivation of stainless steel » Forstex
The passivation process returns stainless steel or other metals back to their original specifications by removing unwanted impurities and oils from the surface.
When a stainless steel part is machined, various particles can penetrate the surface of the base metal, weakening its resistance to corrosion and making the part more susceptible to environmental factors.
Sparks, dirt and other particles and residues such as free iron, grease and processing oils affect the strength of the natural surface and can penetrate the surface during the finishing process. They remain invisible to the human eye and are often the cause of corrosion.
“Passive” is defined as less susceptible to environmental influences. The process improves and cleans the surface of the part. The restored surface acts as a protective coating against environmental factors such as air, water and other extreme conditions. It is important to note that passivation does not change the appearance of the base metal.
Benefits of passivation
Passivation remains an important step in maximizing the corrosion resistance of stainless steel parts and components. The process can significantly increase the life of parts. Improperly performed passivation can actually cause corrosion.
Passivation is a method for maximizing the inherent corrosion resistance of the stainless alloy from which the workpiece is made.
There is no universal method regarding the precise mechanics of how passivation works. But there is no doubt that a protective oxide film is present on the surface of passive stainless steel. This invisible film is said to be extremely thin, less than 0.0000001 inches thick, which is about 1/100,000th the thickness of a human hair!
In practice, contaminants such as dirt or iron particles from cutting tools can be transferred to the surface of stainless steel parts during processing. If not removed, these foreign particles can reduce the effectiveness of the original protective film.
During the machining process, microscopic amounts of free iron can be rubbed off the cutting tool and transferred to the surface of the stainless steel workpiece. Under certain conditions, these particles can develop a thin coating of rust. This is actually corrosion of the steel from the tool rather than the base metal.
Sometimes steel particles from the cutting tool or its corrosion products can cause damage to the part itself.
Likewise, small particles of iron-containing dirt can adhere to the surface of the part.
Although the metal may appear shiny when machined, invisible particles of free iron can cause the surface to rust after exposure to air.
Exposed sulfides can also be a problem. They come from adding sulfur to stainless steels to improve machinability. If the part is not properly passivated, sulfides can act as initiation sites for corrosion on the surface of the product.
In all cases, passivation is required to maximize the natural corrosion resistance of stainless steel.
It will help remove surface contaminants such as ferrous solid dirt and iron particles from cutting tools that can form rust or act as initiation sites for corrosion. Passivation can also remove sulfides exposed on the surface of stainless alloys without mechanical treatment.
A two-step procedure may provide the best corrosion resistance: 1. cleaning, (degreasing, pickling), but in some cases not followed by the procedure 2. acid bath or passivation treatment.
First cleaning
Grease, coolant or other contaminants must be thoroughly removed from the surface to obtain the best corrosion resistance. A commercial degreaser or detergent can be used to clean mechanical oils or coolants. Foreign matter such as thermal oxides may need to be removed by grinding or by methods such as acid etching.
Sometimes an operator may skip basic cleaning, incorrectly assuming that simply immersing the grease in an acid bath will both clean and passivate simultaneously. This does not happen. Instead, the grease contaminant reacts with the acid to form gas bubbles. These bubbles collect on the surface of the workpiece and interfere with passivation.
Even worse, contamination of the passivation solution, sometimes with high chloride content, can cause a corrosion “flare-up”. Instead of producing the desired oxide film with a shiny, clean, corrosion-resistant surface, flash causes a heavily etched or darkened surface—the deterioration of the surface itself that passivation is designed to optimize.
Parts made from martensitic stainless steels [which are magnetic, moderately resistant to corrosion, and have a yield strength of up to (1930 MPa)] per square inch are hardened at high temperature and then annealed to provide the required hardness and mechanical properties.
Precipitation hardenable alloys (which provide a better combination of strength and corrosion resistance than martensitic grades) can be solution processed, partially processed, held at lower temperatures, and then finished with machining.
In such cases, the parts must be thoroughly cleaned with a degreaser or cleaner to remove traces of cutting fluid before heat treatment.
Otherwise, the cutting fluid remaining on the parts will cause excessive oxidation. This condition can cause the underlying layers to remain mottled even after descaling by acid or abrasive methods. Cutting fluids may remain on the parts and harden in a vacuum oven or protective atmosphere, and carburization of the surface may occur, resulting in loss of corrosion resistance.
Etching
Pickling is the removal of the adjacent low chromium metal layer from the surface of stainless steel by chemical means.
Where steel is heated by welding, heat treatment or other means to the point that a colored oxide layer can be seen, there is a chromium-depleted layer on the surface of the steel below the oxide layer. Lower chromium content gives lower corrosion resistance. To restore the best corrosion resistance, the damaged metal layer must be removed, exposing the fully alloyed stainless steel surface.
Mechanical removal may produce abrasive or other particles (corrosion inhibitors) or may be impractical, so chemicals are usually used.
Procedures involving etching solutions of nitric (HNO3) and hydrofluoric (HF) acids remove the scale and chromium-depleted underlayer and restore corrosion resistance. Etching solutions also remove contaminants such as iron and iron particles. Etching solutions other than mixtures of nitric and hydrofluoric acids exist and can be used for specialized applications.
Etching pastes, where a solution is mixed with an inert carrier, are typically used to treat selected areas such as welds. Etching involves the removal of metal and a change in the visual brightness of the metal. Electropolishing is a useful alternative to etching. Metal removal is achieved but usually results in a bright, smooth and more corrosion resistant surface.
Passive baths
After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any of three methods can be used—nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation. Which method to use depends on the class of stainless steel and the specified parameters.
More stable chromium-nickel grades can be passivated in a bath of nitric acid (20% by volume).
Less durable grades of stainless steel can be passivated by adding sodium dichromate to a nitric acid bath to make the solution more oxidized and capable of forming a passive film on the surface.
Another option, used instead of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50% by volume. The addition of sodium dichromate and a higher concentration of nitric acid reduce the likelihood of unwanted corrosion.
Historically, nitric acid was used to passivate stainless steel, but recently a safer and more effective solution using citric acid has become common. Unlike nitric acid, passivation compositions with citric acid have many advantages:
Source: https://forstex.ru/services/travlenie-i-passivacziya-nerzhaveyushhej-stali/
Acid for passivation of stainless steel
Passivation of metals is the transition of the metal surface into an inactive, passive state associated with the formation of thin surface layers of compounds that prevent corrosion.
In engineering, passivation is the technological process of protecting metals from corrosion using special solutions or processes that lead to the creation of an oxide film.
Chemical passivation of stainless steel
In many areas of industry, construction and repair, tools, fasteners and hardware made of stainless steel are used. But despite the fact that this material has increased resistance to corrosion, rust may still appear in some cases. To prevent this, it is necessary to take additional measures - chemical passaging of products.
Why is this necessary?
A stainless steel sheet has a very thin oxide film on its surface. It is this that prevents the formation of rust on parts, fasteners, and hardware made from this material. But the slightest violation of the integrity of this coating leads to the fact that the main anti-corrosion properties of stainless steel are lost. The causes of damage to the oxide film can be very different:
when the material comes into contact with chlorine; when steel interacts with sea water; in case of mechanical or physical damage, including scratches and minor dents.
Therefore, it is important to comply with the operating conditions that are regulated by the manufacturing plants of certain products (cutlery, fasteners, hardware, working tools, solid sheets, etc.). It is prohibited to use detergents containing chlorine and other aggressive chemicals.
But the greatest damage to the oxide film is caused by welding. This is especially harmful in the case of pipe welding. In such a situation, the protective surface is destroyed along the entire seam. Steel passivation is used to restore surfaces and protect products from rust. But here the composition of the stainless steel plays an equally important role.
Stainless steel classification
The anti-corrosion properties of stainless steel directly depend on its composition. Based on this, this steel is marked. The classification allows you to distinguish each type of stainless metal by flexibility, hardness, and degree of anti-corrosion protection. Depending on the composition and purpose, they are distinguished:
martensitic steels. Knives (including those for the food industry) and turbines are usually made from them. This steel, having a large percentage of chromium in its content, is very hard; ferritic materials. The amount of chromium in such steel exceeds the previous value by 3-4%.
This material has high resistance to phosphoric acid, ammonium nitrate and nitric acid; austenitic steels. This type of stainless steel is very ductile. It is often used in mechanical engineering; duplex or ferro-austenitic metals.
These are very durable, but at the same time flexible stainless materials.
Based on the composition of the stainless steel, you can determine whether there is a need for additional processing of the products or not. The likelihood of corrosion on the surface of elements made from this type of steel also depends on this.
Technology and methods
There are various methods for processing stainless steel. But there are two main methods of steel passivation:
Etching with chemical acids (concentrates) in certain areas. This technology is often used for processing welds, but is also allowed in other cases. This process has different processing sequence options. They differ both in the composition of chemicals and in the time of work.
The most common method in this case is electrolytic etching. This technology consists of placing a stainless steel product in a specially prepared bath consisting of concentrated acids. An electric current (alternating or direct) is passed through this composition.
The metal plays the role of either a cathode or anode. The supplied current has a mechanical effect on the steel, resulting in the release of hydrogen or oxygen gas. This helps to separate the oxide film on the surface of the product. Etching with ready-made acid mixtures.
They can be made in the form of pastes, gels, sprays, concentrates. This method is the most convenient.
Regardless of which method is used to passivate stainless steel, it is important to follow the sequence of work.
Stages of chemical passivation
In the process of forming a homogeneous inert film on the surface of stainless steel products, it is important to take into account the characteristics of the steel composition and the degree of damage to the protective coating. Chemical passivation today is an integral part of working with stainless materials. This allows you to extend their service life, get rid of rust and damage, and prevent the formation of corrosion. During passivation work, the following sequence of steps should be followed:
First, the materials are cleaned from contaminants. Grease stains, rust and other deposits are removed. With chemical acid etching technology, the product is immersed in a bath with a mixture of hydrochloric acid and sulfuric acid. At temperatures from 60 to 80 degrees, the steel is kept here for 20-40 minutes.
If the method of etching with ready-made mixtures of acids is used, then special concentrated compositions (pastes, gels, sprays) are used for cleaning, which are applied to the surface of the steel manually. The chemical is left for approximately 30 minutes. Then the products are thoroughly washed with water. The passivation process begins.
In the first case, the steel is immersed in an acid bath. In the second, gels, pastes, sprays and other ready-made chemical compositions are applied to the surface of the product. In the case of ready-made products, one more stage is provided - treatment with a passivator. This allows for the forced formation of an oxide film on stainless steel.
The last stage consists of thoroughly washing the product.
The composition of stainless steel and the grade play an important role in the appearance of the product after chemical passivation. Some species are dark in color, while others are lighter. But regardless of this, this method of steel processing has a whole list of advantages:
improves resistance to corrosion; the surface of the product is uniformly smoothed; burrs, scratches, dents are removed; The service life of the products is significantly increased.
Where can I order the service?
This procedure must be carried out by competent specialists with extensive experience and specific knowledge in this field. Our company employs true professionals in their field. We carry out chemical passivation of stainless steel, taking into account the characteristics of its composition, the degree of damage and the size of the product. All work is carried out in a specially designated place and in compliance with all safety requirements.
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Source: https://Filigrann.ru/uslugi/himicheskoe-passivirovanie-nerzhaveyushhej-stali
Metal passivation: purpose, technology, methods
Although stainless steel is highly resistant to corrosion, the additional protection provided by passivation is desirable. In some cases, when even products made of stainless steel are at high risk of corrosion, the need to perform such a procedure is beyond doubt.
Examples of stainless steel surfaces subjected to corrosion and the results of passivation
What causes the high corrosion resistance of stainless steels?
The essence of such a phenomenon as corrosion is that the surface of the metal begins to deteriorate under the influence of negative external factors and the environment. Typically, corrosion due to constant oxidation affects the metal layer by layer, gradually destroying the internal structure of the steel. In many cases, it no longer makes sense to localize the affected areas of the internal structure of the metal, so steel products have to be replaced with new ones.
Passivation (or passivation), as a technology that allows for reliable protection of steel from corrosion, underlies the creation of such a unique metal as stainless steel. The chemical composition of the vast majority of steels belonging to the stainless category may contain various elements:
However, the main alloying element of such steels, the amount of which in their composition can vary between 12–20%, is chromium. The addition of various alloying elements to the composition of stainless steels makes it possible to give them the required physical and chemical characteristics, but it is chromium that is responsible for the corrosion resistance of the steel alloy.
The effect of chromium on the properties of stainless steel
Stainless steel alloys, which contain 12% chromium, exhibit high corrosion resistance only when interacting with ambient air. If the amount of chromium in the chemical composition of stainless steel is increased to 17%, then products made from it can easily interact with nitric acid without losing their performance characteristics.
To make the metal resistant to even more aggressive environments, which include hydrochloric, sulfuric and other acids, the quantitative content of chromium in it is not only increased, but also elements such as copper, molybdenum, nickel, etc. are added to its composition. In other words , they passivate the metal, that is, increase its passivity to corrosion processes.
During the process of passivation of the weld zone, a strong film is formed
Passivation, in which appropriate alloying elements are added to the chemical composition of stainless steel, is not the only condition for high corrosion resistance of the metal. In order for the protective properties of stainless steel to remain at a high level, the oxide film on its surface, consisting mainly of chromium oxide, must be intact, have a uniform chemical composition and thickness.
Types of corrosion
Despite the fact that the corrosion process leads to almost the same consequences, the reasons that cause it may be different.
The most common cause of corrosion of stainless steel products used in domestic conditions is the use of cleaning products containing a significant amount of chlorine in their chemical composition.
Such agents actively contribute to the destruction of the oxide film on the metal, which leads to the development of a corrosion process over its entire surface (i.e. general corrosion).
Crevice corrosion of stainless steel occurs in cases where parts made of such metal come into contact with each other for a long time. Corrosion of this type, which is typical, often begins to develop in the fastening areas. There is also pitting corrosion, which is often called pitting. It occurs when the oxide film on stainless steel is damaged mechanically.
Corrosion of stainless steel under water manifests itself to a greater extent at the joints of parts
If stainless steel comes into contact with a dissimilar metal in a conductive environment, corrosion begins to develop, which is called galvanic. Products made of stainless steels that are used in sea water and at the same time in contact with metals with a lower degree of alloying are most susceptible to this process.
Intergranular corrosion is a very common phenomenon that occurs when a stainless steel product has been subjected to significant overheating. With strong heating (over 500°), chromium and iron carbides are formed at the boundaries of the crystal lattice of stainless steel, which cause a decrease in the strength of the metal.
Corrosion of stainless steel can occur due to the use of chlorine-containing cleaning compounds
There is also erosive corrosion, which occurs if stainless steel is constantly exposed to an abrasive environment. Constantly affecting the metal surface, particles of such a medium destroy the protective oxide film, which does not have time to recover.
Passivation of stainless steel
It is almost impossible to ensure such operating conditions for stainless steel products that they do not come into contact with other metals and aggressive environments, and are not subject to mechanical damage. That is why the above-mentioned technological operation – passivation – is necessary. The additional degree of protection that passivation provides is often sought to be provided by:
Meanwhile, passivation is not always advisable even for products of similar purposes.
Passivation of a stainless steel weld
Passivation, although a method of treating stainless steel that can provide it with additional protection against corrosion, is in many cases impractical and can even degrade the protective properties of the steel. Therefore, before performing passivation, you should analyze the conditions in which the product will be used in order to clearly decide whether its surface needs additional protection.
Passivation, if the decision to carry it out is made, must ensure the production of a protective layer that is integral and uniform in thickness, which is achieved by strict adherence to the technological process. As a rule, passivation is performed in cases where additional protection is needed on the external rather than internal surface of a stainless steel product.
The essence of a process such as passivation is that the surface of a stainless steel product is treated with a special solution based on nitric and, in some cases, citric acid. Sometimes such a solution can be supplemented with a small amount (2-6%) of sodium bichromate. The chemical composition of such a solution, as well as parameters such as heating temperature and holding time, depend on the grade of stainless steel being processed.
Source: http://met-all.org/obrabotka/himicheskaya/passivacija-metalla-passivirovanie-nerzhavejushhej-stali.html
Passivation
In the modern world, a large number of methods are used to prevent the formation of corrosion on the surface of different types of metals. The substances that are used for this cover the surface with a thin film that prevents the metals from oxidizing.
The thickness of the protective film varies. It depends on the composition applied to the metals. Also, to carry out the procedure for protecting metals from corrosion, methods are used that are based on changing their properties. Passivation belongs precisely to this category of processes.
Passivation of surfaces
Almost all metals are quite durable materials. However, their structure and general condition can be affected by ordinary oxygen or liquid. Under the influence of an aggressive environment, plaque accumulates on the surface of metal products, which is corrosion. It is dangerous because under its influence the structure of the metal is destroyed, and the product made from it becomes unsuitable for further use.
Passivation has found widespread use in the modern world. It is not an easy procedure. It is almost impossible to cope with this without certain knowledge. The procedure is to dissolve the top part of the metal using an anode. In this case, the molecules break down into substances that have different levels of charge. In order for the ions to acquire an ordered appearance, an electric current is applied to the metal at a low voltage level, which is only 6-12 volts.
Ions are divided into positively charged and negatively charged. When an electric current passes through a metal, positively charged particles tend to the cathode, and negatively charged particles tend to the anode. It is at the anode that metal oxides are formed, which are the result of the splitting of the upper metal layer. As a result, a very thin protective film appears on the surface of the processed metal, which has unique protective qualities.
Passivation is aimed at making the metal less active. It becomes passive and is practically not affected by the environment.
In modern industries, this procedure is quite in demand. It helps protect metal surfaces from corrosion. The passivation process is used in situations where there is a need for careful preparation of the surface for applying paint and varnish. Also, this procedure is indispensable in those enterprises where metal objects often have to interact with an aggressive environment.
Passivation of metals is a useful procedure that renders these substances passive. It allows them to retain their properties for a long time. The thin film has an excellent level of protection, which gives metals additional strength and hardness.
Passivation process
The passivation procedure can be carried out at industrial enterprises or at home.
It consists of several stages:
Before any procedures to protect metal surfaces from corrosion, they are prepared. It consists in making the surface as clean as possible, so that it is easier for substances applied to the surface of metals to penetrate their structure. First you need to remove all contaminants from the metal. This can be done by washing and sanding with sandpaper.
At the second stage, it is necessary to prepare a substance that, under the influence of a small current, will contribute to the formation of a thin film that protects against corrosive plaque.
At this stage, it is necessary to conduct an electric current with a low voltage level.
Important: To achieve the best result, the electrical voltage should not be more than twelve degrees.
At the final stage, the metal is tested for strength. Its resistance to environmental influences is checked.
Types of passivation
Currently, according to the method of passivation, the following types of passivation are distinguished:
This type of passivation involves applying salts and acidic solutions along with an electrolyte to the metal surface. As a result of this procedure, charged particles settle on the metal surface, forming a thin film that provides reliable protection against corrosion.
This procedure involves treating metals with chemicals that form a protective film on their surface. For this purpose, solutions are used that consist of nickel, chromium and other elements. They make the metal structure denser and harder.
Metal passivation
By type of metal, passivation can be represented by such processes as:
This type of passivation is used in numerous manufacturing plants. It makes it possible, after degreasing the surface, to apply active substances to the metal surface, which will make the metal passive. Products made from steel processed in this way are strong and durable.
steel passivation.
This type of passivation is characterized by the use of chromium-based solutions. This substance forms a dense film on the surface, which makes the metal more durable.
Currently, zinc passivation is often used. It is worth noting that during this procedure you must be extremely careful to ensure that the coating film is as thin as possible. This is necessary for the reason that for zinc products the thickness of the material is not particularly large. If the film is thick, the thickness of the metal will decrease further.
passivation of zinc.
Iron is one of the most famous metals that are susceptible to corrosion on their surface. It is for this reason that it is recommended to use specialized methods to protect products made from this material. Passivation of this metal is carried out in a solution of sulfuric acid. As a result, a thin protective film is formed on the metal surface.
Source: http://lkmprom.ru/clauses/tekhnologiya/passivatsiya-chto-predstavlyaet-soboy-tekhnologiya/
What is a metal passivator?
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To give products a beautiful appearance, protect them from corrosion and increase their service life, passivation is used. It is produced using special means called passivators.
They process the metal, after which it becomes inactive. The passivator prevents the formation of a corrosive layer on the metal surface.
There is a technological process in which the metal is protected from the formation of an oxide film, which results in the formation of rust.
What is metal passivation?
All products made from metals, except inert ones, under “favorable conditions” react with oxygen and water, resulting in corrosion. During passivation, the chemical activity of metals decreases and they become less susceptible to corrosion.
We can say that passivation is the same coating as rust, but made artificially; it is coated with an oxide, phosphate, sulfate and chloride film. The composition of passivators depends on the type of metal, and passivation itself significantly reduces the rate of corrosion.
To extend the life of the lower layers of metal, it is enough to apply a very thin layer of film to the surface of the product.
Passivation of metals
All metals react differently to a particular environment. Therefore, a certain passivation is carried out for different types of metals. There are weak oxidizing agents that passivate magnesium and titanium, as well as strong ones that have a good effect on aluminum and chromium. For iron, high-intensity sulfuric and nitric acids are used; after a short exposure, a film forms and the reaction stops.
Methods of passivation of metals:
The electrochemical method is used, for example, for passivation of copper. For this purpose, special chromium-containing compounds are used, through which an electric current is passed. For aluminum, a hydrogen fluoride mixture is used, sodium dichromate and sulfuric acid are used to passivate zinc. As you can see, each metal has its own passivators; moreover, they often consist of several components.
If you need to passivate a certain metal, you should not experiment; it is better to buy a ready-made composition. The passivation process is not long, sometimes a few seconds are enough.
It is worth knowing that the longer you carry out passivation, the more characteristic the changes on the surface will become.
But here, too, everything is individual, for example, after immersing a zinc product in a solution for 3-5 seconds, an iridescent film with a greenish tint is formed, and after keeping it in the solution for 30 seconds, you will get a brown coating. Therefore, the holding time during passivation is extremely important.
We should not forget that passivation is a process that occurs on the surface. If the metal is dirty or rusty, the reaction will occur not with the metal itself, but with impurities and foreign particles.
Therefore, before treating with a passivator, the product must be treated, washed or cleaned, depending on the type of contamination. If an electrochemical processing method is used, the necessary conditions must be met, for example, when passivating brass and iron, a certain voltage is required.
You can buy high-quality metal passivators from ADDAPT from the Russo Industrial company by making a request on our website.
Source: https://RussoIndustrial.ru/articles/docs/chto-takoe-passivator-metalla/
Chemical passivation as an optimal coating for heat-resistant steel
Metal passivation is a process as a result of which an oxide film is formed on the surface of the metal, preventing the formation of corrosion. The name of the coating method comes from the word “passivity”. The purpose of passivation is to reduce the chemical reactivity of a metal when interacting with other metals or aggressive environmental conditions.
In its own way, the appearance of a film is the same destruction of metal. But by destroying the top layer of material by several tens of nanometers, passivation saves the lower layers from rust.
Thus, chemical passivation is the interaction of an oxidizing agent with the surface being treated.
Stages of chemical passivation
1. If you do not prepare the metal product first, the oxidizing agent will react not with the alloy, but with foreign elements. Therefore, before passivation it is necessary to clean the surface . Cleaning is carried out in 2 ways: by washing or sanding the product using sandpaper. Now you can start passivation.
2. The process itself involves applying a chemical reagent to the product . A protective film is formed on the alloy, consisting mainly of salts and oxides. The film makes the structure of the product stronger and more durable. The effectiveness of the procedure depends on the following factors:
High-alloy steels, especially chromium-nickel steels, are best suited to chemical passivation. But carbon steels should be treated only for short-term protection, since the level of the protective layer on them is significantly weaker.
3. Cleaning with water . Any salts that may remain on the product may cause corrosion. Therefore, washing should be carried out carefully.
4. Residual acid must be neutralized with a 2-3% ammonia solution or a solution consisting of 25-30 g/l oleic acid and 2-4 g/l sodium hydroxide. Treatment is carried out at 80 - 90 ° C for 2-3 minutes.
What solution is used?
The use of different solutions depends on the properties of the alloy. Let's consider what solutions are used to passivate various classes of ferrous metals:
Highly alloyed alloys resistant to corrosion - nitric and sulfuric acids.
The passivation temperature and time also depend on the alloy class. The temperature ranges from 18 to 90 °C, and the time ranges from 3 to 60 minutes.
The higher the temperature of the solution, the faster the process proceeds.
Application of passivation
Chemical passivation significantly extends the service life of metal products and deserves wide application in a wide variety of fields.
Source: https://metizmash.ru/Articles/Chemical-passivation
