Review of methods for phosphating metals - let's learn in order
The problem of protecting the surface of metals from corrosion has been relevant since the time when humanity began to make the things it needed from ore. Despite the constant improvement of technology, it is not possible to provide complete protection. Phosphating of metal is considered an effective method of protecting against the negative influence of the atmosphere and increasing wear resistance.
Metal phosphating
Phosphating is one of the most effective methods of fighting rust. This processing method
metal coatings are classified as additional. This method is based on the fact that metals, when immersed in a phosphating agent, are coated with its components. They settle on the surface and form an additional protective film.
The procedure for phosphating metal coatings allows you to best prepare them for applying paint and varnish. This measure allows the metal to be less susceptible to corrosion. This method is suitable for additional processing of both ferrous and non-ferrous types of metals.
Phosphating of metals on an industrial scale is carried out by spraying or immersing products in the substance.
It is made from:
These elements form a single substance, which, when interacting with other metals, adheres to them and provides reliable protection against the process of oxidation and corrosion.
At numerous industrial enterprises, this method involves high-quality application of a solution to metal products.
This is done in several ways:
- spraying,
- roller application,
- application with a brush.
The phosphating process does not take much time. After carrying out this procedure, it is necessary to allow products made of processed metals to dry.
Today, this processing procedure is available for various elements. Phosphating steel involves applying a phosphating substance to the surface of a product made from this material. Thanks to this, an additional protective film is formed on the surface of the metal, which is practically not noticeable from the outside.
Water phosphating is used to treat drum boilers. In this case, water with phosphates dissolved in it is introduced into the drum.
Important: This method has a large number of opponents. It is not recommended to use it when the boiler is heating up.
Table 1: Suitability of phosphating as a basis for applying varnish coatings to various metal surfaces
Type of phosphatingMetal surfaces to be treatedSteelGalvanized steelAluminum| Acid alkaline phosphating | + | + | + |
| Zinc phosphating | + | + | + |
| Low zinc phosphating | + | + | + |
| Mn modified low zinc phosphating | + | + | + |
| Nickel phosphating | o | + | — |
| Zinc-calcium phosphating | + | o | — |
Explanations: + - suitable; o - conditionally suitable; — -unsuitable
: What is better for covering a bathtub - enamel or acrylic
Types of phosphating
Today there are a large number of types of phosphating.
The following stand out from them:
This procedure is applied to those metals that do not have a strong structure. Among them are: aluminum, low-alloy steel and magnesium, zinc. One of the subtypes of chemical phosphating is amorphous phosphating. Iron phosphates are used to perform this procedure.
This process of processing metal products belongs to the category of decorative. It involves the formation of a black film on their surface. It is quite dense and gives any product additional strength.
The process of processing metals with zinc phosphates and alloys of this metal is one of the most effective methods of strengthening the structure of any metal. As a result, the coating has optimal thickness and a pleasant silver tint.
Basic processing methods
A protective phosphate film can be produced in a variety of ways, and the specific method chosen is highly dependent on the part to be treated, as well as the application of the part or design. The following phosphating methods are most often used in industry:
- with the help of “Mazhef” preparations;
- using phosphoric acid;
- using zinc monophosphate;
- using phosphating paste.
The drug "Mazhef"
This is nothing more than chemical phosphating, in which the part is dipped into a bath with a special solution. Low carbon steels are subjected to chemical treatment. Most often, this method is used to prepare metal structures and products for painting to obtain reliable anti-corrosion primers.
Majef is a combination of manganese, iron and phosphorus. The product resembles salt or greenish powder.
The concentration of the solution in the bath is no more than 40 g per 1 liter. To obtain a film using a chemical method, the product is placed in a ready-made composition, which is heated and brought to a boiling point. Periodic stirring is recommended. Boil the bath for 15-20 minutes. This is quite enough to cover the steel with a layer of protective film.
In order to obtain a high-quality film with a thickness of 5 to 10 microns using a chemical method, it is necessary to first carefully prepare the surface using abrasive cleaning or using a sandblasting machine.
It is recommended to prepare the composition with some excess, since part of it will evaporate during the heating process. Total acidity is determined by titration with phenolphthalein. The level of free acidity can be determined using methyl orange indicators.
Source: https://wizard-aerosol.com/spetsialnye-materialy/obzor-metodov-fosfatirovaniya-metallov.html
What is phosphating: technology or process of chemical treatment of metal parts before painting, equipment for this and activator
The problem of protecting metals and alloys from corrosion processes has arisen since people learned to extract metal from ore. Production technologies have changed significantly and become more modern, but corrosion still destroys products and structures.
Zinc coating is used to protect metal surfaces, but this does not provide maximum protection. Modern industry uses a more advanced method of protection from adverse effects - metal phosphating.
What is phosphating
Using this technology, you can not only preserve the product or surface, but also obtain increased wear resistance of the material.
The technology involves treating metal surfaces with special solutions based on phosphate salts. As a result, a durable protective film is formed. Among the types of phosphating, the most popular method is the application of phosphating primers. Waterjet and chemical processing of metals are also used.
Phosphate film allows you to improve the protective characteristics and service life of the paintwork several times over. Due to its low electrical conductivity, the film also improves adhesion and prevents under-film corrosion processes. Often the technology is used on products before painting using powder technology.
Phosphate film easily withstands the effects of organic substances - these are various oils, lubricants and hot materials, any gases except hydrogen sulfide.
This technology allows processing cast iron, low-alloy, and carbon steels. Phosphate coatings can be applied to zinc, cadmium, copper alloys, and aluminum. High carbon steel is also phosphated.
But, despite its high protective qualities, the phosphate coating can be destroyed under the influence of alkalis, sea water, water steam, acid, fresh water, and water vapor.
The protective film is formed by dipping the product into a special bath containing a phosphating solution. It is also possible to apply the coating by spraying in a jet chamber. Depending on the composition of the solutions, phosphates with or without a well-defined crystal lattice can form on the surface.
A crystalline film is deposited from a solution with heavy metal cations, and an amorphous film is obtained from a solution of alkali metal acid phosphates or ammonium acid phosphate.
With the help of phosphating, metals can be used for a long time under difficult conditions, such as:
- conditions of high humidity;
- when exposed to fuels and lubricants;
- in organic solvent environments;
- under voltage up to 1000 V.
Phosphoric acid
Phosphoric acid is used to obtain coating using the cold method. In order for the phosphating process to proceed as stably as possible, the solution temperature should be in the range of 18-25 degrees.
To achieve coatings with high quality and durability, you need to strictly adhere to the amount of active ingredients. The following concentrations are used in industry:
- 40 g/l phosphoric acid;
- zinc nitrate – 200 g per 1 l;
- sodium sulfate 8 g per 1 l;
- zinc oxide – 15 g per 1 liter.
In this solution, the part or metal structures are processed for 30 minutes. This is quite enough for phosphates to form on the surface.
The technology is suitable for processing large parts using the jet method. This option, compared to phosphating in baths, makes it possible to significantly reduce the duration of the process, as well as reduce material consumption.
Zinc monophosphate method
This technology is used to protect metals that will be used in the electrical industry, as well as in mechanical engineering. The surface or part is placed in a solution of the following substances:
- zinc monophosphate in the amount of 20 g per 1 l;
- sodium nitrate – 35 g per 1 liter.
The phosphating process takes place at a solution temperature of 60 degrees. It takes about 20 minutes to coat metals with a dense phosphate film. The process also requires a bath.
As for the quality of the coating, the characteristics of phosphate films are similar to those obtained using Majef's solution. This way you can provide a high degree of protection to any metal.
For processing galvanized steel, it is better to use a solution that uses zinc sulfate, zinc nitric acid, phosphoric acid, and sodium fluoride. The process is carried out at temperatures of about 60 degrees, and its duration is up to 20 minutes. This solution can be used to process zinc, carbon steels, and nickel.
Treatment with phosphating pastes
In this case, special phosphating primers are used. The advantage is that phosphating of steel and other alloys can be done at room temperature.
The mixture is applied to the surface of the part using a regular brush. The treatment does not require baths, which means that materials can be subjected to such phosphating at home.
This method is often used by car owners and car manufacturers.
The primer contains a metal pigment, as well as a solvent based on phosphoric acid. Paint most often contains zinc. When interacting with orthophosphoric acid, corrosion products are strengthened, creating a durable protective phosphate layer.
Phosphating paste is widely used for surface treatment of parts made of ferrous and non-ferrous alloys of any size. Primed surfaces are passivated, which also improves their adhesive properties.
Phosphating at home
There is often a need to phosphate various parts at home. Aluminum phosphating is most often used, but different types of steels and more can also be processed.
The technology for producing phosphate coatings at home is slightly different from the industrial one - full-fledged chemical processing is not available to many. Electrochemical processing is predominantly used.
To obtain protective films, you need direct or alternating current. Phosphoric acid solutions or Majef solution are used as an electrolyte. The part to be coated with phosphates is placed on an electrode immersed in an acid bath. Zinc rods are used as the anode, to which an electric current is also connected.
To get a high-quality result, a voltage of 25 V is sufficient. The process of obtaining the film will take about 30 minutes. The method is ideal for various rectilinear parts. It is more difficult to process volumetric geometric products using this method, since the layer will fall unevenly on a complex part, which will reduce its protective qualities.
Using phosphating technology, you can reliably protect metal products from the destructive effects of corrosion. Many methods are available at home, which is very important - many people work with various metals at home and want the parts to have a long service life.
Source: https://xn----8sbna6aihebzq3cl.xn--p1ai/sposoby-borby-s-korroziej/opisanie-togo-chto-takoe-fosfatirovanie.html
Phosphating - reliable protection of metal from corrosion
The problem of protecting metals and alloys from corrosion processes has arisen since people learned to extract metal from ore. Production technologies have changed significantly and become more modern, but corrosion still destroys products and structures.
Zinc coating is used to protect metal surfaces, but this does not provide maximum protection. Modern industry uses a more advanced method of protection from adverse effects - metal phosphating.
Using this technology, you can not only preserve the product or surface, but also obtain increased wear resistance of the material.
What is phosphating
The technology involves treating metal surfaces with special solutions based on phosphate salts. As a result, a durable protective film is formed. Among the types of phosphating, the most popular method is the application of phosphating primers. Waterjet and chemical processing of metals are also used.
Phosphate film allows you to improve the protective characteristics and service life of the paintwork several times over. Due to its low electrical conductivity, the film also improves adhesion and prevents under-film corrosion processes. Often the technology is used on products before painting using powder technology.
Phosphate film easily withstands the effects of organic substances - these are various oils, lubricants and hot materials, any gases except hydrogen sulfide.
This technology allows processing cast iron, low-alloy, and carbon steels. Phosphate coatings can be applied to zinc, cadmium, copper alloys, and aluminum. High carbon steel is also phosphated. But, despite its high protective qualities, the phosphate coating can be destroyed under the influence of alkalis, sea water, water steam, acid, fresh water, and water vapor.
The protective film is formed by dipping the product into a special bath containing a phosphating solution. It is also possible to apply the coating by spraying in a jet chamber. Depending on the composition of the solutions, phosphates with or without a well-defined crystal lattice can form on the surface.
A crystalline film is deposited from a solution with heavy metal cations, and an amorphous film is obtained from a solution of alkali metal acid phosphates or ammonium acid phosphate.
With the help of phosphating, metals can be used for a long time under difficult conditions, such as:
- conditions of high humidity;
- when exposed to fuels and lubricants;
- in organic solvent environments;
- under voltage up to 1000 V.
Cold phosphating of metal
Today, a large number of methods are used to protect metal products from the formation of corrosive plaque. All of them are aimed at creating a thin protective layer on the surface that will protect against the process of metal oxidation for a long time. Treating metals with phosphating solutions is an effective method of combating the formation of rust.
To carry out the phosphating procedure, it is necessary to initially prepare metals or metal products.
In order for the solution substances to adhere better, you need to thoroughly degrease and wash the surface that will be treated.
Only in this case will the coating be of high quality and last for quite a long time. If necessary, the metal material can be sanded with sandpaper before the procedure.
Advantages of phosphating
Phosphating is used in most cases to give metal surfaces additional protection against the formation of corrosive plaque. Thanks to phosphating, metals acquire the following positive qualities:
- hardness. Metals become more resistant to external damage.
- resistance to the influence of electric current.
- the service life of certain metal products that have been treated with phosphating is improved.
- coating strength. Metals are covered with an additional protective film, which gives them special properties.
The phosphating procedure in manufacturing plants is not used as often as anodizing, for example.
Source: https://steelfactoryrus.com/holodnoe-fosfatirovanie-metalla/
Methods and compositions for phosphating metals
Humanity has been concerned with the problem of protecting metal from corrosion destruction since it learned to smelt copper products from ore.
https://www.youtube.com/watch?v=WdbHYdXuSKE
Since then, steel production has improved significantly, and new methods of corrosion protection have been developed. But despite significant achievements in this area, it is almost impossible to ensure 100% non-destruction of such products in the conditions of the earth’s atmosphere.
One of the most advanced ways to protect iron from exposure to unfavorable environments and give its surface increased wear resistance is phosphating.
Phosphating: action of a protective mechanism
Metal phosphating is the process of covering the surface of non-ferrous and ferrous alloys with the thinnest phosphate film, which reliably protects the surface from rust.
In units whose operation involves a constant process of friction, this technology can significantly increase the wear resistance of contacting surfaces. Almost all alloys are amenable to the phosphating process, with the exception of high-alloy steel, on which the phosphate film is formed of very low quality.
This method of protecting metal from destruction makes it possible to operate products for a very long time under the following conditions:
- High humidity.
- Exposure to motor oils.
- In an environment of organic solvents.
- In electrical installations with voltages up to 1000 V.
- As a primer under paintwork.
The phosphate film perfectly protects the base material under the above conditions, but is quickly destroyed in an alkaline and acidic environment. Therefore, before starting to coat metal to protect it from destruction, it is necessary to know exactly the composition of the environment where the product, the surface of which has been subjected to the phosphating process, will be used.
Phosphating methods
Obtaining a protective phosphate film on the surface can be done in various ways, the feasibility of which depends on the dimensions of the workpiece, as well as on the area of application of the metal parts and structures protected in this way.
The following methods of phosphating a metal surface are most often used in industry:
1. Use of the drug "Mazhef".
The most common method of phosphating, which is carried out in special phosphating baths filled with a solution of the drug "Mazhef" in a concentration of up to 40 g/l. To form a stable phosphate film, the metal product is placed in the drug solution, brought to a boil and boiled for 15 - 20 minutes with periodic stirring. This time is enough to cover the metal with a protective layer.
In order for the phosphate film to form of proper quality with a protective layer thickness of up to 5 - 10 microns, the surface of the product must be cleaned using a sandblaster or an abrasive wheel.
Phosphating metal using the drug "Mazhef" can be used to coat low-carbon steel; this method is especially often used to obtain high-quality anti-corrosion primer for painting.
2. Application of phosphoric acid.
This method allows you to obtain cold phosphating of the metal, but the thickness of the protective layer, in this case, will be no more than 5 microns.
For a stable phosphating process to occur using this method, the solution temperature must be in the range from +18 to +25 degrees. To obtain a high-quality protective layer, it is necessary to strictly observe the percentage of active substances included in the solution.
The concentration of chemicals should be as follows:
- Phosphoric acid - 40 g/l.
- Zinc nitrate - 200 g/l.
- Sodium sulfate - 8 g/l.
- Zinc oxide - 15 g/l.
The duration of treatment with this solution is about 30 minutes.
3. Use of zinc monophosphate.
This method is used to protect steel used in electrical and mechanical engineering. The surface to be protected is placed in a solution of the following chemicals:
- Zinc monophosphate - 20 g/l.
- Sodium nitrate - 35 g/l.
The phosphating process is carried out in solution at a temperature of about +60 degrees. The duration of this operation is 15 - 20 minutes.
4. Application of phosphating paste.
This method can be used at room temperature. The working composition of the paste consists of a phosphating solution and filler in a ratio of 3/2. Talc or kaolin can be used as a filler. The solution is applied to the surface to be treated using a brush.
Phosphating at home
At home, metal protection methods can be used that are not widely used in production. One such method of covering a surface with a protective phosphating film is electrochemical treatment.
To apply a protective film to the surface, alternating or direct current is used. A solution of phosphoric acid or the drug "Mazhef" is used as an electrolyte.
The workpiece on which it is planned to apply a protective layer is installed on an electrode, which will be lowered into a bath of electrolyte; zinc rods are used as the anode, to which electricity is also supplied.
For high-quality metal processing, 25 V DC or AC is sufficient. The procedure for applying a protective layer takes about 30 minutes. This phosphating method is ideal for protecting rectilinear parts.
If the geometry of the product subjected to this type of processing is more complex, then the phosphating layer is not applied evenly enough, which significantly reduces the protective properties of this method of applying a phosphate film.
Many phosphating methods that are used in production can be used at home, provided that safety precautions are observed when handling chemical compounds, as well as the exact procedure for applying the protective layer is followed.
The drug "Mazhef" can be used at home. The use of this chemical compound allows you to apply a phosphate film to the surface of the product, which is an ideal primer for painting.
Phosphating the metal before painting will reliably protect the car body from rust, even in those places where the paint will be removed as a result of mechanical impact. Before starting to apply the protective layer, dust and dirt are removed from the surface, and the metal surface must also be thoroughly degreased.
You can do without preparing the working mixture yourself; for this you can purchase ready-made solutions in aerosol packaging, with which you can evenly spray the substance. Painting can only be done after the treated area is completely dry.
Some phosphating compounds to protect metal can be applied with a brush. With this option of applying a protective film, it is necessary to ensure the uniform distribution of the phosphating primer over the surface of the product.
If the workpiece is small in size, then at home you can use the hot method of applying a protective coating. For this purpose, Majef or a mixture of phosphoric acid and zinc nitrate is used. When carrying out such an operation, care should be taken and eye protection should be used, and work should be done in a well-ventilated area.
Source: https://plavitmetall.ru/obrabotka/fosfatirovanie-metalla.html
How to phosphate metal at home?
Most home craftsmen strive to master as many different operations as possible, and in this case [it would be a good idea to learn how to carry out phosphating on your own], which will ensure effective protection of metal surfaces.
Phosphating, as a rule, is carried out at industrial enterprises and allows the formation of a fairly thin layer of film on the metal surface, but well adhered to the base of the workpiece.
This layer in this case consists of phosphoric acid chemical compounds, which include elements such as iron phosphates, as well as manganese and zinc.
The application technology makes it possible to make this zinc layer within five micrometers thick.
Despite the fact that in order to perform phosphating, it is necessary to have certain equipment and various chemical reagents, with the right approach and following certain technological steps, this operation can be performed at home.
At its core, phosphating allows you to organize the protection of a metal surface from the effects of corrosion, which is quite in demand in everyday life.
Any craftsman can master phosphating directly at home if he wishes, of course, if he knows some of the subtleties and nuances of the process.
Features of phosphating
The main and main purpose of phosphating is to provide effective protection of metal surfaces directly from the effects of corrosive processes.
This technology is actively and widely used in industrial areas such as shipbuilding and automotive manufacturing, as well as many other industries.
In addition, it is quite popular in everyday life, where it is used to cover a variety of devices and parts with a protective film. The technology of phosphating metal surfaces serves mainly two purposes.
First of all, as mentioned above, this method provides corrosion protection, which significantly increases the service life of any metal product.
IMPORTANT TO KNOW: Thermal diffusion galvanizing process
In addition, with the help of phosphating it is possible to significantly improve the adhesion of various types of paint and varnish coatings directly to the surface being painted.
It is recommended to perform this protection when painting any metal surface. Phosphate coating has a very high porosity.
Due to this, any paint and varnish composition applied to a metal surface penetrates quite deeply into the internal pores of the phosphate film, which mainly determines the high adhesive properties of this type of coating.
It should also be noted that the film formed in this way passes the metal base and transfers it to a state that can be conditionally called corrosive-passive.
All this leads to the fact that the protective properties of the coating on which phosphating was carried out followed by painting are several times superior in its characteristics to the protective properties of the surface on which the paint and varnish composition was applied without prior preparation.
The use of zinc in this method makes this coating even better and more effective.
Characteristics of phosphating
This method of protection is especially relevant for all kinds of ferrous metals, the use of which is planned in fairly harsh conditions. It should be noted that such processing of various types of non-ferrous metals, including zinc, is carried out much less frequently.
The essence of this method lies mainly in carrying out special treatment of various types of metal surfaces with certain phosphoric acid solutions with the obligatory addition of zinc.
Due to this specific treatment, a continuous film is formed on the surface in a special way from phosphates of a wide variety of metals, including zinc, which must necessarily have either an amorphous or crystalline structure.
In this case, it should be noted that those phosphate compositions that are prepared from phosphate solutions of several groups of metals simultaneously have the highest anti-corrosion properties.
IMPORTANT TO KNOW: How to choose heat-resistant paint for metal?
In this case, to prepare the required mixture, it is allowed to use ready-made liquid concentrates.
At its core, the formation of this type of coating on a metal surface is a rather complex physical and chemical process.
From this we can conclude that at the moment when the phosphating composition itself comes into contact directly with the surface of the steel, electrochemical processes begin to occur.
In this case, the dissolution of iron itself occurs in the anodic areas; accordingly, hydrogen ions are reduced in the cathodic areas of the metal surface.
In addition, directly on the surface of the steel product itself, in the so-called near-electrode space, the pH value begins to increase, and the environment in this space becomes alkaline.
The phosphate film itself is formed due to the deposition of phosphate ions, as well as hydrogen phosphate ions, on the metal surface.
The strength of the coating itself on the metal surface is ensured by crystal chemical bonds, the boundaries of which are determined by the phosphating line.
General properties of coatings
It should be noted that any phosphate solution must be prepared with the addition of zinc.
In this case, the composition is more efficiently fixed to the very base of the metal. During this process, in some cases, a variety of salts can be formed, which, as a rule, precipitate.
During preparation of the composition, salt sediment should be periodically removed. The finished phosphate-based coating must have certain properties.
First of all, the metal coating must have a minimum number of through pores. In addition, its structure must be finely crystalline.
You should also pay attention to the specific gravity of the film, which should not exceed three grams per square meter.
Several different phosphating methods are currently used. First of all, it should be noted normal phosphating, which is carried out at temperatures close to boiling temperatures.
This method makes it possible to obtain a film with a thickness of up to seven micrometers, however, in some individual cases the mechanical properties of the processed material are reduced.
Also, very often, steel is processed using cold phosphating, in which the film has a minimum thickness and, accordingly, has low protective properties.
IMPORTANT TO KNOW: Methods and types of mechanical processing of metals
Protection can also be performed by accelerating phosphating, in which a special accelerator is introduced into the composition, which significantly reduces the overall process time. In any case, when the phosphating line is carried out, the entire metal base is cleaned of corrosion products and all kinds of fatty deposits using various groups of solvents.
Carrying out phosphating at home
Despite the relative complexity of the process, under certain conditions it is possible to perform chemical phosphating of, say, steel at home. The easiest way to carry out this process is the accelerated type.
To do this, you will first need to properly prepare the appropriate composition. It will be necessary to take a special preparation in certain proportions, which is called majef and a zinc nitrate type.
Next, the mixture is thoroughly mixed and heated to a temperature close to the boiling point. After this, the metal product being processed is immersed in the solution for fifteen minutes.
Paint coating can be applied to the treated workpiece only after it has completely dried.
Due to phosphating, it is possible to significantly increase the protective properties of almost any metal coating, including steel. Moreover, this operation can be performed at home without any problems.
In this case, the main thing is to observe the prescribed proportions and follow the appropriate recommendations. Also, when using this method of protecting metal bases, it is necessary to follow safety regulations.
Source: https://rezhemmetall.ru/kak-fosfatirovat-metall-v-domashnix-usloviyax.html
Phosphating
Today, a large number of methods are used to protect metal products from the formation of corrosive plaque. All of them are aimed at creating a thin protective layer on the surface that will protect against the process of metal oxidation for a long time. Treating metals with phosphating solutions is an effective method of combating the formation of rust.
To carry out the phosphating procedure, it is necessary to initially prepare metals or metal products. In order for the solution substances to adhere better, you need to thoroughly degrease and wash the surface that will be treated. Only in this case will the coating be of high quality and last for quite a long time. If necessary, the metal material can be sanded with sandpaper before the procedure.
Phosphating with film formation
This type of phosphating involves the formation of crystalline coatings. Examples include processes carried out by treating with solutions containing: zinc phosphate, zinc-iron phosphate and zinc-manganese phosphate.
In the case of interaction of a steel surface with a zinc phosphate solution, the resulting chemical reaction equation can be written as follows: Fe + 5Zn(H2PO4)2 → Zn3(PO4)2↓ + Zn2Fe(PO4)2↓+ 6H3PO4 + H2
The resulting insoluble salts, crystallizing, capture 4 water molecules each, forming hopeite Zn3(PO4)2*4H2O and phosphophyllite Zn2Fe(PO4)2*4H2O, which mainly form the coating.
Crystallization centers form in certain areas of the surface; the metal surface is gradually covered with crystals growing until they close together. Then the access of the solution to the metal is stopped, which leads to the end of layer growth.
The criterion for the completion of the process is the cessation of film mass growth. The practical value of phosphate layers is maximum when their growth has stopped.
Note that in reality the process of formation of the phosphate layer is much more complicated. According to the “theory of dissolution and primary passivation of metals,” the surface is considered as a set of anodic and cathodic areas, and the dissolution of the metal is the result of the combined occurrence of anodic ionization of the metal and cathodic reduction of the oxidative component of the aggressive environment.
For a number of reasons, the duration of formation of the phosphate layer can reach 40 – 60 minutes or more. The most effective acceleration methods are based on the use of accelerator compounds introduced directly into the phosphating solution.
So-called accelerators are, as a rule, oxidizing agents. In particular, they eliminate mechanical blocking of the metal surface by hydrogen bubbles. Oxidizing agents replace the reaction of hydrogen formation with other processes, directly participating in electrode transformations at the metal/solution interface.
The most common accelerators are nitrates, nitrites, chlorates and molybdates.
In order for the phosphated product to have optimal properties for further application of powder varnish coating, the following requirements must be met:
The phosphate coating must have a uniform fine-crystalline structure over the entire surface of the processed product and thus a constant low weight per unit surface area.
This can be achieved in particular by the following:
Selecting the appropriate phosphating method to suit the organic coating used as well as the metallic composition of the product
Compliance with the established conditions for carrying out the phosphating method in individual pre-treatment areas - such as electrolyte temperature, concentration of components, processing time, pressure when irrigating the product or flow conditions in the electrolyte.
A particularly important role in obtaining phosphate coatings with a fine-crystalline structure is played by the activation of the metal surface before phosphating. By pre-treating the product with special aqueous solutions containing titanium phosphate, particularly favorable conditions for further phosphating are created on the metal surface.
The number of places where crystallization centers can appear increases; as a result, during subsequent phosphating, crystallization takes place in the form of a much denser network than would have occurred without prior activation of the surface.
A more rapid formation of a dense phosphate layer of a closed structure is also achieved, and at the end of the process it is possible to obtain a very small crystal size and thus an optimally small thickness of the phosphate layer.
The corrosion properties of the coating directly depend on proper preparation of the metal surface of the phosphated product. For example, you should thoroughly rinse off all water-soluble substances.
To prevent the water used for rinsing from drying out on the metal surface, as well as the transfer of foreign substances from other baths to the surface, it is necessary to use completely demineralized water at the last stage of rinsing.
Additional rinsing of phosphated surfaces with aqueous solutions containing chromic acid can provide noticeably higher corrosion protection of painted products.
For environmental and health reasons, new chromium- and chromic-acid-free rinses have been developed for additional rinsing of phosphated surfaces, providing an environmentally friendly alternative to previously used chrome solutions. These systems are in most cases based on titanium and/or zirconium compounds.
Amorphous phosphating (“non-opaque phosphating”)
The term “amorphous” in this type of phosphating emphasizes the very weakly expressed crystalline structure of the resulting layer.
As mentioned above, the historically established definition of “uncovering” does not reflect the real situation. The main distinguishing feature in this case is the fact that the process is carried out in solutions of alkali metal or ammonium phosphates (without the use of heavy metal salts). Since such phosphates are well dissociated in aqueous solutions, the cation does not contribute to the coating.
Since the layer created on the surface of the steel consists mainly of iron phosphate, this method is often also called “iron phosphating”.
The mechanism of layer formation is conventionally divided into two reaction stages:
- etching and corrosion reaction
- layer formation reaction
Etching reaction - for example, on a steel metal surface: Fe + 4NaH2PO4 → Fe(H2PO4)2 + 2Na2HPO4 + H2
Primary iron (II) phosphate, in turn, can enter into the following reactions: 2Fe(H2PO4)2 + 2Na2HPO4 + 1/2О2 → 2FePO4 + 4NaH2PO4 + H2O
2Fe(H2PO4)2 + 4Na2HPO4 + 3/2О2 + H2O → 2Fe(OH)3 + 8NaH2PO4
Source: https://alufinish.ru/articles/280-fosfatirovanie
Phosphating metal before painting: features of the process
Preparing metal structures for painting is the most important procedure, the quality of which determines the durability of the future coating. The surface must not only be cleaned of dirt, but also at the final stage the metal must be degreased before painting.
Stages of work execution
Preparing metal is not such a simple process as it might seem at first glance. The work is divided into several stages, the most important of which are:
- removing rust and old paint from the surface;
- performing phosphating and degreasing.
Preparing for painting metal products can be done using various technologies, but first of all, rust and remnants of the previous paint layer should be removed from them.
Removing paint and rust
Cleaning metal from corrosion and old paint layers can be done in three ways:
- chemical;
- mechanical;
- thermal.
Mechanical method
This method, considered the most effective, involves removing rust and paint manually or using a power tool. Processing can be performed:
- wire brushes;
- grinding discs;
- using a sandblasting unit;
- by waterjet method (performed only in industrial enterprises).
Chemical treatment
Chemical treatment is based on the effect of chemical substances sprayed on the surface or applied with a brush to rust.
Rust removing compounds are divided into two types:
The disadvantage of wash-off products is the likelihood of new foci of corrosion appearing on the metal, therefore after treatment the surface must be immediately dried and treated with anti-corrosion compounds.
When treating rust with indelible compounds, a chemical reaction results in the formation of a kind of primer layer on the metal surface that cannot be washed off with water.
Processing of metal structures is most often performed:
- a solution of sulfuric or hydrochloric acid (5%) with the addition of a corrosion inhibitor;
- orthophosphoric acid (15-30% essence), which converts rust into a protective coating;
- mixture 50 gr. hydroxypropionic acid per 100 ml of petroleum jelly, under the influence of which rust turns into salt and is easily cleaned from the surface with a rag.
Thermal method
Removing paint from metal surfaces using the thermal method involves using a blowtorch. The metal is heated until the paint coating gradually peels off, which can be easily removed with a spatula or a metal brush.
The main advantage of this method is significant time savings, but the main disadvantage is a fire hazard and some restrictions on the types of surfaces. It is impossible to process sheet and galvanized material, cast iron using this method - the surface is deformed and the integrity of the structures is compromised.
Metal degreasing
Degreasing of structures is carried out to ensure good adhesion of the metal with the paint and varnish composition and primer.
To degrease metal before painting, in principle, you can use any composition that removes organic substances and fats. But still, it is better to use complex compounds that convert rust into a useful layer and prevent its appearance in the future:
- White Spirit;
- numbered nitro solvents;
- degreaser with complex alcohols;
- kerosene.
It is not recommended to use gasoline as a degreasing agent, since as a result of its exposure, an invisible oil film appears on the surface, which impairs adhesion to the paint.
Degreasing must be carried out in well-ventilated areas with constant air circulation, since the vapors of most chemicals used are very toxic. To avoid poisoning, it is recommended to wear a respirator, work in rubber gloves and safety glasses - if any solvent gets into the eyes, a chemical burn to the mucous membrane cannot be avoided.
Phosphating of metal surfaces
Phosphating is the process of coating the surfaces of ferrous or non-ferrous metals with a thin film that protects it from rust formation and improves adhesion to the paint composition.
The use of this technology can significantly improve the wear resistance of contacting parts in friction units. The method can be implemented for almost all alloys, except for high-alloy steel - a phosphate film of insufficiently high quality appears on it.
Why is phosphating performed?
Phosphating of metal before painting is carried out in order to provide the surface with reliable protection from corrosion processes in places that have been mechanically cleaned of old paint and rust. Before applying a protective layer, metal structures or products must be thoroughly cleaned of dust and dirt, and also degreased.
This method of protecting metal structures allows their operation in the following conditions:
- exposure to automobile oils and fuels;
- in electrical installations up to 1 kV;
- high humidity;
- in environments with organic solvents;
- being under paintwork.
The resulting film is capable of reliably protecting the metal under the above conditions, but is quickly destroyed in aggressive acidic and alkaline environments. Therefore, before performing phosphating, it is necessary to determine the composition of the environment in which the metal product will be used.
Phosphating methods
The formation of a phosphate protective film on a metal surface is achieved in several ways, the possibility and feasibility of which depends on the size of the structure and its area of application.
The most commonly used methods are:
- surface treatment with Majef, which is allowed even for low-carbon steel, resulting in the formation of a high-quality primer with anti-corrosion properties;
- the use of phosphoric acid or “cold phosphating”, in which the thickness of the protection is no more than 5 microns;
- the use of zinc monophosphate, which is used primarily in the mechanical engineering and electrical power industries;
- treatment with phosphating paste.
To prepare metal for painting, it is necessary to perform a number of mandatory procedures, without which high-quality painting and, accordingly, long-term operation of metal structures are impossible.
You may also be interested in knowing which metal fence paint is best for your situation. Read about this in the article about painting metal fences.
Source: https://kraskaton.ru/stroyka-remont/rabota/fosfatirovanie-metalla-pered-pokraskoj/
Metal phosphating - methods and compositions
Phosphating is the treatment of metal with special means based on phosphate salts, as a result of which a protective film appears on the surface. Among the options for protecting metal from corrosion by phosphating, the best known is phosphating primer. Waterjet phosphating and chemical metal processing are also used. In addition to protecting the metal, the film provides increased adhesion (adhesion) of the metal to paints and varnishes.
Waterjet phosphating
Water jetting is considered one of the best ways to protect metal. The composition for metal processing is prepared on the basis of soft water. The parts are dipped for 10-15 minutes in a 10% solution of potassium dichromate. The temperature of the liquid is from 70 to 80 degrees Celsius.
Next, the film is hydrophobized, for which the product is placed in a 10% solution of organosilicon liquid in gasoline for 7 minutes. After this, the gasoline vapor is allowed to evaporate in the open air and the metal is sent to dry at 100 degrees for an hour.
Phosphating primers
To protect the metal, a phosphating primer can be used, consisting of 9/10 metal pigments, as well as a solvent based on orthophosphoric acid. When interacting with an electrolyte, paint containing zinc is strengthened by corrosion products and forms a dense film.
Phosphating primers are used for processing products made of ferrous and non-ferrous metals of any size (from large structures to the thread of a single part). The primed surface leads to passivation of the metal, as well as improvement of the adhesive properties of the material.
Chemical treatment of steel
Chemical phosphating is the dipping of metal into special chemical compounds, as a result of which a protective film appears on its surface.
We put MAZHEF salt into the bath, based on the proportion of 35 grams per liter of water. Pour in the liquid, bring it to a boil and keep it like that for 20 minutes. Next, remove the container from the heat to determine and adjust (if necessary) the acidity level.
The composition is produced in excess, since during heating some of it evaporates. The level of total acidity is determined by titration with phenolphthalein. To titrate 10 milliliters of solution, 30 milliliters of decinormal sodium hydroxide will be needed. Free acidity will be determined by the presence of the methyl orange indicator.
To titrate a 10 ml sample, 4 ml of decinormal sodium hydroxide solution is needed. The amount of alkali spent on titration is indicated in points. Acidity standards: total - 28-30 points, free - 3-4 points (that is, the ratio of different types of acidity to each other can range from 7 to 10).
Phosphating is carried out at a temperature of 98 degrees Celsius for 1-2 hours.
The work can be considered complete when the bubbling of hydrogen stops. Next, the metal is kept in the container for 10-15 minutes. This is necessary for the film to crystallize.
The consumption rate of MAZHEF for phosphating a square meter of surface can range from 120 to 140 grams. The acidity level is adjusted with water or by adding MAGEF salt. The specific amount of salt required to achieve an acidity level of 30 points can be calculated using the formula:
A (kg) = (30-n)*V/1000
The variable V stands for volume, and n stands for the number of solution points. If the steel contains a large proportion of alloying components (copper, chromium, vanadium), it will not be possible to obtain a film of proper quality. The quality of work is reduced by the presence of aluminum, lead, arsenic components, as well as chloride and sulfide impurities in the solution. The proportion of chlorine ions should not exceed 0.3%.
If the film turns out to be of poor quality, it can be removed using a 15% hydrochloric acid solution or a heated 20% sodium hydroxide solution. It should be borne in mind that in the case of repeated phosphating, the film will have a more coarse crystalline structure with less protective qualities.
Accelerated treatment with MAZHEF salt
Phosphating of low-alloy and electrical grades began to be carried out using mixtures including the following components (grams per liter of water):
Option #1
- drug MAZHEF - 30-40;
- zinc nitrate Zn(NO3)26H2O - 50-60.
Option No. 2
- drug MAZHEF - 45-50;
- zinc nitrate Zn(NO3)26H2O - 70-80:
- sodium fluoride NaF.
Phosphating is carried out in just 10-15 minutes, provided the liquid temperature is 97-98 degrees Celsius for solution No. 1 and solution No. 2. Without cleaning the surface, the procedure can be performed by adding zinc oxalate to the composition. This substance will remove traces of corrosion during the formation of a film.
solution (grams per liter):
- zinc monophosphate - 35;
- zinc nitrate - 53;
- phosphoric acid - 14;
- zinc oxalate - 0.1.
The permissible level of total acidity is 70-80 points, free acidity is 12-15 points, liquid temperature is 92-98 degrees Celsius, phosphating time is 20-40 minutes.
Properties, application technology and areas of application of phosphate coatings
Zinc oxalate is prepared from sodium oxalate and zinc nitrate. When the solutions are combined, a precipitate of zinc oxalate forms at the bottom of the container, which must be removed using a filter. The precipitate is then dried and used to create a phosphating solution.
Accelerated treatment with zinc salts
Treating the metal in a solution of zinc salts allows for better surface protection compared to MAZEF salt.
Composition components (grams per liter):
- zinc monophosphate - 37;
- zinc nitrate - 54;
- phosphoric acid - 16.
During phosphating, the composition will need to be adjusted. To do this, you need to add a concentrate, which includes 500 grams of zinc nitrate, 480 grams of zinc monophosphate, 180 grams of phosphoric acid and a liter of water.
A black film with enhanced protective characteristics is obtained by sequentially dipping parts into two compositions. One of them contains 1 gram of soda ash, 23 grams of ferrous phosphate, 8 grams of zinc oxide, 32 grams of orthophosphoric acid (all quantities are indicated per liter of water). The total acidity of the composition is from 56 points, and the free acidity is from 9 to 14 points. The temperature of the liquid is from 92 to 97 degrees Celsius.
After dipping in the above solution, the product is placed in a 9% solution of potassium chromium for 5 minutes at a temperature of 80 to 95 degrees Celsius. Next, the part is washed again in a soap-soda solution, and then in hot water and placed in a container for re-phosphating.
This time the mixture includes 150 grams of zinc nitrate, 30 grams of MAZHEF, 3 grams of carbonic acid. Acidity indicator - from 80 points, free acidity - from 2 to 4 points. The temperature of the composition is from 50 to 60 degrees Celsius, the phosphating period is from 10 to 20 minutes. Next, the product is placed in a soap-soda solution for 2 minutes.
The process is completed by drying the film and treating it with mineral oil.
Cold process
Cold phosphating involves processing the material at temperatures from 20 to 40 degrees Celsius. You can use one of two types of solution.
To work, you will need the following components (calculated in grams per liter):
Solution No. 1. We load the amount of MAZHEF salt into the bath corresponding to the volume of water. Add boiled and infused sodium fluoride and zinc nitrate to the solution. To increase the acidity level of the solution, add 1.5 grams of MAZHEF salt, 2-3 grams of zinc nitrate and 2-3 milligrams of sodium fluoride to each point.
Solution No. 2. To create a solution, we use a concentrate that includes 80 grams of zinc monophosphate, 750 grams of zinc nitrate, 160 grams of phosphoric acid, 40 grams of soda ash and 1 liter of water.
To prepare 100 liters of working solution, add 12 liters of caustic soda concentrate (300 grams per liter) to 85 liters of water, and then add water to the level of 100 liters. We also add 40 grams of sodium nitrous acid. If the acidity value is less than required, add caustic soda little by little.
