Galvanizing and methods
Galvanizing and methods.
Galvanizing is a coating metal layer of zinc for corrosion protection. Galvanizing provides metal both active and passive corrosion protection.
The benefits of galvanizing, as a method of applying a corrosion resistant coating
Types, methods and ways of galvanizing
Hot, cold, electroplating, thermal diffusion and thermal galvanizing
Galvanizing is a coating metal layer of zinc for corrosion protection.
Galvanizing, as a method of corrosion protection, suitable for flat or slight curved surfacesnot subject to mechanical stress.
Galvanizing, as a method of protection against corrosion, based on the following principle. Most metals (e.g., zinc, tin, aluminum) oxidize in air, thus on the surface of the metal formed a dense protective film of oxidized metal compounds. This film prevents the penetration of oxygen deep into the metal and thus stops further oxidation of the metal. However, in the case of iron, the process is different. The resulting oxidized iron compounds (e.g., hydroxides) have a larger volume than the original metal, the result is formed a film of hydroxide immediately breaks down and turns loose. This loose, loose and unstable the film freely passes oxygen back to unoxidized metal, and the process of oxidation continues. Iron and steel are not able to protect themselves from further oxidation, so rust.
If coating iron or steel with a zinc layer, its protective oxide film will not pass the oxygen as the metal coating, and to iron under the coating. Therefore, the iron or steel is protected from corrosion. Zinc and iron form a galvanic couple in which iron is a less active metal, the result of the zinc in the coating composition reacts with the corrosion first, and primary metal (iron) remains almost untouched.
In a galvanic pair of iron-zinc has a higher electronegative charge than iron. This means that the corrosion exposures are primarily zinc and until then, until it is completely dissolved, the metal will be fully protected from corrosion. Zinc does not join iron (steel) in the oxidation reaction. In that case, if any part of the surface of the metal the zinc coating breaks, the zincreacts with oxygen and water to form zinc hydroxide, which in turn, having a good protective properties, protecting the base metal.
The thickness of the zinc layer depends on the technology, method of galvanizing, the temperature and duration of process of galvanizing and ranges from 6 µm for electroplated galvanizing to 1.5 mm.
Galvanizing provides metal both active and passive protection against corrosion. Passive protection is the barrier action of the zinc coating. The barrier action of the zinc coating is to ensure that upon weathering the pure zinc (Zn) reacts with oxygen and forms oxide of zinc (ZnO), followed by reaction with carbon dioxide (CO2) and the formation of zinc carbonate (ZnCO3), normally Matt grey, quite solid material that stops further corrosion of the material.
Active corrosion protection occurs due to cathodic effect of the zinc coating compared to the more noble metals, such as iron (if you look in the electrochemical series of the stress) zinc serves as the anode, which protects the iron underneath from corrosion for as long as it is completely corroded. The cathodic effect of the zinc coating protects exposed even damage to the main metal (for example, if the metal has scratches, scores, cracks or scrapes) or not galvanized cutting edges of metal, placed at a certain distance from the adjacent zinc coating.
Currently, zinc is the most common method of corrosion protection.
A layer of freshly applied zinc are usually brightly lit. Over time, the surface of the zinc due to the corrosion of zinc forms a patina and the zinc layer becomes darker and matte in appearance. Patina is a weatherproof protective layer of zinc oxide and zinc carbonate.
The benefits of galvanizing, as a method of applying anti-corrosion coating:
zinc coating increases the service life of metal up to 50 years or more,
– zinc coating covers the main product of metal a uniform layer,
– zinc coating is resistant to chipping,
– zinc coating of iron and steel provides not only barrier and electrochemical protection,
– zinc coating restores itself to damaged areas,
– zinc coating is more resistant to chipping under impact than similar corrosion resistance polymeric coatings.
Types, methods and ways of galvanizing:
Galvanizing is carried out using the following methods and ways:
– thermal diffusion;
During hot-dip galvanizing iron or steel is immersed in a bath of molten liquid zinc, the temperature of which is about 450 °C. When the hot-dip galvanizing on the border between iron (steel) and zinc forms a stable alloy of these two metals (Fe-Zn), and above it is a very hard layer of pure zinc.
The thickness of zinc layer is usually from 50 to 150 microns, and the zinc tape is from 5 to 40 microns.
Due to the greater thickness of the zinc layer parts, hot dip galvanized, have a lifespan, mostly more than 50 years.
Prior to conducting hot-dip galvanizing , the product is subjected to preliminary preparation to empty the product from extraneous contamination, impurities, etc. and includes the following steps: cleaning and degreasing, etching using acid solutions, rinsing after pickling and fluxing, thorough drying.
After conducting hot-dip galvanizing products last and is blown with compressed air for the purpose of drying the articles and removing from the treated surface excess of zinc.
However, the production process of hot-dip galvanizing has faults, caused by environmental risk, complexity and energy consumption of the process.
Cold galvanizing involves coating the surface of the metal, for example, in the form of finished products, special zinc-containing mixture-paint.
This method is used when you need to zinc the product of a complex configuration, or the product contains a hard-to-reach places, or when the product cannot be galvanized by other methods.
Zinc-containing a mixture of paint contains 90 % zinc in the form zinc powder (the rest is binders) and in the coating of metal resistant to damage and temperature extremes.
Cold galvanizing of metal is performed in a wide temperature range. As in the hot-dip galvanizing the surface of the protected metal must be clean of dirt, impurities, etc.
Cold galvanizing method characterized by ease of application, the ability to paint individual damaged area, scrape or welds, the possibility of using different tools to apply the paint (brush, roller or spray) and low cost compared to other methods.
Galvanizing is an electrochemical process for the deposition of zinc from zinc-containing electrolytic solution (electrolyte) the product connected to the negative pole of the source of electric current. During electroplating galvanizing main metal and zinc combine at the molecular level.
Galvanic zinc coating has high thickness accuracy and high adhesion to the base metal, has an exceptional smoothness, uniformity and decorative. However, the galvanizing has a number of limitations. It does not allow to create a protective layer thicker than 40 microns.
The electroplating process of galvanizing is performed by electrolysis with a consumable anode in the following way. In the electrolytic solution are placed the primary metal product and zinc plates. Zinc plates are connected to the positive pole of the electric current and the metallic product to negative. Thus, the zinc plates are the anode and product is cathode. When you turn an electric current on the surface of the plates and the product is formed, the difference of electrical potentials, the zinc plate begins to dissolve in the electrolyte, the molecules of the zinc move to the surface of metal products, settling on her and forming a homogenous protective zinc layer.
The rate of formation of the zinc coating on the product depends on the composition and density of electrolyte, temperature and the characteristics (density, etc.) of electric current. As in the process of electroplating galvanizing zinc plate acts as a consumable, it is periodically cleaned of oxides and salts are monitored for exhaustion.
As in hot or cold galvanizing the surface of the protected metal product must first be cleaned of dirt, impurities, etc.
However, the production process electroplating galvanizing has faults, caused by environmental risk, complexity and energy consumption of the process, as well as the necessity of disposing of used electrolyte.
Thermal diffusion galvanizing:
Thermal diffusion galvanizing is a process in which the metal product is applied Zn-containing dry mixture, after which it is placed in a furnace or container, which creates a high temperature – about 290-450 and above OS. Under high temperature zinc atoms pass into the gaseous state and penetrate the surface layer of metal products.
The process of penetration of atoms of one substance into another is called diffusion. Hence the name of this method of galvanizing. In the literature, the method of thermal diffusion galvanizing is also called “hereditary” (by the name of its inventor Sherard (Sherard Cowper-Coles)).
Thermodiffusion zinc cover exactly follows the contours of products, it is homogeneous in thickness on the entire surface, including complex shapes and threaded connections.
Thermodiffusion zinc coating is anodic in respect to black metals and electrochemically protects the steel from corrosion. It has a durable adhesion (adhesion) with the base metal due to mutual diffusion of iron and zinc in the surface intermetallic phases, there is no delamination and spallation of the coating under impact, mechanical loads and deformations of the processed products.
Use of thermal diffusion galvanizing mainly in those cases when the surface of the treated metal necessary to form the protective layer, the thickness of which exceeds 6 µm.
This method has some significant advantages compared to other. First, the ecological cleanness of the production process and no waste. Secondly, uniformity, high thickness accuracy, and high adhesion of zinc to the primary metal. Thirdly, the thermal diffusion galvanizing products are subjected to any complicated shape and configuration, having various holes and ledges. Fourth, the ability to adjust the thickness of the zinc coating. Fifth, compared with the electroplating method of thermal diffusion galvanizing does not cause irreversible hydrogen embrittlement of the metal during the process of application (directly in the container).
As in hot, cold galvanizing or electroplating the surface of the protected metal products must be pre-cleaned of dirt, impurities, etc.
Thermal galvanizing is a kind of process of thermal spraying of metal on a substrate and represents a heating process, the dispersion and transfer of condensed particles of the sprayed zinc gas or plasma flow on the surface of the parent metal products to form a protective zinc layer.
Under the General title of thermal spraying have the following methods: flame spraying, high speed flame spraying, detonation spraying, plasma spraying, spraying, melting, electric-arc metallization and activated electroarc metallization. Inherently thermal spraying is very similar to welding, the difference lies in the functional purpose of the transferred material. The purpose of the weld – connection of structural elements of buildings, the purpose of thermal spraying – surface protection against corrosion, wear , etc.
To use this method of deposition in the cases of applying the zinc layer on bulky articles which cannot be subjected to protective treatment in other ways.
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