First, essential difference
Cold galvanizing is also called electrical galvanizing, is the use of electrolytic equipment after the workpiece after oil removal, acid pickling into the composition of zinc salt solution, and connect the cathode of the electrolytic equipment; On the opposite side of the workpiece, the zinc plate is connected to the positive electrode of the electrolytic equipment. When the power is switched on and the current is moved from the positive electrode to the negative electrode, a layer of zinc will be deposited on the workpiece.
Hot dip galvanizing, also known as hot dip galvanizing, is a method of dipping steel components into molten zinc solution to obtain a metal coating, is the workpiece oil removal, pickling, dipping, drying after dipping into the molten zinc solution for a certain time, out.
Two, principle difference
Cold galvanizing USES chemical principle to separate zinc alloy into zinc ions, attached to the surface of steel, generally thin layer of zinc, steel in the general environment is easy to rust. Generally, cold galvanizing is used for anti-corrosion of various steel products and structures. The amount of cold galvanizing is very small, only 10-50g per square meter.
As is known to all, the mechanism of anti-atmospheric corrosion of zinc is mechanical protection and electrochemical protection. Under the condition of atmospheric corrosion, the surface of zinc layer is covered with ZnO, Zn(OH)2 and basic zinc carbonate protective film, which can slow down the corrosion of zinc to a certain extent. When this protective film (also known as "white rust") is damaged, a new film will be formed. When the zinc layer is badly damaged and endangers the iron matrix, the zinc produces electrochemical protection on the matrix. The standard potential of zinc and iron is -0.76V, while the standard potential of iron is -0.44V. Hot dip galvanized layer formation process is the most out of pure iron substrate and formed between the zinc layer of iron, zinc alloy, the process of workpiece surface formed in hot dip plating of iron - zinc alloy layer, which can make the iron with good combination between pure zinc layer, the process can be simply described as follows: when the blacksmith a immersed in molten zinc liquid (usually molten liquid zinc is about 455 ℃), the first in zinc and alpha iron solid solution is formed on interface. This is the matrix metal iron in a solid state dissolved by zinc atoms formed a crystal, the two metal atoms are fused between the atoms, the attraction between the atoms is relatively small. Therefore, when zinc in solid solution after reaching saturation, two kinds of zinc iron atom diffusion, spread to (or called "into") of the iron substrate zinc atom in the lattice matrix migration, gradually formed and iron alloy, and spread to molten zinc liquid FeZn13 formation of iron and zinc metal compounds, sank into the hot dip galvanized pot is zinc slag. When the workpiece is removed from the zinc leaching solution, a pure zinc layer is formed on the surface, which is hexagonal crystal. Its iron content is not more than 0.003%, hot dip galvanized coverage, coating fine, no organic inclusion.
Three, appearance difference
Cold galvanized appearance is smooth, bright, even, flat, using color passivating process of electrodeposit is also yellow and green main color, showing seven colors. The electrodeposit with white passivation process is bluish white or greenish white, and the electrodeposit with white passivation process is slightly colorful with sunshine at a certain Angle. There is only a little zinc layer at both ends of the cold-plated steel tube, and there is no galvanized layer in the cold galvanized steel tube. The two ends of the cold-galvanized steel tube are as smooth as before, and there is absolutely no zinc tumor, and the appearance of the steel tube does not follow the standard.
It is easy to produce "electric firing" in the corner of the complex workpiece, which is dark, and the zinc layer is thick. It is easy to form the dead Angle of current in the negative corner and produce the dark area of undercurrent. The zinc layer in this area is thinner. The workpiece as a whole without zinc tumor, caking and other phenomena.
Hot dip galvanized appearance and cold galvanized, the color is darker, slightly rough, no cold galvanized smooth, the appearance of silver white, easy to produce process water and a few drop tumor, especially in one end of the workpiece is more obvious, and the surface is not bright, not too reflective. Hot dip galvanized steel tube with blue ink printed on both ends of the hoop, the whole body of the standard and specifications. At one end of the hot dip galvanized steel pipe there is a zinc needle or a few nodules hanging, inside and outside there is a complete galvanized layer.
In a word, the zinc layer of hot dip galvanizing is several times thicker than that of cold galvanizing, and the corrosion resistance is also several times of cold galvanizing.
At present, many enterprises choose steel grille board, grille board, steel grille board products, for the factory internal platform pedals, ceiling ceiling and other construction projects. Depending on the project, customer requirements for product quality are also different, especially in galvanized quality requirements.
Domestic galvanizing method is divided into cold galvanizing and hot galvanizing. What's the difference:
Principle: hot dip galvanized covering ability, coating density, no organic inclusion. As is known to all, the mechanism of anti-atmospheric corrosion of zinc is mechanical protection and electrochemical protection. Under the condition of atmospheric corrosion, the surface of zinc layer is covered with ZnO, Zn(OH)2 and basic zinc carbonate protective film, which can slow down the corrosion of zinc to some extent. When this protective film (also known as white rust) is damaged, a new film will be formed. When the zinc layer is badly damaged and endangers the iron matrix, the zinc produces electrochemical protection on the matrix. The standard potential of zinc is -0.76V and the standard potential of iron is -0.44V. It is obvious that hot - dip galvanizing has better anti - atmospheric corrosion ability than electric galvanizing.
Galvanized layer formation process is the most out of pure iron substrate and formed between the zinc layer of iron, zinc alloy, the process of workpiece surface formed in hot dip plating of iron - zinc alloy layer, and makes good combination between iron and pure zinc layer, the process can be simply described as follows: when the blacksmith a immersed in molten zinc liquid, zinc first formed in the interface and alpha (heart) solid solution. This is the matrix metal iron in a solid state dissolved by zinc atoms formed a crystal, the two metal atoms are fused between the atoms, the attraction between the atoms is relatively small. So when zinc in solid solution after reaching saturation, two kinds of zinc iron atom diffusion, spread to (or called infiltration) iron matrix of zinc atom in the lattice matrix migration, gradually formed and iron alloy, and spread to molten zinc liquid iron and zinc in the formation of intermetallic compounds FeZn13, sank into the hot dip galvanized pot is zinc slag. When the workpiece is removed from the zinc leaching solution, a pure zinc layer is formed on the surface, which is hexagonal crystal. Its iron content shall not exceed 0.003%
The cold galvanizing process is used to protect the metal from corrosion. For this purpose, the coating of zinc filler is used. It is coated on the protected surface by any coating method. Suitable for repair work (that is, in the repair work, only in the area of the steel surface damage, repair surface can be recoated). Cold galvanizing process is used for the anticorrosion of various steel products and structures. (cold galvanizing is electric galvanizing, galvanized amount is very small, only 10-50g/m2, its own corrosion resistance than hot dip galvanizing difference. The use of electric galvanized relatively cheap.
Actually, the effect of cold galvanization, at most and besmear prevent embroider oil is equal, be inferior even!! Structure stress key parts, be careful, especially exposed parts!
Steel is galvanized on the surface under the condition of cooling, while hot dip galvanized is galvanized on the surface of steel tube under the condition of hot dip, which has strong adhesion and is not easy to fall off. Although hot dip galvanized pipe also appears the phenomenon of corrosion, it can meet the technical and sanitary requirements in a long period.1. The electroplating layer is even and thin; The hot coating is thicker and the thickness is uneven.
2. Bright appearance of electroplating and dark color of hot plating.
3. The coating is thinner and has poor corrosion resistance. Hot coating because of the thicker, and with the matrix metal formation of infiltration layer, corrosion resistance is good
Process flow chart of anticorrosive hot dipping of workpiece:
Leaching zinc piece of check - in addition to oil, acid cleaning, leaching solvent - drying - leaching zinc - cold water exposure, passivation, cleaning, repair, inspection, packaging, warehousing.
Detailed process of strip galvanizing:
Cold rolled coil inspection - to bundle - uncoiler to flat machine, shear - welding machine - alkaline cleaning section - cleaning section - entry looping to annealing furnace, zinc pot - air knife - zinc - empty cold - water - pull machine to light the whole machine, roller chromium to drying and cooling device, the export looping to surface defect detection device, the trimmer and waste processing plant - electrostatic oiler - reel to bundling machine, packing and warehousing
1. Rust removal (18% ±)
2. Cleaning (flowing water < 30 m3/ day)
3. Plating aid (ammonium chloride, zinc chloride, water < 50℃±)
4, drying
5. Zinc dipping (zinc rare earth alloy and aluminum alloy 455℃±)
6. Cooling and passivation (sulfuric acid and chromic acid)
7. Afterprocessing
4. Monthly production
1000 t - 1500 t
V. sewage discharge is less than 30 m3 / day
Composition in sewage: ferrous chloride ≥3%;
Chromic acid 0.05% or higher
Sulfuric acid acuity 0.03 ‰
Zinc plating is divided into hot dip (hot plating) and electroplating (cold plating), hot dip steel to go through the pickling water drying into the zinc pot, and then after the process of water cooling passivation drying, here the zinc pot in the zinc is liquid at high temperature, directly with the steel surface formed zinc ferroalloy, general zinc layer thick, steel corrosion resistance.
During electroplating, chemical principles are used to separate zinc alloy into zinc ions, which are attached to the surface of steel.
Hot dip galvanized steel tube with blue ink printed on both ends of the hoop, the whole body of the implementation of standards and specifications; At one end of the hot-dip galvanized steel pipe, there are zinc needles or a few nodules hanging, inside and outside there are complete galvanized layer, the appearance is not cold galvanized smooth; Cold galvanized coating is bright, and the sun into a certain Angle of this bright into seven colors, cold galvanized steel tube hole only at both ends of a little zinc layer, and then to the inside of no galvanized layer, cold galvanized steel tube at both ends of the same smooth, absolutely no zinc tumor.
