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Questions and answers on zinc plating
What are the main uses of hot-dip galvanized steel sheets?
Answer: hot dip galvanized sheet is mainly used in building, household appliances, automobile, machinery, electronics, light industry and other industries.
What kinds of galvanizing methods are there in the world?
Answer: there are three kinds of methods, such as galvanizing, hot dip galvanizing and zinc plating.
Hot galvanizing can be divided into two types according to the different annealing methods.
Answer: it can be divided into two types: inner line annealing and off-line annealing. They are called protective gas method and flux method respectively.
4. What are the commonly used steel grades of hot-dip galvanized steel sheet?
Answer: Commodity coil (CQ), structure galvanized sheet (HSLA), deep drawing galvanized sheet (DDQ), bake hardening galvanized sheet (BH), dual phase steel (DP), TRIP steel (phase transformation induced plasticity steel), etc.
5. What are the forms of galvanized annealing furnace?
Answer: there are three kinds of vertical annealing furnace, horizontal annealing furnace and vertical horizontal annealing furnace.
6. What are the usual cooling methods for cooling towers?
Answer: there are two kinds: air cooling and water cooling.
7. What are the main defects of galvanizing?
Answer: There are mainly: shedding, scratches, passivation spots, zinc grains, thick edges, air knife notches, air knife scratches, exposed steel, inclusions, mechanical damage, poor performance of steel base, wave edge, ladle curve, size, imprint, zinc layer thickness, roll printing, etc.
 It is known that the production specifications are not 0.75 x 1050 mm and the coil weight is 5 tons. What is the length of the coil? (zinc plated weight 7.6g/cm3)
Solution: L = G/ (H x B * P) = (5 * 1000) / (0.75 x 1.050 * 7.6) = 834.42m
Answer: the coil length is 834.42m.
 what are the main reasons for zinc layer falling off?
Answer: The main reasons of zinc layer falling off are: surface oxidation, silicon compound, cold tie emulsion too dirty, NOF oxidation atmosphere and protective gas dew point too high, unreasonable air-fuel ratio, low hydrogen flow rate, oxygen infiltration into the furnace, low strip temperature, low RWP furnace pressure, low door suction, NOF furnace temperature, grease The evaporation is not enough, the aluminum content of the zinc pot is low, the speed of the unit is too fast, the reduction is insufficient, the residence time in the zinc solution is too short, and the coating is too thick.  what are the causes of white rust and black spots?
Answer: black spots are formed by further oxidation of white rust. The main causes of white rust are:
(1) poor passivation and insufficient or uneven thickness of passivation film.
(2) no oil on the surface or residual moisture on the surface of the strip;
(3) moisture is found on the surface of strip when coiling.
(4) passivation is not completely dried.
(5) wet or wet in transportation or storage;
(6) the storage time of finished products is too long.
(7) galvanized sheet is contacted or stored together with other corrosive media such as acid and alkali.
 how long is the longest storage allowed in the library? Why?
Answer: allow storage for three months to avoid oxidation for too long.
 what is the length tolerance of galvanized steel sheet?
Answer: tolerance should not be allowed to exceed +6mm.
13. What is the big zinc flower? What is little zinc flower?
Answer: The big zinc flower is the normal zinc flower, according to the normal production process can produce the big zinc flower, its crystal nucleus diameter is not less than 0.2 mm; the crystal nucleus diameter is less than 0.2 mm called the small zinc flower, this kind of zinc flower can not be generally recognized by the naked eye.
 protection principle of hot-dip galvanized coating?
Answer: because zinc can form a good corrosion resistant film on the surface in corrosive environment. It not only protects the zinc layer itself, but also protects the steel base.
15. What is the passivation principle of hot dip galvanized steel sheet?
Answer: Chromate passivation of galvanized sheet can form passivation film. Its chemical reaction formula is as follows: Zn + H2GrO4 - ZnGrO2 = H2
Trivalent chromium in solution passivation group is difficult to dissolve in water, its chemical property is not active and plays a skeleton role. And hexavalent chromium in solution is easy to dissolve in water. It can play a role of re-passivation when passivation film scratches, and has the curative effect of passivation film. Therefore, to a certain extent, the passivation film can prevent the direct erosion of galvanized sheet by steam or humid air, and the Dunhuang zinc layer plays a protective role.
16. What are the ways to test the corrosion resistance of hot-dip galvanized steel sheets?
Answer: there are three ways to test the corrosion resistance of hot-dip galvanized sheet.
(1) salt spray test; (2) moisture test; (3) erosion test.
 why should galvanizing products be rust treated?
Answer: When the hot-dip galvanized sheet is in the humid air, especially when the air contains acidic substances such as SiO2, CO2, NO2 and NO, the surface of the galvanized layer will soon form loose white rust. The main components of white rust are ZnO and Zn (OH) 2. This kind of white rust not only affects the appearance, but also brings great difficulties to the future use. Zinc pot
 what are the causes of white rust?
Answer: the causes of white rust are:
(1) hot dip galvanizing is combined with other corrosive media such as acid, alkali and salt.
(2) the passive film or film is damaged.
(3) the effect of passivation or oiling is not good.
(4) warehouse ventilation is poor and humid.
(5) galvanized sheet is dripping water during transportation.
(6) transport at low temperature and store at high temperature to form condensed water.
19. How to calculate the thickness of the coating?
Answer: The coating thickness formula is: Dz = Gz (Sz * dz) in which: Dz - coating thickness mm, Gz - coating weight per square meter g,
Sz -- the area of mm2, DZ - zinc in the plating area.
The above formula can be simplified to: Dz = 0.000139Gz
 what are the causes of zinc particles?
Answer: (1) the original board problem is mainly the cleanliness of the original board and the content of the original plate iron powder.
(2) the temperature of the zinc solution is too high to float the bottom slag.
(3) the high content of Al affects the solubility of iron.
What is the significance of finishing of hot-dip galvanized steel sheet?
Answer: With the continuous expansion of the use of hot-dip galvanized sheet, in the modern continuous hot-dip galvanizing line, strip finishing can achieve the following objectives:
(1) To improve the flatness and flatness of the sheet, but also to flatten the surface of zinc particles, so that the surface of the strip smooth, which is particularly beneficial for later deep drawing and other occasions with high accuracy.
(2) The smooth roll used has been shot-peened beforehand, so the smooth surface of the plating plate has certain roughness. It can improve the adhesion of the coating, but also can store a certain amount of oil, in the deep-drawing process, beneficial to the lubrication of the die;
(3) For the plated sheet after painting treatment, although the surface is controlled into small zinc flowers, the small zinc flowers can still be exposed through the paint layer. Therefore, for some galvanized sheets with high demand, the surface of the small zinc flower needs to be treated again. This will make the galvanized steel get a more uniform and uniform silver appearance.
(4) By smoothing, the lower yield point can be reduced, the yield platform disappears or is not obvious, and the slip line can be prevented from appearing in the later stretching or deep drawing process, thus improving the possibility of deep drawing.
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