Basic knowledge of the hottest galvanizing 0

Basic knowledge of hot dip galvanizing

what is hot dip galvanizing

hot dip galvanizing is also called hot dip galvanizing. It melts the zinc ingot at high temperature, puts some auxiliary materials, and then immerses the metal structure into the galvanizing tank, so that a layer of zinc layer is attached to the metal components. The advantage of hot-dip galvanizing is its strong anti-corrosion ability, and the adhesion and hardness of the zinc coating are good. The disadvantage is that the price is high, requiring a large number of equipment and sites, the steel structure is too large to be put into the galvanizing tank, the steel structure is too thin, and hot plating is easy to deform. Zinc rich coating generally refers to the anticorrosive coating containing zinc powder. Zinc rich coatings on the market contain zinc

formation process of hot-dip galvanized layer

the formation process of hot-dip galvanized layer is the process of forming iron zinc alloy between the iron substrate and the outermost pure zinc layer. The iron zinc alloy layer is formed on the surface of the workpiece during hot-dip plating, which makes the iron and pure zinc layer work well. The unsuitable position of the guide wheel of the platform will also cause the increase of the friction force with the 1 side column. The process can be simply described as: when the iron workpiece is immersed in molten zinc, First, zinc and α Iron (body center) solid melt. This is a crystal formed by zinc atoms dissolved in the matrix metal iron in the solid state. The two metal atoms are fused, which will replace the talc based reinforcement in polypropylene composites made by rhetech, an automotive supplier located in Whitmore lake, Michigan. The attraction between the two is relatively small. Therefore, when zinc reaches saturation in the solid melt, the atoms of zinc and iron diffuse each other, and the zinc atoms diffused (or infiltrated) into the iron matrix migrate in the matrix lattice, and gradually form an alloy with iron, while the iron diffused into the molten zinc liquid forms an intermetallic compound fezn13 with zinc, and sinks into the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc immersion solution, a pure zinc layer is formed on the surface, which is hexagonal crystal. Its iron content is not more than 0.003%

protective performance of hot-dip galvanized coating

generally, the thickness of galvanized coating is 5 ~ 15 μ m. The hot-dip galvanized coating is generally 65 μ M or more, even up to 100 μ m。 Hot dip galvanizing has good coverage, dense coating, and no organic inclusions. As we all know, the mechanism of zinc's resistance to atmospheric corrosion includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are ZnO, Zn (OH) 2 and basic zinc carbonate protective films on the surface of the zinc layer, which can slow down the corrosion of zinc to a certain extent. This protective film (also known as white rust) is broken. At the same time, it can provide users with a variety of experimental schemes and data processing methods according to various experimental standards of GB, ISO, JIS, ASTM and din. If it is bad, it will form a new film. When the zinc layer is seriously damaged, endangering the iron matrix, zinc produces electrochemical protection for the matrix. The standard potential of zinc is -0.76v, and the standard potential of iron is -0.44v. The formation of micro it between zinc and iron is an important factor for the smooth progress of the experiment and the accuracy of the experimental results. When the battery is used, zinc is dissolved as an anode, and iron is protected as a cathode. Obviously, the atmospheric corrosion resistance of hot-dip galvanizing to base metal iron is better than that of electro galvanizing

formation control of zinc ash and zinc slag in hot galvanizing process

zinc ash and zinc slag not only seriously affect the quality of zinc coating, but also cause rough coating and zinc tumor. Moreover, the cost of hot galvanizing is greatly increased. Generally, 80 ~ 120kg zinc is consumed per 1t of workpiece plated. If the zinc ash and zinc slag are serious, the zinc consumption will be as high as 140 ~ 200kg. Controlling the zinc carbon is mainly to control the temperature and reduce the dross produced by the oxidation of the surface of the liquid zinc. Some domestic manufacturers cover with refractory sand, carbon ash, etc. In foreign countries, ceramic or glass balls with low thermal conductivity, high melting point, low specific gravity and no reaction with molten zinc can be used to reduce heat loss and prevent oxidation. This kind of ball is easy to be pushed away by the workpiece and has no adhesion to the workpiece

the formation of zinc slag in molten zinc is mainly the zinc iron alloy with poor fluidity formed when the iron content dissolved in molten zinc exceeds the solubility at this temperature. The zinc content in zinc slag can be as high as 95%, which is the key to the high cost of hot dip galvanizing

it can be seen from the solubility curve of iron in molten zinc that the amount of iron dissolved, that is, the amount of iron loss, is different at different temperatures and different holding times. At 500 ℃, the iron loss increases sharply with heating and holding time, almost in a linear relationship. Below or above the range of 480 ~ 510 ℃, the iron loss increases slowly with time. Therefore, 480 ~ 510 ℃ is called malignant dissolution zone. Within this temperature range, the molten zinc will erode the workpiece and zinc pot most seriously, and the iron loss will increase significantly when it exceeds 560 ℃. When it reaches above 660 ℃, zinc will erode the iron matrix destructively, and the zinc slag will increase sharply, so the plating cannot be carried out. Therefore, at present, plating is mostly carried out in the areas of 450 ~ 480 ℃ and 520 ~ 560 ℃

5.2 control of the amount of zinc slag

to reduce zinc slag, we should reduce the content of iron in the zinc solution, that is, we should start from reducing various factors of iron dissolution:

⑴ plating and heat preservation should avoid the peak area of iron dissolution, that is, do not work at 480 ~ 510 ℃

⑵ zinc pot materials shall be welded with steel plates with low carbon and silicon content as far as possible. High carbon content will accelerate the corrosion of molten zinc to iron pot, and high silicon content can also promote the corrosion of molten zinc to iron. At present, 08F high-quality carbon steel plate is mostly used. It contains 0.087% carbon (0.05% - 0.11%), silicon ≤ 0.03%, and contains elements such as nickel and chromium that can inhibit the erosion of iron. Do not use ordinary carbon steel, otherwise the zinc consumption is large and the service life of the zinc pot is short. It was also suggested that using silicon carbide to make molten zinc bath can solve the iron loss, but the molding process is also a problem

⑶ always dredge the slag. First, raise the temperature to the upper limit of the process temperature so that the zinc slag can be separated from the zinc liquid, and then reduce it to below the process temperature to make the zinc slag sink at the bottom of the tank and scoop it up. The plated parts falling into the zinc solution should also be salvaged in time

⑷ iron in the plating aid should be prevented from being brought into the zinc bath with the workpiece. Reddish brown iron containing compounds will be generated when the plating aid is used for a certain time, and it should be filtered regularly. The pH value of the plating aid should be maintained at about 5

⑴ less than 0.01% aluminum in the plating solution will accelerate the formation of sediment. An appropriate amount of aluminum will not only improve the fluidity of the zinc solution and increase the brightness of the coating, but also help to reduce zinc slag and zinc ash. A small amount of aluminum floating in the liquid surface is beneficial to reduce oxidation, too much will affect the quality of the coating, resulting in spot defects

⑥ heating and temperature rise should be uniform to prevent explosion and local overheating

executive standard of hot dip galvanized steel pipe

welded steel pipe, hot dip galvanized steel pipe, square and rectangular pipe, foreign common standards TM A53, ASTM A500, ASTM A513, Si c80.1, ANSI c80.3 1387, BS1139 10219, en10220, en10240, en10255 n2440, din2444, din2458 1163, as1074, as2556