The difference between hot-dip galvanizing and cold-dip galvanizing for surface treatment of malleable cast iron pipe fittings

urface treatment of malleable cast iron pipe fittings. Difference between hot-dip galvanizing and cold-dip galvanizing. Malleable cast iron pipe fittings are important fittings used in water pipes. Since they are often in contact with water and air, their corrosion resistance is very important. Malleable cast iron pipe fittings are generally treated with galvanizing on the surface of the product for rust prevention. Among them, galvanizing is divided into hot-dip galvanizing and cold-dip galvanizing. So, how to distinguish between hot-dip galvanizing and cold-dip galvanizing?

From the appearance, since the cold-dip galvanizing coating is uniform, its surface is close to the blank, and it has the surface bumps of the blank product, and looks smooth and bright. Due to the electric shielding phenomenon, cold-dip parts generally have no galvanized layer inside. Hot-dip pipe fittings have a thicker zinc layer on the surface because the hot-dip galvanizing layer is later and the fluidity of the zinc liquid is poor. In addition, since the hot-dip galvanizing layer will oxidize to produce a white protective layer, the color is generally darker, and the surface smoothness is far less than that of cold-dip products.

Now some manufacturers use some ungalvanized black parts to spray a layer of silver powder paint on the surface and then pretend to sell them as galvanized pipe fittings. The method of distinguishing them is also very simple. Use two pipe fittings to knock against each other, and the silver paint on the surface of the pipe fittings will fall off. In addition, from the color, if the silver paint pipe fittings do not fall off, the paint layer is very bright, the cold-dip galvanized pipe fittings are dark and bright, and there is generally no galvanized layer inside, and hot-dip galvanizing will turn dark gray after a period of time.

So which performance is better between cold-dip galvanized pipe fittings and hot-dip galvanized pipe fittings?

Specific analysis from the galvanizing process of the product:

Hot-dip galvanizing, also known as hot-dip galvanizing, is a method of immersing pipe fittings in molten zinc liquid to obtain a metal coating.

Principle: Hot-dip galvanizing has good covering ability, dense coating, and no organic inclusions. As we all know, the mechanism of zinc's anti-atmospheric corrosion is mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there is a protective film of ZnO, Zn(OH)2 and basic zinc carbonate on the surface of the zinc layer, which slows down the corrosion of zinc to a certain extent. This protective film (also known as white rust) will be damaged and a new film will be formed. When the zinc layer is severely damaged and endangers the iron matrix, zinc provides electrochemical protection to the matrix. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as the anode and iron is protected as the cathode. Obviously, hot-dip galvanizing has better atmospheric corrosion resistance to the base metal iron than electroplating.

The formation process of the hot-dip galvanizing layer is the process of forming an iron-zinc alloy between the iron matrix and the outer 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 layers well combined. The process can be simply described as follows: when the iron workpiece is immersed in molten zinc liquid, a solid solution of zinc and α iron (body-centered) is first formed on the interface. This is a crystal formed by the base metal iron dissolving zinc atoms in a solid state. The two metal atoms are fused and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid solution, the zinc and iron atoms diffuse into each other. The zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix lattice and gradually form an alloy with the iron. The iron diffused into the molten zinc liquid forms an intermetallic compound FeZn13 with zinc and sinks to the bottom of the hot-dip galvanizing pot, which is zinc slag. When the workpiece is removed from the zinc bath, a pure zinc layer is formed on the surface.