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Malleable steel pipe fitting is an important part used in waterway pipe, due to constant contact with water and air, its corrosion resistance is very important. Malleable steel pipe fittings are generally treated with galvanizing on the surface of the product. Galvanization includes hot galvanization and cold galvanization. So, how to differentiate them?
From the appearance to distinguish, for the cold galvanizing coating is uniform, its surface is close to the blank, with unique surface concave and convex feeling, looks smoother and brighter. Due to the phenomenon of electrical shielding, cold plated parts generally have no galvanizing layer inside (except for special galvanizing techniques). However, as the hot-dip galvanizing layer is later and the liquid zinc is less fluidity, the surface zinc layer is thicker, and because the surface of the hot galvanizing layer oxidizes to produce a white protective layer, the color is generally darker, and the surface smoothness is far less than that of the cold plating product.
There are still some manufacturers spray a layer of silver powder paint to impersonate galvanized pipes for sale. The differentiation method is also very simple, tap with two pipes, the surface of the silver paint will fall off. In addition, from the color to distinguish, if silver paint pipes do not fall off, the paint layer is very bright. Cold galvanized pipes are dark bright, and generally have no galvanized layer inside, and hot galvanizing will become dark grey after a period of time.
So which is better, cold - plated and hot - plated fittings?
Hot galvanizing, also known as hot dip galvanizing, is a method of immersing tube parts in molten zinc to obtain a metal coating.
Principle: hot galvanizing has good covering ability, compact coating, no organic inclusion. It is well known that the mechanism of atmospheric corrosion resistance of zinc is mechanical protection and electrochemical protection. ZnO, Zn(OH)2 and basic zinc carbonate protective film are on the surface of Zn layer under atmospheric corrosion conditions, which slow down the corrosion of zinc to some extent. While this layer of protective film (also known as white rust) is damaged it will form a new one. When the zinc layer is seriously damaged, which endangers the iron matrix, the 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 microcell, zinc is dissolved as anode and iron is protected as cathode. Obviously, the atmospheric corrosion resistance of hot galvanizing to the base metal iron is better than that of electric galvanizing.
The formation process of hot galvanizing layer is the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The surface of the workpiece formed an iron-zinc alloy layer during hot dip plating, so that the iron and pure zinc layer can be well combined. The process can be simply described as: When the iron workpiece is immersed in molten zinc, a solid solution of zinc and cast iron (body core) is formed at the interface first. This is a crystal formed by the molten zinc atoms of the base metal iron in a solid state, the two metal atoms are fused, and they have less gravitational force.
Thus, when zinc is saturated in a solid solution, the zinc and iron atoms diffuse to each other, spreading (or infiltrating) the zinc atoms in the iron matrix migrating in the matrix lattice, gradually alloy with iron, the iron that diffuses into the molten zinc liquid forms the intermetallic compound FeZn13 with the zinc and sinks to the bottom of the hot galvanizing pot, that is, zinc slag. The surface of the workpiece forms a pure zinc layer when it is removed from the zincification solution.
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