First, the methods of cold galvanizing and hot galvanizing of galvanized steel pipes are different:
1. Cold plating: It is a process of anti-corrosion treatment on the surface of workpieces through electrochemical principles.
2. Hot-dip galvanizing: also known as hot-dip galvanizing, is a method in which steel components are immersed in a molten zinc solution to obtain a metal coating.
Second, the principles of cold galvanizing and hot galvanizing of galvanized steel pipes are different:
1. Cold plating: Use electrolytic equipment to put the pipe fittings into a solution composed of zinc salt after degreasing and pickling, and connect the negative electrode of the electrolytic equipment, place a zinc plate on the opposite side of the pipe fittings, and connect it to the positive electrode of the electrolytic equipment. The power supply uses the directional movement of the current from the positive pole to the negative pole to deposit a layer of zinc on the pipe fittings. The cold-galvanized pipe fittings are processed first and then galvanized.
2. Hot dipping: When the iron workpiece is immersed in the molten zinc solution, a solid solution of zinc and α-iron (body center) is first formed on the interface. This is a crystal formed by dissolving zinc atoms in the base metal iron 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 atoms of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix lattice and gradually form an alloy with iron.
1. The entire steel surface is protected, and the molten zinc can be easily and uniformly covered no matter inside the pipe fitting in the depression or any other corner where the coating is difficult to enter.
2. The hardness value of the galvanized layer is greater than that of steel. The uppermost Eta layer has a hardness of only 70 DPN, so it is easily dented by collisions, but the lower Zeta layer and delta layer have hardness values of 179 and 211 DPN respectively, which are higher than iron’s hardness value of 159 DPN, so it is impact resistant and Both wear resistances were quite good.
3. In the corner area, the zinc layer is often thicker than in other places and has good toughness and wear resistance. The other coatings are often the thinnest, the most difficult to apply, and the most vulnerable to damage at this corner, so they often need to be maintained again.
4. Even if it is caused by a great mechanical injury or other reasons. A small part of the zinc layer falls off, exposing the iron base. At this time, the surrounding zinc layer will function as a sacrificial anode to protect the steel here from corrosion. The opposite is true for other coatings, rust will form immediately and quickly spread to the bottom of the coating, causing the coating to peel off.
5. The consumption of the zinc layer in the atmosphere is very slow, about 1/17 to 1/18 of the corrosion rate of steel, and it is predictable. Its lifespan far exceeds that of any other coating.
6. The life of the coating depends on the thickness of the coating in a specific environment. The thickness of the coating is determined by the thickness of the steel, that is, the thicker the steel, the thicker the coating, so the thicker steel parts in the same steel structure must also get a thicker coating to ensure a longer life.
7. For aesthetics, art, or when used in a specific severely corrosive environment, the galvanized layer can be painted with a duplex system. As long as the paint system is selected correctly and the construction is easy, its anti-corrosion effect is better than that of painting alone and hot-dip galvanizing. The combined life expectancy is 1.5~2.5 times better.
8. In addition to the hot-dip galvanizing method, there are several other methods to protect steel with a zinc layer. Generally, the most widely used, anti-corrosion effect and economical benefit is the hot-dip galvanizing method.