The process principles of hot-dip galvanizing and cold-dip galvanizing are very different, and the appearance of the galvanized layer is also very different.
Author: Marisa
Principles of hot-dip galvanizing and cold-dip galvanizing:
1.Hot-dip galvanizing is to make molten zinc metal react with iron matrix to produce alloy layer, so that the matrix and coating are combined. It is to immerse the steel products with cleaned and activated surface in molten zinc liquid, and through the reaction and diffusion between iron and zinc, a zinc alloy coating with good adhesion is plated on the surface of the steel products.
2.Cold galvanizing is also called electro-galvanizing. It uses electrolytic equipment to put the pipe fittings into a solution of zinc salt after degreasing and pickling, and connects the negative pole of the electrolytic equipment. A zinc plate is placed opposite the pipe fittings, connected to the positive pole of the electrolytic equipment and connected to the power supply. The directional movement of the current from the positive pole to the negative pole will deposit a layer of zinc on the pipe fittings.
Comparison between hot-dip galvanizing and cold-dip galvanizing:
1.The coating of hot-dip galvanizing is thicker, generally 35-60 microns, and the coating has a higher anti-corrosion ability, which can achieve anti-corrosion protection for more than 50 years without repair. Due to the hot-dip galvanizing method, steel products can only be fully plated. The cost of hot-dip galvanizing is lower than that of other protective coatings, but it is more expensive than cold-dip galvanizing (due to the thickness of the galvanizing layer). The reason is simple. Other protective coatings such as sandblasting, rust removal and painting are labor-intensive processes. On the contrary, the process of hot-dip galvanizing is highly mechanized and can be produced in factories.
2.The coating of cold-dip galvanizing is thinner, generally 5-15 microns, and its anti-corrosion ability is equivalent to that of anti-rust paint, so it cannot be used as an anti-corrosion protection measure for important components and projects with anti-corrosion years. Cold-dip galvanizing can be plated on only one side or all sides.
Hot-dip galvanized square tubes emit fewer pollutants, mainly waste acid from pickling workpieces. The pollutants of cold-dip galvanizing mainly include waste acid from pickling workpieces, electroplating waste liquid, waste passivation liquid, etc. The types and quantities of emissions are much greater than hot-dip galvanizing.
After comparison, we found that the thickness of the galvanizing layer of hot-dip galvanizing is several to dozens of times different from that of cold-dip galvanizing, and the anti-corrosion ability is completely not at the same level. In architectural design, there are design service life requirements for structural or construction steel products. Therefore, the anti-corrosion protection measures for steel products should choose a solution that matches the design service life, and hot-dip galvanizing anti-corrosion protection can meet this requirement.
How to distinguish between hot-dip galvanizing and cold-dip galvanizing?
1.Because the process principles of hot-dip galvanizing and cold-dip galvanizing are very different, the appearance of the galvanized layer is also very different.
Cold-dip galvanizing: The cold-dip galvanizing coating has a uniform texture, a very thin thickness, a smooth and flat surface, and a shiny surface. The overall appearance of the workpiece has no zinc nodules, lumps, and process ripples.
Hot-dip galvanizing: The hot-dip galvanizing coating is thick and uneven, with a rough texture, and there are slight unevenness on the surface. There is no cold-dip galvanizing brightness. The appearance of the workpiece is prone to process water marks and a few drips, especially at one end or edge of the workpiece.
2.The coating thickness of hot-dip galvanizing and cold-dip galvanizing is very different, which can be checked by a coating thickness detection instrument. When measuring, 5 detection points can be selected on a single workpiece, and the average value of the 5 measured values can be calculated.
Combined with the appearance inspection, the galvanizing method can be basically determined. If the building design has coating thickness requirements, it can also be determined whether the galvanized material meets the design requirements.