What is the difference between spray and galvanizing?
Release date：2018-06-28 author： click：
Spraying is also the electrostatic powder coating that we often talk about. It is used to charge, adsorb on the surface of the iron plate, and then baked at 180~220 °C to melt and adhere the powder to the metal surface. The sprayed products are mostly used indoors. The cabinet has a flat or matte finish. The spray powder mainly includes acrylic powder, polyester powder and the like.
Zinc Plating Zinc plating is a surface treatment technique that coats the surface of a metal, alloy or other material with a layer of zinc for aesthetics and rust prevention.
The main method currently used is hot-dip galvanizing.
Hot-dip galvanizing is a result of the older hot-dip method. Since the application of hot-dip galvanizing in France in 1836, it has been a hundred and forty years old. However, the hot-dip galvanizing industry has developed on a large scale with the rapid development that has accompanied the past three decades.
The production process mainly includes: preparation of the original board → pre-plating treatment → hot dip plating → post-plating treatment → finished product inspection. According to the habit, it is often based on the pre-plating method.
Different hot dip galvanizing processes are divided into two categories: off-line annealing and in-line annealing, namely wet method (single sheet hot-dip galvanizing method), off-line annealing (single sheet hot-dip galvanizing method), hot-dip galvanizing Huilin (Wheeling) method (continuous hot-dip galvanizing method for strip steel), in-line annealing Sendzimir method (protective gas method), modified sensation method, U.S. steel method (with Japanese law), (Selas) method and Sharon law.
1. Out-of-line annealing: Before hot rolling or entering the hot-dip galvanizing line, first in the bottom-of-the-line annealing furnace or in the middle, so that there is no annealing process in the galvanizing line. The steel sheet must maintain a clean, pure iron active surface free of oxides and other contaminants prior to hot dip galvanizing. In this method, the annealed surface iron oxide scale is first removed by pickling, and then coated with a solvent consisting of zinc chloride or a mixture of ammonium chloride and zinc chloride to prevent the steel sheet from being oxidized again. .
(1) Wet hot-dip galvanizing: The solvent on the surface of the steel sheet is subjected to hot-dip galvanizing without entering the dried (i.e., the surface is still wet) into the zinc liquid whose surface is covered with the molten solvent. The disadvantages of this method are:
a. It can only be galvanized in a lead-free state, and the alloy layer of the plating layer is thick and the adhesion is very bad.
b. The generated zinc slag is accumulated at the interface between the zinc liquid and the lead liquid and cannot be deposited on the bottom of the pot (because the specific gravity of the zinc slag is greater than that of the zinc liquid and less than the lead liquid), so that the steel sheet contaminates the surface by passing through the zinc layer. Therefore, this method has been basically eliminated.
(2) Single sheet steel: This method generally uses hot rolled rolled sheet as a raw material. First, the annealed steel sheet is sent to a pickling workshop, and the hot-dip galvanized iron sheet on the surface of the steel sheet is removed with sulfuric acid or hydrochloric acid. The steel plate after pickling is immediately immersed in a water tank for galvanizing, which prevents the steel plate from reoxidizing. After pickling, water washing, squeezing, drying, and entering the zinc pot (the temperature is always maintained at 445-465 ° C), hot-dip galvanizing, and then oiling and chromizing. The hot-dip galvanized sheet produced by this method has a significantly improved quality than the wet-galvanized product, and has a certain value only for small-scale production.
(3) Huilin method heat: The continuous galvanizing production line includes a series of pre-treatment processes such as lye degreasing, hydrochloric acid pickling, water rinsing, solvent coating, drying, etc., and the original plate needs to be hood before entering the galvanizing line for galvanizing. Furnace annealing. This method has a complicated production process and high production cost. More importantly, the products produced by this method often have solvent defects and affect the corrosion resistance of the coating. Moreover, the AL in the zinc pot often acts on the surface of the steel sheet to form aluminum trichloride and is consumed, and the adhesion of the plating layer is deteriorated. Therefore, although this method has been published for nearly 30 years, it has not been developed in the world hot-dip galvanizing industry.
2. In-line annealing: The original plate with hot coiling is provided directly from the cold rolling or hot rolling workshop, and gas protection recrystallization annealing is performed in the hot dip galvanizing line. Hot-dip galvanizing methods belonging to this category include: Sendzimir Act, Modified Sendzimir Act, US Steel Law (with Kawasaki Act of Japan); Silas Act; Sharon Law.
Sendzimir method: It combines the annealing process and the hot-dip galvanizing process. The in-line annealing mainly consists of an oxidation furnace and a reduction furnace. The strip steel is directly heated to about 450 degrees in the oxidation furnace, and the residual rolling oil on the strip surface is burned off to purify the surface. After that, the strip is heated to 700-800 degrees to complete the recrystallization annealing. After the cooling section is controlled, the temperature before entering the zinc pot is about 480 degrees. Finally, the zinc pot is galvanized without contact with the air. Therefore, the Senjimir method yields. High and good galvanizing quality, this method has been widely used.
U.S. Steel: It is a variant of the Sendzimir method. It simply replaces the degreasing of the oxidizer with an alkaline electrolytic degreasing tank. The rest of the process is basically the same as the Sendzimir method. After the original board enters the line, it is first electrolytically degreased, then washed, dried, and then recrystallized by a reducing furnace with a protective gas, and finally into the zinc pot by hot-dip galvanizing in the case of sealing. In this method, since the strip steel is not heated by the oxidation furnace, the oxide film on the surface is thin, and the hydrogen content of the shielding gas in the reduction furnace can be appropriately reduced. This is beneficial to furnace safety and reduced production costs. However, since the strip steel is not preheated, it enters the reduction furnace, which undoubtedly increases the heat load of the reduction furnace and affects the life of the furnace. Therefore, this method has not been widely used.
Silas method: Also known as direct flame heating method; firstly, the strip is eluted with alkali by alkali, and then the scale of the surface is removed with hydrochloric acid, washed with water, dried, and then passed into a vertical in-line annealing furnace directly heated by a gas flame. By strictly controlling the ratio of the flame of the gas and air in the furnace, it is subjected to incomplete flame burning in the case of excess gas and insufficient oxygen, thereby causing a reducing atmosphere in the furnace. It is rapidly heated to a recrystallization temperature and the strip is cooled under a low hydrogen protective atmosphere, and finally immersed in a zinc liquid in a sealed state to perform hot dip galvanizing. The method is compact, with low investment costs and high output (up to 50/hour). However, the production process is complicated, especially when the unit is stopped, in order to avoid burning the strip steel, it is necessary to adopt a method in which the furnace traverses away from the steel strip, so that there are many operational problems, so the hot-dip galvanizing industry adopts this method very little.
Sharon Law: In 1939, the United States Sharon Company put into production a new type of hot-dip galvanizing unit, so it is also called Sharon Law. This method is to inject a hydrogen chloride gas into the strip in an annealing furnace and bring the strip to a recrystallization temperature, so it is also called a gas pickling method. Pickling with hydrogen chloride gas not only removes the scale on the surface of the strip, but also removes the grease on the surface of the strip. Since the surface of the strip is corroded by oxidizing gas to form a pockmark, the coating obtained by the Sharon method is adhered. Very good. However, due to the serious corrosion of the equipment, it causes high equipment maintenance and renewal costs. Therefore, this method is rarely used.
Improved Sendigmi: It is a superior hot-dip galvanizing process; it connects the separate oxidizers and reduction furnaces in the Sendzimir method from a smaller cross-section aisle, including preheating furnaces and reduction furnaces. The entire annealing furnace, including the cooling section, forms an organic whole. Practice has proved that the law has many advantages: high quality, high yield, low consumption, safety and other advantages have gradually been recognized. It has developed very fast. Almost all of the new lines have been used since 1965. In recent years, the old Sendzimir units have also been modified in this way.