The most perfect galvanizing science guide

• Posted by Hongqi An May 23, 2022 Filed in Business #  #galvanizing machine  134 views
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  Over the years, China's economy has maintained a high growth rate, which has provided a good basis for the development of China's galvanizing industry, and China has become a major producer and consumer of hot-dip galvanized steel. Recently, China's economic development has entered a period of steady growth, and the galvanizing industry has entered a new period of development.

   

  What is galvanizing?

  Galvanizing is a surface treatment technique in which the surface of a metal, alloy or other material is coated with a layer of zinc for aesthetic and rust prevention purposes. The main method used is hot-dip galvanizing. Scholars and experts suggest that hot-dip galvanizing is a traditional industry, and in today's situation of overcapacity and increasing environmental protection, there is a need to increase technological innovation, actively promote common technologies, and use Internet technology to promote the construction of smart factories. Research and development to promote the collection and treatment technologies of waste gas, waste liquid and waste slag, energy-saving and emission reduction technologies, corrosion-resistant materials and alloys. Accelerate the development of industry standards and regulate the market with standards. Create a favorable environment for the sustainable development of the hot-dip galvanizing industry.

   

  Zinc is soluble in acid and alkali, so it is called an amphoteric metal and hardly changes in dry air. In humid air, zinc produces a dense film of alkaline zinc carbonate on its surface. Zinc is less resistant to corrosion in sulfur dioxide, hydrogen sulfide and marine atmospheres, especially in high temperature and high humidity atmospheres containing organic acids. The standard electrode potential of zinc is -0.76 V. For steel substrates, zinc plating is an anodic layer, which is mainly used to prevent corrosion of steel, and its protective performance is very much related to the thickness of the coating. The protective and decorative properties of zinc plating are significantly improved by passivation, dyeing or coating with light protectors.

   

  What are the different types?

   

  Cold Galvanized

   

  Cold galvanizing, also known as electro-galvanizing, is a process in which fittings are degreased, pickled, and placed in a solution of zinc salts using electrolytic equipment, and the negative electrode of the electrolytic equipment is connected to the negative electrode.

   

  Mechanical Galvanizing

   

  In a rotating barrel containing the plated parts, glass (1841, -3.00, -0.16%) balls, zinc powder, water and accelerator, the glass balls, as the impact medium, rotate with the barrel and generate mechanical and physical energy by friction and hammering with the surface of the plated parts, and "cold welding" the coated zinc powder to the plated parts with the action of the chemical accelerator. It forms a smooth, uniform and detailed plating layer with a certain thickness.

   

  The technical standards are ASTM B695-2000 and Military C-81562 for mechanical galvanizing.

   

  Hot-dip galvanizing

   

  The main method of galvanizing the surface of steel sheets today is hot-dip galvanizing.

   

  Hot-dip galvanizing was developed from the older method of hot-dip galvanizing, which has been used in industry for nearly 200 years since it was introduced in France in 1836. However, the hot-dip galvanizing industry has developed on a large scale in the last three decades, along with the rapid development of cold-rolled strips.

   

  The production process of hot-dip galvanized sheets mainly includes: raw sheet preparation → pre-plating treatment → hot-dip plating → post-plating treatment → finished product inspection. It is customary to divide the hot-dip galvanizing process into two categories, i.e. wet (single sheet hot-dip galvanizing method), off-line (single sheet hot-dip galvanizing method), hot-dip galvanizing (Wheeling) method (continuous hot-dip galvanizing method for strip steel), in-line annealing (Sendzimir) method (protective gas method), modified Sendzimir method The Sendzimir method, the modified Sendzimir method, the American Iron & Steel method (same as the Kawasaki method), the Sales method, and the Sharon method.

   

  

   

  Off-line annealing

   

  This is where the hot-rolled or cold rolled steel sheet is first recrystallized and annealed in a draw-bottom annealing furnace or hooded annealing furnace before entering the hot-dip galvanizing line so that the galvanizing line does not have an annealing process. The steel sheet must maintain a clean, pure iron-active surface free of oxides and another dirt present prior to hot-dip galvanizing. This method consists of removing the annealed surface iron oxide by pickling and then applying a layer of protection consisting of zinc chloride or a solvent consisting of a mixture of ammonium chloride and zinc chloride, thus preventing the steel sheet from being re-oxidized.

   

  (1) Wet hot-dip galvanizing

   

  The surface of the steel plate is hot-dip galvanized without drying (i.e., the surface is still wet) into a zinc solution with a molten state solvent covering the surface. The disadvantages of this method are

   

  a. Galvanizing can only be done in a lead-free state, and the alloy layer of the plating is very thick and has bad adhesion.

   

  b. The generated zinc slag is deposited at the interface between the zinc and lead solution and cannot be deposited at the bottom of the pan (because the specific gravity of the zinc slag is greater than that of the zinc solution and less than that of the lead solution) so that the steel plate is contaminated with the surface by passing through the zinc layer. Therefore, this method has been basically eliminated.

   

  (2) Single steel plate

   

  This method generally uses hot-rolled stacked plates as raw materials, and firstly sends the annealed steel plates into the pickling workshop to remove the iron oxide on the surface of the plates with sulfuric acid or hydrochloric acid. Immediately after pickling, the steel plate is immersed in a water tank to prevent re-oxidation. After pickling, water washing, squeezing, and drying, the steel is hot-dip galvanized in a zinc pot (the temperature is always maintained at 445-465°C), and then oiled and chromated. This method produces hot-dip galvanized sheets with significantly higher quality than wet galvanized finished products and is only of value for small-scale production.

   

  (3) Huilin method hot

   

  This continuous galvanizing line includes a series of pre-treatment processes such as alkaline degreasing, hydrochloric acid pickling, water rinsing, solvent coating, drying, etc. Moreover, the original plate needs to be covered with an overhead furnace before entering the galvanizing line. This annealing method is complex and costly, and the main reason is that the products produced by this method often have solvent defects that affect the corrosion resistance of the coating. In addition, the AL in the zinc pot often interacts with the solvent on the surface of the steel plate to produce aluminum trichloride, and the adhesion of the plating layer deteriorates. Therefore, although this method has been introduced nearly 30 years, it has not been developed in the world's hot-dip galvanizing industry.

   

  In-line annealing

   

  The cold-rolling or hot rolling shop directly provides the strip coil as the raw plate for hot-dip galvanizing, which is then annealed in the hot-dip galvanizing line under gas-protected recrystallization. Hot-dip galvanizing methods that fall under this category include the Senghimir method, the modified Senghimir method, the American Iron and Steel method (same as the Kawasaki method in Japan); the Sellars method; and the Sharon method.

   

  (1) Sengimil method

   

  It is the combination of the annealing process and hot-dip galvanizing process, its in-line annealing mainly includes an oxidation furnace, and reduction furnace two components. The strip is heated directly by a gas flame in the oxidation furnace to about 450 degrees to burn off the remaining rolling oil on the surface of the strip and purify the surface. After that, the strip is heated to 700-800 degrees to complete recrystallization annealing, and the temperature is controlled by the cooling section before entering the zinc pot at about 480 degrees, and finally entering the zinc pot for galvanizing without contacting the air. Therefore, the Sengiamir method has a high yield and better galvanizing quality, and this method had been widely used.

   

  (2) Meilian method

   

  It is a variation of the Sengiemir method, which simply uses an alkaline electrolytic degreasing tank to replace the decreasing role of the oxidation furnace, and the rest of the process is basically the same as the Sengiemir method. After the raw sheet enters the operating line, it is first electrolytically degreased, then washed and dried, then recrystallized and annealed in a reducing furnace with protective gas, and finally, hot-dip galvanized in a zinc pot under sealed conditions. In this method, the strip is not heated by the oxidizing furnace, so the oxide film on the surface is thin and the hydrogen content of the protective gas in the reducing furnace can be reduced appropriately. This is beneficial for furnace safety and lower production costs. However, since the strip is not preheated before entering the reduction furnace, this undoubtedly increases the heat load of the reduction furnace and affects the life of the furnace. Therefore, this method is not widely used.

   

  (3) Cyrus method

   

  Also known as the flame direct heating method; first of all, the strip is degreased by alkali washing, and then remove the surface of the oxide skin with hydrochloric acid, and after washing, drying, and then into the vertical line annealing furnace directly heated by the gas flame, through strict control of the proportion of gas and air flaming in the furnace, so that it is incomplete flaming in the case of excess gas and oxygen deficiency so that the furnace caused by the reduction atmosphere. The strip is heated rapidly to the recrystallization temperature and cooled in a low hydrogen protective atmosphere, and finally dipped into the zinc solution under closed conditions for hot-dip galvanizing. This method has compact equipment, 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 off the strip, it is necessary to use the method of furnace traverse away from the strip, which has many operational problems, so the hot-dip galvanizing industry uses this method rarely.

   

  (4) Sharon method

   

  In 1939, a new hot-dip galvanizing unit was put into operation by the American Sharon Company, so it was also called the Sharon method. This method is also known as the gas pickling method, as it involves injecting hydrogen chloride gas into the strip in the annealing furnace and bringing the strip up to the recrystallization temperature. This method not only removes the oxide from the strip surface but also removes the grease from the strip surface, which is corroded by the oxidizing gas and forms a pockmarked surface. However, the serious corrosion of the equipment, results in high maintenance and renewal costs of the equipment. Therefore, this method is rarely used.

   

  

   

  (5) Modified Senjimi

   

  It is a superior hot-dip galvanizing process; it connects the separate oxidizing and reducing furnaces of the Senjimir method by a small cross-sectional aisle, so that the entire annealing furnace, including the preheating furnace, reducing furnace, and cooling section, form an organic whole. The practice has proved that the method has many advantages: high quality, high yield, low consumption, safety, and other advantages have gradually been recognized. Its development is so fast that almost all new lines built since 1965 have adopted this method, and in recent years most of the old Senjimir units have been modified according to this method.

   

  Principle

   

  The hot-dip zinc layer is formed by zinc in a liquid state in three steps: the surface of the iron base is dissolved by the zinc solution to form a zinc and iron alloy phase layer; the zinc ions in the alloy layer further diffuse into the base to form a zinc and iron intercalation layer, and the alloy layer is wrapped with a zinc layer on the surface.

   

  Performance characteristics

   

  With a thick and dense pure zinc layer covering the surface of steel fasteners, it can avoid contact between the steel substrate and any corrosive solution, protecting the steel fastener substrate from corrosion. In the general atmosphere, the zinc layer surface forms a very thin and dense zinc oxide layer surface. It is very difficult to dissolve in water, so it plays a protective role for the steel fastener matrix. If zinc oxide is insoluble with other components in the atmosphere, it is ideal for corrosion prevention; it has a zinc-iron alloy layer, which is densely bonded and has unique corrosion resistance in marine salt spray atmosphere and industrial atmosphere; due to the strong bonding, zinc-iron is mutually soluble and has strong wear resistance; due to the good ductility of zinc, the alloy layer adheres to the steel substrate. The alloy layer is firmly attached to the steel substrate, so hot-dip galvanizing can be used for cold-punching, rolling, drawing, bending and other molding processes without damaging the coating; the hot-dip galvanizing of steel parts is equivalent to the annealing treatment, which can effectively improve the mechanical properties of the steel substrate and eliminate the stresses in the forming and welding of steel parts, and is conducive to the turning process of steel parts; the surface of fasteners after hot-dip galvanizing is bright and beautiful; the pure zinc layer is the most plastic layer of hot-dip galvanizing. The pure zinc layer is the most plastic layer of hot-dip zinc, and its nature is close to that of pure zinc, with good ductility.

   

  Application range

   

  This plating method is especially suitable for strong corrosive environments such as strong acids and alkaline mists.

   

  Electroplating zinc

   

  1、Principle

   

  Since zinc is not easy to change in dry air, but in humid air, the surface can generate a very dense zinc carbonate film, and this film can effectively protect the interior from corrosion. And when for some reason the plating is damaged and the lesser substrate is exposed, zinc forms a microcell with the steel substrate so that the fastener substrate becomes the cathode and is protected. In automotive transportation and other industries is widely used, but what is needed is a trivalent chromium passivation layer, zinc-nickel alloy plating closed coating, reduce hexavalent chromium passivation harmful, toxic layer.

   

  2、Performance characteristics

   

  The zinc plating layer is thicker, crystalline, meticulous, uniform and non-porous, with good corrosion resistance; the zinc layer obtained by electroplating is pure, slow corrosion in acid, alkali and other mists, can effectively protect the fastener substrate, galvanized layer by chromic acid passivation to form white, color, military green, etc., beautiful, with a certain degree of decorative, because the galvanized layer has good ductility, so it can be cold punching, rolling, bending and other kinds of molding without damage The galvanized layer has good ductility, so it can be cold-punched, rolled, bent, etc. without damaging the coating.

   

  3、Application range

   

  Electro-galvanization is involved in an increasingly wide range of fields, fastener products have been used throughout the machinery manufacturing, production of galvanized hook and loop fences, electronics, precision instruments, chemical industry, transportation, aerospace, etc. in the national economy has great significance.

   

  What are the processes?

   

  There are two main types of galvanizing processes: hot-dip galvanizing and cold-dip galvanizing. Cold galvanizing is also known as electro-galvanizing. Here we mainly discuss electro-galvanizing.

   

  There are many different types of electro-galvanizing. However, in terms of the pH value of the galvanizing solution, there are two main categories: alkaline galvanizing and acid galvanizing.

   

  Alkaline Galvanizing

   

  The alkaline galvanizing process means that the pH value of the plating solution is alkaline. However, depending on the complexing agent, there are two types of galvanizing: (CN)2 chloride galvanizing and zincate galvanizing. (CN)2 chloride galvanizing is a very old type of galvanizing. There are three main components in the plating solution: the main salt zinc oxide, the complexing agent sodium (CN)2ide, and the conductive salt sodium hydroxide (commonly known as kerosene). In the early days, there was no brightening agent in the (CN)2 chemistry plating solution. As people's aesthetic requirements increased, a brightening agent was added to the (CN)2 solution for galvanizing. (CN)2 galvanizing process is stable and the plating layer is detailed. The plating solution has a good dispersion ability. Depending on the amount of sodium (CN)2, there are three types of galvanizing: high (CN)2, medium (CN)2, and low (CN)2. （The biggest disadvantage of (CN)2 galvanizing is that it is too toxic and harmful to the environment.

   

  Zincate galvanizing is a fast-growing galvanizing process in the last 30 years. Its main components are zinc oxide, the main salt, and sodium hydroxide, a complexing agent and conductive salt (commonly known as kerosene). A brightening agent is added in order to obtain a fine, well-dispersed bright coating. The main period of development of domestic zincate galvanizing was the 1970s when there was a great deal of (CN)2-free galvanizing, and the famous DPE galvanizing process and DE galvanizing process have been used until now. This process is not as stable and detailed as the (CN)2 galvanizing process. The biggest advantage is that it is (CN)2-free and much less harmful to the environment. Nowadays, zincate galvanizing has been developed to overcome the defects such as blistering and brittleness, and the dispersion ability has been greatly improved, which is comparable to (CN)2 galvanizing.

   

  Acid galvanizing

   

  The second result of the big (CN)2-free galvanizing in the 1970s was the (CN)2-free ammonium chloride galvanizing. It goes hand in hand with zincate alkaline galvanizing. It uses zinc chloride as the main salt and ammonium chloride as the complexing agent and conducting salt. Citric acid and NH3 tri acetic acid are used as auxiliary complexing agents. Polyethylene glycol and thiourea are used as plating refiners. The plating obtained from this solution is fine and well dispersed. The plating solution is less toxic. However, the main disadvantage is that the plating solution is unstable and the ammonium chloride gas emitted is very corrosive to the plating equipment. Current efficiency is also low. High sensitivity to impurities. Narrow operating temperature range. It has been largely eliminated by the potassium chloride galvanizing process.

   

  Potassium Chloride (or Sodium Chloride) Galvanizing Process

   

  The potassium chloride (or sodium chloride) galvanizing process is a new (CN)2-free galvanizing process developed in the 1980s. Its main components are zinc chloride as the main salt, generally at a content of 70-90 g/L. Potassium chloride is used as a conductive salt, at a content ranging from 140-280 g/L, which can be adjusted according to different needs. Boric acid is used as a pH buffer to stabilize the pH value between 4.6 and 5.6. Since this plating solution works at room temperature, the solubility of boric acid is not high and the content is generally controlled at 25-30g/L.

   

  These three components alone will not give a good zinc plating. Some additives must be added to obtain a bright, detailed zinc coating. The advantages of potassium chloride galvanizing are stable plating solution, bright and detailed coating, low cost, high current efficiency, and non-toxicity. The disadvantage is that the dispersion of the solution is slightly worse than that of alkaline galvanizing, and the brittleness of the coating is greater. Nevertheless, the introduction of potassium chloride galvanizing has been universally welcomed. It has developed very rapidly and has taken over half of the galvanizing industry. Currently, there are two main galvanizing processes: alkaline galvanizing and acid galvanizing with potassium chloride. All other galvanizing processes have been dwarfed and are slowly receding from the stage of history.

   

  Among the acid galvanizing processes is the sulfate galvanizing process. Its main component is zinc sulfate, the main salt. The content is between 250-500g/L. Alum or aluminum sulfate is used as the conductive salt. The content is between 30-50g/L. Conductive salt is also added with sodium sulfate or sodium chloride. In addition to the above, some additives are added to make the plating detailed. Dextrin or gum was first used, but later some special brightening agents were invented to make the coating brighter and more detailed. The biggest advantage of sulfate galvanizing is that it can be used at higher current densities (1-5A/dm2) and at faster plating speeds. However, the disadvantage is that the dispersion ability of the plating solution is very poor and it is not suitable for plating more complex workpieces.