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The main reasons for the corrosion of galvanized sheet during use

Galvanized sheet refers to a steel sheet with a layer of zinc plated on the surface. Galvanized sheet can be divided into ordinary spangle, fine spangle, flat spangle, no spangle and phosphating treatment according to the surface state. Galvanized sheet is divided into two types according to packaging: cut-to-length galvanized sheet and coiled galvanized sheet. Galvanized sheets are generally packaged with iron sheets, and the galvanized sheets should be bundled firmly to prevent mutual friction between the galvanized sheets. HUNGTHAI METAL has many years of production experience, providing customers with high-quality galvanized steel coils, color-coated steel coils.

The main reasons for the corrosion of galvanized sheet in use are mainly reflected in the following three aspects:

  1. During the entire service life of the galvanized sheet, the first corrosion that occurs is the oxidation of the surface galvanized layer, resulting in white rust. After a long time, the white rust on the surface further reacts with impurity gases such as carbon dioxide in the humid air to form black spots. When the galvanized sheet is used for a long time and the corrosion of the galvanized layer is serious, the steel base loses the sacrificial anti-corrosion effect of zinc, and begins to oxidize, resulting in red rust. Once the steel base begins to oxidize, the corrosion rate becomes rapid and the galvanized sheet ends its life.
  2. In addition, there are two special corrosion conditions of galvanized sheet called blackening. One is that after the galvanized sheet has been used for a period of time, due to the influence of elements such as lead in the galvanized layer, the dull part of the zinc flower will turn black, and the blackened galvanized layer still has a protective effect on the steel base. It just has a certain effect on lifespan.
  3. The corrosion resistance of galvanized sheet in dry environment or pollution-free environment is very good. However, in a polluted environment, the service life will be greatly shortened, and it must be pre-coated to form a color plate before use.
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Analysis of the Causes of Leakage Defects in Galvanized Steel Sheets

Galvanized steel has excellent corrosion resistance and bright appearance, and is widely used in construction, home appliances, automobiles and other industries. Leakage plating is a common surface defect of hot-dip galvanizing, which seriously affects the corrosion resistance and appearance of galvanized sheets, and even affects the formability of galvanized sheets.

When the surface of the galvanized steel sheet is not completely exposed to the base metal or the dipping flux is insufficient, leakage plating will occur during dipping. The reasons for this quality defect are as follows:

  1. Leakage plating caused by raw materials and influencing factors in the manufacturing process. The influencing factors are:

Excessive oil or rust (often localized corrosion); paint marks; weld skin at the weld; defects in the steel; scars, slag inclusions;

  1. The pretreatment process is not perfect and the leakage plating caused by improper operation.

The workpiece clamping is unreasonable, there is overlap, the phenomenon of air pockets and the lack of activity in pickling;

Under-pickling caused by acid aging and short pickling time;

Poor degreasing, there are individual dirt;

The composition of the fluxing solution is reduced, resulting in insufficient fluxing effect.

  1. If it stays in the air for too long before drying, new corrosion products are formed on the surface of the workpiece.
  2. The drying temperature is too high, causing the local flux layer to burn.
  3. There is a problem with the composition of the zinc solution. For example, if the aluminum content is too high (>0.05%), it is easy to cause leakage plating.
  4. The immersion speed is too slow or pauses in the middle, and the flux layer is burned or the complex workpiece has pockets. During the immersion plating, individual parts of the workpiece cannot be contacted with the zinc solution and leak plating.

In short, according to the reasons for the defects of galvanized steel sheet, targeted measures are taken, and the problem of leakage plating can be solved. In addition to taking corresponding countermeasures for the above reasons, the most important thing is to set up an inspection process for pretreatment. That is, after taking it out of the flux tank, carefully inspect the surface of the galvanized steel sheet, find out the individual parts that are not up to the standard in the pretreatment in time, and take measures to eliminate them.

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Galvanized sheet classification and common post-treatment process

HUNGTHAI METAL has many years of experience in producing steel coils and is a professional manufacturer dedicated to providing customers with professional steel solutions.

This article will introduce the classification of galvanized sheets and the commonly used post-treatment processes:

  1. Classification by manufacturing and processing

(1) Electro-galvanized sheet: The electro-galvanized sheet is produced by electrifying the steel sheet through an electrolyte with zinc ions, so that the zinc ions obtain electrons, which are converted into zinc and deposited on the surface of the steel sheet.

(2) Hot-dip galvanized sheet: A galvanized sheet produced by hot-dip galvanizing by infiltrating cold-rolled or hot-rolled steel strips into molten zinc liquid in a continuous hot-dip galvanizing production line.

  1. Classification by coating form

1) Hot-dip pure cold-rolled steel sheet: The zinc content in the molten zinc solution exceeds 99%.

2) Hot-dip galvanized iron alloy sheet: The zinc content in the molten zinc solution exceeds 99%. A zinc-iron alloy layer is produced on the coating through an alloy strengthening treatment process, and the iron content in the alloy coating is generally between 7 and 15%.

3) Hot-dip galvanized sheet: the content of aluminum in the molten zinc solution is about 55%, the content of silicon is about 1.6%, and the other components are zinc.

  1. According to the form of post-treatment, according to the form of post-treatment, the galvanized sheet is divided into passivation, oiling, passivation oiling, paint sealing, phosphating or no treatment.

(1) Passivated galvanized sheet. Passivation galvanized sheet is to reduce white rust under humid conditions, and a passivation protective film is further applied on the surface of the product after galvanizing. According to the type of passivation film, it is divided into hexavalent chromium passivation, trivalent chromium passivation, chromium-free passivation and other products, which are generally not used for zinc-iron alloy plates.

(2) Oil coated galvanized sheet. The purpose of oiling is also to prevent the product from getting wet and causing white rust. But since anti-rust oil is extremely volatile or wiped off, it only works before it is used. Oiling methods include electrostatic oiling and roller oiling. If you want to prevent the product from producing white rust during its use period, it is best to use passivation and oiling. The color-coated substrate is more suitable for light oiling treatment, and the applied oil can be cleaned and removed in the pre-treatment of the color-coated line.

(3) Paint-sealed galvanized sheet. Paint seals are commonly known as fingerprint-resistant products. In order to prevent galvanized products from leaving fingerprints during processing, a very thin anti-fingerprint protective film is applied to the surface of galvanized products, which can also improve the corrosion resistance and performance of products. The coating properties of the product. Suitable for special needs of home appliances.

(4) Phosphating galvanized sheet. Due to the poor bonding force between the galvanized sheet and the organic coating, the coating requirements cannot be met. The phosphating treatment of the galvanized sheet before coating can effectively improve its corrosion resistance and enhance its bonding force with the organic coating. .

  1. Post-treatment process combination and application
    The surface treatment of galvanized sheet is also in order, and different surface treatment processes have different uses.
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THE Use, processing and installation of pre-painted steel coils

For high-quality HUNGTHAI METAL color-coated steel coils, it is very important to be familiar with its use and installation methods.

  • The mechanical properties (such as yield strength, elongation) and coating properties (such as pencil hardness, T-bend value, impact energy) of color-coated sheets usually change with the increase of storage time, which affects the processing and forming, so it is generally recommended Process and use within 1 year.
  • In order to avoid excessive deformation in one forming and destroying the adhesion of the coating, parts with complex shapes should be formed in multiple passes to reduce the amount of deformation in a single pass. Reasonable processing speed and appropriate clearance should be set according to equipment conditions, process conditions, part shape and other factors, and the thickness of the coating should be considered when setting the clearance; if the temperature is too low, the coating flexibility of the color-coated sheet will be affected, and low-temperature processing should be avoided. .
  • Most coatings can be used as lubricants to meet the lubrication requirements of most forming processes. If the lubricity of the coatings is insufficient, the lubricity can be improved by applying oil, waxing, and covering with a peelable protective film. However, it should be noted that wet lubricants are easy to absorb dirt and should be removed before installation. The peelable protective film should also be removed as soon as possible after processing.
  • Pay attention to ensure the cleanliness and integrity of the processing equipment. In order to avoid damage to the coating, the cutting metal particles generated during processing should be cleaned in time; the wear on the surface of the forming roll and the mold should be reduced; the scissors for shearing should be sharp, and the cutting edge should be sharp There must be no burrs.
  • Machining fractures and incision surfaces are prone to corrosion, so burrs should be minimized when cutting or punching, and incisions, openings, etc. should be designed in hidden places as much as possible during processing, or protective coatings, caulking strips, etc. are used to process the edges. Take protective measures.
  • Check the color and surface quality of the color-coated coils before installation. If the coating of the color-coated board has been damaged in the early processing, please use it with caution. During the installation process, it is necessary to avoid dragging, scraping, scratching and scalding the coating to prevent bending and denting. When walking on color-coated parts, you should wear shoes with smooth and soft soles, and there should be no small stones, metal chips and other debris on the soles. The parts are dented and deformed.
  • When using fasteners such as bolts and rivets to connect the color-coated parts, it should be ensured that the corrosion resistance of the fasteners matches that of the color-coated panels, so as to avoid premature corrosion of the fasteners and become a source of accelerated corrosion; Considering the potential difference between the fastener material and the color-coated plate to avoid electrochemical corrosion, it is recommended to use galvanized or aluminum fasteners. Carefully select and use installation tools to avoid scratching the color-coated panels. Reasonably design the tightening position, and the tightening degree should be moderate when the bolts are fixed, so as to avoid too loose or too tight, resulting in the deformation of the color-coated plate.
  • When using chemical products such as adhesives and sealants to process color-coated boards, pay attention to the acidity and alkalinity of chemical products. Neutral chemical products should be used to avoid acid and alkali substances corroding the coating and affecting the use of color-coated boards. life.
  • Pay attention to the installation angle and position of the color-coated board parts. Long-term water retention will reduce the adhesion of the coating. Therefore, the design and installation should ensure that the water can be drained in time to avoid water accumulation. Copper, lead, graphite and unprotected steel plates shall not be used in direct contact with color-coated boards, nor shall logs, untreated wood, wet cement, etc. be used in direct contact with color-coated boards. Or the coating reacts and reduces the service life of the color-coated board.
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What are the environmental factors that affect color-coated steel coils?

Temperature.

At high temperature, the coating is easy to soften, and the corrosive medium is easy to adhere. It is easy to penetrate into the substrate, the oxygen content of the water will increase at high temperature, and the corrosion rate will be accelerated at a certain temperature.

Humidity.

The corrosion of the substrate at the incision and processing damage of the color-coated plate belongs to electrochemical corrosion, and it is not easy to form a corrosion battery (ie, an electrochemical circuit) when the humidity is low.

The temperature difference between day and night.

A large temperature difference is easy to condense, forming galvanic corrosion conditions on the exposed metal. In addition, a large temperature difference also leads to frequent hot and cold deformation of the coating, which will accelerate the aging of the coating and loosen, and the external corrosive medium will easily penetrate into the substrate.

Sunshine time and sunshine intensity.

Orientation and slope affect the duration of sunlight, which in turn affects the durability of the coating. The slope also affects the settling time of corrosive media or dust on the steel plate. Sunlight is an electromagnetic wave, which is divided into gamma rays, X rays, ultraviolet rays, visible light, infrared rays, microwaves and radio waves according to the level of energy and frequency. Waves and radio waves have low energy and do not interact with matter. Infrared is also a low-energy spectrum, and it can only stretch or bend the chemical bonds of substances, not break them. Visible light gives everything a rich color. UV-ultraviolet spectrum is a high-frequency ray, which has greater destructive power than low-energy spectrum. Skin dark spots and skin cancer as we know are caused by the sun’s ultraviolet rays. Likewise, UV can break the chemical bonds of substances, breaking them. It depends on the wavelength of the UV and the chemical bond strength of the substance. X-rays have a penetrating effect, and gamma rays can break chemical bonds and generate free charged ions, which are deadly to organic matter. Fortunately, these rays are rare in sunlight. Therefore, it can be seen from the above that the duration and intensity of sunlight affects the structural stability of the coating, especially in areas with strong UV rays.

Rainfall and rainfall acidity.

Rainfall acidity is undoubtedly detrimental to corrosion resistance. However, rainfall has a dual effect. For wall panels and roof panels with large slopes, rainfall has the effect of cleaning the surface of steel plates and washing away surface corrosion products, but for low-slope roof panels and parts with poor drainage, heavy rainfall It is easy to cause corrosion aggravation.

Wind direction and wind speed.

The effect of wind direction and speed is similar to that of water, and it is often accompanied by it. It is a test for the connection parts of materials, because the wind will cause the connection parts to loosen and rainwater will penetrate into the interior of the building.

Corrosive sediment.

Such as chloride ions, sulfur dioxide, etc., all have an accelerated effect on corrosion, and these sediments mostly occur in the seaside and areas with serious industrial pollution (such as power plants, smelters, etc.).

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Maintenance of color-coated steel plates

The maintenance of color-coated steel plates is a very important part. HUNGTHAI METAL, which provides high-quality steel, has a complete quality supervision system and provides perfect after-sales service, which can eliminate customers’ worries.

1. Metal shavings, leftovers, unused fasteners, etc. generated during the installation of the color-coated board should be cleaned up in time after the installation work to avoid being left on the color-coated board as a source of corrosion. If damage to the coating is found during installation (not yet rusted), repair paint maintenance should be carried out in time, and attention should be paid to the matching of paint to prevent accelerated corrosion due to coating damage during subsequent use (if conditions permit, it is recommended not to use the coating that has already been used damaged pre-painted parts).

2. In the subsequent use of the color-coated board, regular inspection and maintenance are also required. Repainting can be carried out for the aging and damaged parts of the coating. Before repainting, use a neutral cleaner to remove the dirt on the surface, then sand to remove the rust layer on the surface, then apply the primer in a clean and dry state, and finally apply the topcoat. Before the official primer is applied, the dirt, polished iron powder and rust spots must be cleaned up. It is not recommended to use spray paint to repair large areas.

3. Regularly clean the dust, residues and accumulated water on the surface of the color-coated board. It is recommended that the cleaning cycle be once every six months. circumstances increase the frequency of cleaning. Usually, it can be rinsed with clean water. If there are difficult-to-clean stains, you can use a mild soap solution or neutral detergent solution with a soft cloth, sponge, soft brush, etc. to clean, and immediately use clean water after the stain is removed. wash. When using detergents, pay attention to the concentration and method of use. Follow the instructions for use of detergents. Do not mix multiple detergents and bleaches without permission. Improper use of detergents will damage the color-coated boards. Seriously reduce the service life of the color-coated board. When using the detergent, you can test it on a small area first, and then use it on a large area after confirming that there is no damage.

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Anti-fingerprint treatment of galvanized steel coil

Fingerprint-resistant sheet is a composite coated sheet obtained by fingerprint-resistant treatment on the surface of galvanized sheet. Fingerprint-resistant steel plate is the first material to pass environmental protection certification. It is named “fingerprint-resistant” because the surface will not leave fingerprints when touched with clean fingers. The research and production of fingerprint-resistant plates originated in Japan and entered commercial production in the early 1980s. The original purpose of anti-fingerprint treatment is to meet the requirements of users in the field of home appliances. In the production process of home appliances, many parts are touched many times by workers due to the needs of the process, and the sweat stains on the hands of the workers will pollute the surface of the parts and affect the appearance. Therefore, a fingerprint-resistant board has been researched and developed.

Features

The initial anti-fingerprint treatment is to form an inorganic series film, generally a silicate film, on the zinc-plated chromate passivation film, thereby making the surface of the steel sheet resistant to fingerprints. Users have put forward higher requirements for coating products. In addition to fingerprint resistance, they also require grounding, higher fingerprint resistance and coating performance. As a result, a coated sheet in which a thin-film organic composite film is formed on a chromate film on a galvanized steel sheet has been developed. The properties of organic and inorganic fingerprint-resistant steel plates are different. The corrosion resistance and grounding properties of organic fingerprint-resistant steel plates are better than those of inorganic fingerprint-resistant steel plates, but the fingerprint resistance index is worse. However, the corrosion resistance of these two fingerprint-resistant steel plates can meet the requirements of the home appliance industry.

Type

One is that after the galvanized sheet is passivated, an organic coating (fingerprint-resistant film) is applied on the passivation film, and the coating method is mainly roller coating.

The other is to add organic resin and colloidal silica to the passivation solution of galvanizing. After the galvanized strip passes through the passivation tank, a composite coating with corrosion resistance and fingerprint resistance is formed on its surface. Coating methods mainly include electrolytic coating and roller coating. At present, the first process is generally used to produce fingerprint-resistant boards at home and abroad.

The traditional anti-fingerprint board basically uses electro-galvanized sheet as the substrate. With the continuous improvement and development of hot-dip galvanizing technology and process, the surface quality of hot-dip galvanized sheet has reached a level comparable to that of electro-galvanized sheet, while hot-dip galvanized sheet is cheap and has better corrosion resistance. The use of hot-dip galvanized sheet as a fingerprint-resistant substrate is gradually becoming the mainstream. In addition, in 2007, organic composite coated boards with various properties (such as fingerprint resistance, electrical conductivity, self-lubrication, paintability, etc.), which can meet various requirements at the same time, are developing rapidly.

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Development Direction of Color Coated Steel Plate Production

In recent years, the production of color-coated steel plates is still developing, and the production and application of color-coated steel plates in Europe are increasing year by year. From the overall development trend, the development process of color-coated steel plate production is continuously rising. At present, although the production capacity of color-coated steel sheets in the world has been greatly improved, it still has development potential.

The main application market of color-coated steel sheets is the construction industry. In addition to the application of industrial buildings, developing countries provide a huge market for color-coated steel sheets.

The products produced by HUNTHAI METAL are suitable for industrial and civil construction purposes. The main building materials market for color-coated steel sheets is newly decoupled buildings. The market is currently dominated by high-rise and industrial buildings, where large buildings require high-quality, high-performance products. The transportation industry and the home appliance industry are the largest markets other than the construction industry, which can account for 15%-20% of the market.

Color-coated steel plates are mainly divided into three types: coated plates, plastic-coated plates, and printed plates. Coated steel plates are mainly used in the construction industry, while plastic-coated plates and printed plates are mainly used in the home appliance industry.

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Factors Affecting the Printing Quality of Color Coated Steel Coils Part 2

  1. Design and matching of curing and drying curves of each layer of paint
    There are special requirements for the curing process matching between each layer. When setting the oven temperature and determining the final PMT, the adhesion of the ink and the primer, the non-adhesion of the ink and the varnish, and the curing requirements of the high gloss varnish should be considered.
  2. Precise installation of knife device and equipment adjustment
    First of all, the installation and adjustment of the device should be completed after the installation of the printing roller is completed, and the surface of the printing roller should be carefully checked to ensure that there is no serious damage. When adjusting, pay attention to the surface protection of the printing roller to minimize the wear of the knife. Carefully check the cleanliness of the reinforcement device to ensure that there are no iron filings, ink, dust and other debris.
  3. The choice of each roller speed
    The formation process of the embossed pattern requires the same speed rotation between the embossing roller and the transfer roller, and between the transfer roller and the strip. If the speed is purely different, the pattern of the pattern will be messy, and ghosting and stains will easily appear. This results in an unsightly pattern on the belt.
  4. Roll gap and pressure adjustment
    Minimize roll stitching pressure as much as possible, otherwise the print production may fail.
  5. Process adjustment of the knife
    To adjust the printing effect through the knife and the printing roller, it is necessary to slowly adjust the related process parameters of the knife and each roller at low speed; when the knife is in contact with the printing roller, the printing roller must not run without ink, otherwise it will cause excessive wear of the printing roller, It will also cause possible damage to the Doctor’s knife. At the same time, when contacting the printing roller, the pressure between the printing roller and the knife should be as small as possible.
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Factors Affecting the Printing Quality of Color Coated Steel Coils Part 1

HUNGTHAI METAL’s color-coated steel coil products have the characteristics of strong pattern texture, good quality, rich layers and bright colors, which can meet the production requirements of various patterns, and have multiple production lines.

  1. The quality of the printing roller
    The printing roller must have a suitable elastic coefficient, good inking performance, strong solvent resistance, suitable hardness and uniform hardness change in all directions, roundness and cylindricity must be accurate and the amount of deformation should be as small as possible.
  2. Selection of printing substrate
    Since the printing process adopts the transfer method, the requirements for the transfer roller and the printing substrate are higher. In the production process, a key item is the pressure adjustment of the transfer roller and the substrate. At the same time, the surface roughness and flatness of the substrate are required to be high. If the roughness is beaten or the thickness is uneven, the thickness of the film will be uneven, resulting in color difference.
  3. The choice of paint
    The type selection and color matching of each layer of paint is very important. Since the printing layer of the printed product is ink, in addition to the conventional requirements for each layer of paint, the printed product also requires high ink particle fineness and dispersion, good impact resistance, strong adhesion, heat resistance, light resistance, fluidity, Excellent drying, solvent resistance and processing resistance; at the same time, the selection of varnish is also strict. Because the quality of the paint will affect the surface quality and performance of the overall print.
  4. Coating film thickness selection control and viscosity selection
    Appropriately increase the ink viscosity, and on this basis, the dry film thickness should be reasonably controlled by the wet film thickness. In addition, pay attention to the matching relationship between the film thickness and color of each layer.
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