Zinc Coating
Zinc metal has a number of characteristics that make it well suited for use as a coating for protecting iron and steel products from corrosion. Its excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions. The excellent field performance of zinc coatings results from their ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials, some 10 to 100 times slower, depending upon the environment. While a fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, greatly reducing the rate of further corrosion. Click the link above to view a chart depicting the expected service life to first maintenance (5% red rust) of iron and steel based on the zinc coating thickness and the environment.
Zinc metal has a number of characteristics that make it well suited for use as a coating for protecting iron and steel products from corrosion. Its excellent corrosion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions. The excellent field performance of zinc coatings results from their ability to form dense, adherent corrosion product films and a rate of corrosion considerably below that of ferrous materials, some 10 to 100 times slower, depending upon the environment. While a fresh zinc surface is quite reactive when exposed to the atmosphere, a thin film of corrosion products develops rapidly, greatly reducing the rate of further corrosion. Click the link above to view a chart depicting the expected service life to first maintenance (5% red rust) of iron and steel based on the zinc coating thickness and the environment.
A number of different types of methods of applying zinc coatings to steel are commercially available, each of which has unique characteristics. The products produced by each of these processes have different uses depending on their applicability, relative economics and expected service life. To find out more about the various zinc coatings click one of the topics below.
In-line, Continuous (sheet) Galvanizing
Electrogalvanizing
Zinc Plating
Metallizing
Mechanical Plating
Painting
Selection of Zinc Coatings
Electrogalvanizing
Zinc Plating
Metallizing
Mechanical Plating
Painting
Selection of Zinc Coatings
High Temperature Galvanizing
Production Process
High-temperature galvanizing is very similar to batch hot-dip galvanizing at a conventional temperature (830ºF). The steel is run through a batch process beginning with multiple chemical pretreatment tanks used to clean residues from the steel. The pretreatment of the steel begins with a degreasing to remove dirt and oils from the steel. This stage is then followed by an immersion in an acid pickling tank that removes oxides and mill scale from the surface of the steel. After the steel has been adequately cleaned, it is submerged in a tank containing a flux solution, which provides protection against oxidation prior to entering the galvanizing kettle.
The difference between this process and the normal hot-dip galvanizing process exists in the galvanizing kettle. Not only is it run at higher temperatures, typically 1020 to 1040ºF (550 to 560ºC), but also the vessel holding the zinc is usually much smaller than the conventional process. Due to the high temperatures that the holding vessel is operated at, it must also be constructed of ceramics. Due to the poor heat conductivity of the ceramic, the vessel is normally top-heated using fossil fuel burners, electric radiant elements, induction heating, or immersion burners.
The high temperature galvanizing process is commonly used for small hardware items such as nuts and bolts. The coatings produced are extremely uniform and allow for easy assembly without zinc buildup on the threads. Another advantage of the high temperature process is the ability to control coating thickness on silicon-rich or reactive steels. The coatings produced on these steels are thinner and more tightly adherent than coatings produced in the conventional galvanizing process. Size limitations on steel that can be galvanized at high temperatures exist due to the smaller size of the galvanizing kettle.
Coating Characteristics
The coating produced by high temperature galvanizing is very similar to that of the conventional hot-dip galvanizing process, but with the absence of the Eta layer. The coatings produced are typically duller than conventional galvanized coatings because the reaction between iron and zinc goes to completion, mostly forming a coating of intermetallic iron-zinc alloy. The thickness of the coating is highly dependent on the temperature. Coatings produced at temperatures of approximately 1030ºF (555ºC) are typically 4 mils thick, depending on the immersion time in the galvanizing bath. The corrosion performance of this coating is similar to the coatings produced by conventional galvanizing and is strictly dependent on the coating thickness. To view a service-life chart on the performance of zinc coatings, click here.
In-line, Continuous (Sheet) Galvanizing
Production Process
The continuous sheet galvanizing process is also a hot-dip process. Steel sheet, strip or wire is cleaned, pickled, and de-oxidized on a processing line 500 feet in length, running at speeds of over 300 feet per minute. In the coating of sheet or strip, the zinc bath contains larger amounts of aluminum than used in conventional hot-dip galvanizing (0.15 to 0.25%). The aluminum (Al) suppresses the formation of the zinc-iron alloys, resulting in a coating that is mostly pure zinc. In-line heat treatment can be used to produce a fully alloyed (Fe-Zn) coating, called galvannealed steel.
Sheet products continuously coated with zinc-aluminum alloys are also commercially available. Two alloy compositions currently in use are 55% Al-43.6% Zn-1.4% Si and a 95% Zn-5% Al-trace mischmetal (cerium, lanthanum). These coatings are used to enhance the product life for certain applications.
Coating Characteristics
After galvanizing, the continuous zinc coating is physically wiped using air knives to produce a uniform coating across the width of the strip. A variety of coating weights and types is available, ranging up to just under 2 mils (50 µm) per side. One of the most common coatings is Class G90, which has 0.9 oz./ft2 of sheet (total both sides) or about 0.75 mils (18 µm) thickness per side.
Continuously galvanized sheet steels are used to make cars, appliances, corrugated roofing and siding, and culvert pipe. The coated product can be suitably treated for painting for aesthetics or to increase service life. Because of the thin coating, this product normally is used for interior applications or where exposure to corrosive elements is mild.
Electrogalvanizing
Production Process
Electrogalvanized coatings are applied to steel sheet and strip by electro-deposition. Electrogalvanizing is a continuous operation where the steel sheet is fed through suitable entry equipment, followed by a series of washes and rinses, into the zinc plating bath.
The most common zinc electrolyte-anode arrangement uses lead-silver, or other insoluble anodes, and electrolytes of zinc sulfates. Soluble anodes of pure zinc are also used. In this process, the steel sheet is the cathode. The coating is developed as zinc ions in the solution are electrically reduced to zinc metal and deposited at the cathode. Grain refiners may be added to help produce a smooth, tight-knit surface on the steel.
Coating Characteristics
The electrodeposited zinc coating consists of pure zinc tightly adherent to the steel substrate. The coating is highly ductile and the coating remains intact even on severe deformation. The coating is produced on strip and sheet materials to coating weights up to 0.2 oz/ft2 (60 g/m2), or thickness of up to 0.14 mils (3.6 µm) per side. On wire, coating weights may range up to 3 oz/ft2 (915 g/m2). Heat-treated and electro-coated wire can be cold drawn to about 95% reduction in area, depending on the chemical composition of the wire, heat treatment, and diameter.
The electrogalvanized coating is paintable with suitable treatment, and the sheet product is used in automobile and appliance bodies. Due to the extremely thin zinc coating on the sheet, painting or other topcoating is recommended to improve the service life.
Zinc Plating
Production Process
Zinc plating is identical to electrogalvanizing in principle in that both are electrodeposition processes. Zinc plating is used for coatings deposited on small parts such as fasteners, crank handles, springs, and other hardware items. The zinc is supplied as an expendable electrode in a cyanide, alkaline non-cyanide, or acid chloride salt solution. Cyanide baths are the most operationally efficient, but they potentially create a pollution and hazardous material problem.
After alkaline or electrolytic cleaning, pickling to remove surface oxides, and rinsing, the parts are loaded into a barrel, rack, or drum and immersed in the plating solution. Various brightening agents may be added to the solution to add luster, but careful control of the bath and brightener is needed to ensure a quality product. Post-plating treatments may be used to passivate the zinc surface and at the same time impart various translucent colors to the coating. These post-plating treatments may be used to provide a desired color or to extend the life of the plated coating.
Coating Characteristics
The normal zinc-plated coating is dull gray in color with a matte finish, although whiter, more lustrous coatings can be produced, depending on the process or agents added to the plating bath or through post-treatments. The coating is thin, ranging up to 1 mil (25 µm), restricting zinc-plated parts to very mild (indoor) exposures. ASTM Specification B 633 lists four classes of zinc plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25. The number indicates the coating thickness in microns. The coating finds application in screws and other light fasteners, light switch plates and other small parts. Materials for use in moderate or severe applications must be chromate conversion coated.
The coating is entirely pure zinc, which has a hardness about one-third to one-half that of most steels.
Metallizing
Production Process
The sprayed zinc coating is rough and slightly porous, with a specific gravity of 6.4, compared to zinc metal at 7.1. Zinc corrosion products tend to fill the pores as the zinc corrodes in the atmosphere. The coating adherence mechanism is mostly mechanical, depending on the kinetic energy of the sprayed particles of zinc. No zinc-iron alloy layers are present.
Coating Characteristics
The coating can be applied in the shop or field; it gives good coverage of welds, seams, ends and rivets, and can be used to produce coatings in excess of 10 mils (250 µm). Coating consistency is dependent on operator experience and coating variation is always a possibility. Coatings may be thinner on corners or edges and the process is not suitable for coating recesses and cavities.
http://www.galvanizingcost.com :مرجعProduction Process
High-temperature galvanizing is very similar to batch hot-dip galvanizing at a conventional temperature (830ºF). The steel is run through a batch process beginning with multiple chemical pretreatment tanks used to clean residues from the steel. The pretreatment of the steel begins with a degreasing to remove dirt and oils from the steel. This stage is then followed by an immersion in an acid pickling tank that removes oxides and mill scale from the surface of the steel. After the steel has been adequately cleaned, it is submerged in a tank containing a flux solution, which provides protection against oxidation prior to entering the galvanizing kettle.
The difference between this process and the normal hot-dip galvanizing process exists in the galvanizing kettle. Not only is it run at higher temperatures, typically 1020 to 1040ºF (550 to 560ºC), but also the vessel holding the zinc is usually much smaller than the conventional process. Due to the high temperatures that the holding vessel is operated at, it must also be constructed of ceramics. Due to the poor heat conductivity of the ceramic, the vessel is normally top-heated using fossil fuel burners, electric radiant elements, induction heating, or immersion burners.
The high temperature galvanizing process is commonly used for small hardware items such as nuts and bolts. The coatings produced are extremely uniform and allow for easy assembly without zinc buildup on the threads. Another advantage of the high temperature process is the ability to control coating thickness on silicon-rich or reactive steels. The coatings produced on these steels are thinner and more tightly adherent than coatings produced in the conventional galvanizing process. Size limitations on steel that can be galvanized at high temperatures exist due to the smaller size of the galvanizing kettle.
Coating Characteristics
The coating produced by high temperature galvanizing is very similar to that of the conventional hot-dip galvanizing process, but with the absence of the Eta layer. The coatings produced are typically duller than conventional galvanized coatings because the reaction between iron and zinc goes to completion, mostly forming a coating of intermetallic iron-zinc alloy. The thickness of the coating is highly dependent on the temperature. Coatings produced at temperatures of approximately 1030ºF (555ºC) are typically 4 mils thick, depending on the immersion time in the galvanizing bath. The corrosion performance of this coating is similar to the coatings produced by conventional galvanizing and is strictly dependent on the coating thickness. To view a service-life chart on the performance of zinc coatings, click here.
In-line, Continuous (Sheet) Galvanizing
Production Process
The continuous sheet galvanizing process is also a hot-dip process. Steel sheet, strip or wire is cleaned, pickled, and de-oxidized on a processing line 500 feet in length, running at speeds of over 300 feet per minute. In the coating of sheet or strip, the zinc bath contains larger amounts of aluminum than used in conventional hot-dip galvanizing (0.15 to 0.25%). The aluminum (Al) suppresses the formation of the zinc-iron alloys, resulting in a coating that is mostly pure zinc. In-line heat treatment can be used to produce a fully alloyed (Fe-Zn) coating, called galvannealed steel.
Sheet products continuously coated with zinc-aluminum alloys are also commercially available. Two alloy compositions currently in use are 55% Al-43.6% Zn-1.4% Si and a 95% Zn-5% Al-trace mischmetal (cerium, lanthanum). These coatings are used to enhance the product life for certain applications.
Coating Characteristics
After galvanizing, the continuous zinc coating is physically wiped using air knives to produce a uniform coating across the width of the strip. A variety of coating weights and types is available, ranging up to just under 2 mils (50 µm) per side. One of the most common coatings is Class G90, which has 0.9 oz./ft2 of sheet (total both sides) or about 0.75 mils (18 µm) thickness per side.
Continuously galvanized sheet steels are used to make cars, appliances, corrugated roofing and siding, and culvert pipe. The coated product can be suitably treated for painting for aesthetics or to increase service life. Because of the thin coating, this product normally is used for interior applications or where exposure to corrosive elements is mild.
Electrogalvanizing
Production Process
Electrogalvanized coatings are applied to steel sheet and strip by electro-deposition. Electrogalvanizing is a continuous operation where the steel sheet is fed through suitable entry equipment, followed by a series of washes and rinses, into the zinc plating bath.
The most common zinc electrolyte-anode arrangement uses lead-silver, or other insoluble anodes, and electrolytes of zinc sulfates. Soluble anodes of pure zinc are also used. In this process, the steel sheet is the cathode. The coating is developed as zinc ions in the solution are electrically reduced to zinc metal and deposited at the cathode. Grain refiners may be added to help produce a smooth, tight-knit surface on the steel.
Coating Characteristics
The electrodeposited zinc coating consists of pure zinc tightly adherent to the steel substrate. The coating is highly ductile and the coating remains intact even on severe deformation. The coating is produced on strip and sheet materials to coating weights up to 0.2 oz/ft2 (60 g/m2), or thickness of up to 0.14 mils (3.6 µm) per side. On wire, coating weights may range up to 3 oz/ft2 (915 g/m2). Heat-treated and electro-coated wire can be cold drawn to about 95% reduction in area, depending on the chemical composition of the wire, heat treatment, and diameter.
The electrogalvanized coating is paintable with suitable treatment, and the sheet product is used in automobile and appliance bodies. Due to the extremely thin zinc coating on the sheet, painting or other topcoating is recommended to improve the service life.
Zinc Plating
Production Process
Zinc plating is identical to electrogalvanizing in principle in that both are electrodeposition processes. Zinc plating is used for coatings deposited on small parts such as fasteners, crank handles, springs, and other hardware items. The zinc is supplied as an expendable electrode in a cyanide, alkaline non-cyanide, or acid chloride salt solution. Cyanide baths are the most operationally efficient, but they potentially create a pollution and hazardous material problem.
After alkaline or electrolytic cleaning, pickling to remove surface oxides, and rinsing, the parts are loaded into a barrel, rack, or drum and immersed in the plating solution. Various brightening agents may be added to the solution to add luster, but careful control of the bath and brightener is needed to ensure a quality product. Post-plating treatments may be used to passivate the zinc surface and at the same time impart various translucent colors to the coating. These post-plating treatments may be used to provide a desired color or to extend the life of the plated coating.
Coating Characteristics
The normal zinc-plated coating is dull gray in color with a matte finish, although whiter, more lustrous coatings can be produced, depending on the process or agents added to the plating bath or through post-treatments. The coating is thin, ranging up to 1 mil (25 µm), restricting zinc-plated parts to very mild (indoor) exposures. ASTM Specification B 633 lists four classes of zinc plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25. The number indicates the coating thickness in microns. The coating finds application in screws and other light fasteners, light switch plates and other small parts. Materials for use in moderate or severe applications must be chromate conversion coated.
The coating is entirely pure zinc, which has a hardness about one-third to one-half that of most steels.
Metallizing
Production Process
The sprayed zinc coating is rough and slightly porous, with a specific gravity of 6.4, compared to zinc metal at 7.1. Zinc corrosion products tend to fill the pores as the zinc corrodes in the atmosphere. The coating adherence mechanism is mostly mechanical, depending on the kinetic energy of the sprayed particles of zinc. No zinc-iron alloy layers are present.
Coating Characteristics
The coating can be applied in the shop or field; it gives good coverage of welds, seams, ends and rivets, and can be used to produce coatings in excess of 10 mils (250 µm). Coating consistency is dependent on operator experience and coating variation is always a possibility. Coatings may be thinner on corners or edges and the process is not suitable for coating recesses and cavities.
continue in part 2
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