Hot Dip Galvanization
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Hot-dip galvanization 1 Hot-dip galvanization Crystalline surface of a hot-dip galvanized handrail A street lamp in Singapore showing the characteristic spangle Hot-dip galvanization is a form of galvanization. It is the process of coating iron, steel or aluminium with a layer of zinc by immersing the metal in a bath of molten zinc at a temperature of around 860 °F (460 °C). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O 2 ) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO 2 ) to form zinc carbonate (ZnCO 3 ), a usually dull grey, fairly strong material that stops further corrosion in many circumstances, protecting the steel below from the elements. Galvanized steel is widely used in applications where corrosion resistance is needed, and can be identified by the crystallization patterning on the surface (often called a "spangle"). Galvanized steel can be welded; however, one must exercise caution around the resulting zinc fumes. Galvanized steel is suitable for high-temperature applications of up to 392 °F (200 °C). The use of galvanized steel at temperatures above this will result in peeling of the zinc at the inter metallic layer. Electrogalvanized sheet steel is often used in automotive manufacturing to enhance the corrosion performance of exterior body panels; this is, however, a completely different process which tends to achieve lower coating thicknesses of zinc. Like all other corrosion protection systems, galvanizing protects steel by acting as a barrier between steel and the atmosphere. However, zinc is a more electronegative metal in comparison to steel, this is a unique characteristic for galvanizing which means that when a galvanized coating is damaged and steel is exposed to the atmosphere, zinc can continue to protect steel (often within an annulus of 5 mm above which electron transfer rate decreases).
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Hot Dip Galvanization
Transcript of Hot Dip Galvanization
Hot-dip galvanization 1
Hot-dip galvanization
Crystalline surface of a hot-dip galvanized handrail
A street lamp in Singapore showingthe characteristic spangle
Hot-dip galvanization is a form of galvanization. It isthe process of coating iron, steel or aluminium with alayer of zinc by immersing the metal in a bath ofmolten zinc at a temperature of around 860 °F(460 °C). When exposed to the atmosphere, the purezinc (Zn) reacts with oxygen (O2) to form zinc oxide(ZnO), which further reacts with carbon dioxide (CO2)to form zinc carbonate (ZnCO3), a usually dull grey,fairly strong material that stops further corrosion inmany circumstances, protecting the steel below fromthe elements. Galvanized steel is widely used inapplications where corrosion resistance is needed, andcan be identified by the crystallization patterning on thesurface (often called a "spangle").
Galvanized steel can be welded; however, one mustexercise caution around the resulting zinc fumes.Galvanized steel is suitable for high-temperatureapplications of up to 392 °F (200 °C). The use ofgalvanized steel at temperatures above this will resultin peeling of the zinc at the inter metallic layer.Electrogalvanized sheet steel is often used inautomotive manufacturing to enhance the corrosionperformance of exterior body panels; this is, however, acompletely different process which tends to achievelower coating thicknesses of zinc.
Like all other corrosion protection systems, galvanizingprotects steel by acting as a barrier between steel andthe atmosphere. However, zinc is a moreelectronegative metal in comparison to steel, this is aunique characteristic for galvanizing which means thatwhen a galvanized coating is damaged and steel isexposed to the atmosphere, zinc can continue to protect steel (often within an annulus of 5 mm above which electrontransfer rate decreases).
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ProcessThe process of hot-dip galvanizing results in a metallurgical bond between zinc and steel with a series of distinctiron-zinc alloys. The resulting coated steel can be used in much the same way as uncoated.A typical hot-dip galvanizing line operates as follows:• Steel is cleaned using a caustic solution. This removes oil/grease, dirt, and paint.•• The caustic cleaning solution is rinsed off.• The steel is pickled in an acidic solution to remove mill scale.•• The pickling solution is rinsed off.• A flux, often zinc ammonium chloride is applied to the steel to inhibit oxidation of the cleaned surface upon
exposure to air. The flux is allowed to dry on the steel and aids in the process of the liquid zinc wetting andadhering to the steel.
•• The steel is dipped into the molten zinc bath and held there until the temperature of the steel equilibrates with thatof the bath.
•• The steel is cooled in a quench tank to reduce its temperature and inhibit undesirable reactions of the newlyformed coating with the atmosphere.
Lead is often added to the molten zinc bath to improve the fluidity of the bath (thus limiting excess zinc on thedipped product by improved drainage properties), helps prevent floating dross, makes dross recycling easier andprotects the kettle from uneven heat distribution from the burners.[1] Lead is either added to primary Z1 grade zinc oralready contained in used secondary zinc. A third, declining method is to use low Z5 grade zinc.[2]
Steel strip can be hot-dip galvanized in a continuous line. Hot-dip galvanized steel strip (also sometimes looselyreferred to as galvanized iron) is extensively used for applications requiring the strength of steel combined with theresistance to corrosion of zinc. Applications include: roofing and walling, safety barriers, handrails, consumerappliances and automotive body parts. One common use is in metal pails. Galvanised steel is also used in mostheating and cooling duct systems in buildingsIndividual metal articles, such as steel girders or wrought iron gates, can be hot-dip galvanized by a process calledbatch galvanizing. Other modern techniques have largely replaced hot-dip for these sorts of roles. This includeselectrogalvanizing, which deposits the layer of zinc from an aqueous electrolyte by electroplating, forming a thinnerand much stronger bond.
HistoryIn 1742, French chemist Paul Jacques Malouin described a method of coating iron by dipping it in molten zinc in apresentation to the French Royal Academy. In 1836, French chemist Stanislas Sorel obtained a patent for a methodof coating iron with zinc, after first cleaning it with 9% sulfuric acid (H2SO4) and fluxing it with ammoniumchloride (NH4Cl).
SpecificationA Hot Dip Galvanized coating is relatively easier and cheaper to specify than an organic paint coating of equivalentcorrosion protection performance. The British, European and International standard for Hot Dip Galvanizing is BSEN ISO 1461 which specifies a minimum coating thickness to be applied to steel in relation to the steels sectionthickness e.g. a steel fabrication with a section size thicker than 6 mm shall have a minimum galvanized coatingthickness of 85 µm.Further performance and design information for galvanizing can be found in BS EN ISO 14713-1 AND BS EN ISO14713-2. The durability performance of a galvanized coating depends solely on the corrosion rate of the environmentin which it is placed. Corrosion rates for different environments can be found in BS EN ISO 14713-1 where typicalcorrosion rates are given with a description of the environment in which the steel would be used.
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Media
A hot-dip galvanizing 'kettle'with fume hood
Galvanized I-beam Galvanized angle iron Steel tube ready to be galvanized
References[1] Lohse et al., Adaptation to Scientific and Technical Progress of Annex II Directive 2000/53/EC, January 2008 (http:/ / rohs-elv. exemptions.
oeko. info/ fileadmin/ user_upload/ Background/ Final_report_ELV_Annex_II_revision. pdf)Lead addition described on page 12
[2] European Commission, Reference Document on Best Available Techniques in the Non Ferrous Metals Industries, December 2001 (ftp:/ / ftp.jrc. es/ pub/ eippcb/ doc/ nfm_bref_1201. pdf)Primary zinc grade table on page 15
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Article Sources and ContributorsHot-dip galvanization Source: http://en.wikipedia.org/w/index.php?oldid=589521907 Contributors: AgadaUrbanit, Alistair1978, Andy Dingley, Avanu, B137, Ben Boldt, BenFrantzDale,Blanchardb, Canterbury Tail, Cmglee, Cycotic, Diffusionteki, Dogears, Doodle77, Dthomsen8, Element16, Fru1tbat, Gaurangkumar.88, Helpful4sure, Hqb, Huw Powell, Idacoolboy, Iepeulas,Jonico61, Josve05a, Julesd, Kolbasz, L314t, Materialscientist, Maury Markowitz, Melonkelon, Michael Devore, Mmxsaro, Mmxx, Mogism, MrBell, MrOllie, Muscarella, Observer29,Omegatron, P199, Perskyro, Phasmatisnox, Plasticspork, Possum, R'n'B, R6144, Randrag, Retodon8, Rettetast, Rifleman 82, Ronhjones, Salmanazar, Securiger, Semperf, Shaddack, Slakr, SnowBlizzard, Stone, The Yeti, Thumperward, Toddst1, Trkiehl, Twinsday, TwoTwoHello, Vadim Makarov, Vgeraskin, Wizard191, Yerocus, York3k2, 125 ,علی ویکی anonymous edits
Image Sources, Licenses and ContributorsFile:Feuerverzinkte Oberfläche.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Feuerverzinkte_Oberfläche.jpg License: Public Domain Contributors: TMgFile:Lamppost-singapore.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Lamppost-singapore.jpg License: Creative Commons Attribution 2.5 Contributors: Chaerani (MeutiaChaerani – Indradi Soemardjan).Image:galvanizing01.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Galvanizing01.jpg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Ies,PhasmatisnoxImage:galvanizing02.JPG Source: http://en.wikipedia.org/w/index.php?title=File:Galvanizing02.JPG License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Ies,PhasmatisnoxImage:galvanizing03.JPG Source: http://en.wikipedia.org/w/index.php?title=File:Galvanizing03.JPG License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Ies,PhasmatisnoxImage:galvanizing04.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Galvanizing04.jpg License: Creative Commons Attribution-ShareAlike 3.0 Unported Contributors: Ies,Phasmatisnox
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