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Definition of Galvanizing
A process for rust proofing iron and steel by applying a zinc coating
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Barrier Protection Zn + ½O2 ® ZnO ZnO + H2O ® Zn(OH)2
Zn(OH)2 + CO2 ® ZnCO3 + H2O
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Galvanic (Cathodic) Protection
Galvanic Series of Metals and Alloys Corroded End - Anodic (Electronegative) Magnesium Zinc Aluminum Cadmium Iron or Steel Stainless Steels (active) Lead Tin Copper Gold (Electropositive) Protected End - Cathodic (Most Noble)
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Corrosion Rate vs. pH Corrosion of zinc in aqueous solutions is primarily determined by the pH of the electrolyte.
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Service Life
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Galvanizing Methods Hot-dip galvanizing Batch Continuous
Electrogalvanizing Zinc Spraying Sheradizing
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Batch Galvanizing Thick & brittle alloy layers cause coating to flake if base metal bent. Source: “Zinc Coatings”, 2008, American Galvanizers Association,
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Substrate for Continuous Galvanizing
CS low carbon steel FS SS carbon steel DDS extra or ultra low carbon steel EDDS ultra low carbon stabilized steel HSLAS micro-alloyed low carbon steel AHSS advanced high strength steels Includes solution (SHS) and bake hardening (BHS)
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The Coated Sheet Steel Family
Source: AISI
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Continuous Hot-Dip Galvanizing Line
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Cleaning
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Alkaline Cleaning
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Cleaning using Direct Fire Furnaces
Atmosphere within the DF section is reducing to slightly oxidizing. Furnace typically has 3 to 4 zones. Strip temperature exiting the DF section is from 590°C to 700°C. Air deliberately controlled to incompletely burn the fuel Reactions effectively remove carbon residues and may produce a light oxide film to be reduced by the anneal furnace. Surface degreased by direct heating Typical reactions in the DF section: C + 2H CH4 C + 2H2O CO2 + 2H2 CO2 + C CO CO + 2H2O CO2 + H2 and FeO + H Fe + H2O
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Furnace Function A galvanizing furnace is required to:
Heat treat the strip (specified thermal trajectory) to obtain desired mechanical properties Clean the strip in the case of DFF (direct fired furnace) Remove oxide by reduction Adjust strip temperature down to that required for entry into the galvanizing bath Remove microscopic iron oxide films and produce a clean surface which will react correctly with the galvanizing bath.
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Continuous Annealing Furnaces
Horizontal Vertical
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Typical Heating Cycle
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Annealing Iron Carbon Phase Diagram Most galvanized sheet is
annealed in the intercritical region just above 720°C. Deep drawing (IF) steel is annealed just above the intercritical region.
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Galvanizing Bath Chemical Components
Strip entry Top dross Air knives Strip exit Fe2Al5 Zn (Al, Fe) Guide rolls Sink roll FeZn7 Bottom dross
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Iron-Aluminum Solubility Curves
wt%Al 0.050 0.045 0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.10 0.08 0.16 0.12 0.14 0.22 0.20 0.18 0.28 0.26 0.24 0.30 wt%Fe Galvanneal FeZn7 (bottom dross) Galvanize Fe2Al5Znx (top dross) Fe/Al solubility curve Liquid Zinc Fe and Al in solution (no intermetallics) 460°C
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Coating Weight Control
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Gas Wiping - The Few Critical Inches
The fluid process by which excess Zn is removed is known as “jet stripping” or “gas wiping.” Strip Zinc Layer The jet acts on the layer through both pressure and shear forces. Gravity, pressure, and viscous shear within the fluid determine the Zn velocity profile within the fluid layer. At the strip boundary Zn moves at the same velocity as the strip. Knife Jet
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Galvanneal Process Cooling Soaking zone Fixed & mobile
Inductor furnace Touch-rolls Wiping Zinc bath
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Galvanize (GI) versus Galvanneal (GA)
Metallic appearance Thin to thick coatings Very thin alloy Coating very adherent
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Galvanize (GI) versus Galvanneal (GA)
Reaction controlled by Reheating time and temperature Bath Al content Steel grade
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GI GA Zinc (h) Fe2Al5-XZnX z d G
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Temper Rolling & Tension Leveling
2013 Tension leveling and temper rolling units are used to control shape, remove discontinuous elongation and, in the case of temper rolling, condition the galvanized surface. R1 R4 R2 R3 Strip Travel Direction Leveler Unit Exit Bridle Entry Bridle Temper rolling and especially tension leveling are facilitated by high tensions. Hence, these units are “isolated” from the rest of the line by powerful bridles. HDG Temper Rolling/Tension Leveling
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Tension Leveling R1 R4 R2 R3 Strip Travel Direction Bending Roll Unit Exit Bridle Entry Bridle Within the leveler there are usually two independent bending roll sets - each of which contains an upper and lower bending roll cartridge. (A larger anti cross bow roll is frequently installed downstream). Tension is very important in in this process and is provided by bridles in front of and behind the leveler unit.
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Improving Flatness
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Non Tempered/Non Leveled Sheet
Fluting due to Dead Soft/Ageing Discontinuous Bend
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Tempered or Leveled Sheet
When no yield point Smooth Bend Leveled - no fluting fill
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Chemical Treatment of Galvanized
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Passivation (Chemical Treatment)
Primary Objective Prevent “Humid Storage Stain” or “White Rust” Improve service life Improve paintability Secondary Objectives Serve as a ‘lubricant” Maintain “brightness” of sheet in use Types of Treatments: Chromate (Cr+6) surface treatments (spray/squeegee or roll applied) - being phased out Cr+3 and non-chrome surface treatments (spray/squeegee or roll applied) Rust inhibitive oils Acrylic & other polymer types - roll applied Phosphates, etc. as paint pretreatments
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Types of Hot-Dip Coatings
Coating Name Coating Composition ASTM Specification Galvanize Zinc A653/A653M A1063/A1063M Galvanneal Zinc-10% Iron Aluminum-Zinc 55% Aluminum-Zinc A792/A792M Zinc-Aluminum Zinc-5% Aluminum A875/A875M Zinc-Aluminum-Magnesium Zn-5/13% Al-2/4% Mg A1046/A1046M Aluminized Al-5/11% Si, or pure Al A463/A463M Terne Lead-8% Tin A308/A308M General Requirements for all hot-dip coatings - ASTM A924/A924M
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HSS & AHSS
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Tailor Welded Blanks
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Hydroforming
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Additional Information
More technical information on steel sheet metallic-coated with zinc-based coatings can be obtained at: GalvInfo Center For more and detailed information on the topics covered in this presentation see the following page:
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