Definition of Galvanizing A process for rust proofing iron and steel by applying a zinc coating

Barrier Protection Zn + ½O2 ® ZnO ZnO + H2O ® Zn(OH)2 Zn(OH)2 + CO2 ® ZnCO3 + H2O

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)

Corrosion Rate vs. pH Corrosion of zinc in aqueous solutions is primarily determined by the pH of the electrolyte.

Service Life

Galvanizing Methods Hot-dip galvanizing Batch Continuous Electrogalvanizing Zinc Spraying Sheradizing

Batch Galvanizing Thick & brittle alloy layers cause coating to flake if base metal bent. Source: “Zinc Coatings”, 2008, American Galvanizers Association, www.galvanizeit.org

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)

The Coated Sheet Steel Family Source: AISI

Continuous Hot-Dip Galvanizing Line

Cleaning

Alkaline Cleaning

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 + 2H2 CH4 C + 2H2O CO2 + 2H2 CO2 + C 2CO CO + 2H2O CO2 + H2 and FeO + H2 Fe + H2O

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.

Continuous Annealing Furnaces Horizontal Vertical

Typical Heating Cycle

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.

Galvanizing Bath Chemical Components Strip entry Top dross Air knives Strip exit Fe2Al5 Zn (Al, Fe) Guide rolls Sink roll FeZn7 Bottom dross

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

Coating Weight Control

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

Galvanneal Process Cooling Soaking zone Fixed & mobile Inductor furnace Touch-rolls Wiping Zinc bath

Galvanize (GI) versus Galvanneal (GA) Metallic appearance Thin to thick coatings Very thin alloy Coating very adherent

Galvanize (GI) versus Galvanneal (GA) Reaction controlled by Reheating time and temperature Bath Al content Steel grade

GI GA Zinc (h) Fe2Al5-XZnX z d G

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

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.

Improving Flatness

Non Tempered/Non Leveled Sheet Fluting due to Dead Soft/Ageing Discontinuous Bend

Tempered or Leveled Sheet When no yield point Smooth Bend Leveled - no fluting fill

Chemical Treatment of Galvanized

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

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

HSS & AHSS

Tailor Welded Blanks

Hydroforming

Additional Information More technical information on steel sheet metallic-coated with zinc-based coatings can be obtained at: GalvInfo Center www.galvinfo.com For more and detailed information on the topics covered in this presentation see the following page: http://www.galvinfo.com/galv_info_notes.htm