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

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

4. 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)

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

6. Service Life

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

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

9. Substrate for Continuous GalvanizingCS
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)

10. The Coated Sheet Steel Family
Source: AISI

11. Continuous Hot-Dip Galvanizing Line

12. Cleaning

13. Alkaline Cleaning

14. 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

15. 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.

16. Continuous Annealing Furnaces
Horizontal
Vertical

17. Typical Heating Cycle

18. 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.

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

20. 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

21. Coating Weight Control

22. 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

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

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

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

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

27. 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

28. 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.

29. Improving Flatness

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

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

32. Chemical Treatment of Galvanized

33. 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

34. 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

35. HSS & AHSS

36. Tailor Welded Blanks

37. Hydroforming

38. 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: