1. GALVANIZING
METHODS
HOT DIPPING, ELECTROPLATING , GALVANNEALING

2. ABOUT GALVANIZING
• Galvanization is the process of applying a
protective zinc coating to steel or iron, in order to
prevent rusting.
• The protection is two fold:
• Physical protection from moisture and air
• Electrochemical cathodic protection.
• Patina - a layer of zinc oxides, salts, and other
compounds formed when Zn reacts with salts and
moisture(protective and impermeable in nature)
• Spangle – snow flake like glittering crystallites on the
surface
• Degree of galvanizing expressed in weight of Zn per unit
surface area.

3. GALVANIZING METHODS
• Hot dipping in molten Zn. (HDGI)-thick coat
• Continuous sheet galvanizing(run a sheet/wire
through molten Zn or Zn alloy ) –thin coat done
before forging and can withstand bending and
reshaping.(done without bracing)
• Zn paint or Zn spray – thin coat, weak bonding with
steel surface.
• Electro-galvanizing – thin coat , strong bonding
better finish

4. STEPS IN HDGI PROCESS
i. The material generally used is Aluminum and silicon( and
phophorous) killed steel so that no oxidation occurs during
solidification.
ii. Abrasive blasting : the process of using a forceful stream of
particles, available in varying hardness, to remove residue
and contaminants from steel surfaces to prepare for
galvanizing.
iii. Caustic cleaning to remove organic impurities
iv. Passing the metal through a molten bath of zinc at a
temperature of around 460 °C).
v. Further reactions: The pure zinc (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 in
many circumstances, protecting the steel.

5. HDGI PROCESS
• In case of HR, Si should be low and in case of CR P
should be low. Else it will get irregular coating with low
adherence.
• Zn grades that can be used: Z1 to Z5 (Z1 is purest)
• Lead is often added to the molten zinc bath to improve
the fluidity(but not eco friendly), generally required in Z1
grade Zinc.
• Aluminium added for shining
• Chromate quenching (a layer of tri and hexavalent
chromium compound) is done to prevent reaction of
zinc with concrete/H2O , H2 evolution and wet storage
stains.
• Phosphating is done prior to any painting to increase
adhesion of paints.

6. DUPLEX COATING
• Duplex coating is liquid or powder paint over galvanized
surface done to provide for synergistic protection.
• Is 10 times more abrasion resistant
• Is 8 times more chip resistant
• Offers 20 times more edge protection
• Very good long-term corrosion protection
• Attractive appearance, unlimited color variety
• High level of mechanical resilience.
• High level of cost effectiveness due to its long service life
• Environmentally-sound process.

7. LAYERS OF Zn COATING
• The outer Eta layer that is composed of pure zinc.
• The Zeta layer composed of an alloy that is 94% zinc
and 6% iron
• Delta layer : The second layer of zinc-iron alloy
growth from the base steel formed during the
galvanizing process;
• The Delta layer’s chemical composition is approximately
90% zinc and 10% iron
• Delta layer is 60% harder than the base steel , it protects
from abrasion and corrosion
• The thin Gamma layer composed of an alloy that is
75% zinc and 25% iron

8. ADVANTAGES AND DISADVANTAGES
OF HDGI
• Advantages:
• Smooth and uniform coating.
• Sharp edges and corners are protected optimally.
• Stress release (internal)
• The coating is very robust and can withstand minor
bumps and bruises in connection with the
mounting, transportation etc.
Disadvantages :
• Limitation of the product size.
• Risk of distortion due stress release esp. in forged
products.

9. APPLICATIONS OF HDGI STEEL
• Roofing and walling
• Safety barriers
• Handrails,
• Consumer appliances
• Automotive body parts
• Metal pails in most heating and cooling duct systems in
buildings .
• Cannot be used in places of extreme temperature
conditions , peeling starts at 200˚C
• According to the naming conventions - G90 finish would
have 0.90 oz. of zinc per square foot.

10. ABOUT ELECTRO-GALVANIZED STEEL
• Electrolytic deposition of Zn on the surface.
• often used in automotive manufacturing to enhance the
corrosion performance
• This coating cannot stand high temperatures.
• Zinc coatings, whether electrogalvanized or hot dip
usually require chromate conversion coatings to deter
white rust.
• For fabrications that need to operate at high
temperatures sulfamate nickel plating is used.
• electrogalvanized coatings are susceptible to zinc
whiskers
• zinc coating in electro galvanization is mentioned in
terms of thickness of the layer.

11. DIFFERENCE BETWEEN HOT DIPPED
AND ELECTROPLATED ZINC :
• Hot dip galvanized coatings are far thicker than electro-
galvanized (usually up to 10x as thick)
• Electroplated zinc coatings are smooth and shiny, and
preferable for aesthetic reasons, whereas galvanizing may be
spangled, or gray and drippy.
• Zinc electroplating is thin and usually does not cause any
problems with fasteners, whereas galvanized coatings are
heavy and will interfere with fastener threads unless they are
specially dimensioned to take the coating thickness into
account.
• Electro-galvanized coatings are susceptible to zinc whiskers
• Zinc coating in electro galvanization is mentioned in terms of
thickness of the layer.In HDGI it is mentioned in terms of weight
of deposit.

12. GALVANNEALING
• Annealing is a process of heating metals to above
the critical temperature, maintaining a suitable
temperature, and then cooling .
• Annealing is used to induce ductility, soften
material, relieve internal stresses, refine the structure
by making it homogeneous, and improve cold
working properties hardness and ductility.
• Galvannealing is a process of galvanizing and
annealing to produce specialized sheets of steel
with an iron alloy layer.

13. GALVANNEALED STEEL
• Galvannealed steel sheets have medium-light gray,
smooth appearance.
• They offer good paintability, weldability,
• These are more corrosion resistant, and formable.
• Galvannealing is commonly referred by its trade name
of Jet-Kote.
• Painting of galvannealed steel is important because it
shows white to dark grey marks if it comes in contact
with water.
• Application: It is extensively used in the
automotive, signage, electric equipment, and other
industries requiring a metal with good paintability and
long reliable service life.

14. GALVALUME
• Trademark for a special type of hot-dip galvanized
steel sheet with a coating consisting of 55%
aluminum, 43.4% zinc and 1.6% silicon
• Applications :
• Galvalume sheets are used where it’s unique blend
of corrosion resistance and high heat reflectivity is
beneficial
• In roofing and siding applications as well as
unexposed automotive parts, appliances, furniture,
outdoor cabinetry, computer cases, gutters, pipe,
and in other non-construction applications

15. ADVANTAGES OF GALVANIZNG STEEL
• Zinc is extremely durable. Esp. in hard water where it
forms a passive layer of carbonates.
• Galvanized surface is scratch resistant.
• Satiny appearance that give aesthetic values to
the product.

16. ALUMINIZED STEEL
• Hot dipping of steel in either :
• Al –Si alloy (5% to 11% silicon) : Type I
• Pure Al : Type II
• Silicon improves adherence.
• Type I : Common in industrial products ,better
adherence , intended principally for high temperature
corrosion resistance.
• Possible end uses: mufflers, furnaces, ovens, ranges,
heaters, water heaters, fireplaces, and baking pans.
• Type II : It is intended principally for applications
requiring atmospheric corrosion resistance.(not high
temperatures)
• Possible end uses :corrugated roofing and siding, grain
bins, air conditioner housings and drying ovens.

17. ALUMINIZED STEEL
• The basic structure of aluminized steel is a thin
aluminum oxide layer outside.(Al is always oxidised)
• An intermetallic layer that is a mix of aluminum,
silicon, and steel,
• Finally a steel core.
Aluminized Steel is highly resistant to corrosion and
shows high levels of heat reflectivity.(mainly due to
the intermediate layer)
Difference between Al and Zinc protection is that
aluminium provides just the physical protection. So ,if
aluminium layer breaks/cracks corrosion will start, but
not in case of zinc.