Hot-Dip Galvanizing Resource Library

Hot-Dip Galvanizing Resource Library

Click on the links below to read more about each topic:


1. WHAT IS HOT-DIP GALVANIZING (HDG)?

Hot-Dip Galvanizing or HDG is a process in which zinc bonds to steel at the molecular level to produce a four-layer corrosion-resistant finish. The first three layers are zinc-iron alloy layers that are actually harder than the base steel. The fourth or top layer is pure zinc.

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2. WHY IS ZINC USED IN THE HOT-DIP GALVANIZING PROCESS?

Although there are several metals that could be used to galvanize steel zinc has properties that make it the best choice by far. Zinc is a natural element and is essential for healthy human life. Zinc is included in vitamins, used in the prevention of sun burn, used to promote healing of wounds and minimizing diaper rash. Zinc is also 100% recyclable without any loss of characteristics or elemental change. Zinc also acts as an anode to steel electrochemically meaning the zinc will sacrifice itself to prevent corrosion of steel. (See Question #4 for more information on the electrochemical process.)

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3. WHY IS HDG SUPERIOR TO PAINT FOR CORROSION PROTECTION?

Although the paint industry has made great improvements in paint systems HDG is still superior to paint for corrosion protection providing decades not just for years. Here’s why:

NOT JUST A COATING: HDG is not just a coating like paint. HDG metallurgically bonds zinc to the steel.

MORE THAN BARRIER PROTECTION: in addition to the metallurgical bond HDG provides two other types of protection - Cathodic Protection and the Zinc Patina. Paint does not provide these extra protections.

PAINT REQUIRES REPAIR WHEN SCRATCHED DOWN TO BARE STEEL:
Despite improvements in paint in recent years the most serious drawback of paint systems is that if the coating is damaged and bare steel is exposed it will immediately begin to corrode. As the rust increases it grow under the remaining paint coating separating it from the steel and causing the paint to flake, then peel off. As the paint peels more steel is exposed and the corrosion grows in all directions under the paint causing more and more paint to flake and peel off. The paint system fails. Repair to paint can be expensive, difficult to make, hard to remove the rust from the surface and harder still to verify that the surface was prepped and painted properly for the new paint to stick.

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4. WHAT TYPE OF CORROSION PROTECTION DOES HDG PROVIDE?

HDG Offers Three Types of Protection Against Corrosion

BARRIER: The four-layer galvanized coating provides superior barrier protection to prevent corrosion. The harder alloy intermetallis layers mean the coating is very difficult to compromise.

CATHODIC: The electrochemical relationship between zinc and steel in the presence of an electrolyte, like moisture, means that if the galvanized coating is ever compromised down to the bare steel that the electrons in the zinc will “sacrifice” themselves to prevent corrosion. A “flash” rust will occur on the surface. But then the corrosion stops. Over time zinc from the top layer will flow into a cut up to ¼” in the coating to “heal” it.

ZINC PATINA: The zinc patina is a hard, thin film that forms on the galvanized coating when it reacts with the oxygen, moisture and carbon dioxide in the air. The zinc patina acts a barrier for the galvanized coating and must wear off before the galvanized coating can begin to wear.

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5. ARE ALL ZINC-COATING SYSTEMS THE SAME AS HOT-DIP GALVANIZING?

Not at all! There are several ways to apply zinc to steel but none offers the protection of HDG:

  • Metallizing is an expensive process in which zinc in wire or powder form is sprayed onto the steel. Because the zinc is sprayed onto the steel instead of migrating into the steel like HDG it only provides barrier and limited cathodic protection.
  • Mechanical Galvanizing is a process sometimes used for fasteners or other small items in which the items are placed in a large rotary drum along with zinc powder. The mechanical energy generated by the drum’s rotation causes the zinc powder to be mechanically “welded” to the material.
  • Zinc-rich Paint is a paint that contains up to 78% zinc by weight and is one of the approved repair methods for HDG in ASTM A780, the repair specification for HDG. However, as with most paint coatings, the quality of the application is a major factor in determining the long-term performance of the coating. Consideration must be given to the initial steel surface condition (is it new, rusty or contaminated?), proper surface preparation, weather conditions (properly controlled temperature and humidity), the skill of the painter, the curing process and handling.
  • Continuous Galvanizing is typically used for very thin sheet products. Coils of steel sheet metal are fed as ribbon through a molten metal bath where it reacts to leave a protective surface coating. It has similar properties to hot-dip galvanizing, but the coating is thin since one of the factors affecting how much zinc will bond to the steel is the thickness.
  • Electroplate Galvanizing is the process whereby zinc is electrochemically applied to the steel. The coating thickness is very thin.



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6. IF I AM CONSIDERING HDG IS THERE A PARTICULAR TYPE OF STEEL I SHOULD USE?

The quality of the galvanized coating is affected by the chemistry of the steel. When considering HDG choose steels with the following targets:

Carbon < 0.25%
Phosphorus < 0.4%
Manganese < 0.5%
Silicon < 0.04% **
** > 0.15% but < 0.25% is also permissible



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7. IS THERE ANYTHING I CAN DO TO MAKE SURE MY MATERIAL IS READY TO BE GALVANIZED?

Yes, there is. Following these simple steps:

  • REMOVE ALL PAINT, LACQUER, EXCESSIVE RUST AND MILL SCALE
    Because oil-based paints are not removed during the galvanizing process you should order uncoated pipe or have your material sand blasted to remove all paint, lacquer, excessive rust and mill scale.
  • ADD CROPS OR HOLES TO GUSSETS, WEBS OR END PLATES
    To prevent zinc from pooling on your material make sure the zinc will flow off by adding crops or holes to gussets, webs or end plates.
    Hot Dip Galvanized
  • ADD VENT AND DRAIN HOLES TO YOUR HOLLOW MATERIAL
    Because hollow material is galvanized inside and out make sure you add vent and drain holes. Closed ends or sealed hollow sections can explode in the kettle injuring employees and ruining your material.

    Hot Dip Galvanizing
    The size of the hole in hollow pipe or tube
    needs to be 25% of the diameter of the
    material or 3/16” whichever is greater.
  • ALWAYS USE GALVANIZER-FRIENDLY ID TAGS OR MARKERS
    Galvanized Coating
    Oil-based paints do not come off in the galvanizing process and some tags can melt or burn off in the kettle so use metal tags and wires designed to survive the galvanizing process or an approved paint-stick marker. Your IG sales staff can point you in the right direction.
  • LET US KNOW WHEN YOU ARE COMING
    If we know when you are coming we can more efficiently schedule your material for galvanizing.