Stacked Galvanized Metal Bars

GALVANIZING

WHAT IS GALVANIZING?

Hot-dip galvanizing is the process of coating iron, steel, or aluminum structures with a zinc layer, protecting the base metal from weather damage and providing protection against corrosion. Galvanized steel is widely used in applications where rust resistance is required. The metal is coated in zinc by first passing the metal through a molten bath of zinc at a temperature of around 860° F (460° C). Once the metal is exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO). Zinc oxide then reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3). This produces a dull grey, strong material that stops further corrosion, and protects the steel from the elements. Galvanized steel can be identified by the crystallization pattern on the surface, often referred to as a “spangle”.

WHY GALVANIZE?

Due to the strength of the metallurgical bond, along with its ability to withstand very aggressive handling, galvanized coatings continue to be unmatched to date.

The metal is first cleaned, and then the zinc is adhered to the steel using heat as a catalyst. Zinc, unlike other surface coatings, will form a metallurgic bond to the steel, outperforming any other mechanically bonded coating system on the market. Once coated, the zinc will corrode preferentially to the steel using what is called “cathodic” or “galvanic protection”, so that over time, if a scratch appears on the surface, the zinc will react to the elements before the steel will be affected. Bare metal up to one-quarter inch will also be protected by the bonded zinc immediately adjacent to the bare area.

PROCESS

01.

Caustic Cleaning

Similar to hot soap, the alkaline mixture dissolves and removes grease and some paints commonly used in the rolling and fabrication process.

02.

Acid Cleaning

Sulfuric acid is used to remove any rust, mill scale, or any other metallic impurity. Once completed, this process leaves the steel chemically clean and ready to allow the galvanizing reaction to take place. Three 7800-imperial gallon acid pickle tanks provide adequate pickling capacity to meet our customers’ requirements.

03.

Flux

The final step in steel preparation, fluxing, safeguards the steel against premature oxidization, which would otherwise allow rust formation and other surface contamination. This contamination, if allowed to occur, would greatly impede or even halt the galvanizing reaction.

04.

Galvanizing

Lowering the material into liquid zinc will cause the steel to rise in temperature, matching that of the zinc bath. This temperature, sufficient enough to act as a catalyst, initiates the galvanizing reaction. Once complete, the material is slowly withdrawn from the zinc kettle, allowing the excess zinc to drain, leaving a properly galvanized product. At Red River Galvanizing Inc., four overhead cranes, all with 26 feet of working height and 10 tons lifting capacity, provide ample flexibility to produce a superior galvanized product.

05.

Quench

Once galvanized, the material may be quenched in water to reduce its temperature and to allow safe handling. The material may also be immersed in a chromate solution effective in reducing the formation of zinc oxide, or what is commonly referred to as white rust or wet storage stain.

06.

Inspection

Once galvanized, all material is inspected using both visual inspection and electromagnetic induction testing to ensure conformance with the specifications CSA G164-1981M and ASTM A123.

SPECIFICATIONS

SpecificationsDescription
CSA G164-1981MHot Dip Galvanizing of Irregularly Shaped Articles.
ASTM A123Zinc (Hot Galvanized) Coatings on Products Fabricated from Rolled, Pressed, and Forged Steel Shapes, Plates, Bars and Strip
ASTM A143Safeguarding Against Embrittlement of Hot Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement
ASTM A153Zinc Coating (Hot Dip) on Iron and Steel Hardware
ASTM A767Zinc Coated (Galvanized) bars for concrete reinforcement
ASTM A780Repair of Damaged Hot Dip Galvanizing Coatings

GALVANIZING VS. PAINT

Hot-dip galvanized steel is coated with zinc, which is metallurgically bonded to the steel. Removal of the zinc coating can only be accomplished through substantial force. Unlike paint, zinc will not allow the environment to “creep” under the coating, causing premature coating failure. Zinc bonds perpendicular to the steel surface, causing an even layer throughout – even over sharp corners. In contrast, paint tends to recede from sharper areas, effectively reducing the coating value in these regions.