A primer on galvanizing

May 1, 2007
THE American Galvanizers Association (AGA), a non-profit trade association dedicated to providing technical support on today's innovative applications

THE American Galvanizers Association (AGA), a non-profit trade association dedicated to providing technical support on today's innovative applications and state-of-the-art technological developments in hot-dip galvanizing for corrosion control, provides answers to the most common questions.

How does galvanizing protect steel from corrosion?

Zinc metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfides to attack the steel. Zinc is more importantly anodic to steel — meaning it will corrode before the steel, until the zinc is entirely consumed.

How long can I expect my galvanized steel project to last in service?

Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions, without the need for maintenance.

Does the galvanized steel coating of zinc resist abrasion?

The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and have excellent abrasion resistance.

Why would you want to paint over galvanized steel?

Called duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately two times the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings make for easy repainting, excellent safety marking systems, and good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.

What are the specifications governing hot-dip galvanized steel?

Structural steel (plate, wide-flange beams, angles, channels, pipe, tubing) are galvanized to ASTM A 123/A 123M. Fasteners and small parts that fit into a centrifuging basket are galvanized to ASTM A 153/A 153M. Reinforcing steel is galvanized to ASTM A 767/A 767M.

Isn't galvanizing more expensive than paint?

Depending on the product mix, square feet per ton, and condition of the steel surface, galvanizing is often less expensive on an initial cost basis. However, as with any purchase, the lifetime costs should be considered when making a project decision on the corrosion prevention system to utilize. And, with galvanizing, the life cycle cost, i.e. the cost per year to maintain, is almost always less than a paint system. Paint systems require maintenance, partial repainting and full repainting several times over a 30-year project life. The costs can be staggering, making the decision to paint a costly one in the long run.

How long will hot-dip galvanizing protect my steel from corrosion?

The corrosion rate of zinc and how long it will provide protection is a function of the coating thickness and the amount of corrosive elements in the atmosphere. For example, in rural settings where there is less automotive/truck exhaust and plant emissions, galvanized steel can easily last for 100 to 150 years without maintenance. Industrial and marine locations contain significantly more aggressive corrosion elements such as chlorides and sulfides and galvanized steel may last for 50 to 100 years in those cases. The relationship between coating thickness and atmospheric conditions is contained in a popular graph developed by the AGA. Please see the publication Hot-Dip Galvanizing for Corrosion Protection: A Specifier's Guide.

Can I paint right over the galvanized coating? If so, what procedure should be followed?

Galvanized coatings can be easily and effectively painted, not only for aesthetics but also to extend the structure's service life. The age and extent of weathering of the galvanized coating dictate the extent of surface preparation required to produce a quality paint system over galvanized steel. ASTM D 6386, Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting, should be consulted for suggested surface preparation methods for galvanized coatings of varying ages.

How much weight will my material gain from galvanizing?

As an average, the weight of the article will increase by about 3.5% due to zinc picked up in the galvanizing process. However, that figure can vary greatly based on numerous factors. The fabrication's shape, size, and steel chemistry all play a major role, and other factors like the black weight, the different types of steel that get welded together, and the galvanizing bath chemistry can also have an effect.

What is the difference between hot-dip galvanizing after fabrication and continuous hot-dip galvanized sheet?

The process steps are similar but the production equipment is very different. After fabrication galvanizing is a more manual process where structural steel (fabricated plate, wide-flange beams, angles, channels, tube, pipe, fasteners) is suspended by wire, chain or hook from crane hoists and immersed in the cleaning solutions and zinc. Continuous sheet galvanizing involves uncoiling cold-rolled sheet, passing it through the cleaning steps and molten zinc bath at speeds up to 500 feet per minute, drying and recoiling.

Facts about hot-dip galvanizing:

  • Using zinc to protect steel from corrosion (hot-dip galvanizing) is a 150-year-old practice.
  • Corrosion is caused by the inherent tendency of metals, when subjected to air and moisture, to revert to their original earthly forms, usually an ore state. They do this through a chemical or electrochemical reaction with the environment.
  • Galvanizer's kettles (GalvaSource) are set at temperatures ranging between 815 degrees F and 850 degrees F.
  • A galvanizer knows that a piece of steel should be immersed for a specific amount of time in order for the metallurgical reaction between zinc and iron to reach completion. The completion of the metallurgical reaction is observed when bubbling of the molten zinc in the kettle stops. At this point, the galvanizing is complete and the steel is removed from the kettle to cool.
  • The largest kettle in the world is located in Kansas. The kettle measures 82'4" long × 10'4" wide × 12'4" deep.
  • Galvanizers can hot-dip galvanize a piece of steel that is larger than the kettle dimensions using a procedure called progressive dipping.
  • Zinc seals the underlying steel from contact with its environment. If the steel is exposed to the elements due to mechanical damage of the zinc coating, the surrounding zinc corrodes sacrificially, protecting the underlying steel from corrosive attack.
  • The zinc coating on galvanized steel is uniform: inside, outside, corners, and edges.
  • The hot-dip galvanized reinforcing steel bond with concrete is at least as great as the bond of bare steel to concrete.
  • When the Brooklyn Bridge was built, over 14,500 miles of hot-dip galvanized wire were used for its four main cables. Over 100 years later when the bridge underwent massive rehabilitation, the hot-dip galvanized wire was in excellent condition.
  • Hot-dip galvanized steel lasts longer today than it did 20 years ago. Because of environmental laws, our air is cleaner and less contaminated with corrosive emissions.
  • A reddish-brown staining infrequently develops on the surface of a newly galvanized piece of steel that is composed entirely of intermetallic layers. The steel is not rusting; there is just a very small amount of iron in the zinc-iron alloy layers that is oxidizing, causing the staining to occur. This does not cause any adverse effects on the corrosion performance of the galvanized steel.
  • Corrosion annually costs the US economy 3.2% of the gross national product, over $279 billion. Indirect costs to the public could raise the percentage to as much as 6%. Some indirect costs of corrosion are: lost productivity due to traffic delays, accidents caused by corroded hand and guardrails, excessive use of nature's raw materials, and energy to replace corroded steel.
  • Based on a study by NACE International (The Corrosion Society), members of Congress, and the Department of Transportation (DOT), better corrosion management can be achieved using preventive strategies at every level of involvement (owner, operator, user, government, Federal regulators, and general public).

More information is available at the AGA's Web site at www.galvanizeit.org