The Dry Galvanizing Process
At Galvcast, we use a process called hot dip galvanizing. This process can be broken down into three basic steps:
Surface Preparation
The most important step when applying a coating of any kind, including galvanizing. If the surface is not properly prepared, the zinc will not adhere to the steel and uncoated portions of steel will be apparent when withdrawing the steel from the molten zinc. This is recognized immediately and corrective measures are taken to fix the problem.
There are three steps in the surface preparation when galvanizing:
- Caustic Cleaning -
A hot Alkali solution is used to remove organic contaminants such as dirt, paint markings, grease and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag must be removed before galvanizing by grit-blasting, sand-blasting or other mechanical means.
- Acid Pickling -
Scale and rust are removed from the steel surface by pickling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid.
- Fluxing -
This is the final surface preparation step in the galvanizing process. Fluxing removes oxides and prevents further oxides from forming on the surface of the metal prior to galvanizing. The steel or iron is dipped in an aqueous solution of zinc ammonium chloride. The material is then dried prior to immersion in molten zinc.
One of the advantages of galvanizing is that the surface preparation can be done in any weather conditions. Painting the surface however, can only be done in the proper weather conditions. This can lead to costly errors, delays, and disputes.
Galvanizing
In this step, the material is completely immersed in a bath consisting of a minimum of 98% pure molten zinc at a temperature of about 840o F (449o C). The zinc metal then reacts with the iron on the steel surface to form a zinc/iron inter-metallic alloy. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining, vibrating and/or centrifuging. The articles are cooled in either water or ambient air immediately after withdrawal from the bath. Because the galvanizing process involves total material immersion, it is a complete process; all surfaces are coated. Galvanizing provides both outside and inside protection for hollow structures. Hollow structures that are painted have no interior corrosion protection. Galvanizing is performed at the factory under any weather or humidity conditions. Most brush-applied and spray-applied coatings depend upon proper weather and humidity conditions for correct application. This dependence on atmospheric conditions often translates into costly construction delays. The galvanizer’s ability to work in any type of weather allows a higher degree of assurance of on-time delivery. Working under these circumstances, galvanizing can be completed quickly and with short lead times. Two-day or three-day turnaround times for galvanizing are common.
Click here to see pictures of the galvanizing process
Inspection
The two properties of the hot-dip galvanized coating
that are closely scrutinized after galvanizing are
coating thickness and coating appearance. A variety of
simple physical and laboratory tests may be performed
to determine thickness, uniformity, adherence and
appearance.
Products are galvanized according to long-established,
well-accepted, and approved standards of
ASTM, the Canadian Standards Association (CSA), and
the American Association of State Highway and
Transportation Officials (AASHTO). These standards
cover everything from minimum required coating thicknesses
for various categories of galvanized items to the
composition of the zinc metal used in the process.
The inspection process for galvanized items is simple
and fast, and requires minimal labor. This is important
because the inspection process required to assure
the quality of many brush-applied and spray-applied coatings is
highly labor-intensive and uses expensive skilled labor.
A finished load ready to go outside of the new plant
Additional Information
If you would like additional information on issues concerning galvanizing, from the process of galvanizing to painting on galvanized surfaces to performance reports of galvanized structures around North America, click here.