GALVANIZING PROCESS

The Hot Dip Process and Surface Preparation

Surface Preparation:
Surface preparation is by far the most important step of any coating process.  In most cases if a coating fails before the end of its expected service life it is because the surface preparation was done incorrectly.  With galvanizing, the surface preparation process contains its own built-in means of quality assurance and quality control in that zinc will simply not react with a steel surface that is not perfectly clean.  Surface preparation for galvanizing typically consists of three steps:

1. Caustic Cleaning
2. Acid Pickling
3. Fluxing

The galvanizing process consists of the following basic steps:

Oil and Grease Removal -- A hot alkaline cleaner is used to remove dirt, oil, grease and soluble markings.  This process is known as caustic cleaning.  This cleaning will NOT, however, remove epoxies, paint, and welding slag.  Those contaminants must be removed by sandblasting, which we can arrange for you.

Pickling -- A dilute solution of hot sulfuric acid is used to remove surface rust and mill scale to provide a chemically clean metallic surface, readying the metal for the hot-dip process.  This portion of the process can take anywhere from 30 minutes to 1-1/2 hours, depending on the complexity or the thickness of the part.

Fluxing -- Fluxing 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.

Galvanizing -- This is where the steel actually gets the zinc coating.  The steel is immersed into a bath of molten zinc, which is heated between 820 and 850 F.  During the galvanizing process, the steel comes up to the bath temperature and the zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, topped by an impact resistant pure zinc layer.  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  The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining, vibrating and/or centrifuging.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.

Inspection -- The final step in the hot-dip process is to ensure compliance with the specifications.  Coating thickness, uniformity, adherence and appearance are evaluated.  Electronic testing gauges are used to give quick and convenient measurements of the coating.  Additionally, Certificates of Compliance can be issued to satisfy the acceptable workmanship.  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.