The Art Galvanizing Works, Inc. Cleveland, Ohio

Galvanizing Process

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     Hot dip galvanizing is the process of applying a zinc coating to fabricated iron or steel material by immersing the material in a bath consisting primarily of molten zinc.

    The process is described in more detail below, but the process is inherently simple. The simplicity of the galvanizing process is a distinct advantage over other methods of corrosion protection.

    The recorded history of galvanizing goes back to 1742 when a French chemist named Melouin, in a presentation to the French Royal Academy, described a method of coating iron by dipping it in molten zinc. In 1836, Sorel, another French chemist, obtained a patent for a means of coating iron with zinc after first cleaning it with 9% sulfuric acid and fluxing it with ammonium chloride. A British patent for a similar process was granted in 1837. By 1850, the British galvanizing industry was using 10,000 tons of zinc a year for the protection of steel. 

    Galvanizing is found in almost every major application and industry where iron or mild steel is used. The utilities, chemical process, pulp and paper, automotive, and transportation industries, to name just a few, historically have made extensive use of galvanizing for corrosion control. They continue to do so today. 

    For over 150 years, galvanizing has had a proven history of commercial successas a method of corrosion protection in myriad applications worldwide.

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    Barrier protection is perhaps the oldest and most widely used method of corro­sion protection. It acts by isolating the metal from the electrolytes in the environment. Two important properties of barrier protection are adhesion to the base metal and abrasion resistance. Paint is one example of a barrier protection system. 

    Cathodic protection is an equally important method for preventing corrosion. Cathodic protection requires changing an element of the corrosion circuit, introduc­ing a new corrosion element, and ensuring that the base metal becomes the cathodic element of the circuit. 

    Hot dip galvanizing provides excellent barrier protection as well as cathodic protection. 

    There are two major variations of the cathodic method of corrosion protection. The first is called the impressed current method. In this method an external current source is used to impress a cathodic charge on all the iron or steel to be protected. While such systems generally do not use a great deal of electricity, they often are very expensive to install. 

    The other form of cathodic protection is called the sacrificial anode method in which a metal or alloy that is anodic to the metal to be protected is placed in the cir­cuit and becomes the anode. The protected metal becomes the cathode and does not corrode. The anode corrodes, thereby providing the desired sacrificial protection. In nearly all electrolytes encountered in everyday use, zinc is anodic to iron and steel. Thus, the galvanized coating provides cathodic corrosion protection as well as barrier protection.

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    The galvanizing process consists of three basic elements: 

• Surface Preparation

• Galvanizing

• Inspection

    Surface preparation is the most important step in the application of any coating. In most instances where a coating fails before the end of its expected service life, it is due to incorrect or inadequate surface preparation. 

    The surface preparation step in the galvanizing process has its own built-in means of quality control in that zinc simply will not react with a steel surface that is not perfectly clean. Any failures or inadequacies in surface preparation will be imme­diately apparent when the steel is withdrawn from the molten zinc because the unclean areas will remain uncoated. Immediate corrective action is taken. 

    On-site painting or other field-applied systems of corrosion protection may involve the use of different subcontractors and/or work groups to prepare the surface and apply the coating. This can result in problems in coordinating activities, leading to costly and time-consuming delays, errors, and disputes concerning responsibility and financial liability. Once a job has been delivered and accepted at the galvanizer’s plant, there is one point of responsibility for ensuring that the material leaves the plant properly galvanized. That point of responsibility is the galvanizer.

    Surface preparation for galvanizing typically consists of three steps: caustic cleaning, acid pickling, and fluxing. 

    Caustic Cleaning - A hot alkali solution often 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. 

    Pickling - Scale and rust normally are removed from the steel surface by pick­ling in a dilute solution of hot sulfuric acid or ambient temperature hydrochloric acid. 

    Surface preparation also can be accomplished using abrasive cleaning as an alternative to or in conjunction with chemical cleaning. Abrasive cleaning is a process whereby sand, metallic shot, or grit is propelled against the steel material by air blasts or rapidly rotating wheels. 

    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 and promotes bonding of the zinc to the steel or iron surface. The method for applying the flux depends upon whether the particular galvanizing plant uses the wet or dry galvanizing process. 

In the dry galvanizing process (see Figure 5), the steel or iron materials are dipped or pre-fluxed in an aqueous solution of zinc ammonium chloride. The materi­al is then thoroughly dried prior to immersion in molten zinc. In the wet galvanizing process, a blanket of liquid zinc ammonium chloride is floated on top of the molten zinc. The iron or steel being galvanized passes through the flux on its way into the molten zinc.

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  In this step, the material is completely immersed in a bath consisting of a min­imum of 98% pure molten zinc. The bath chemistry is specified by the American Society for Testing and Materials (ASTM) in Specification B 6. The bath temperature is maintained at about 850 F (454 C). 

    Fabricated items are immersed in the bath long enough to reach bath tempera­ture. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by draining, vibrating, and/or centrifuging.  Art Galvanizing specializes in the Centrifuge Process for Small and Difficult Items.

    The chemical reactions that result in the formation and structure of the galva­nized coating continue after the articles are withdrawn from the bath as long as these articles are near the bath temperature. The articles are cooled in either water or ambi­ent air immediately after withdrawal from the bath.

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    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 the ASTM. the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO). These standards cover everything from the minimum required coating thicknesses for various categories of galvanized items to the composition of the zinc metal used in the process. 

    Testing methods and interpretation of results are covered in the publication, The Inspection of Products Hot Dip Galvanized After Fabrication, published by the AGA and available from theArt Galvanizing or  AGA.

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