The estimated time to first maintenance for a hot-dip galvanized coating that experiences common atmospheric exposure can be seen in Figure 7. Time to first maintenance is defined as the time to 5% rusting of the substrate steel. The time to first maintenance of hot-dip galvanized steel is directly proportional to the zinc coating thickness.
The term hot-dip galvanizing is defined as the process of immersing iron or steel in a bath of liquid zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. The coating is produced as the result of a metallurgical reaction between the liquid zinc and the iron in the steel. The coating forms an equal thickness on all surfaces immersed in the galvanizing kettle. This process, similar to the one seen in Figure 1, has been in use since 1742 and has provided long-lasting, maintenance-free corrosion protection at a reasonable cost for many years. The three main steps in the hot-dip galvanizing process are surface preparation, galvanizing, and post-treatment, each of which will be discussed in detail.
The first step in the hot-dip galvanizing process is intended to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. This is achieved by first hanging the steel using chains, wires, or specially designed dipping racks, as seen in Figure 3, to move the parts through the process. There are three cleaning steps to prepare the steel for galvanizing.
Once the steel has been completely cleaned, it is ready for immersion in the liquid zinc. The galvanizing kettle contains zinc specified to ASTM B 6, a document that specifies any one of three different grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the zinc melt in order to promote certain desirable properties in the galvanized coating.
There are many other types of corrosion protection, such as coating steel with oil, grease, tar, asphalt, polymer coatings or paints, or corrosion protection materials such as stainless and weathering steel, sacrificial anodes, plating systems and impressed current systems. These are some of the most commonly used corrosion protection materials and systems and are sometimes used together with hot-dip galvanized steel. Most of these materials rely on barrier protection, while some of them rely on cathodic properties to prevent corrosion of the steel. The most effective type of corrosion protection that provides both barrier and cathodic protection is hot-dip galvanizing.
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