Hot Dip Galvanizing
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, hot-dip galvanizing has had a proven history of commercial success as a method of corrosion protection in myriad applications worldwide.
Hot-Dip Galvanizing or HDG is a known, proven method of providing corrosion protection preventing steel from rusting. It does so in two ways. First, galvanizing provides an extremely tough, tenacious barrier preventing moisture and other corrosive agents from attacking the steel. Second, Zinc (Zn), the metal used in galvanizing, cathodically protects the steel by sacrificing itself before the steel rusts. The combination of these two actions provide an extremely long lived protection for steel products.
Metallurgic Bond
- Galvanizing is a process in which zinc metallurgicaly bonds to steel at the molecular level to produce a four-layer corrosion-resistant finish. The first three layers are zinc-iron alloy layers that are actually harder than the base steel. The fourth or top layer is pure zinc.
Cathodic Protection
- All metals, ores once refined, want to passivate to their natural state. Steel, for example, rusts to form ferrous and ferric oxides, the basel state of iron in nature. However, metals passivate at different rates. That is, some metals give up their electrons more easily than others, Zinc will give up its electrons before steel or iron. This is called cathodic protection and has been known for a very long time. The ancient Romans observed that when zinc and iron were placed together, the iron didn’t rust. Today, zinc ingots are hung over the sides of ships when they are being maintained so that the zinc passivates before the ship’s steel rusts.
- Metals have been arrayed in order of their “willingness” to give up their electrons. This is called the Galvanic Series of Metals. Silver, platinum, and gold are very stingy in giving up their electrons. They corrode slowly and are very stable. This is why they are used in jewelry and as money. They are called the noble metals. On the other end of the spectrum, aluminum, magnesium and zinc give up their electrons easily. They will sacrifice themselves and protect any metal that is more noble. Therefore, zinc will protect steel when they are in contact with each other.
Longevity
- HDG’s longevity has been proven by examination of projects going back decades and not estimated on the basis of extrapolation of accelerated tests. It has been around for a long time. The recorded history of galvanizing goes back to 1742 when a French chemist named P.J. Malouin, described a method of coating iron by dipping it in molten zinc. By 1850, the British galvanizing industry was using 10,000 tons of zinc a year for the protection of steel.
- HDG longevity has been determined by examination of thousands of projects. These projects have been categorized according to their atmospheric environment. Five such environments a been identified: Rural being the most benign, and in order of aggressiveness, Suburban, Temperate Marine, Tropical Marine, and finally Industrial which the most harsh. These have been arrayed on the following chart. The longevity is estimated based on the thickness of the zinc layer in mils. The minimum thickness required by ASTM A 123, the standard governing HDG, is 3.9 mils and is highlighted on the chart. Therefore a new product placed in a Suburban environment can be expected to last at least 95 years.
