WHAT IS HOT DIPPED GALVANIZING?
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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 inherently simple. The simplicity of the galvanizing process is a distinct advantage over other methods of providing corrosion protection. The history of galvanizing goes back to 1742 when a French chemist named Melouin described in a presentation to the French Royal Acade3my, a method of coating iron by dipping it in molten zinc. In 1836 another French chemist, Sorel. Obtained a patent for a means of coating iron with zinc after first cleaning it with 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 can be found in almost every major application and industry where iron or mild steel is used. The utilities, chemical process, pulp, paper, automotive and transportation industries, to name jus a few have historically made extensive use of galvanizing for corrosion control. They continue to do so today. For over 140 years galvanizing has had a proven history of commercial success as a method of corrosion protection in a myriad of application worldwide. Several other zinc methods attempt to prevent rusting of mild steel, first sacrificial coating knowing as cathodic protection, and second zinc rich paint. Neither method can compete with hot dipped galvanizing process. Hot dipped galvanizing provides protection for steel far superior to that provided by any other method. Hot dipped galvanizing or (barrier protection is perhaps the oldest and widest used method of corrosion protection. It acts by providing by providing a barrier protection between the electrolytes in the environment. The two most important properties of barrier protection are adhesion t the base metal and abrasion resistance. Paint is one example of barrier protection and can be ok if the paint coating is not nicked or broken. However, once the paint is nicked or the paint has been chipped off, then water and oxygen are free to attack the steel and rust blisters grow rapidly. This is death for dump bodies which are under constant attack by rocks and concrete slabs provide the jack hammer to breakdown painted coatings. The rusting process is accelerated as as water and oxygen seep between the paint coat and the steel surface. The paint coating now has turned against the owner accelerating the rusting process as rust is formed in a damp moist area behind the paint coating, protected from drying by the paint coat itself. The resulting RUST BUBBLE grows very rapidly, and when it breaks many times reveals a complete rust through of the steel requiring a replacement or repair of the dump body. This explains why paint-based coating systems fail, so quickly. When a nick in the paint surface allows oxygen, and water, behind the paint surface usually within the first year the battle is lost and the truck chassis is still new. The solution to this problem is the galvanizing coating which is metallurgically bonded to the steel at the molecule level through a series of zinc-iron alloy layers capped by a free zinc layer. When an engineer studies the galvanized process the engineer discovers the coating is more than just a barrier coating; it actually becomes part of the steel surface and is anodic to the steel substrate. Because zinc reacts with the atmosphere, the galvanized coating is constantly changing. Zinc reacts with the environment to form protective zinc oxides, zinc hydroxides and zinc carbonates. The zinc carbonates over time provide a stable base to apply paint for aesthetic reasons. These zinc reaction products are known as the zinc patina. The zinc patina actually helps protect the free zinc layer, thus providing additional corrosion protection. These changes and reactions affect how paint will adhere to the galvanized surface. (22011) |
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