Hot dipped galvanizing gives a cathodic zinc protection to structural steel from rust. Steel put through the process of hot dipped galvanising goes through a series of chemical cleaning steps that lead up to the last step of being fully immersed in an 830-Fahrenheit bath of molten zinc. The viscosity properties of zinc and the cleaning solutions, the heat effect on the zinc, and the metallic zinc finish, require specific design parameters be met in order to ensure the steel after hot dipped galvanising precisely performs as the engineer intends it to.
The Hot-Dip Galvanising Process
Hot dipped galvanising is a factory-controlled process where high- or mild-strength steel is cleaned progressively in causic, acid (sulfuric or hydrochloric) and flux solutions before immersed in a bath of 830 Fahrenheit of molten zinc. Because of a metallurgical reaction, the molten zinc bath provides a coating on the steel. Such coating has 4 layers, 3 of which are zinc alloy, and the 4th is a top layer of pure zinc. A tight adherent, abrasion-resistant coating is created as a result of such reaction. In fact, the 3 zinc alloy layers are harder than the steel itself and have a bond to the steel of roughly 3,600 psi, creating a very tough coating, very difficult to damage when erected and exposed to tear and the harsh weather conditions. Typical coatings on steel are in excess of four mils, but could vary depending on the type and thickness of the steel. Unlike many steel coatings where there's a specified thickness, the engineer should provide minimum thickness, depending on what coating specification is utilized.
How Zinc Protects Steel
The nature of the zinc coating is metallic, therefore it cannot be penetrated by moisture. The coating serves as a barrier. It isolates the steel it's protecting from electrolyte solutions such as salt water, dew, water, and rain. The zinc coating that's exposed to air will sacrificially rust before any of the steel it's protecting will corrode. This included when gouges or scratches occur when steel products are erected.
The rate of corrosion of zinc in most environments is very low, meaning the hot dipped galvanised coating will protect the steel from rusting for 75 years or more. In the process of zinc corrosion, the zinc finish progressively forms a luster of zinc oxide, zinc hydroxide, and zinc carbonate. The carbonate coating is bound tightly to the zinc underneath, slow to react to the corrosive elements present in the air, and isn't soluble in water.
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