Introduction to Galvanization
How Galvanizing Metal Parts Increases Part Life
Galvanization is a metal coating process in which a ferrous part is coated with a thin layer of zinc. The zinc coating seals the surface of the part from the environment, preventing oxidation and weathering from occurring.
The primary method of galvanization is “hot dip galvanization”, which has been in use for over 150 years. While the idea of coating a part in molten zinc was first proposed by chemist Paul Jacques Malouin in 1742, the process was not put into practice until patented by chemist Stanislas Sorel in 1836. Sorel’s process has changed little since then, and still involves coating a part in molten zinc after cleaning it with an acid solution and coating the part in flux.
Preparing a Part to be Galvanized
Cleaning the part before galvanization is extremely important. Surface contaminants such as grease, dust, or dirt can compromise the galvanization process. Generally, parts are cleaned in a two step process. In the first step, the part is cleaned in a hot alkaline solution to remove any surface contaminants. In the second step, the part is immersed in a weak acid solution to remove rust and scale, resulting in a clean and smooth surface.
After cleaning, the part is coated in flux to encourage metallurgical bonding during the galvanization process. The flux is a zinc ammonium chloride solution. In “dry galvanization”, the part is dipped into a bath of flux, then removed and let to dry before the galvanization process. In “wet galvanization”, the flux floats on top of the molten zinc, so the part passes through the flux before it touches the molten zinc.
The Hot Dip Galvanization Process
In both dry and wet galvanization, the part is immersed in a molten zinc bath with a temperature of about 860ºF(460ºC). The zinc metallurgically bonds to the steel part, resulting in a transition layer of steel/zinc alloy between the steel part and the zinc coating. The galvanization process results in a dull gray crystalline surface finish, often referred to as “spangle”.
Within 24-48 hours after galvanizing, the outer surface of the zinc coating becomes zinc oxide. Any post-galvanization coating operations, such as powder coating, need to be performed before this oxidation occurs. After short term atmospheric exposure, some of the outer layer of zinc oxide becomes zinc hydroxide. After extended atmospheric exposure, the zinc oxide is converted to zinc carbonate. This “patina” provides an additional layer of protection to the steel part, helping it to withstand weathering.
Benefits of Galvanizing Metal Parts
Galvanization helps to extend the life of steel parts by providing a barrier between the steel and the atmosphere, preventing iron oxide from forming on the surface of the steel. Galvanization also provides superior corrosion resistance to parts exposed to the environment.
Galvanization provides a cost-effective solution for coating steel parts, specifically those that will receive significant environmental exposure over their lifetime.
Galvinized Screw
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