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Galvanization Process

Rajib Singha Jun 6, 2019
The process of galvanization has been in practice for about 200 years. It is basically used for protecting metals from corrosion. More follows...
Metals such as iron and steel are prone to corrosion, and to protect them from this problem, we make use of a process called galvanization. The process is a metallurgical one, wherein, zinc is melted and used to coat iron or steel.
It is also known as the hot dip galvanizing process, where the coating of corrosion protection is given from one mil to just over four mils thick. Once applied, the zinc reacts with the coated metal and forms what is known as zinc carbonate.
This process not only makes a metal resistive of corrosion, but adds to its strength as well. This is the reason why a galvanized metal tends to be thicker than an uncoated one.
Most house sidings bear unpleasant staining of nails and screws. This is because the metals are not galvanized. It is obvious for such non-galvanized metals to cause dark stains on the siding, when they are attacked by corrosion.
It is not only about unsightly staining, corroded nails and screws may also lead to deteriorating metal fasteners. So, one simple solution for all such problems is galvanization.
So, how does this hot dip galvanizing process go about. Here, the metal is given a hot bath of molten zinc, and kept liquefied at a temperature of about 860 degrees F. The metal is simply given a dip into this molten zinc or fed through the zinc bath. But, this step is preceded by some other steps, and again followed by some finishing procedures.
The very first step in preparing the metal to be galvanized comprises removal of oil, grease, dirt, and soluble markings from its surface. For this cleaning procedure, a hot alkaline cleaner is employed.
Following this is a step known as pickling. Here, the metal is chemically cleaned and prepped for the hot dip, by using a diluted solution of hot sulfuric acid. This gets rid of the rust and mill scale on the surface.
Then comes fluxing. On top of the zinc bath, there floats the flux (which is zinc ammonium chloride solution). So, the metal is made to pass through this flux. The aim of this process is to remove oxides, and prevent any oxidation of the metal before it gets galvanized.
Having gone through the fluxing part, the metal is now ready to be galvanized. Post this, the galvanized metal is dipped into a dilute chromate solution. This is to provide a long lusting luster to the treated metal. This process, known as quenching, also helps reduce the temperature to encourage immediate handling and inspection.
Then comes quality check, or in other words, inspection. Coating thickness, uniformity, adherence, and appearance, are the parameters which are inspected in order to ensure compliance with the specifications.
Similar to this is what is known as an electro galvanizing process. Here, the primary metal is virtually impregnated with zinc. The metal is coated with a thinner, and a tighter-bonding coat of zinc with the help of electroplating (coat with metal by electrolysis).
The zinc compound is exposed to an electric current, thereby, making the zinc ions positively charged. These ions then establish a more secure bond with the primary metal, thus providing a somewhat stronger tensile strength. However, this process tends to be inefficient for materials which are constantly exposed to elements like salt water.