What is Galvanizing?

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Hot-dip galvanizing

Galvanization is the process by which zinc is coated over corrosive (easily rusted) metals. Galvanizing involves coating corrosive metals, such as steel and iron, with a non-corrosive metal. The process of galvanizing, not only protects from corrosion of various ‘soft metals’ but also adds strength of the original, uncoated metal. Apparently, galvanized metal is thicker than the uncoated metal and so fittings and fastenings are generally measured with the additional galvanizing layer height specs in mind. American Society for Testing and Materials (ASTM) specifications provide guidelines for the thickness of galvanized metals based on the underlying metal and the type of application. Galvanization refers to any of several electrochemical (branch of chemistry that studies chemical reaction which take place in a solution) processes named after the Italian scientist Luigi Galvani. Now the term generally refers to an electro deposition (electroplating) process used to add a thin layer of another metal to a ferrous substrate, in order to prevent rusting.

What is the invention history of galvanization?

The process of galvanization was patented by Stanislas Sorel, a French engineer of Paris, France in December 1837. Sorel filed a patent for a “galvanic” method of protecting iron from rust by either coating it in a bath of molten zinc or by covering it with so-called “galvanic-paint”. This was the precursor to the modern day hot-dip galvanizing.

 

What is the mechanism involved in Galvanizing? 

Galvanizing is the practice of immersing clean, oxide-free steel into molten zinc to from a protective coating over the metal. The coating is bonded metallurgically to the steel and this coating helps to protect the surface against corrosion. In recent use, the term refers to the coating of steel or iron with zinc. This is done to prevent corrosion (specifically rusting) of the ferrous item. The zinc is consumed as a sacrificial anode (metallic anode or sacrificial rod used in cathodic protection), so that it protects the exposed steel. Cathodic protection is a technique of preventing metal from corroding. So, in case of scratches through the zinc coating, the exposed steel will be cathodically (or sacrificially) protected by the surrounding zinc coating, unlike an item which is painted with no prior galvanizing where the scratched surface would rust. Additionally, galvanizing to protect the surface of iron and steel is favored due to its low cost, ease of application, and the extended maintenance-free service that it provides. The zinc coating protects the steel in 2 ways

  • The base metal is protected from the atmosphere by the zinc coating, which is corrosion resistant
  • The Zinc coating provides cathodic (or sacrificial) protection. This is because zinc has greater electro negativity (measure of a substance’s ability to attract electrons) than iron or steel, so it will corrode in preference to the base metal in the case of a scratch (which is not the case in the case of regular paints)

 

What are common methods of Galvanizing?

  • Hot-dip Galvanizing: Hot-dip galvanizing is a form of galvanizing. It is the process of coating iron, steel, or aluminum with a thin zinc layer by passing the metal through a molten bath of zinc at a temperature of around 860º F. When exposed to the atmosphere, the pure zinc reacts with oxygen to form zinc oxide, which further reacts with carbon-di-oxide to form zinc carbonate, a dull grey, fairly strong material that stops further corrosion in many situations, protecting the steel below from the elements
  • Electro-Galvanizing: It is an electrolytic process for galvanizing, where a thinner, tighter bonding coat of zinc is applied to a metal through “electroplating”. Electro plating is a process of running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor. In this process, an electric current is passed through a zinc compound, which positively charges the zinc ions to adhere on more securely to the conductive primary metal. Electro-galvanizing provides better surface coating on the metal by virtually saturating the metal with zinc.

 

What are the Uses of Galvanizing?

  • Galvanizing of nails and screws is the most common method of preventing the unattractive staining on many types of house siding.
  • Magnesium is added into the hot dip solution in hot-dip galvanizing process, where the galvanized metals are used in a marine environment to build resistance to ocean salt water
  • Electro-galvanizing process is most commonly employed with galvanizing iron or steel beams, angle irons, etc., that are mostly used in building construction.
  • Galvanized steel is widely used in applications where rust resistance is needed, and can be identified by the crystallization (process of formation of solid crystals) patterning on the surface (often called a “spangle”)

 

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