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Galvanising

History

The recorded history of galvanizing goes back to 1742 when a French chemist named P.J. Malouin, in a presentation to the French Royal Academy, described a method of coating iron by dipping it in molten zinc. In 1836, Stanilaus Tranquille Modeste Sorel, another French chemist, obtained a patent for a means of coating iron with zinc, after first cleaning it with 9% 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 is found in almost every major application and industry where iron or mild steel is used. The utilities, chemical process, pulp and paper, automotive, and transportation industries, to name just a few, historically have made extensive use of galvanizing for corrosion control. They continue to do so today. For over 150 years, hot-dip galvanizing has had a proven history of commercial success as a method of corrosion protection in myriad applications worldwide.

Zinc…

  • Zinc has a self-healing mechanism in it. The zinc coating sacrifices itself slowly by galvanic action to protect the base steel. This sacrificial action continues as long as any zinc remains in the immediate area. 
  • Zinc melts at 787 F (420 C), and boils at 1,665 F (907 C). 
  • Zinc comprises an estimated 0.004% of the Earth’s crust. 
  • Zinc ranks 25th in order of material abundance in the Earth. 
  • Zinc is essential for the growth and development of almost all life: between 1.4 and 2.3 grams of zinc are to be found in the average, healthy adult.
Zinc and Steel

Zinc’s most remarkable quality is its natural capacity to protect. By protecting steel against corrosion, zinc protects buildings, automobiles, ships and steel structures of every kind from corrosion by the atmosphere, water, and soil. Galvanizing for protection of iron and steel is favored because of its low cost, the ease of application, and the extended maintenance-free service that it provides.

Facts About Hot-Dip Galvanizing:

  • Using zinc to protect steel from corrosion (hot-dip galvanizing) is a 150-year-old practice!
  • Corrosion is caused by the inherent tendency of metals, when subjected to air and moisture, to revert to their original earthly forms, usually an ore state. They do this through a chemical or electrochemical reaction with the environment.
  • Galvanizer’s kettles are set at temperatures ranging between 815 F and 850 F (435 C to 454 C).
  • A galvanizer knows that a piece of steel should be immersed for a specific amount of time in order for the metallurgical reaction between zinc and iron to reach completion. The completion of the metallurgical reaction is observed when bubbling of the molten zinc in the kettle stops. At this point, the galvanizing is complete and the steel is removed from the kettle to cool.
  • The largest kettle in the world is located in Kansas. The kettle measures 82’4″ long x 10’4″ wide x 12’4″ deep.
  • Galvanizers can hot-dip galvanize a piece of steel that is larger than the kettle dimensions; it’s called progressive dipping.
  • Zinc seals the underlying steel from contact with its environment. If the steel is exposed to the elements due to mechanical damage, the surrounding zinc corrodes sacrificially, protection the underlying steel from corrosive attack.
  • The zinc coating on galvanized steel is uniform: inside, outside, corners and edges.
  • The hot-dip galvanized reinforcing steel bond with concrete is at least as great as the bond of bare steel to concrete.

Hot- Dip Galvanising

Hot-dip galvanized steel has been effectively used for more than 150 years. The value of hot-dip galvanizing stems from the relative corrosion resistance of zinc, which, under most service conditions, is considerably better than iron and steel. In addition to forming a physical barrier against corrosion, zinc, applied as a hot-dip galvanized coating, cathodically protects exposed steel

Steps in the Galvanizing Process

Though the process may vary slightly from plant to plant, the fundamental steps in the galvanizing process are:  

Soil and grease removal – A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings. Pickling – Dilute solutions of either hydrochloric or sulfuric acid remove surface rust and mill scale to provide a chemically clean metallic surface.  

Fluxing – Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the molten zinc bath. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats atop the molten zinc and the item passes through the flux immediately prior to galvanizing.  

Galvanizing – The article is immersed in a bath of molten zinc at between 815-850 F (435-455 C). During galvanizing, the zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.  

Finishing – After the steel is withdrawn from the galvanizing bath and excess zinc is removed. The galvanized item is then air-cooled or quenched in liquid.  

Inspection – Coating-thickness and surface-condition inspections complete the process. measurement of coating thickness can be made with a magnetic thickness gauge.  

Externally exposed galvanizing on heavy steelwork is now lasting an average of 40 years in the UK, with lifetime continuing to increase as acid rain concentrations decline. Over 800,000 tonnes of steel were hot dip galvanized in the UK alone in 2002.