Hot-dip galvanized steel Is the process of immersing a steel product in a bath of molten zinc. The process is inherently very simple., what distinguishes it from other methods of protecting steel against corrosion.
For the first time, the hot-dip galvanizing method was invented by the Frenchman Malouin back in 1742 year. Thirty years later, the Italian Galvani discovered electrochemical processes, which happen between metals. Galvani's research was continued by the Englishman Faraday, which in 1829 discovered the sacrificial role of zinc when interacting with most other metals. IN 1836 the Frenchman Sorel patented an early galvanizing process. Already in 1850 year the British electroplating industry spent about 10 thousand tons of zinc to protect steel from corrosion.
Corrosion protection of steel
Today, zinc coatings can be found almost everywhere., where steel products and structures are used. Hot-dip galvanized steel has a long and successful history of applications in millions of tons of diverse products and is currently the most popular method of protecting steel against corrosion worldwide..
Hot-dip galvanizing technology
The hot-dip galvanizing process consists of three main steps (picture):
– surface preparation;
– galvanizing itself and
– quality control.
Preparation of steel surface before galvanizing
Surface preparation is the most important step in any coating.. In most cases, deterioration of protective coatings before the expected service life is caused by improper or inadequate surface preparation..
The galvanizing process of steel has its own, built-in tool for quality control of surface preparation - zinc simply will not react to "dirty" steel. Any errors or inconsistencies in surface preparation of the steel will immediately manifest, when the steel is removed from the molten zinc bath, poorly prepared surfaces will be left without zinc coating.
Preparation of the steel surface before hot-dip galvanizing includes three steps:
Heated alkaline solutions are used to degrease the steel surface., acidic solutions or biological detergents to remove organic contaminants from the metal surface, such as dirt, traces of paint, greases and oils. Contaminants such as various resins, bitumen or welding slag, which are not removed by degreasing, must be removed prior to galvanizing by shot blasting, sand or other mechanical methods.
A heated solution of sulfuric acid or a solution of hydrochloric acid at room temperature removes mill scale and rust from the steel surface. Instead of etching, or together with etching, abrasive cleaning or cleaning with sand or metal shot is used.
The last step in the preparation of steel before hot-dip galvanizing is processing it in a solution of a double salt of zinc ammonium chloride.. This operation serves two purposes.. It removes all remaining oxides and applies a protective layer to the steel, to prevent the formation of steel on the surface before immersing it in a bath of molten zinc.
Processing steel in a molten zinc bath
During its own hot-dip galvanizing, the steel product is completely immersed in a bath of molten zinc. Bath contains at least 98 % pure zinc and has a temperature of about 449 ºS.
Molten zinc reacts with iron in steel to form a series of layers of iron-zinc intermetallic alloys. When the thickness of the coating on the steel product reaches the specified thickness, they are slowly removed from the hot-dip galvanizing bath., and excess zinc is removed by draining, vibrating or processing in a special centrifuge.
The metallurgical reaction will continue until then, while the product remains at a temperature, which is close to the bath temperature. Products are cooled or by immersion in a passivating solution, or into the water, or just left to cool outdoors.
Zinc coating quality control
Quality control of hot-dip zinc coating is quick and easy. Of greatest importance are two characteristics of hot-dip zinc coating - the thickness of the coating and its appearance.. To determine the thickness of the coating, its uniformity, adhesion strength and appearance are applied by various physical and laboratory tests.
Hot zinc coating according to GOST 9.307-89
In Russia and neighboring countries, requirements for protective coatings, applied by hot-dip galvanizing is established by GOST 9.307-89. The applied zinc coating is subjected to visual inspection, thickness and bond strength.
Appearance control produce on 100 % products. The surface of the zinc coating must be smooth or rough, the coverage must be continuous. Coating color - from shiny silver to matte dark gray.
Coating thickness should be at least 40 μm and no more 200 μm. The thickness of the zinc coating is measured magnetically, as well as metallographic or gravimetric methods.
For adhesion strength control coatings apply the following methods:
- scratch grid application method;
- brushing method (scratching with steel brushes);
- heating method to 180-200 ºS;
- swivel hammer impact method 212,5 r.