Galvanizing is a process in which protective coating is offered by zinc to prevent corrosion of base metal. The protective coating offered by zinc is dual-in-nature. It provides a protective barrier layer that insulates the steel from corrosive attacks, and also sacrificial/ cathodic protection to the exposed area of the steel. The rapid corrosion of steel by de-icing salts has brought the steel users & manufacturers of automobiles & appliances inclined towards coated products.
Hot dip galvanizing is the most commonly used process today as it takes care of all the problems of corrosion. Also zinc coating by hot dip galvanizing has been found to be more stable & tough even in the most aggressive environments. Continuous strip galvanizing by hot dip method is done in series of processes. These are preparation of strip, annealing of strip, controlled zinc coating, cooling, skin passing, imparting flatness & passivation.
The CR strip to be galvanized needs to be cleaned of scale, oil, iron fines & other surface contaminants is passed through alkaline solution subsequent cleaning action is done by rotating brushes. The strip then enters the non-oxidation furnace where it is directly exposed to a set of burners.
The burners are preset on a fixed air : gas ratio to enable an oxidizing atmosphere in the first zone & slightly reducing atmosphere in the subsequent zones. The strip is degreased as soon as it passes through the first zone of non-ox furnace. These oxides are reduced in the last part of the non-ox furnace where the ambient temperature is maintained between 1200 and 1300 degrees Celcius depending upon the strip thickness & line speed.
The strip then passes through a radiant tube furnace where the strip is taken to annealing temperature in the presence of protective atmosphere of nitrogen & hydrogen. The strip temperature is maintained between 550 & 750 degrees Celcius, depending on end use of the strip. The strip then passes through a precisely controlled gas fired soaking zone to achieve the desired properties in the material followed by controlled jet cooling section to bring down the temperature of the strip very near to the zinc bath temperature prior to entering the zinc bath. The strip is then immersed in molten zinc bath and zinc iron alloy layer formation takes place over the strip in the form of zinc