GALVANISING and ZINC PLATING
Galvanised steel is steel that has been coated with zinc in order to prevent rusting / corrosion. Sometimes the galvanising process is referred to as hot dip galvanising. The zinc forms a barrier against corrosion in that the steel underneath
does not come into contact with water / moisture in the air.
PRE GALVANISING PROCESSES -
A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag, which cannot be removed by degreasing, must be removed before galvanizing by grit-blasting, sand-blasting, or other mechanical means.
A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel.
The final surface preparation step in the galvanising process, a zinc ammonium chloride solution, serves two purposes. It removes anyremaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc.
Material is loaded onto jigs, before being immersed in molten zinc at a temperature of around 450 degrees until the temperature of the work is the same as the zinc. During this process the molten zinc reacts with the surface of the steel to form a series of zinc/iron alloys. As work is removed from the zinc, the zinc on its surface will begin to solidify, as this happens excess zinc is removed to ensure a smooth finish. Work is then usually transferred to a quench tank where it is cooled to allow handling.
CENTRIFUGE / SPIN GALVANISING
This follows the same process as general galvanising, except that the work is placed in baskets to be galvanised and rapidly removed from the bath before the zinc on the work solidifies, the basket is then placed in a centrifuge and spun for several seconds. This removes excess zinc from the surface and prevents the items sticking together to ensure a smooth finish. Work is then transferred to a quench tank where it is cooled to allow handling.
In industry electroplating is often the process used to galvanise steel products as it is more efficient and more cost effective than the hot dipping process. Also, it is easier to control the entire process. For instance, the thickness of the coating can be determined with great accuracy.
A good example of this can be seen in car manufacturing. Galvanising of steel car bodies is now standard practise although twenty years ago only the must prestigious of cars such as Rolls Royce had galvanised bodies.
The steel car body is electrically charged as negative and is suspended in a conducting solution known as the electrolyte. As the car body is negatively charged it is known as the cathode. Rods of pure zinc are positively charged and it is the zinc from these rods that is eventually deposited on the surface of the steel. The rods are suspended in the electrolyte.
A current passing through the electrolyte causes the zinc already in the solution to migrate towards the cathode (the car body). At the same time zinc from the anode (the zinc rods) pass into the electrolytic solution. The rods eventually need replacing.
Chromating is often performed on galvanized parts to make them more durable. The chromate coating acts as paint does, protecting the zinc from white corrosion, thus making the part considerably more durable, depending on the chromate layer's thickness. It cannot be applied directly to steel or iron, and does not enhance zinc's cathodic protection of the underlying steel from rust.
The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black.
Darker coatings generally provide more corrosion resistance. However, the coating color can also be changed with dyes, so color is not a complete indicator of the process used.