What is Galvanizing?
Galvanizing is one of the most important processes in manufacturing good quality fencing. It is the most important tool against corrosion. Following are some details on different kinds of galvanizing used in the Fencing industry.
Hot Dip Galvanizing:
The best known way of coating is hot dip galvanizing according to BS EN ISO 1461. In the process, as the name suggests, the component to be galvanized is dipped in a bath of hot, liquid Zinc. Zinc melts at 419ºC, while Zinc in molten bath usually has a temperature between 440 and 460ºC. The parts to be galvanized are degreased, pickled and fluxed (coated with an adhesive liner), after which they are dipped in the molten Zinc bath. By dipping, the insides, welds and hard to reach corners are well covered. The minimum layer of thickness for the materials used outdoors according to BS EN ISO 1461, depending on the thickness of the steel, is 70 µm or over. This way of Zinc coating offers maximum protection from corrosion, but is very laborious. Because of the heat involved, welds can pull loose or the material can deform. In case of cavities (like in hollow sections), dipping must be done very carefully, as to not let the trapped air expand and make the object explode. Furthermore objects often need to be polished and buffed, because the layer of Zinc is thick and rough.
This way of galvanizing is often used in our industry of tubes or hollow sections. In the process the strip steel, straight from the coil is led through a preheating oven, pre-treatment baths and the molten Zinc baths at high speeds, before the sections or tubes are rolled from it. When exiting the Zinc bath the excess Zinc is skinned off with compressed air and blown back to the bath. In this way the thickness of the layer can be determined fairly accurately, usually it is between 15 and 20 µm. The last station in the process makes the surface much smoother than the hot dip galvanizing does, while the adhesion of the Zinc on the steel is just as good. The disadvantage on the other hand is, because the tubes and sections are rolled and therefore welded after the Zinc plating, the welds are not always well protected against corrosion. Although, by welding the Zinc on both sides of the welding line heats up, melts again and partially flows into the weld; this is not enough to completely protect the weld from corrosion. Some rolling mills thereforeoffer a follow-up treatment with a cold Zinc coating, but even this way of Zinc coating is effective only when used on materials that are powder coated after being galvanized. Because both follow up treatment with cold galvanizing coating and powder coating are only possible on the outside of the section, the weld on the inside of the section is protected from corrosion only minimally.
Third way of Zinc coating is centrifugal galvanizing. This procedure is often used for small parts, which are difficult to hang up or have small holes that would fill up with Zinc in standard hot dip galvanizing. In centrifugal galvanizing the parts are deposited into a crate. This crate is then dipped in a bath of liquid Zinc. The temperature of this bath is usually 530ºC, so the Zinc is thinner. By swiftly rotating the crate as soon as it leaves the bath, the parts are centrifuged, therefore reducing the layer thickness. By adjusting the time of centrifuging the layer thickness can be adjusted very precisely.
Electroplating or electrolytic galvanizing is the system that consists of fumigation of metal ions on another metal. It's the exact same technique that is used for chromium-plating or nickel-plating objects. The technique works as follows – Zinc chloride is dissolved in a bath of water. It disintegrates in Zn2 cations and Cl anions. The object to be Zinc plated, is hooked up to the negative pole of a current source, thereby becoming the cathode. Another metal object is hooked up to the positive pole, becoming the anotde. As soon as both objects are immersed into the Zinc chloride bath, an electric current will flow from positive to negative. The Zinc ions now absorb electrons and fumigate on the cathode (the object to be galvanized) as Zinc. Only the chlorine solution remains in the bath. The big advantage of electroplating is that you need minimum quantity of Zinc (eventually a layer is only 7 or 8 µm) to create a very smooth surface that perfectly seals in the steel, thus protecting it from corrosion. Because the layer is so thin, it is sensitive to damaging.