Pickling in acid removes any deposits, iron oxides and ferrous residues on the products from millwork. The products are immersed in a solution of hydrochloric acid (HCl) diluted with water (H2O). The galvanizing plant has a series of pickling baths with varying concentrations of acid. This helps to improve product management and optimise the process. To prevent over pickling steel parts and to protect the tanks, inhibitors are added to reduce acid attack on the metal. Under certain circumstances, products may be sandblasted rather than pickled. This may be at the customer’s request; required under the applicable legislation or deemed necessary by Zitac further to their inspection of the material. Zitac will entrust this procedure to a qualified supplier.
After pickling, products are immersed in water to remove any acid residue from the previous operation. This is to help prevent any acid, iron or iron salts from the pickling process being transferred to the fluxing bath and then contaminating the rest of the procedure.
Fluxing involves immersing the products in a “double salt” flux solution of zinc chloride and ammonium chloride (ZnCl2.2NH4Cl). This coats the surfaces of the product with a protective layer that prevents any oxides forming on the part until it is immersed in the molten zinc. The coating also improves the reaction between the iron and the zinc.
Drying and preheating
The products are heated in a drying oven at approximately 60–80°C in order to reduce the thermal shock when the work is dipped in the galvanizing bath. This also improves the reaction conditions for layer formation, and thus reduces the time spent in the zinc bath.
Once the parts come out of the drying oven, they are immersed in a bath of molten zinc (99.995% purity) at a controlled temperature between 440 and 465°C. During immersion, fluxing salts are released from the surface of the iron and a metallurgical reaction takes place on the surface of the products to form a protective metal coating. It is during this stage that the steel reacts with the zinc, forming a series of layers of iron/zinc alloy. The time spent in the bath will depend on the thickness of the products. Items that have higher thicknesses of steel will have a higher thermal inertia. This means that it will take longer to bring the steel to the temperature of the bath. Other things being equal (for example, the same composition of steel), thicker steel will have a thicker coating as it will be kept at a high temperature for longer.
The small parts, such as brackets, nails, washers, bolts, nuts etc., undergo the same chemical surface cleaning treatments and are immersed in zinc baths, but they are held in perforated baskets. After galvanizing, the basket is removed from the bath and placed in a centrifuge for a few seconds. This removes excess zinc to give a smooth finish and ensure that the products comply with size requirements set out in the applicable standards. The centrifuge may remove the outer layer of pure zinc. This results in a duller surface than the galvanized work, also because the temperatures of the molten zinc are usually higher for galvanizing small parts.
The galvanized parts are left to cool naturally in room temperature air or in warm water.
Finishing and quality control
When the work is extracted from the tank, the molten zinc may drip and form slightly thicker streaks when it hardens. This does not affect the quality of the galvanizing. However, if the galvanized parts are needed for precision fittings, these small spikes can be ground off. In any event, the imperfections will be removed when they are sharpened. A visual inspection is then carried out to make sure that the work is completely coated with zinc and that no parts have been missed. The work is also checked for drips or spikes that could impair its use, and for any excess zinc on the threads. Finally, the thickness of the coating is checked using a magnetic gauge. The galvanized work must meet certain technical standards, which set out the minimum and average values for the thickness of the coating depending on the thickness of the galvanized material.
Packaging and storing of materials in closed environments that are protected from the elements
The work is packaged in accordance with the customer’s requirements, and we can even handle customised packaging.