Hot dip galvanizing is the process of coating iron or steel objects with zinc by immersing the fabricated steel product in a bath of molten zinc at a temperature of about 450°C. During the process, a metallurgically bonded layer is formed that protects the steel. Most galvanizing plants can handle a wide variety of shapes and weights of steel products.

Stages of the process

In the Benelux countries, (hot dip galvanizing) plants operate as service providers applying a galvanized coating to workpieces supplied by their clients. The various steps of the process are described below, although there may be slight differences between individual galvanizing companies. In the initial stages of the process, fabrications or castings delivered to the galvanizing plant are subjected to initial examination and sorting to enable the galvanizing plant to organize its work program as efficiently as possible and to optimize employee safety.

Degreasing

If the steel delivered to the galvanizing plant shows traces of light grease or oil, these are removed in a degreasing bath. Usually an acidic degreaser is used but sometimes an alkaline variant is used, and after degreasing the steel it may be rinsed in a rinse bath to prevent the degreasing agent from being carried over (=transferred) to the next stage in the process.

Chemical cleaning

Dry cleaning removes rust and mill scale, the most common corrosion products that contaminate the surface of steel. The cleaning process is usually performed with an acid solution containing ferric chloride at ambient temperature, and the goal is to obtain a chemically clean steel surface before immersing it in molten zinc. The cleaning time depends on the degree of rust on the article. The workpiece is rinsed in a rinsing bath after cleaning to prevent chemicals from entering the flux.

Flux treatment

Flux serves to remove the last traces of oxides and provides a final intensive cleaning of the steel surface. Flux increases the irradiation of the steel surface with the molten zinc, thus promoting the reaction between iron and zinc. The flux usually consists of an aqueous solution of chlorides, mainly a mixture of zinc chloride and ammonium chloride. Flux is applied to the materials to be galvanized in various ways. In most galvanizing plants, for example, the flux is in an aqueous solution in a bath and the workpiece to be galvanized is dipped into it. The latter is also known as dry galvanizing.

Drying

In dry galvanizing, after immersion in the flux bath, the steel objects to be galvanized are dried in a drying oven and are then covered with a protective thin layer of flux.

Galvanize

Galvanizing takes place in a bath of molten zinc, the temperature of which in most plants is between 445- 455°C (zinc has a melting point of 419°C and is a solid below that). In galvanizing plants using the high-temperature process, the temperature of the zinc will be 540°C or higher. The analysis of the metal in the bath must meet relevant international standards and contain at least 98% zinc. The galvanizing process causes the formation of a coating on the objects consisting of multiple layers of zinc-iron alloys formed by the reaction of zinc with the iron in the steel. When the galvanized objects are removed from the molten zinc, a layer of almost pure zinc is formed over the alloy layers, also called solidification layer. The immersion time for the steel objects depends on the weight and thickness of the steel profiles. As the galvanizing process takes place, the surface of the molten zinc in the galvanizing bath is covered with a thin layer of oxides and flux residues. The surface of the zinc bath is manually cleaned of this (“chipping”) to ensure that the galvanized object does not come into contact with oxide and flux residues when it is removed from the zinc bath.