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Pressure Die-Casting Processes
Published in Frank Porter, Zinc Handbook, 1991
Zinc alloy can be recovered from the oxide skimmings taken off the melting pots if they are added to the scrap and the combined furnace load is treated with a flux (not needed in normal melting operations). Numerous proprietary fluxes are sold for use with zinc alloys, based on zinc chloride or zinc ammonium chloride. The metal is held at about 450°C and the recommended quantity of flux is added gradually, with stirring, until the dross is fine and powdery and can be removed with a perforated ladle. The metal is then left for 15 minutes to let any aluminides rise to the surface and be skimmed off. The skimmings are kept for return to the next batch of scrap.
Industrial Methods of Preventing Galvanic Corrosion: Part I
Published in Susai Rajendran, Gurmeet Singh, Titanic Corrosion, 2019
Susai Rajendran, Gurmeet Singh
During the process of hot-dip galvanization, a metallurgical bond is formed between zinc and steel. The use of materials before and after galvanization remains the same. However, the corrosion resistance of the materials increases after coating [2]. Hot-dip galvanization includes the following steps (Fig. 4.2): Cleaning/decreasing: Steel is treated with concentrated sodium hydroxide to remove paint, oil, and grease [2].Rinsing: The material is washed with a minimum amount of pure water [2].Pickling: When the material is treated with concentrated HCl, scales are removed (Fig. 4.3) [2]. This is called “pickling.” During the pickling process, rust and scales are removed (https://en.wikipedia.org/wiki/Pickling_(metal)). Rinsing: The material is again washed with a minimum amount of pure water to remove the pickling solution [2].Fluxing: The cleaned surface may be oxidized by air. To prevent this, a flux is applied on the metal surface. A well-known flux is zinc ammonium chloride [2]. A flux is a chemical cleaning agent, a flowing agent, or a purifying agent.Drying: The flux is dried on the steel. This helps in the deposition of zinc on the metal [2].Zinc bath: The steel is dipped into a molten zinc bath. Lead is often added to a molten zinc bath to control the amount of zinc deposited on steel (Fig. 4.4). Environmental regulations in many countries disapprove the use of lead [2]. Cooling and inspection: Finally, the steel is cooled in a quench tank, reducing its temperature and preventing undesirable reactions between the newly formed coating and the atmosphere [2].
Effect of elemental additions on hot-dipping galvanization behavior
Published in Cogent Engineering, 2023
Murtadha A. Jabbar, Mohammed Y. Yousif, Nuha H. Jasim
In order to dip the sample in zinc, it is first divided into 4 × 4 cm pieces and then cleaned with a metal brush to remove the oxides and rust from the sample’s surface. After polishing the sample’s surface with various grades of sandpaper, residual oxidation is removed with acid. It is then immersed in a flux solution prepared by 30% flux powder and 70% pure water for 4 min. Depending on the manufacturer’s recommendations of flux powder, the solution temperature must be between 40 and 60 ºC to function properly. Afterward, it is hot dipped at 100°C until a saline layer forms on its surface. The zinc ammonium chloride flux with chemical formula Cl4H8N2Zn is used to remove the oxide film that forms after acid cleaning and to deposit a protective layer to prevent further oxides from forming on the steel surface before immersion in molten zinc. Fluxing is the final step before immersion in molten zinc after cleaning samples or products in general. The melting temperature of zinc is 419.5 ºC. To make molten zinc sufficiently fluid for immersion in water, factories use zinc kettles to melt it and maintain its temperature between 435 and 455 ºC. A zinc kettle at the Basrah galvanizing factory uses natural gas to heat it. Since these possibilities were not available, an electric furnace and a crucible were used to melt the zinc, as well as for the immersion process of the samples. As shown in Table 3, the galvanizing process has been repeated several times with different additive percentages.