ANS:C - pickling

Pickling is a chemical process used to remove surface impurities, stains, and scale from metal surfaces, particularly steel and iron, by immersing the metal in an acidic solution. The acidic solution dissolves the surface oxides, rust, and other contaminants, leaving behind a clean surface. Here's how the pickling process works:

1. Preparation: Before pickling, the metal surface may be cleaned to remove any loose debris or oils that could interfere with the pickling process.
2. Immersion: The metal is then immersed in a pickling solution, typically an acidic solution such as hydrochloric acid (HCl) or sulfuric acid (H2SO4). The concentration of the acid solution and the duration of immersion depend on the type of metal being pickled and the extent of contamination on its surface.
3. Chemical reaction: The acid in the pickling solution reacts with the surface oxides, rust, and scale on the metal surface, converting them into soluble compounds that can be easily rinsed away. The reaction may produce hydrogen gas as a byproduct, which is safely vented from the pickling tank.
4. Rinsing: After the desired pickling time, the metal is removed from the pickling solution and rinsed thoroughly with water to neutralize any remaining acid and wash away the dissolved contaminants.
5. Passivation (optional): In some cases, the pickled metal may undergo a passivation treatment, which involves immersing it in a solution of nitric acid or other chemicals to create a protective oxide layer on the surface, preventing future corrosion.

Pickling is commonly used in various industries, including steel manufacturing, metal fabrication, and metal finishing, to prepare metal surfaces for further processing, such as painting, plating, or welding. It helps improve the appearance, cleanliness, and surface quality of metal products, ensuring better adhesion of coatings and enhancing their corrosion resistance.