To protect ferrous metal sheets from corrosion, they are dipped in a bath of molten zinc to add a protective layer of it on the sheet. This process is called as:
🔬 Understanding Corrosion Protection for Steel
Ferrous metals, like iron and steel, are prone to rust (corrosion) when exposed to oxygen and moisture. To prevent this, a protective layer is often applied. The question describes a specific method: dipping the steel into a bath of molten zinc.
Galvanizing: A process of applying a protective zinc coating to steel or iron to prevent rusting. The most common method is hot-dip galvanizing, where the parts are submerged in a bath of molten hot zinc. This is precisely the process described in the question.
⚖️ Detailed Analysis of Coating Processes
Let's examine the different surface treatment processes to understand why galvanizing is the correct answer.
(b) Galvanizing
Process: Submerging a ferrous metal in a bath of molten zinc at a temperature of around 450 °C. This creates a thick, durable, and metallurgically bonded layer of zinc on the surface.
Protection Mechanism: Zinc protects steel in two ways: it provides a physical barrier against moisture, and it offers sacrificial protection. Zinc is more reactive than iron, so if the coating is scratched, the zinc will corrode first, "sacrificing" itself to protect the underlying steel.
(a) Parkerising
Process: This is a method of phosphating, where a steel part is treated with a phosphoric acid solution. This converts the surface of the metal into a non-metallic, crystalline phosphate layer.
Protection Mechanism: It provides mild corrosion resistance and creates an excellent porous surface for paints and oils to adhere to. It is not a metallic coating.
(c) Electroplating
Process: This uses an electric current to deposit a thin layer of one metal onto another. The part to be plated is the cathode in an electrolytic cell.
Protection Mechanism: It can be used to apply a layer of zinc (electrogalvanizing), but the process is different from the hot-dip method described. Electroplating typically produces a thinner, more uniform coating.
(d) Enamelling
Process: This involves fusing a layer of vitreous (glass-like) material onto a metal surface by firing it at very high temperatures.
Protection Mechanism: It creates a hard, smooth, durable, and chemically resistant surface. It is a glass coating, not a zinc coating.
💡 Study Tips for Metal Coatings
- Galvanizing = Molten Zinc Bath: This is the key association. If the question mentions dipping steel in molten zinc, the answer is galvanizing.
- Electroplating = Electric Current: This process always involves electricity to deposit the metal layer.
- Enamelling = Glassy Finish: Think of enamel pots and pans; it's a hard, glass-like coating.
- Parkerising = Phosphate: This is a chemical conversion process, not a metallic coating.