Corrosion resistance of steel is increased by addition of chromium and nickel.

Explanation:
- Corrosion resistance: Corrosion is a process in which metals are gradually deteriorated by chemical reactions with their surroundings. Corrosion can lead to the weakening and failure of metal structures. Therefore, it is important to enhance the corrosion resistance of steel, which is a commonly used material in various industries.
- Chromium: Chromium is a key element in stainless steel, which is known for its excellent corrosion resistance. When chromium is added to steel, it reacts with oxygen in the air to form a thin, invisible layer of chromium oxide on the surface of the steel. This layer acts as a barrier, preventing further corrosion and protecting the underlying steel from environmental factors such as moisture and chemicals.
- Nickel: Nickel is another element commonly added to steel to improve its corrosion resistance. Nickel enhances the passivity of steel by promoting the formation of a stable oxide layer on the surface. This oxide layer acts as a protective barrier against corrosion, similar to the chromium oxide layer.
- Synergistic effect: The combination of chromium and nickel in steel creates a synergistic effect, resulting in significantly improved corrosion resistance compared to steel without these alloying elements. The chromium and nickel work together to enhance the formation and stability of the oxide layer, providing long-lasting protection against corrosion.
- Other elements: While other elements such as sulphur, phosphorus, lead, vanadium, aluminium, tungsten, molybdenum, and zinc can also influence the properties of steel, they do not have the same significant impact on corrosion resistance as chromium and nickel. Some of these elements may even have detrimental effects on corrosion resistance or other mechanical properties of steel.
- Conclusion: In summary, the addition of chromium and nickel to steel greatly improves its corrosion resistance. These alloying elements promote the formation of a protective oxide layer on the surface of the steel, preventing further corrosion and ensuring the durability and longevity of steel structures.