Which of the following materials is most commonly used for protective coatings?

Nickel and tin-plating are the most commonly used materials for protective coatings due to their effective properties in preventing corrosion. Nickel plating provides a hard, corrosion-resistant surface that can withstand high temperatures and is often used in harsh environments, making it ideal for avionics applications. Tin plating, on the other hand, offers excellent solderability and protects underlying metals from corrosion as well. The combination of these two metals enhances durability and protection against environmental factors that could lead to corrosion.

While aluminum can also be used for coatings, it does not provide the same level of protection as nickel and tin in many applications. Lead is generally avoided due to its toxicity and environmental concerns, and gold, although it has excellent corrosion resistance and conductivity, is more commonly used for specialized applications rather than as a widespread protective coating material due to its high cost. Thus, nickel and tin plating stand out as the most practical and widely utilized option for coatings in the avionics field.