Which of the following metals is primarily used for sacrificial anodes?

The metal primarily used for sacrificial anodes is zinc. Sacrificial anodes are a cathodic protection method used to prevent corrosion in metal structures, such as pipelines or underground storage tanks. Zinc is chosen for this purpose due to its electrochemical properties. Specifically, zinc has a higher electrochemical potential compared to most other metals found in these structures.

When zinc is connected to a iron-based structure, it corrodes (sacrificing itself) before the iron can, effectively protecting the iron from rusting or deterioration. This process is vital in environments where moisture or electrolytes can accelerate corrosion.

Using other metals like steel or lead would not be effective as sacrificial anodes because they do not provide the same level of protection; they either corrode too slowly or are less electrochemically favorable compared to zinc. Copper, while often used in electrical applications, is also not suitable as it can actually facilitate corrosion of the structure it's meant to protect rather than inhibit it. Therefore, zinc’s properties in providing cathodic protection make it the correct choice for sacrificial anodes.