Which of these materials is the most cathodic?

The most cathodic material among the options listed is zinc. In the context of corrosion and cathodic protection, cathodic materials are those that resist corrosion and are capable of providing electrons to prevent the corrosion of more anodic metals.

Zinc is often used as a sacrificial anode in various applications, including marine environments and on underground pipelines. It has a more negative electrochemical potential compared to other metals, which allows it to corrode preferentially and protect other metals from corrosion.

Magnesium is also a highly reactive metal and commonly used for cathodic protection, but it is more anodic than zinc. Stainless steel, while it has good corrosion resistance due to the presence of chromium, typically does not serve as a sacrificial anode and is not as cathodic as zinc.

Copper, although known for its excellent conductivity, is generally more noble than zinc and would not act as a sacrificial anode in cathodic protection systems.

Understanding the electrochemical series helps clarify the behavior of these metals in cathodic protection applications. In this series, the more negative the potential, the more cathodic the material is considered, so among the choices, zinc stands out.