Cathodic protection of metals is based on

Analyze the question: Cathodic protection is a way of preventing corrosion (rusting) of a metal such as iron or steel pipes, ship hulls, and underground tanks. We need to identify the principle behind it.

Explain the concept: Corrosion is an oxidation process: the metal loses electrons.

Fe → Fe²⁺ + 2e⁻

Cathodic protection makes the metal we want to protect act as the cathode (the site of reduction), so it does not lose electrons. There are two main methods:

1. Sacrificial protection: Iron is connected to a more reactive metal such as zinc or magnesium. Zinc is more easily oxidised than iron (it has a more negative electrode potential), so zinc corrodes instead and the iron is protected. This is the principle behind galvanising and the magnesium blocks fixed on ship hulls.
2. Impressed current: An external power source supplies electrons to the metal, making it the cathode.

Solution:

• A. Standard electrode potential of hydrogen: Hydrogen is only the reference electrode (0.00 V), so this is not the basis of the protection.
• B. Electrical conductivity: Conductivity is needed for the circuit, but it is not the principle.
• C. Nature of oxides formed: This applies to protection by a passive oxide layer, as in aluminium, which is a different concept.
• D. Relative tendencies of oxidation: The more reactive metal (the one with the greater tendency to be oxidised) is oxidised in preference to the metal being protected. This is the principle of cathodic protection.

Answer: D

Tip: Remember "Cathodic = Cathode is safe." The metal that is more easily oxidised (higher in the reactivity series) is sacrificed to protect the less reactive one. Whenever you see "sacrificial", "galvanising" or "zinc/magnesium bars", think of the relative tendency to lose electrons.