A ship's hull is made of steel. Blocks of magnesium are bolted to the outside of the hull below the waterline to prevent the steel from corroding. Explain why attaching magnesium blocks protects the steel hull from corrosion.

Magnesium is more reactive than iron/steel, so it has a lower reduction potential and will preferentially oxidise.

When the magnesium blocks are attached to the hull they act as sacrificial anodes. The magnesium gives up electrons to the steel, forming Mg²⁺ and Fe²⁺/Fe³⁺ ions.

Because the magnesium corrodes first, the steel is protected; the corrosion current flows to the magnesium rather than to the hull.

Even if the magnesium is not in contact with every part of the hull, the galvanic current can still reach the steel through the conductive seawater, so protection continues until the magnesium is consumed.