Which of the following describes ferrous metals?

Ferrous metals are defined by their iron content, which makes them susceptible to corrosion, especially when exposed to moisture and air. This characteristic arises from the use of iron in their composition, making them prone to rusting if not treated or protected adequately. Common examples of ferrous metals include steel and cast iron. Protective measures, such as galvanization, painting, or applying oil, are often necessary to mitigate this corrosion risk.

The other descriptions do not apply to ferrous metals: they do contain iron, they are not mixtures of metals in a way that would define them as non-ferrous (which typically refers to metals without significant iron content), and they are generally heavier than non-ferrous metals, which often have lightweight properties. Hence, the correct answer emphasizes the critical need for protection against corrosion due to the presence of iron in these materials.