Final answer:
Intergranular corrosion is primarily caused by the exposure of metal to oxygen and water, leading to rust formation. Small scratches in protective coatings can create galvanic cells, accelerating the corrosion process.
Step-by-step explanation:
One of the primary causes of intergranular corrosion is the exposure of the metal to oxygen and water. When iron is exposed to these elements, it can rust and deteriorate through oxidation. Figure 20.9.2 shows that small scratches in a protective paint coating can lead to rapid corrosion of iron by allowing oxygen to be reduced at the exposed surface (cathode), while metallic iron is oxidized at the less exposed site (anode). This galvanic cell formation can result in the formation of rust.