Steel and copper are used in electrical contact in seawater. In which case is galvanic damage to the steel likely to be most severe?

Answer: B. A large copper plate fastened with small steel rivets

When dissimilar metals are in contact, the active steel becomes the anode and the noble copper the cathode, accelerating corrosion of the steel (galvanic corrosion). The amount of oxygen reduced at the cathode is roughly proportional to the cathode area, and the anodic dissolution current equals it, so the corrosion current is set mainly by the cathode area. With a large copper cathode plate and small steel anode rivets, a large current concentrates on a small area, and the rivets corrode severely in a short time. Conversely, with small copper rivets in a large steel plate, the corrosion current is spread over a wide steel surface, so the corrosion rate (per unit area) is small. The choice saying the area ratio has no effect is therefore also wrong. When joining dissimilar metals, the rule is to make the fasteners the more noble metal or to insulate the joint.