When the concentration of dissolved oxygen varies from place to place on steel in water, a differential aeration cell forms. Where does corrosion tend to concentrate?

Answer: A. Where the oxygen supply is low (deep in crevices, under deposits and so on)

Where oxygen is plentifully supplied, the cathodic reaction of oxygen reduction occurs readily and the potential is higher. A cell therefore forms in which the oxygen-poor regions become anodes where iron dissolves, and the oxygen-rich regions become cathodes. This is the differential aeration (oxygen concentration) cell, and corrosion concentrates where oxygen has difficulty reaching: deep in crevices, under deposits, just below the waterline and so on. The choice naming the oxygen-rich regions reverses anode and cathode. An oxygen concentration difference alone creates a potential difference, so the choice saying the whole surface corrodes almost uniformly is also wrong. Inside a crevice, hydrolysis of the dissolved metal ions acidifies the solution and chloride ions also collect, accelerating corrosion further (crevice corrosion). Bolted joints and areas under gaskets are typical sites.