The formation of a pit in the surface of a boiler tube is most likely to occur when __________.

Correct Answer: Option D — the tube metal acts as an anode

Electrochemical corrosion drives pitting on waterside boiler tube surfaces. When local galvanic cells form due to differential aeration, deposits, or stressed metal, the specific tube area acting as the anode suffers rapid loss of metal electrons, culminating in deep pit formation per standard marine engineering corrosion theory.