In this work, the effect of the size and geometrical shape of coating defects on pipeline corrosion under alternating current (AC) interference in a deoxygenated, near-neutral pH bicarbonate solution was studied by direct current (DC) potential analysis, polarization curve measurements, and surface characterization. The results demonstrate that the size of the defect is critical to AC corrosion of steel occurring at the defect base. When a defect is small, the threshold AC to induce pitting corrosion is low. Moreover, both anodic and cathodic current densities decrease, which is attributed to the blocking effect of corrosion product accumulating at the defect. The shape of the defect affects corrosion only when the defect is small, e.g., 5 mm in diameter. The effect becomes undetectable when the defect is large, e.g., 10 mm in diameter. Generally, circular and triangular defects are associated with the most and least negative DC potentials as well as the largest and smallest anodic current densities, respectively, while square-shaped defects fell somewhere in between in terms of both potential density and current density values.