Résumé : Under-deposit corrosion (UDC) is one of the main factors leading to perforation or leakage of gathering pipeline in the oilfield. In this paper, galvanic corrosion of mild steel under CaCO3 deposit was investigated by using wire beam electrodes (WBE) in the oilfield-produced water. Potential and galvanic mappings indicated that the wire electrodes covered by CaCO3 deposit mainly acted as anode while the rest bare wire electrodes acted as cathode at the beginning of corrosion, causing apparent UDC. However, the deposit-covered electrodes were finally transformed from anode to cathode along with the bare electrodes from cathode to anode, indicative of intensive galvanic polarity reversal. It is supposed that FeCO3 corrosion products could fill into the deposit layer and form a physical diffusion barrier to block the transportation of aggressive ions from the bulk solution to the metal substrate.