[en] Immersion tests, electrochemical measurements and surface characterizations (Scanning electron microscopy, Energy dispersive spectroscopy, Laser scanning confocal microscope) were used to study the corrosion behavior of a 3Cr low alloy pipeline steel welded joint in CO₂-saturated oilfield formation waters. The formation mechanism of the corrosion scale was discussed with in situ pH monitoring of various regions in the welded joint. In the initial stage of corrosion, the lowest potential and the highest corrosion rate were found on the base metal (BM). After the immersion of 120 h, the heat affected zone (HAZ) underwent the slightest corrosion, which may be related to the formation of a dense compact outer layer (mainly composed of FeCO₃) on the HAZ. In pH monitoring tests, the HAZ showed a higher pH level than the BM, which favored the nucleation of FeCO₃ and led to much more FeCO₃ deposition on the HAZ. (paper)