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The characteristics of grooving corrosion at a welding seam of QT800 coiled tubing were investigated under different conditions by using the conventional immersion test and constant potential accelerated corrosion test, respectively. The results showed that the sensitivity of grooving corrosion of the welded tube, represented by the grooving corrosion factor, was within the acceptable range even after the long-term immersion test. The reasons of the grooving corrosion were analyzed from the microstructure and electrochemical performance of the weld. Compared with the matrix and the heat-affected zone, the fusion line had coarse grains, large corrosion current, and low self-corrosion potential, which played a part in the main reason to induce the groove corrosion effect.