<p>S13Cr tubing occurred sever pitting corrosion failure during the operation of a gas well in Bohai oilfield. In order to identify the root cause and reveal the pitting corrosion mechanism, a systematic failure analysis was conducted on the failed S13Cr tubing. The research methods included macro-surface morphology observation, fluorescent magnetic particle inspection, metallographic structure analysis, microscopic corrosion morphology characterization, and corrosion product composition testing. Additionally, high-temperature and high-pressure autoclave experiments were performed to simulate the downhole conditions and the weight loss corrosion behavior of S13Cr tubing was evaluated. Results showed that, although the chemical composition and mechanical properties of S13Cr tubing conform to API Spec 5CT standards, metallurgical defects are identified as the root cause of the pitting failure. Non-metallic AlN inclusions up to 32.5&#xa0;μm in size are found in the S13Cr tubing, and a pitting corrosion rate of 0.2740&#xa0;mm/a is measured under simulated annulus conditions. Pitting corrosion originated at non-metallic inclusions on the outer wall, where pitting nucleation preferentially occurred in the annular protection fluid with high content of Cl⁻. With the prolonged service time, the matrix was continuously eroded by corrosive ions in the solution medium, leading to pit growth and radial propagation. Notably, the distinct intergranular microcracks propagating were formed at the pit bottom where corrosion products had accumulated. After the grain boundaries are damaged, severe grain detachment occurs, and the substrate undergoes a repeated cycle of corrosion, detachment, and further corrosion until perforation. It is suggested that the tubing material quality should be strictly controlled and the annular protective fluid should be prepared with freshwater to improve the purity and reduce pitting damage to the tubing.</p>