Abstract: A physical and chemical inspection was conducted on a thin-walled stainless steel pipeline with medium leakage in one power plant. The cause of the leakage was analyzed using methods such as intergranular corrosion test, scale sample XRD analysis, microstructure inspection, and leakage point analysis. The results showed that the leakage was caused by pitting corrosion perforation caused by residual water (industrial water) in the pipeline. Based on the characteristics of pitting corrosion, a corresponding phased array ultrasonic testing process was designed and developed, and an ultrasonic phased array transverse wave self focusing transducer was used to detect pitting corrosion defects at welded joints. The experimental results showed that this process can effectively detect typical pitting corrosion defects at the tube sample, with efficient detection and intuitive results.