Since December 2022, the pipe wall thinning or leakage phenomenon has frequently occurred in a power plant. The failure parts mostly occurred at the lower part of the horizontal pipes. The failure position is 30 mm–60 mm before and after the pipe weld. The initial leak point is small. After being tapped by the hammer, a large area of the pipe section near the leak point is broken. At this time, two TP316L ammonia pipes’ wall of a 600 MW power plant unit were found thinner after more than two years of operation. Causes of thinning were analyzed by macroscopic examination, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis, and X-ray diffraction test (XRD). Results show that a large number of parallel-distributed second phases in the matrix of the base metal, which indicates an inadequate heat treatment of the original pipe. Corrosion products were found on the inner wall of the pipe, which are generated by the reaction of the pipe with NH3, CO2 and H2O, and no other abnormal phase is found. The results show that the main reason for the failure of the ammonia pipeline is methylammonium corrosion, and the secondary reason is the original metallographic microstructural defect of the pipeline material.