Abstract: A leak occurred in the weld of austenitic stainless steel instrumentation pipe for the essential service water system in a nuclear power plant.Macroscopic observation,chemical composition analysis,hardness test,metallographic inspection and scanning electron microscope analysis were used to analyze the reasons for the leakage of the instrumentation pipe.The results showed that the welding heat-affected zone on the inner wall of the instrumentation pipe formed a serious temper color,that was,a chromium-poor oxide layer.Under the action of chloride ions in seawater,pitting pits were formed on the inner wall of the pipe,the pitting pits continued to expand,and finally penetrate the entire pipe wall,causing the instrumentation pipe to leak.