Failure analysis of the leakage in girth weld of bimetal composite pipe

Abstract

A bimetal mechanically-composite pipe leaked at the entrance of processing station. The failure location was the 6o'clock direction of the girth weld connecting the gathering pipe and pipe in the station. In order to figure out the root cause of the failure, a series of experiments were carried out. The macroscopic analysis and metallographic test results show that the leakage point is located at the 6o'clock direction of the girth weld, the root welding and the base pipe of the bimetallic composite pipe are corroded, resulting in perforation. The end of the bimetallic composite pipe is not seal-welded, the girth weld has misalignment and at 6o'clock, the root weld was concave, and the wall thickness of the root weld and the lining is<1.5 mm. The FeCl3 pitting corrosion test results of the girth weld show that the pitting corrosion resistance of the root weld is lower than that of the liner and pure stainless-steel pipe. The galvanic corrosion test shows that the galvanic corrosion grades of weld-pure material, weld-liner, carbon steel-weld, carbon steel-liner are all severe corrosion, and the galvanic corrosion of carbon steel-weld is the most serious. The root cause of the failure of girth weld puncture leakage: the end of the bimetallic composite pipe is not seal-welded. At 6o’clock, the girth weld has misalignment, and the root weld is concave, resulting in that the thickness connecting the root weld and the liner being small. In the medium environment containing Cl- and CO2, local corrosion occurs, and then the medium enters the base pipe and the liner interlayer, and the galvanic corrosion aggravates the corrosion of the base pipe. To avoid the reoccurrence of such failure, the mitigation measures were proposed.