Failure of a Type 304 Stainless Steel White Liquor Transfer Pipeline

A stainless steel pipe transferring hot white liquor solution of sodium hydroxide and sodium sulfite, developed leaks adjacent to the welds within four years of service. The stainless steel pipe was AISI type 304 and welded with E308 weld electrodes. The service temperature was 190 deg C (375 deg F) and the solution contained approximately 700 ppm chlorides. Liquid penetrant inspection of the pipeline showed the leaks were numerous and confined adjacent to the welds. A metallographic specimen from the circumferential weld showed the cracks initiated at the inside surface. In addition to the base metal, SCC also had initiated at a notch at the weld root due to improper welding procedures. Failure was attributed to chloride-induced SCC with secondary contributory factors, including improper welding procedures. It was recommended that the pipeline be replaced with a material more resistant to SCC. The candidate materials are commercial grade unalloyed titanium or Inconel 600, which have superior resistance to SCC compared to austenitic stainless steels.