The Mechanism of Knife-Line Attack In Welded Type 347 Stainless Steel

Recent failures because of knife-line attack occurred in welded Type 347 stainless steel drums handling fuming nitric acid. Knife-line attack is intergranular corrosion in a narrow band adjacent to the weld. This attack occurs under specific conditions. The mechanism is based on the solid solubility of niobium (columbium) in 18-8 stainless steels at high temperatures with subsequent formation of grain boundary chromium carbide during the sensitizing treatment. This type of corrosion was observed in fuming nitric acid and also in boiling 65 percent nitric acid. Experiments were conducted on regular Type 347 and Extra Low Carbon Type 347 stainless steels. The latter appeared to be less susceptible to knife-line attack.