Sensitization and Tunneling Corrosion of Austenitic Type 347 Stainless Steel

Sensitization of type 347 (UNS S34700) austenitic stainless steel (SS) samples removed from forged bars was investigated using the electrochemical potentiokinetic reactivation (EPR) method and the weight-loss technique of ASTM A 262, Practice B. A normal and a low-carbon steel were investigated. After solution-annealing at 1,050°C, the two steels were submitted to sensitization treatments at 550°C, 670°C, 790°C, and 910°C for times varying from 1 h to 130 h. The steel with normal carbon content also was solution-annealed at 1,140°C and submitted to the same sensitization treatments for times varying from 1 h to 62 h. Correlation between results obtained by the two techniques was very poor. The lack of correlation was ascribed to tunneling corrosion, which is typical of forged steels, in addition to intergranular corrosion resulting from sensitization. The electrochemical test was most sensitive to corrosion by sensitization. The Practice B test did not discriminate between the two types of attack. The steel solution-annealed at higher temperature was more susceptible to sensitization.