Mechanism of the so-called “Active Path Corrosion Type Stress Corrosion Cracking”

Abstract

Advanced acoustic emission (AE) monitoring and signal processing have been attempted to study the mechanism of the so-called APC-type intergranular SCC of the sensitized AISI 304 steel in 1000ppm NaF solution and 0.1wt% acidified tetrathionic solution at ambient temperature. AE signals (out-plane displacement) from the Mode-I cleavage-like fracture were monitored during both the CERT and the constant load test of a compact tension (CT) and a plate specimen, respectively. Generation of the cleavage-like fracture showed following stress dependancy which was characteristic to the material-environment combination, i.e., AE signals in the fuloride-SCC were mainly observed at the stresses which induced small scale yielding at the tip of the notch or SCC. However, the fracture size was limited to about 20μm. On the contrary, cleavage-like fractures larger than 300μm were observed at higher stresses for tetrathionic-SCC.
Generation velocity of the cleavage-like fracture estimated by the AE source inversion processing was found to be almost 10⁷ to 10⁹ times that of SCC propagation rate for the fluoride and the tetrathionic SCC, respectively. Feasible mechanism for such fast fracture; stress sorption grain boundary decohesion fracture for the fluoride SCC and hydrogen assisted fracture of the martensitic phase as well as the stress sorption fracture for the tetrathionic SCC, were discussed.