Initiation and propagation steps in pitting corrosion of austenitic stainless steels: monitoring by acoustic emission

Acoustic emission (AE) technique was used to study the development of pitting corrosion on AISI 316L austenitic stainless steel, in a 3% NaCl solution acidified to pH 2. The initiation and the propagation steps of the pits were separately studied owning to a specific polarization procedure. It appears that the initiation step of pitting corrosion is not significantly emissive, whereas the propagation step is characterized by the emission of resonant signals. This kind of AE signals is representative of the development of the pits in the form of occluded cells, in which the evolution of hydrogen bubbles appears to be the emissive phenomenon. A subsequent change in the mode of corrosion, i.e. the transfer to uniform corrosion, can be detected by the AE technique.