Study of Stainless Steel Localized Corrosion in Sodium Hypochlorite Solution by Detection and Analysis of Electrochemical Noise

摘要

Different types of AISI 316 stainless steel were tested in a sodium hypochlorite solution (7% active chlorine). Specimens, obtained from a 3 mm thick rolled sheet, were made according to ASTM E8 and mounted in constant load tension machines. A fraction of the immersed surface was covered by an epoxy resin in order to limit a 1 cm~2 free surface.Applied load was calculated in order to reach in the specimen gauge length a stress equal to 70% of the material yield strength (σ_(y_(0.2%)).Potential noise was collected between two identical specimens with a sampling rate of 1 sample/s. For each run 1024, 2048 or 3072 points were sampled to obtain the spectra. For each sample potential monitoring was carried out up to 240 hours. Potential noise was then analyzed in the frequency domain and correlated to corrosion attack. Corrosion developed initially as crevice under epoxy coating. Based on stainless steel composition different corrosion morphology was evidenced: generalized corrosion, cavernous pits or SCC phenomena.The power spectral density (PSD) of the potential noise reduced with time increasing as a consequence of an increase of the number of corrosion events. The slopes of the PSD curve in the medium frequency range were correlated to the type of localized corrosion phenomena: higher values being connected to pitting corrosion. The presence of a significant peak in the power spectra was probably due to the occurrence of a steady state crevice corrosion, however it was not yet possible to establish if there was a typical frequency for such phenomenon.