Extreme-Value Distribution of the Metastable-Pit Depth Estimated by Potential Noise Method
Hiroyuki INOUE, Riki MORIYA and Rokuro NISHIMURA
Abstract:The extreme-value distribution of metastable-pit depths on SUS304 stainless-steel electrodes was examined. The electrode with the surface area of 10mm2 was immersed in a 10mass% NaCl solution at 60, 70 or 80ºC, and measured its potential noises for 48hours. The depth of the metastable pits was calculated from the electric quantity of the local anodic currents that were estimated from the measured potential noises by using the RPS method, assuming the shape of the metastable pits as true hemispheres. The maximum pit depths for 7 to 8 samples at 70 and 80ºC were fairly approximated to a Gumbel distribution. The minimum surface area required for forming the stable pit was estimated about 10000 mm2, under the conditions employed in this study. From the return period equivalent to this surface area, the critical pit depths at 70 and 80ºC were estimated 6.4 and 9.4μm, respectively. Key Words:Electrochemical noise analysis, ENA, pitting, monitoring, extreme-value statistic, EVAN, critical pit radius