Quantitative evaluation of pit sizes for high strength steel: Electrochemical noise, 3-measurement, and image-recognition-based statistical analysis

In this study, pit depths, diameters, and locations of high strength X80 pipeline steel in aerated sodium chloride solutions are quantitatively investigated by electrochemical current noise (ECN) measurements as well as confocal laser scanning microscopy (CLSM). From the results obtained via ECN signals and SEM images, pits are believed to be initiated immediately, and current transients are strongly affected by the cathodic process. The computer simulation of ECN reveals that overlapping processes lead to both increase and decrease of transient amplitude depending on the time delay of original transients. Furthermore, the pit sizes estimated from the current transients are far lower than the actual pit sizes observed from SEM images. Hence, current transients obtained from ECN correspond to nucleation instead of the propagation process of pits. An image recognition technique is introduced for the purpose of recognizing corrosion pits from optical images, which significantly increases the efficiency of statistical analysis of pit diameters and pit locations. Results obtained from the statistical analysis of high strength pipeline steel in aerated NaCl solutions indicate that pit diameter follows lognormal distribution, and pit sites are completely spatial random.