The Study Of Pitting Dynamics For304Stainless Steel In3.5%NaCl Solution

The pitting corrosion behaviors of304stainless steel in3.5%NaCl solution werestudied by dynamic potential electrochemical impedance spectroscopy (DEIS), timescan electrochemical impedance spectroscopy (TSEIS), electronic speckle patterninterferometer (ESPI), electrochemical noise (EN), ultraviolet spectrophotometry (US)and three-dimensional video microscope. Dynamic information and mechanisms ofpitting were investigated. The main research contents and results are as follows:Electrochemical signal characteristics at different pitting stages were investigatedby DEIS and TSEIS, via this to characterize the pitting mechanism. The results showedthat metastable pits emerged at the potential (0.02V(SCE)) which was more negative thanthe pitting potential (0.15V(SCE)), and the generation and passivation of metastable pitswere stochastic. The steady pitting potential which got from DEIS was negative thanbreakdown potential which got from dynamic potential polarization by0.05V. Pittingnucleation can happened just when the thickness of passivation film reduced to a certaindegree.the metastable pitting behavior was studied combining constant potentialpolarization and three-dimensional video microscope. The results indicated that activedissolution rate of single metastable pit increased gradually with time. Nucleation rateof metastable pits was increased with polarization potential increasing. The dissolutionrate and average peak current of metastable pits were increased with polarizationpotential rising. The expansion rate of pits mouth was greater than the expansion rate ofpits depth, and formed a “bowl” structure. Pitting current density of single pit wasbasically unchanged at pitting active dissolution stage. This indicated that thedevelopment of metastable pits were controlled by diffusion of corrosion products.Pitting rate of a single pit and pitting mechanisms of the304stainless steel wereinvestigated by utilizing ESPI, EN, US and three-dimensional video microscope. Theresults showed that under0.05V(SCE)polarization, the pitting corrosion process can bedivided into four stages: Firstly, drastic fluctuations of the current noise occured at740s, which means the passivation film was breaking, thus it can be concluded that thespan of pitting incubation period is740s; Secondly, a speckle occured on the ESPIimage at750s, thus the span of initiation period of the steady pitting is about10s;Thirdly the growth rate of the pit increased during750～780s, which indicates that thepit corrosion is in active dissolution period; Since then, the growth rate of the pit declined rapidly which means the pit was repassivated. After793s, the growth rate ofthe pit raised again as secondary pits emerged. The pit morphology was observed andits volume was measured by three-dimensional video microscope, and the results werein agreement with those which were obtained by corrosion product concentrationanalysis method. Some secondary pits were found in the bottom of the pit inthree-dimensional reconstruction images.Steady pitting behavior, the geometrical characteristic of pits and the mechanism ofpits sustainable development were studied by constant potential polarization andthree-dimensional video microscope. The result showed that when the polarizationpotential upon0.35V(SCE)would obtain open type pits, and would get the enclosed typepits below0.35V(SCE). The metal plates covered upon pits mouth would cracked withpolarization potential increasing. Pitting current, pits depth, pits mouth diameter andpits volume were increased with polarization potential increasing. The development ofsingle pit controlled by corrosion products diffusion at every polarization potential. Thediameter expanding speed of enclosed type pits mouth would reduced with time,however the depth expanding speed would increased with time. The enclosed type pitsshape would changed from “cone” to “bottle”. The ratio of pits mouth diameter and pitsdepth was steadily in3～4for open type pits, and the pits mouth diameter and pitsdepth increased linearly with time. The shape of open type pits were half ellipsoid.