Corrosion Initiation in High Purity Nickel under Cathodic Polarization

Abstract

Corrosion is an electrochemical phenomenon that usually occurs on metal anodes. However, localized corrosion on the cathode has been reported many times, and its corrosion mechanism is still unclear. To gain a better understanding of the localized corrosion that occurs on the cathode, we will mainly study the localized corrosion initiation that occurs on high-purity Ni (99.9945 wt.% pure) which is cathodically polarized in an electrolyte solution of 1M H2SO4 + 1g/L NaCl, and the potential is -600mV vs the open current potential. In this thesis, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to study the relationship between the surface phase change of the sample and corrosion initiation, and Electron Backscatter Diffraction (EBSD) was used to determine the initial site of corrosion. It was finally found that this localized corrosion occurs when the surface phase coverage reaches a maximum of about 30%, at this time, the charging time is 100 minutes. Furthermore, the corrosion pits occurs preferentially at the grain boundaries of the Ni samples. The hypotheses of surface microgalvanic corrosion and H enrichment at grain boundaries are given.