EIS and Mott-Schottky to study the passive film properties of 316L and its susceptibility to pitting corrosion in CO<inf>2</inf>environments

Abstract

© 2016 Australasian Corrosion Association. All rights reserved. In the production of oil and gas under anaerobic conditions at high temperature and partial pressure of CO2, the use of corrosion resistant alloys, such as 316 stainless steel, are being increasingly used to replace carbon steel with inhibition. However, stainless steel may suffer pitting corrosion. This study investigates the relationship between the passive film properties in relatively mild CO2environments and the susceptibility to pitting corrosion. Electrochemical impedance spectroscopy (EIS) and capacitance measurements (Mott-Schottky) are used to investigate the semiconductor properties of the passive films formed on 316L stainless steel in 3 wt.% NaCl aqueous solution in CO2containing environment at different temperatures (30 C - 80 C) as well as different NaCl concentrations (3 wt.% - 8 wt.%) at 30 C. Its susceptibility to pitting corrosion is also studied with cyclic polarization technique. The results indicate that the pitting susceptibility of 316L stainless steel increase with increasing temperature and NaCl concentration. The properties of the passive film are influenced by temperature as well as NaCl concentration as identified by the 2 AC techniques. Bode phase plots of specimen at 30 C and 40 C show one time constant whereas two time constants are observed at high temperatures 60 C and 80 C. The two time constant feature observed at high temperature can be attributed to an inhomogeneous film consisting of a compact inner layer and a less compact (porous) outer layer. Capacitance measurements based on the Mott-Schottky relationship reveals a p-n type heterojunction in all conditions. The point defects as determined by the donor (ND) and acceptor (NA) densities increase with increasing temperature and NaCl concentrations indicating that the passive film becomes weaker and hence more prone to pitting corrosion.