An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part I. Establishment of equivalent circuits for EIS data modelling

Abstract

Electrochemical impedance spectroscopy (EIS) is a powerful analysis technique, which can provide a wealth of information on the corrosion reactions, the mass transport and the electrical charge transfer characteristics of physical vapour deposition (PVD) ceramic coated steels in an aqueous solution. Although a huge amount of potentially useful data can be generated using the EIS technique, these data need to be carefully interpreted. This is usually done using an ‘equivalent circuit’ which comprises an assembly of electrical circuit elements that model the physicoelectric characteristics of the electrode/solution interface. A systematic study of PVD ceramic (TiN and CrN) coated mild steel and AISI 316L stainless steel was carried out using the EIS technique as the coated systems were immersed in 0.5 N NaCl solution. The relevant equivalent circuits (ECs) are developed by non-linear least square curve fitting to the exponential data to build up a description of the influence of different coatings deposited on steels on the temporal evolution of corrosion in such systems. Constant phase elements describing the non-ideal (e.g. capacitive) characteristics of the electrochemical interface, designated as Q, are introduced to achieve a more accurate simulation of electrochemical corrosion. The mass transport behaviour is also dealt with, through the introduction of diffusion-related elements such as Warburg (designated as W) and cotangent–hyperbolic (designated as O) impedance. The use of these elements significantly improves the quality of fit of the simulation to the EIS data. Finally, the physical validity of the proposed models is discussed in terms of the current–frequency response of the coated steel electrode, during extended corrosion degradation over a period of immersion of up to one week.