Content area
Full Text
Introduction
A theoretical electrochemical impedance spectroscopy (EIS) study was made to simulate several cases that can occur on metallic coatings that are exposed to sour media, in order to predict the corrosion behaviour of low‐carbon steel (Azuma et al., 2004). In one case, the simulation was made by a nickel thin film configuration with two different surface coatings. In another case, the presence of a passive layers using NiO2 (nickel oxide) to protect against the corrosive media was used. The techniques for the deposition of these thin films could form an adhesion to the substrate, or have some textural and mechanical properties, yielding the anticorrosive barrier. The main reason for using the oxide thin film was for its corrosion resistance.
Simulation and discussion
A simulation of the EIS technique with the metallic coatings was assessed in the electrolyte NaCl 3 wt% + H2S (saturated) by Liu et al. (1999), Barbosa et al. (1998), Cheng et al. (1999, 2000) and Ma et al. (2000). The frequency range proposed for this work was from 10,000 to 0.01 Hz, with a voltage amplitude of 10 mV, and ten integration cycles, as per ASTM Designation: G106‐89 (1999). The simulation of the electrochemical processes was carried out by the fitting equivalent circuits to illustrate the effects of different electrochemical processes that could occur on the coated material when exposed to the electrolytes. This scope of the study was limited to the consideration of the different possible processes and only then to propose their equivalent circuit.
All the nomenclature used was based on the Zview program version 2.7 which was utilized for the construction of the different equivalent electrical circuits (ECs), taking the electrolyte resistance as R1 and the different capacitors such as Cn, with n the element which describes the system that appears (Cottis and Turgoose, 1999). The simulations were carried out considering seven different systems, setting the R1 (Rs) values and fitting the rest of the parameters to obtain the theoretical EIS plots. The values of R1 were different because all equivalent circuits represented different systems as well; for example, some of the EC described films free of defects, whereas others described oxide films with macro‐pores or some other defect. Table I shows the values of the...