Impedance Analysis for Steel Material in Low Electrolyte Solution Environment
Authors: Sawanobori Koyo , Yagi Yuta , Suzuki Ryoji , Noda Kazuhiko 
Published: February 2020

There is fresh water is only 0.04% of the water present on the earth, abundance as compared with sea water is extremely small. However, fresh water is an essential presence. This is because, from the fact that the human race in this water have built a civilization by committing their lives. Despite being the most familiar corrosive environment at the same time, a small electrochemical data in the fresh water environment. This is because, fresh water is present in rivers, lakes, a large number of types such as underground water, ion species containing, the amount is also abundant. In other words, fresh water is difficult to simulate freshwater environment because it is rich in diversity. Moreover, fresh water as compared to the seawater able chloride ion is very small. The conductivity is much lower, presumably because the electrochemical measurement is difficult. In this study, to investigate the corrosion behavior of metal materials such as pure iron, stainless steel SUS304, pure zinc, pure copper which is often used under a fresh water environment. And an object thereof is to consider impedance behavior at low electrolyte solution.
A test material pure iron as (99.5%), stainless steel SUS304 is used as a surface treatment material, pure zinc (99.5%), using four kinds of added practical with many pure copper. After polishing the test surface (10mm × 20mm) by emery paper (# 1000), what was subjected to ultrasonic cleaning was used by ethanol. The test solution was used NaCl solution. The three types of concentration was adjusted 0.01M, 0.001M, 0.0001M. AC impedance measured in two types of measurement method of the two-electrode methode and a three-electrode methode. In two electrode method using a sample of the same kind to the working electrode and the counter electrode, the distance was fixed at 50 mm, was immersed in aqueous solutions. The three-electrode methode using platinum as a counter electrode, the reference electrode (Ag / AgCl / KCl sat.). The distance between the sample surface and the luggin capillary is fixed to 2 mm, it was immersed in aqueous solutions.
After both the immersion potential of the two-electrode methode and a three-electrode methode was measured between the 1800s, the measurement was started the measurement frequency 100kHz ~ 10mHz, the applied potential amplitude as ± 10mV.
From the results of the AC impedance measurement. Pure iron (99.5%), pure copper, pure zinc are, there were confirmed that the reaction resistance is increased as the solution concentration is reduced from the result by three electrode methode and two electrode methode. Since the solution was determined that the corrosion reaction is suppressed as the sparse, Cl- is considered to have an impact as an electrolyte. Impedance behavior by the three electrode methode in stainless steel SUS304 is it was not possible to accurately determine the difference by the NaCl solution concentration. This is because the current flows in addition to direct measurement circuitry is believed to be difficult to analyze.
In contrast with two electrode method, it was found to detect differences in distinct reaction resistance. Many metal materials can be confirmed that the reaction resistance is increased as a solution concentration decreases, the solution is considered to corrosion reaction as the dilute was suppressed. However, it can be said that the reaction resistance due to the increase of the NaCl solution concentration was a increasing in SUS304. From this fact, the reaction resistance of SUS304 is considered to be affected by the Cl-.. Further, the results measured using a three-electrode method as the concentration becomes thin, inductive behavior to the high-frequency region has been confirmed. As a cause of this phenomenon, The solution resistance increases in accordance. With the concentration of the electrolyte is reduced, current is presumed to be because the current flows in the absence of potential measurement circuit be flowing. In the two electrode methode, without expression induction behavior at high frequency range, it is considered that it is an accurate measurement. From the results of this experiment, it can be said that in the extremely low environment electrolyte concentration is effective in impedance measurement by the two-electrode method is to analyze the rate of the reaction.