Study of Measuring Method for Corrosion Behavior Analysis in Low Concentration Solution
Authors: Yagi Yuta , Suzuki Ryoji , Noda Kazuhiko 
Published: February 2020

1. Introduction
Many metal materials are used in construction that is water treatment plant, power plant, dam and gate etc. And these metal materials are exposed in low concentration solution like river water, drainage water, ground water and lake water etc. These water is called fresh water, and fresh water is water that is concentration 1000ppm or less and is electric conductivity 200mS･m-1 or less (insulation resistivity is 5Ω･m or more). It is generally thought that corrosion does not occur in low concentration solution. But there are serious damages to construction by corrosion in low concentration solution. In fact, the flash water is accessible corrosion environment. Nevertheless corrosion behavior analysis in low concentration solution is not tested by electrochemical method and electrochemical data for corrosion in low concentration solution is very small. The reason is presumed that it is difficult to measure with DC method like polarization curve measurement in low concentration solution. Corrosion current for pure corrosion reaction is not measured by DC method because high solution resistance has big impact to measurement result. While on the other hand, AC impedance spectroscopy which is able to separate to solution resistance and charge exchange resistance is effective method of corrosion in low concentration solution. Therefore, the object of this study is consideration of optimal AC impedance spectroscopy method for corrosion behavior analysis in low concentration solution.
2. Experimental Methodology
Test materials were several different types of metals. And the test materials are casted epoxy resin after the test materials were adhered lead cable by Ag paste. Test surface was 10mm×20mm. Test surface was grinded in water by emery paper (#1000). Test piece was cleaned by ultrasonic cleaning in ethanol before measurement. Test solution was NaCl solution of ten concentrations (0.0001M~5M). The solution temperature was ambient temperature, and in atmosphere. Measurement was three electrodes method and two electrodes method. In three electrodes method, the test surface was fixed while keeping distance (2mm) between tip of luggin capillary. And the counter electrode was Pt plate, the reference electrode was silver-silver chloride electrode saturated KCL. In two electrodes method, working electrode and counter electrode were same metals. After immersed in solution for 1800 s, natural potential was measured, and impedance spectroscopy was measured. AC potential amplitude was ±10mV, periodicity range was 100kHz~10mHz.
3. Result
The inductive loop appeared at high frequency side in 0.05M or less Cl- concentration solution in nyquist plot measured three electrodes method. it is necessary that the inductive loop is cleared because the inductive loop is not behavior of corrosion reaction. And so impedance spectroscopy was measured by two electrodes method. The inductive loop disappeared when impedance spectroscopy was measured by two electrodes method. Measurement with the use of three electrodes cell was suitable measurement method in 0.05M or more Cl- concentration solution, and measurement with the use of two electrodes cell was suitable measurement method in 0.05M or less Cl- concentration solution. The charge exchange resistance of pure iron in 0.0001M～0.5M NaCl solution diminishes along lessening of Cl- concentration. And the charge exchange resistance of pure iron in 1M～5M NaCl solution increases along increase of Cl- concentration. Diminish of the charge exchange resistance results from breaking of oxide layer by Cl-. In contrast, increase of the charge exchange resistance results from diminish of dissolved oxygen in solution and absorb of Cl- on surface of iron. Therefore, the charge exchange resistance of pure iron in low concentration solution is higher than the charge exchange resistance in high concentration solution. But, in low concentration solution, concentration behavior of Cl- heavily affect to the charge exchange resistance.