Copper is used for tubes of absorption-refrigerator machines because it has good corrosion resistance and thermal conductivity. However, it has been reported that a leakage accident occurs due to pitting corrosion inside copper tubes in flowing fresh water[1]. The occurrence of pitting corrosion of copper is caused by the presence of bicarbonate ions which affect scale formation and aggressive anions such as chloride and sulfate ions. Therefore, the corrosion diagnosis method of copper using electrochemical measurements in fresh water has been reported in Japan[2]. This method can discriminate the corrosion type of copper by using a potentiodynamic polarization and galvanostatic polarization, and the corrosion map 3D can be created based on the results of electrochemical measurements and the pH of test solution. In the previous report[2], the current density at 0.2 V vs. SSE in the potentiodynamic polarization curve can separate the corrosion type of copper. Therefore, in this study, a new corrosion diagnosis method of copper by using corrosion map 3D was proposed by combining potentiostatic polarization of 0.2 V vs. SSE and galvanostatic polarization since the corrosion map 3D can be easily created by using the current density obtained by the potentiostatic polarization of 0.2 V vs. SSE.

In this study, simulated test waters with eight compositions were made using a pure water and analytical grade of sodium bicarbonate, sodium chloride and sodium sulfate. As a measuring device, a small potentiogalvanostat (TOHO TECHNICAL RESEARCH, PPS1) was used. Experiments were carried out with a three-electrode system. The copper electrode was used as working electrode, the platinum wire was used as counter electrode and the saturated silver/silver chloride electrode (DKK-TOA, HS-205C) was used as reference electrode. The procedures for measurement is described as follows: the potentiostatic polarization at 0.2 V vs. SSE for 10 min, the galvanostatic polarization at 500 µA cm^-2 for 10 min and the electrode surface was observed by optical microscope (KEYENCE, VHX-200).

From the observation results of the electrode surface using the optical microscope, pitting corrosion was observed at No.2 and No.7, general dissolution at No.3, No.4, No.6 and No.8 and no corrosion at No.1 and No.5. Fig. 1 shows the corrosion map 3D created by the electrochemical measurements. The axes of corrosion map 3D were the potential of copper electrode after 10 min by galvanostatic polarization at 500 µA cm^-2, the current density by potentiostatic polarization at 0.2 V vs. SSE and pH of the test solution. The plots of corrosion map 3D agreed with the observation results of the electrode surface, indicating the possibility of simple corrosion diagnosis by combining amperometry and potentiometry.

References

[1] E.Mattsson, A.-M. Fredriksson, British Corrosion Journal, (1968), 3, 246-257.

[2] Y. Hoshi, R. Kobayashi, I. Shitanda, M. Itagaki, Y. Hirata and T. Hishinuma, Zairyo-to-Kankyo, 65, 159-164 (2016).