Corrosion Degradation Behavior of Bending Aluminum and Copper Materials for Air Conditioning Refrigerant Piping
Authors: Uchiyama Seiji , Uemura Satoshi , Hayashi Takeharu , Kimura Yuji , Seki Shiro 
Published: August 2024

Introduction

Many types copper alloys of refrigerant piping are used for practical use owing to their high workability, thermal conductivity, and weather ability. However, there is concern about depletion, due to consume in large quantities via emerging country and new devices such as electric vehicle and so on. Recently, we focused on aluminum as light metal, and applied for refrigerant piping of air-conditioning equipment. We focused on corrosion factors of piping for inside (refrigerant passage) and outside (atmospheric exposure) and investigated simulated practical used straight sections. As a result, almost equal practicalities were confirmed for both cases of aluminum and copper [1].
In actual refrigerant piping, bending sections are always appared utilized alongside straight pipes. When pipes are bent using a bender, tensile stress occurs on the outer surface and compressive stress on the inner surface. Stress corrosion cracking (SCC) can be induced under tensile stress in corrosive environments. Evaluating SCC potential in aluminum piping due to bending is crucial for its long-term use in refrigerant piping. Therefore, in this study, mechanical, electrochemical characteristics, and weather resistance characteristic of bending aluminum and copper piping under tensile stress were investigated to confirm practicality of Al refrigerant piping.

Experiments

A6063-T83(Al-T) and C1220-1/2H(Cu-H) were used for the aluminum materials and utilized copper materials, respectively. To confirm the electrochemical characteristics in order to investigate the difference in the bending curvature, natural electrode potential measurements were examined at eight bent points (four on the tensile side, four on the compressive side) and one straight segment. The natural potential of the samples was measured using a three-electrode method, with the samples, Pt wire, and Ag/AgCl as working, counter and reference electrodes, respectively, immersed in a 3.0 mol L-1 NaCl solution. Measurements were carried out using a potentiostat (HZ-5000, Hokuto Denko) at 303 K maintained by a constant-temperature water bath. To investigate the corrosion resistance of bending aluminum and copper piping, cyclic degradation test was performed according to the Japanese Automotive Standard JASO M610-92. Each cycle, lasting 8 hours, comprised three steps: 1. neutral saltwater spray (35°C, pH 6.5-7.2, 2h), 2. drying (60°C, 25% RH, 4h), and 3. humidification (50°C, 95% RH, 2h). Specimens were removed at 8h, 24h, 48h, 96h, 192h, 384h, and 496h during the cyclic degradation test for appearance observation. In addition, Samples at 496h, cross-sectional observations using SEM (JSM-7200F, JEOL) were performed after etching the samples at the tensile and compressive sides of the bent sections and the straight segment.

Results & Discussion

Fig.1 shows the time dependence in the natural electrode potential of tensile side of Cu-H. At all measurement points, stable natural potential of -0.27 V vs. Ag/AgCl(V) and no significant natural electrode potential changes were confirmed during the measurement time. Fig.2 shows the time dependence in the natural electrode potential of tensile side of Al-T. At all measurement points, the average natural potential value remained approximately -0.75 V vs. Ag/AgCl(V) across all measurement points, and no significant natural electrode potential changes were observed during the measurement time. However, deviations in the natural electrode potential values were also slightly observed at measurement points c and d with a large curvature in the bent part. These results suggested that regions with increased curvature due to bending might be occured to destroying to the oxide film on the surface of the Al piping, indicating subtle yet progressing oxidation. Fig 3. shows cross-section SEM images of the tensile and compressive sides of the bent sections and the straight segment of Cu-H after 496h cyclic degradation test. No significant were observed in all observation points.The results of this measurement indicate that the no evidence of corrosion of the copper tube due to tensile stress. Fig 4. shows cross-section SEM images of the tensile and compressive sides of the bent sections and the straight segment of Al-T after 496h cyclic degradation test. Thin oxide observed at all the measurement points, but there were no significant differences observed in its production at each of the measurement locations. In an environment of constant exposure to seawater, such as that encountered in the natural electrode potential measurements, chlorine can cause severe damage to the Al bending piping. On the other hand, cyclic degradation tests ,which simulated actual use for severe weather resistance performances, conditions suggested that the effects of Al bending are minimal, as the oxide film is re-formed during the wet-dry process. Thus we demonstrated that Al bending piping must use in practical useful.
[1] S. Uchiyama, et., al., Mater Corros.(Wiley), 74, 344-351 (2023).

Figure 1