Corrosion and Degradation Behavior Using Aluminum and Copper As Refrigerant Piping
Authors: Uchiyama Seiji , Kimura Yuji , Hiraoka Koji , Uemura Satoshi , Seki Shiro 
Published: February 2024

Introduction

Many types of refrigerant piping use copper alloys owing to their high workability, thermal conductivity and weatherability. However, copper is the 25th place in Clark number (relative abundance of a chemical element, typically in Earth’s crust), and have the relative low value (ca. 0.01). Therefore, there is concern about depletion, if that is consumed in large quantities via emerging country and new devices such as electric vehicle and so on. Recentry, we focused on aluminum as light metal, which is the 4th place in Clark number (ca. 7.6), and applied for refrigerant piping of air-conditioning equipments, and the practical application has been achieved in the some case. Moreover, in the case of practical use for pipework of actual equipment, long-term (more than 20 years) performance evaluation by corrosion is neccesary. Therefore, in this study, we investigated corrosion-resistant characteristic of various aluminum alloys by accelerated degradation tests with changing storage tempetature and atmosphere. In general, the corrosion factor of the piping materials depends on their contact atmosphere, and differs inside (refrigerant) and outside (e.g., air) of the piping. In concrete terms, there are concerns about the acid of refrigerant oil and salt corrosion of sea water at inside / outside of the piping. According to the usage condition of piping, we proposed immersion tests and cyclic corrosion tests such as intermittent cycles between salt mist and drying, and chemically analyzed by microscope, weight observation and corrosion potential measurements [1].

Experiments

All experimental copper and aluminum samples were obtained as commercially used refrigerant piping of Japan. To evaluate the corrosion resistance of the materials in contact with the refrigerant (inside piping corrosion), immersion tests were performed in an electrolyte which was a mixture refrigerant fluid R-410A (a difluoromethane / pentafluoroethane mixture 50:50 by mass) and refrigeration machine oil (synthetic oil that consisted of polyvinyl ether, an antioxidant, and a wear-resistant agent).
Immersion tests were conducted at 393K (maximum temperature for practical use) for 20 days. The appearance, sample mass, and self-electrode potential of the test samples were investigated before and after the immersion tests, as fundamental evaluations.
To investigate the corrosion resistance of the materials in the field environment (outside piping condition) a cyclic degradation test was performed according to JASO M610-92. One cycle of this Japanese Automotive Standard lasts 8 hours contains 3 process steps:

a tridimensionality spray of saltwater (5.0 wt.% sodium chloride, pH 6.5–7.2, 308 K) for 2 h

a dry process (333 K, humidity: 25 %) for 4 h

a wet process (323 K, humidity: 95 %) for 2 h

The samples were removed from the testing process at specific times (8, 16, 24, 96, 192, 384, and 496 h), and were investigated in terms of appearance, mass loss and depth of pitting corrosion.

Results & Discussion

Immersion tests results show negligible damage to copper and aluminum alloys by chemical and thermophysical effects up to 393 K, the maximum temperature for practical use and they confirm useful piping properties for the inside of aluminum and copper alloys.
Fig. 1 shows the explored time dependences of corrosion weight loss for two-types copper alloy in cyclic corrosion tests. Both logarithmic of corrosion weight loss for two copper alloy increased with the logarithmic of explored time until 496 h. The passivation film formation at copper alloy should not be occurred by the cyclic corrosion conditions for outside piping. Fig. 2 (a) shows the explored time dependences of corrosion weight loss for two-types aluminum alloy in cyclic corrosion tests. Weight loss of A6063-T83 was relatively larger than that of A3003-O. Although the corrosion weight loss of A3003-O linearly increased up to 96 h and showed stable weight passage of ca. 0.5 g m-2, that loss of A6063-T83 also linearly increased up to 96 h and exhibited stable and huge weight passage of ca. 1.5 g m-2 resulting from the difference of passivation film formations. We also observed each depth profiles of each corrosion pitch, and confirmed favorable accordance with the mass loss (Fig.2(b)). The accelerated degradation results for 496 h of cyclic corrosion tests are corresponding to 1.5 years of exposure on the Okinawa Prefecture coastline and to 20 years of exposure in Kariya city, Aichi Prefecture, an inland area. Therefore, we experimentally demonstrated the high weathering resistivity of aluminum alloy piping, which can exceed that of copper alloy. We have also demonstrated the 20 years durability of industrial piping in inland areas that do not experience salt-induced corrosion.
[1] S.Uchiyama, et., al., Mater Corros.(Wiley), 74, 344-351 (2023).

Figure 1