Corrosion and degradation behavior of aluminum and copper refrigerant piping

Abstract

Many types of refrigerant piping use copper alloys because of such alloys' high workability, thermal conductivity, and weatherability. However, copper is the 25th most abundant element in Earth's crust (as indicated by its Clark number: a relatively low value, ca. 0.01). Therefore, there is concern about depletion, if emerging countries and new devices such as electric vehicles use large quantities of copper. Recently, we have focused on aluminum as a lightweight metal, which is 4th place in terms of Clark number (ca. 7.6). Refrigerant piping also uses aluminum in air conditioning equipment; thus, practical applications are available. Aluminum alloys used as material for piping also contain other elements such as manganese, silicon, magnesium, copper, and iron. Regarding practical use for pipework, long-term evaluation (more than 20 years) by corrosion is necessary. Therefore, in this study, we investigated the corrosion-resistant characteristics of various aluminum alloys by accelerated degradation tests with changing storage conditions, such as temperature and atmosphere. In general, the corrosion of the piping materials depends on the contact atmosphere and differs between inside (refrigerant) and outside (e.g., air) piping. Specifically, there are concerns about the acid of refrigerant oils (inside piping) and corrosion by the salts in seawater (outside piping). The results from immersion tests in refrigerant and cyclic corrosion tests (intermittent cycles between salt mist and drying) proved that aluminum alloys can be used for refrigerant piping in the same way as copper materials.