The alternation of an air-conditioner refrigerant from a freon-based refrigerant to CO 2 is proposed. A maximum temperature in the CO 2 heat-exchange cycle approaches to 180°C after brazing, which might affect susceptibility to intergranular corrosion. In this work, the intergranular corrosion susceptibility of Al-Mn-Cu alloys heat treated at 180°C after brazing was investigated. In both of the heat treated and non-treated Al-Mn-Cu alloys, Al-Cu intermetallic compounds were observed at grain boundaries, while in the non-treated Al-Mn-Cu alloys, Al-Mn intermetallic compounds also existed at the grain boundaries, being in contact with the Al-Cu compounds. The observation indicates that the Al-Mn compounds promote precipitation of the Al-Cu compounds at the grain boundaries. It also means development of Cu depleted zone along the grain boundaries. However, the intergranular corrosion susceptibility was decreased when the heat-treatment time become longer because Cu solid solubility in the grains decreased down to the same level of the Cu depleted zone. With the Mn concentration, the decreasing rate is accelerated and the time needed to show no susceptibility is shortened.

In an effort to improve the tensile strength of aluminum silicon (Al Si) alloys used in heat exchangers, we investigated the influence of Si concentration and heat treatment at 453 K on the susceptibility of Al Si alloys to intergranular corrosion. It was found that the susceptibility to intergranular corrosion increased with an increase in Si concentration. It also initially increased with heat treatment at 453 K, but then decreased with long term heat treatment at 453 K. The addition of Mg and Mn, which affect the precipitation of Si, promoted precipitation and reduced the susceptibility of the Al Si alloys to intergranular corrosion. With longer heat treatment at 453 K, large Si precipitates were observed in the grains and at the grain boundaries, which reduced the susceptibility to intergranular corrosion. Short term heat treatment at 453 K formed a continuous Si depleted layer along the grain boundaries, which increased the susceptibility to intergranular corrosion. It is suggested that the susceptibility to intergranular corrosion was dependent on the addition of Mg and Mn.