1993 年 42 巻 479 号 p. 917-922
It is a great problem for air conditioning units that leakage accidents of refrigerant happen in an extremely short running time after installation. Such a trouble may be caused by ant nest corrosion of copper tube used for a heat exchanger. This type of corrosion is characterized by complicatedly divided microscopic caverns, and can be observed in the cross section of the corroded copper tube by using an optical microscope. The surface of the corroded copper tube usually becomes discolored greyish-brown or blue-violet without showing any corrosion product. This corrosion is produced by organic acid vapor originating from the inside or outside of copper tube.
This study reports the investigation of the growth mechanism of ant nest corrosion. The ant nest corrosion could be reproduced in the heating cycle condition of (298K×12hr_??_313K×12hr) for 30 days in a humid oxygen atmosphere containing formic acid vapor. Porous accumulation was observed in the corrosion caverns. The chemical analysis of the corrosion caverns by an electron probe microanalyzer confirmed that the porous accumulation was copper oxide and no formic acid complex existed in the corrosion caverns. This result means that the formic acid complex dissolves in the solution of inner corrosion caverns.
Consequently, the growth mecanism can be explained as follows. A cathodic reaction occurs as O2 +4H++4e-→2H2O on the wet surface of copper tube, and an anodic reaction occurs as Cu→Cu+ +e- in the inner corrosion caverns. Cu+ in formic acid solution is complexed by CO2H-: Cu++CO2H-→Cu(I)(HCO2). Then Cu(I)(HCO2) changes to Cu2O rapidly. Cu2O takes the form of porous oxide. This oxide is dampened with formic acid solution. It is thought that the ant nest corrosion grows as the oxidation reaction of copper at the interface of the porous oxide and metallic copper. Therefore, the more the porous oxide becomes wet, the more corrosion caverns widen. The complicated divided pattern of corrosion caverns is caused by the cracks of porous oxide that are formed by the change in volume of the porous oxide.