Ant's Nest-like pitting attack on copper tube and its analysis

Electrochemical trials and efforts have been made to clarify mechanism of the ant's nest-like pitting attacks which had been observed on the copper tube used in air-conditioning units. It has been known and pointed out that the attacks require some conditions such as wet atmosphere, oxygen and formic acid. Experimental researchs were made on corrosion variables of corrosion potential and pH values in 102 ppm formic acid solution for about one month exposure. The corrosion reactions composed the ant's nest-like pitting attacks were thermodynamically estimated and its process was conveniently considered as two stages; in early corrosion stage and in steady stage. A kinetic study by polarization resistance curves revealed that reactions in alkaline atmosphere played important role and showed characteristics with fast rate reaction system. It is also indicated that the pitting attacks will occur not only on phosphorus-deoxidized copper tubes but also oxygen-free high purity copper tubes.