Interactive effect of ant nest corrosion and stress corrosion on the failure of copper tubes。Among all the metals, copper has been one of the most common metals used for industrial and domestic purposes due to its excellent electrical conductivity, good mechanical workability, low cost and other relatively noble properties. It is a material commonly used in heating and cooling systems, and it is also used for electricity transportation due to its high conductance. A variety of environmental factors can easily cause corrosion of copper. Scale and corrosion products have a negative effect on heat transfer, and they cause a decrease in heating efficiency of the equipment.As for current situation, despite a lot of premature failure had been prevented effectively, a large amount of failures still exist.Formicary corrosion is also called ant nest corrosion. The term comes from the similarity of the morphology to an ant’s nest.
Method
Copper tube specimens of 40 mm in length, suspended at 45° in glass test tube, were exposed to the concentration of 1 vol% formic acid vapor in airtight glass vessels for different days. At the same time, thermal cycling experiments were conducted, holding the temperature by immersing the airtight glass vessels in a thermostatically controlled water bath at 40 °C for 16 h followed by 8 h at 25 °C to cover the first 24 h cycle. Thermal cycling was continued for up to 30 days. An extra sample of pure copper powder was also prepared by the same process. These tubes containing the leakage points were sawn from the copper tube for microscopic test. Then the areas of leakage points were investigated by scanning electron microscope (SEM), EDS to analyze the composition of corrosion products.
Several micro-cracks can be seen obliviously. Actually, these micro-cracks are the laminations/folds which could have been produced during tube manufacturing.The pitting holes are distributed regularly, almost all pitting distribute on the bottom of internal groove. The edges of the ant nest corrosion tunnels propagate along grain boundaries and the edges of the stress corrosion tunnels exhibit transgranular fracture.The stress corrosion cracking process normally propagates by intergranular fashion with multiple branching. EDS was used to analyze two representative points in the cross section of the leakage tube, and the main corrosion products identified were Cu2O.Cu2O was identified as the main corrosion product, and the presence of Cl ions was identified as a significant factor in stress corrosion. The interaction of ant nest corrosion and stress corrosion that lead to the failure of copper tubes. The stress corrosion accelerates the corrosion rates of ant nest corrosion.Owing to stress corrosion accelerate the corrosion rates, the corrosion products don’t have enough time to accumulate and expand leading to the morphology of interactive corrosion is thinner than single ant nest corrosion.The corrosion starting point also exist difference. The single ant nest corrosion starting point is irregular. In contrast, starting point of the interactive corrosion tends to distribute at the bottom of internal groove where the tensile stress is concentrated.
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