Zairyo-to-Kankyo
Online ISSN : 1881-9664
Print ISSN : 0917-0480
Conference Publication
Effect of P Contents and Tube-shape on Stress Corrosion Cracks in Hair-pin Bending Copper Tube
Hirokazu TamagawaKoji KanamoriKozo KawanoYoshihiko KyoYoshiyuki Oya
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2018 Volume 67 Issue 6 Pages 251-255

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Abstract

The susceptibility of copper to stress corrosion cracking is increased by the addition of Zn, P, As, Sb, Si and other elements. Among these, P, As and Sb can lead to substantial increases even when present in extremely small amounts. P is an important additive element used in copper tubes, recently found to enhance resistance to ant-nest corrosion in Cu-P alloys with concentrations increased 10-fold. The problem of stress corrosion cracking in copper heat transfer tubes, typically used in air conditioner heat exchangers, is known rarely phenomenon. But its relationship to high-phosphorus alloys is not yet understood. In this study, the relationship between highphosphorus copper tubes and stress corrosion cracking was analyzed by investigating the effects of P concentration and tube-shape on stress corrosion cracking in hair-pin bending copper tube.

We confirm that alloys with small amounts of added P (0.027%) were prone to grain boundary corrosion. However, no clear difference in the degree of corrosion progression between Cu-0.027%P and Cu-0.38%P alloys was observed. While the segregation of P to grain boundaries in high-P alloys is a clear cause for concern, no segregation was observed in alloys with concentrations from 0.02% to 0.4% in our FE-EPMA analysis. We conclude that residual stress in hairpin bent copper tubes is a rare phenomenon. Similar results were found for thin-walled copper tubes. But the stress corrosion cracking in Cu-P alloys tended to occur when there were abnormalities in the bending process. As the shape of hairpin bent tubes became non-uniform, stress concentrated around local area, resulting in residual stress and stress corrosion cracking.

In summary, it was found that while shape defects in hairpin bending process reduce the resistance to stress corrosion cracking in Cu-P alloy tubes, variations in P concentration from 0.02 - 0.4% produced no significant effect on stress corrosion cracking.

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© 2018 Japan Society of Corrosion Engineering
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