Effect of Expansion Process on Corrosion Resistance of Copper Tubes Inner Surface

Abstract

Residual carbon on the inner surface of copper tubes is known to be a cause of pitting corrosion. We showed previously that the rapid filling test was useful to evaluate the pitting corrosion resistance of copper tubes. Immersion tests using the rapid evaluation test solution showed that corrosion occurs on the entire surface of copper tubes with low residual carbon amounts, while those with high residual carbon amounts show pitting corrosion. Therefore, it was necessary to improve the corrosion resistance of copper tubes with high residual carbon amount, which was expected to undergo pitting corrosion. As pitting corrosion occurs when anodes were locally concentrated on part of the metal surface, it had been suggested that anodes be dispersed over the entire surface by the processing of the metal surface. Metal processing had various purposes, including changing the shape and properties of metals, and in this case, leading to desirable surface properties (such as expansion and drawing processes). Here, we focused on the expansion process and its effects on corrosion resistance of copper tubes. As a result, no corrosion was observed in the expanded copper tubes, which was expected to improve corrosion resistance. The composition analysis of the inner surface of the copper tubes showed a peak intensity of Cu₂O only in the expanded copper tubes (no processing), suggesting that an oxide film was formed by the expansion process.