Mechanism of So-called Erosion-Corrosion and Flow Velocity Difference Corrosion of Pure Copper

摘要

Piping tubes of pure copper are often used for a household plumbing or a coil for cooling water and so on, since it is excellent in corrosion resistance as well as in thermal conductivity. Leakage accidents, however, due to corrosion are sometimes reported. The cause of them may be a so-called erosion-corrosion or flow velocity difference corrosion, which are together categorized in flow-induced localized corrosion. So-called erosion-corrosion is originated through the turbulence in fluid flow, which breaks the protective film on the metal surface so that the area exposed to the fluid flow of higher velocity is possibly damaged. In contrast, the surface area exposed to the fluid flow of lower velocity is corroded by a flow velocity difference corrosion, which seemingly occurs due to differential aeration. In this study, corrosion tests were carried out on pure copper specimen in a 3 percent NaCl solution using a jet-in-slit apparatus that has two test cells of the same structure. In a test cell, so-called erosion-corrosion was easily reproduced. In another cell, flow velocity difference corrosion was successfully reproduced on the specimen surface by simply reversing the flow direction of test liquid. Polarization curves as well as cyclic voltammograms were measured to find that the character of protective film determines the type of flow-induced localized corrosion.