Corrosion of Pure Copper Caused by Vortex

摘要

A jet-in-slit testing apparatus is suitable for evaluating so-called erosion-corrosion resistance of pure copper. The followings are primary features. (1) Strong turbulence, caused by the rapid slowdown in flow velocity, occurs in the fluid flow over the specimen surface. (2) The location of maximum turbulence is different from the position where maximum shear stress occurs. Eventually, it was found that the inlet tube attack of copper alloys was caused not by shear stress, but rather by turbulence. In this study, the apparatus was modified to examine the effect of flow velocity difference on corrosion in the absence of turbulence. At a lower flow rate in a 1 percent CuCl_2 (II) solution, specimens showed the exactly expected morphology the damage depth was shallower in a higher velocity region, and deeper in a lower velocity region. At a higher flow rate, however, a deep pit was observed in the center of the specimen where the velocity is higher. Visualization of the flow conditions near the center of the specimen revealed the presence of a vortex at this location. Polarization curve measurements indicated that the deep pit under the fixed vortex occurred by the same mechanism as that of differential aeration cell corrosion. This proposed mechanism was confirmed by macro cell current measurements.