Mechanism of erosion-corrosion on copper alloys

Erosion-corrosion damage on 6 - 4 yellow brass in a 3 % sodium chloride solution was brought about by using a jet-in-slit testing facility. In this facility, a narrow gap was made between two disks. A jet was impinged from a nozzle located at the center of the upper disk against the center portion of the lower disk (specimen). Then the solution radially flew through the gap. Two types of concentric circular damage were observed on the specimen surface. Firstly, it has been proved that the inner circular damage was caused by the shear stress of the fluid, as has been already pointed out by the literature, which mechanically took away the protective films on the specimen surface. On the other hand, it has been made clear that the outer circular damage was caused by the turbulence of the flow which broke the films away. The inlet-tube corrosion was created by injecting the 3 % sodium chloride solution into an 8 mm-bore tube of 6 - 4 yellow brass. The distributions of both the shear stress and the turbulence intensity were measured. The shear stress scarcely varied within the portion where the damage occurred. The turbulence intensity, on the contrary, reached a maximum at the damaged portion. Based on the foregoing test results, it has been concluded that the principal cause of inlet-tube corrosion is the turbulence of the flow which mechanically breaks the protective films away from the brass surface. (author)