The rate of diffusion-controlled corrosion of 90° Copper Elbow by acidified dichromate has been investigated in relation to the following parameters: effect of solution velocity in the absence and presence of drag reducing polymer in case of liquid, gas, and solid flows on the rate of diffusion-controlled corrosion. The result has been obtained according to those equations: kα v0.44 in the absence of drag reducing polymer, k α v0.33 in case of liquid–solid and the presence of drag reducing polymer, k α v0.36 in case of liquid–gas and the presence of drag reducing polymer