Step 1: Corrosion may occur due to difference in physical condition of the same metal at different regions.
Step 2: In bent pipes, some portions of the metal are under greater mechanical stress.
Step 3: The stressed region behaves differently from the unstressed region.
Step 4: This difference creates an electrochemical cell called a stress cell.
Step 5: Hence, the cell formed in bent pipes is:
\[
\boxed{\text{Stress Cell}}
\]