When a metal pipe is bent, the bent portion experiences mechanical stress.
This stressed portion and unstressed portion behave differently electrochemically.
Due to difference in stress, a potential difference is developed between the stressed and unstressed regions.
This forms a stress cell.
In such a cell, corrosion is more likely to occur at the stressed region.
Bent pipes, cold-worked metals, and stressed machine parts are common examples of stress cells.
Therefore, the cell formed in bent pipes is:
\[
\text{Stress Cell}.
\]