Problem 7.8: A small carbon steel bolt is threaded into a

From Chapter 7 Problem Set. Level 3: System Integration.

Question

A small carbon steel bolt is threaded into a large copper header. In a separate system, a large carbon steel pipe is connected to a small copper instrument line. Which configuration produces more severe corrosion of the steel? Explain using current density.

Answer

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The small steel bolt in a large copper header is far worse. The large copper cathode drives a high total cathodic current (oxygen reduction over a large area). That entire current must be balanced by anodic dissolution concentrated on the tiny bolt surface. High current density on a small anode means rapid, localized metal loss. In the reverse case, the large steel anode spreads dissolution over a much larger area, reducing current density and slowing the rate of attack.