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, leading to 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.