Problem 7.4: A gasket creates a crevice on a carbon steel

From Chapter 7 Problem Set. Level 1: Conceptual Mastery.

Question

A gasket creates a crevice on a carbon steel flange in oxygenated water. Where does the anode develop? Where does the cathode develop? Why does the attack accelerate over time?

Answer

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Anode: inside the crevice (low oxygen). Cathode: exposed surface outside the crevice (high oxygen). Oxygen differential creates voltage. Over time, chloride migrates into the pit to balance charge, acidity rises locally, and repassivation becomes impossible. The pit worsens.