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). The oxygen differential creates voltage. Over time, chloride migrates into the pit to balance charge, acidity rises locally, and repassivation becomes impossible - the pit reinforces itself as a feedback loop.