Problem 7.2: The chapter says passivation is not immunity

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

Question

The chapter says “passivation is not immunity – it is a ceasefire.” Name four mechanisms (film saboteurs) that can break the ceasefire and briefly state how each works.

Answer

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Mechanical disruption (flow/erosion): shear forces strip the oxide layer at elbows, tees, and pump impellers. Chemical dissolution (low pH): hydrogen ions protonate the oxide, converting it back to soluble species. Chloride penetration: small, mobile Cl⁻ ions pierce the film and form soluble metal–chloride complexes, preventing repassivation. Sulfate’s two paths: Sulfate can interfere with protective film stability by forming soluble metal-complexes. It also aids in microbiologically influenced corrosion (MIC) when SRB use sulfate as an electron acceptor to consume electrons associated with iron oxidation.