⟵ 07 Corrosion (Problem Set) · Contents · 08 Scale ⟶
Corrosion: Answer Key
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Anode (metal dissolves), cathode (electrons consumed by oxidizer), electrolyte (ionic conduction through water), metallic path (electron conduction through metal). In an open cooling tower, the cathodic reactant (oxygen) is most difficult to remove because it is continuously replenished from the atmosphere.
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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. Biological attack (MIC): sulfate-reducing bacteria produce sulfides and acids under biofilm deposits, chemically attacking the metal surface.
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Carbon steel corrodes. It is less noble than copper on the galvanic series. When both metals share an electrolyte and metallic path, the less noble metal (steel) becomes the anode and loses electrons preferentially.
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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.
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W = (12.4500 − 12.1500) × 1,000 = 300 mg. T = 90 × 24 = 2,160 hr. MPY = (534 × 300) / (7.87 × 3.5 × 2,160) = 160,200 / 59,497.2 = 2.69 MPY. Classification: “Very Good” for carbon steel in an open cooling tower (1–3 MPY range).
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Cl⁻ = 250 / 35.5 = 7.04 meq/L. SO₄²⁻ = 120 / 48 = 2.50 meq/L. P-alk = 0, so all alkalinity is bicarbonate: HCO₃⁻ = 300 / 50 = 6.00 meq/L. LS = (7.04 + 2.50) / 6.00 = 1.59. Interpretation: LS > 1.2 indicates high corrosion risk - aggressive anions significantly outweigh alkalinity protection. Passive film destabilization, pitting, and under-deposit corrosion are likely.
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(a) Cathode - removes the cathodic reactant (oxygen). (b) Anode - strengthens the passive film, locking the lattice. (c) Cathode - increases resistance via precipitating a protective film. (d) Electron path - breaks the metallic circuit between dissimilar metals.
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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.
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Chloride, low pH, and biofilm deposits weaken or remove passive films, which lowers resistance. Biofilms also create oxygen differential cells, which increase voltage by making the covered area anodic and the exposed area cathodic. Corrosion localizes because a small anode is driven by a large cathode, increasing current density and causing pitting rather than uniform metal loss.
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When the passive film is breached, resistance drops because the protective barrier no longer impedes electron transfer and ion migration. Simultaneously, voltage increases because the exposed bare metal has a much lower reduction potential than the surrounding passivated surface, creating a large potential difference. By Ohm’s Law, lower resistance and higher voltage both drive higher current. That current concentrates on the small exposed anode, producing the intense localized dissolution that defines pitting.