⟵ 06 Destructive Oxidation (Problem Set) · Contents · 07 Corrosion ⟶
Destructive Oxidation: Answer Key
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Transactional oxidation transfers electrons without destroying a useful structure. Sodium and chlorine exchange electrons to form a stable salt, but no engineered material or living structure has lost its function. Destructive oxidation strips electrons from a working material, forcing it into a new and less useful form. Rusting converts iron metal into iron oxide; the pipe wall does not spontaneously rebuild itself under normal system conditions.
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The same electrochemical force (electron theft) that destroys bacterial membranes, proteins, and DNA also strips electrons from metal lattices. An oxidizer added to kill biology will also corrode metal if the metal is exposed. This creates a fundamental tension: the oxidizer needed for biological control is the same force that drives corrosion.
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Anode (a material capable of being oxidized), cathode (an oxidizing species capable of accepting electrons), electrolyte (the water, which allows dissolved ions to move), and metal path (allows electrons to move). Remove the anode: no material to oxidize. Remove the cathode: no electron acceptor. Remove the electrolyte: no ionic pathway to complete circuit. Remove the metal path: electrons cannot flow between anode and cathode. Without all four, electrochemical corrosion in water comes to a halt.
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Warming decreases oxygen solubility but increases reaction kinetics. In warm water, there may still be enough dissolved oxygen present for the faster kinetics to dominate, producing higher net oxidation rates.
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Linear interpolation: 100°F is 23/63 of the way from 77°F to 140°F. Δ = 8.3 − 4.4 = 3.9 mg/L. Decrease: 3.9 × (23/63) = 1.4 mg/L. Estimated DO at 100°F ≈ 8.3 − 1.4 = 6.9 mg/L.
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20°C → 50°C = +30°C = three 10°C steps. Rate multiplier = 2³ = 8×.
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Mass fraction of Fe in Fe₂O₃: (2 × 55.8) / 159.6 = 111.6 / 159.6 = 0.699 (≈70%). 10 lb rust × 0.699 = 6.99 lb of iron destroyed.
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Tower A has stronger oxidizing pressure (+650 mV vs. +200 mV). Tower A is more likely to suppress biofilm because the water is more strongly oxidizing. All else equal, Tower A also poses a greater corrosion risk to exposed metals because the environment has a stronger tendency to accept electrons.
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At 140°F, dissolved O₂ ≈ 4.4 mg/L = 4.4 ppm. 1 ppm = 1 lb O₂ per million lb water. 50,000 lb/hr ÷ 1,000,000 × 4.4 = 0.22 lb O₂/hr entering the boiler.
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Oxygen’s two unmatched advantages: (1) it is dissolved in almost every water system on Earth, and (2) it is constantly replenished by contact with air. Applied oxidants like chlorine or ozone must be dosed and maintained. Oxygen requires no addition - it is always present by default. Combined with its respectable reduction potential (~1.23 V) and continuous supply, oxygen becomes the most persistent corrosion threat because it never runs out and never needs to be fed.
⟵ 06 Destructive Oxidation (Problem Set) · Contents · 07 Corrosion ⟶