Choosing a Sacrificial Anode for a Buried Steel Main
A water board is laying a 3.2 km buried mild-steel main through irrigated farmland. The soil is moist, aerated and chloride-rich (resistivity about 20 Ω m), and the pipe carries a coal-tar coating that is damaged at several joints. The engineer must fit a sacrificial anode bed that will protect the steel for 10 years without being dug up, at a design current of 0.50 A (F = 96500 C mol⁻¹). Three materials are on the store list: zinc (E°(Zn²⁺/Zn) = −0.76 V, M = 65.4 g mol⁻¹), magnesium (E°(Mg²⁺/Mg) = −2.37 V, M = 24.3 g mol⁻¹) and scrap copper (E°(Cu²⁺/Cu) = +0.34 V). The accountant prefers copper because it is free. The maintenance crew reports that on an earlier section, a magnesium bed corroded away in under three years and the coating blistered where the anodes had been. Steel in this soil is known to pit badly under chloride attack.
1. Calculate the anode mass required for 10 years at 0.50 A, for both zinc and magnesium, and explain why the answer is not simply the calculated mass.
Read the worked answer
Charge over the design life: t = 10 × 365 × 24 × 3600 = 3.15 × 10⁸ s. Q = I t = 0.50 × 3.15 × 10⁸ = 1.58 × 10⁸ C.
Moles of electrons: n(e⁻) = 1.58 × 10⁸ / 96500 = 1.63 × 10³ mol.
Zinc (Zn → Zn²⁺ + 2e⁻): n(Zn) = 817 mol → 817 × 65.4 ≈ 5.3 × 10⁴ g ≈ 53 kg. Magnesium (Mg → Mg²⁺ + 2e⁻): 817 × 24.3 ≈ 2.0 × 10⁴ g ≈ 20 kg.
Why it is only a starting figure: the current will not be a constant 0.50 A (soil moisture and temperature vary seasonally), anodes self-corrode even while delivering no useful current, and the anode must never be exhausted before replacement, so a safety factor of about 1.5 is normal. That takes the zinc requirement to roughly 75–80 kg, split across several anodes in a bed rather than one lump. Magnesium looks lighter, but mass is not the deciding variable - see the next task.
2. The accountant argues that copper is free and therefore preferable. Explain what would happen to the pipe if copper were bonded to it, and rule on the proposal.
Read the worked answer
Compare electrode potentials with the metal being protected. For iron, E°(Fe²⁺/Fe) = −0.44 V; for copper, E°(Cu²⁺/Cu) = +0.34 V.
Copper sits above iron in the electrochemical series, so in a soil electrolyte the copper is the cathode and the steel becomes the anode. The galvanic cell therefore drives current out of the pipe, exactly the opposite of cathodic protection:
- anode (corroding): Fe → Fe²⁺ + 2e⁻
- cathode (protected): O₂ + 2H₂O + 4e⁻ → 4OH⁻
Worse, the coating is intact over most of the length and damaged only at joints, so the bare steel is a small anode coupled to a large cathode. That geometry concentrates the whole corrosion current on a tiny area and produces rapid, deep pitting - precisely the failure mode the chloride soil already encourages.
Add that copper ions leaching into farmland soil are toxic and that the "free" scrap has no sacrificial function at all.
Ruling: reject copper outright and remove it from the store list. A metal can only protect steel if it is more negative than iron; metallic cost is irrelevant next to that criterion.
3. Explain why the magnesium bed failed early, and state how you would confirm that the chosen zinc installation is actually working.
Read the worked answer
Why magnesium failed: the driving voltage of Mg relative to steel is about −2.37 − (−0.44) = 1.9 V, far more than the small bare area at the damaged joints needs. The surplus current has to go somewhere, and on a coated pipe it generates hydrogen at the steel surface:
2H₂O + 2e⁻ → H₂ + 2OH⁻
The gas lifts and disbonds the coal-tar coating, and the OH⁻ makes the local environment alkaline enough to attack the coating bond. Once coating is lost, the exposed area grows, the current demand rises, and the anode is consumed even faster - a self-accelerating loop. That is exactly the observed pattern: anode gone in under three years, blistering where the anodes were. Zinc's modest potential (−0.76 V) delivers just enough current without overprotecting.
Verification: measure the pipe-to-soil potential against a copper/copper-sulfate reference electrode; protection is conventionally accepted at about −0.85 V or more negative, with a more negative criterion in chloride-bearing or anaerobic soils. Supplement with close-interval potential surveys along the 3.2 km, current-output checks on each anode, and excavation/inspection of one anode bed - because chloride pitting can proceed at a rate that a single spot reading will miss.