Choosing a sacrificial anode for a steel tank
A village water tank is made of steel, essentially iron, and holds well water containing dissolved oxygen and salts. To slow corrosion, the maintenance team plans to bolt a block of a more active metal to the tank and connect it electrically, so that the block rather than the tank is oxidised.
The engineer proposes zinc. A supplier, noting that "tin plate protects steel cans", offers tin instead at a lower price. Standard electrode potentials are Mg²⁺/Mg = −2.37 V, Zn²⁺/Zn = −0.76 V, Fe²⁺/Fe = −0.44 V, Sn²⁺/Sn = −0.14 V, all against the hydrogen electrode.
When the tank is full and the block new, the protection current is measured as 0.050 A. The team wants the block to last at least 180 days without replacement and asks whether a 0.30 kg zinc block is sufficient, and whether magnesium would be better. The inside of the tank is painted, and the paint is known to blister in strongly alkaline conditions.
1. Decide between zinc, tin and magnesium on electrochemical grounds, explaining for each whether the steel is protected or attacked.
Read the worked answer
Protection requires the attached metal to be the anode, that is, to have the more negative electrode potential, so that it is oxidised in preference to iron.
- Zinc (−0.76 V) is more negative than iron (−0.44 V). In the damp, conducting environment the zinc dissolves (Zn → Zn²⁺ + 2e⁻) and the electrons it releases keep the steel surface cathodic, where oxygen is reduced instead of iron dissolving. Even where the zinc merely coats the steel, a scratch still leaves zinc as the anode.
- Tin (−0.14 V) is more positive than iron, so the polarity is reversed: the tin is cathodic and the steel becomes the anode. At every pore and scratch the iron dissolves faster than it would unprotected, and the small anodic area concentrates the attack. Tin plate works for food cans only because the tin layer is continuous and the contents are not an aggressive electrolyte.
- Magnesium (−2.37 V) does protect the steel, but with a driving voltage of about 1.9 V against iron, far more than zinc's 0.32 V.
2. Using the measured current of 0.050 A, calculate the mass of zinc consumed in 180 days and decide whether the 0.30 kg block is adequate. Show how the margin should be interpreted in practice.
Read the worked answer
Charge passed: Q = It = 0.050 A × 180 × 86 400 s = 0.050 × 15 552 000 = 777 600 C.
Moles of electrons: 777 600 / 96 485 = 8.06 mol. Since Zn → Zn²⁺ + 2e⁻, moles of zinc = 8.06/2 = 4.03 mol, and mass = 4.03 × 65.4 = 264 g, i.e. about 0.26 kg.
So 0.30 kg is formally sufficient, with only about 12 % spare. That margin should not be trusted, for three reasons. The 0.050 A is a single measurement made with new, clean metal; current depends on water conductivity, temperature and oxygen supply, and rises in warm, well-aerated water. As zinc corrosion products coat the surface, the current may fall, which protects the block but also weakens protection of the steel. And the end of life is not when the block vanishes but when it can no longer deliver enough current - so a sensible specification is 0.4–0.5 kg, with the block inspected annually.
3. The team is tempted by magnesium because the same calculation gives only about 98 g of metal. Explain why they should still hesitate, referring to the potentials, the paint and the steel itself.
Read the worked answer
For magnesium the electron count is the same 8.06 mol and Mg → Mg²⁺ + 2e⁻, so mass = 4.03 × 24.3 ≈ 98 g, a fifth of the zinc mass. The saving is real but the electrochemistry brings costs.
The driving voltage against iron is 2.37 − 0.44 = 1.93 V, six times that of zinc. At the steel cathode this drives hydrogen evolution, 2H₂O + 2e⁻ → H₂ + 2OH⁻, much harder. The result is overprotection: hydroxyl ions raise the local pH sharply, and the setup states that the paint blisters in strongly alkaline conditions, so the coating fails and a larger bare steel area is exposed. Hydrogen absorbed by high-strength steel can also cause embrittlement and cracking.
Magnesium is the right choice in high-resistance media such as soil or fresh water where the current is hard to sustain, precisely because it pushes hard. In this tank, where a modest 0.050 A is enough, zinc's gentler 0.32 V driving voltage is the better engineering judgement, and the extra 0.17 kg of metal is cheap insurance. In either case the block must be in solid electrical contact with the steel - an insulating gasket or plastic bush would make the whole calculation worthless.