Why Iron Rusts Faster Near the Sea

A torch cell, a car battery and a rusting iron gate are all electrochemistry. Batteries harness redox reactions to deliver current on demand, while corrosion is the same kind of reaction happening where we do not want it.

This part covers primary cells, rechargeable batteries, fuel cells, and the corrosion of iron with ways to prevent it.

A primary battery cannot be recharged once its reaction is complete, while a secondary battery can be recharged by passing current the opposite way; the dry cell and the mercury cell are both primary cells.

Dry (Leclanche) cell:

- Anode — the zinc container
- Cathode — a graphite rod surrounded by powdered manganese dioxide and carbon
- Electrolyte — a moist paste of ammonium chloride and zinc chloride

$$\text{Anode: }\text{Zn}\to {\text{Zn}}^{2+}+2e^{-}$$

$$\text{Cathode: }{\text{MnO}}_2+{\text{NH}}_4^{+}+e^{-}\to \text{MnO(OH)}+{\text{NH}}_3$$

It gives about $1.5$ V.

Mercury cell — zinc–mercury amalgam anode, a paste of HgO and carbon as cathode, and a paste of KOH and ZnO as electrolyte:

$$\text{Zn(Hg)}+2{\text{OH}}^{-}\to \text{ZnO}+{\text{H}}_2\text{O}+2e^{-}$$

$$\text{HgO}+{\text{H}}_2\text{O}+2e^{-}\to \text{Hg}+2{\text{OH}}^{-}$$

Its voltage stays steady at about $1.35$ V, because the overall reaction involves no ion whose concentration changes.

Worked example. A dry cell's zinc can loses $1.3$ g as it runs down:

$$Q=\frac{1.3}{65}\times 2\times 96487\approx 3.86\times 10^3\text{ C}$$

At $0.1$ A, that lasts about $3.86\times 10^4$ s, or roughly $10.7$ hours.

An everyday example. Wall clocks and TV remotes run on dry cells, while hearing aids and watches have used mercury-type button cells for their steady voltage.

The substance. A dry cell's voltage drifts down as it is used because ion concentrations inside it change — unlike the mercury cell.

In a lead storage battery, lead is oxidised and lead dioxide reduced, both forming lead sulphate as sulphuric acid is used up; charging reverses both reactions, and the nickel-cadmium cell uses a similar reversible reaction with a longer life.

Lead storage battery — lead anode, lead dioxide cathode on a lead grid, and about $38$ percent sulphuric acid as electrolyte:

- Anode: $\text{Pb}+{\text{SO}}_4^{2-}\to {\text{PbSO}}_4+2e^{-}$
- Cathode: ${\text{PbO}}_2+{\text{SO}}_4^{2-}+4{\text{H}}^{+}+2e^{-}\to {\text{PbSO}}_4+2{\text{H}}_2\text{O}$

$$\text{Overall: }\text{Pb}+{\text{PbO}}_2+2{\text{H}}_2{\text{SO}}_4\to 2{\text{PbSO}}_4+2{\text{H}}_2\text{O}$$

Charging reverses these, regenerating Pb, PbO${}_2$ and sulphuric acid.

Nickel-cadmium cell:

$$\text{Cd}+2{\text{Ni(OH)}}_3\to \text{CdO}+2{\text{Ni(OH)}}_2+{\text{H}}_2\text{O}$$

It lasts longer than a lead battery but costs more to make.

Worked example. The overall equation uses one mole of H${}_2$SO${}_4$ per mole of electrons. A battery delivering $10$ A for $1$ hour passes

$$n_{e^{-}}=\frac{10\times 3600}{96487}\approx 0.373\text{ mol},m_{{\text{H}}_2{\text{SO}}_4}=0.373\times 98\approx 36.6\text{ g used}$$

An everyday example. Home inverters that keep fans running during a power cut usually rely on lead storage batteries.

The substance. A hydrometer can test a lead battery's charge, because the acid's density falls as sulphuric acid is consumed.

A fuel cell turns the energy of a continuously supplied fuel directly into electricity: in a hydrogen-oxygen cell, hydrogen is oxidised at the anode and oxygen reduced at the cathode through concentrated KOH, producing only water.

Construction. Porous carbon electrodes containing finely divided platinum or palladium catalysts dip into concentrated aqueous KOH, with hydrogen and oxygen bubbled through them.

$$\text{Anode: }2{\text{H}}_2+4{\text{OH}}^{-}\to 4{\text{H}}_2\text{O}+4e^{-}$$

$$\text{Cathode: }{\text{O}}_2+2{\text{H}}_2\text{O}+4e^{-}\to 4{\text{OH}}^{-}$$

$$\text{Overall: }2{\text{H}}_2+{\text{O}}_2\to 2{\text{H}}_2\text{O}$$

Advantages:

- High efficiency — chemical energy becomes electrical energy directly, without burning fuel to drive a generator
- Pollution-free — the only product is water
- Continuous — it runs as long as fuel is supplied, with no recharging

Worked example. With $E_{cell}^{\circ }=1.23$ V and $n=4$:

$${\Delta }_r{G}^{\circ }=-4\times 96487\times 1.23\approx -475\text{ kJ}$$

for every $2$ mol of H${}_2$, or about $-237$ kJ per mole of water formed.

An everyday example. Hydrogen fuel-cell buses give off only water vapour from their exhaust.

The substance. A fuel cell is not recharged like a battery — its reactants come from outside, so it never runs down while fuel lasts.

**Rusting happens because different spots on an iron surface act as tiny anodes and cathodes: iron is oxidised to Fe${}^{2+}$ at anodic spots while oxygen is reduced at cathodic spots in the presence of water, and the Fe${}^{2+}$ is then oxidised further to hydrated iron(III) oxide, which is rust.

The reactions:**

$$\text{Anode: }2\text{Fe}\to 2{\text{Fe}}^{2+}+4e^{-}$$

$$\text{Cathode: }{\text{O}}_2+4{\text{H}}^{+}+4e^{-}\to 2{\text{H}}_2\text{O},E^{\circ }=1.23\text{ V}$$

$$\text{Overall: }2\text{Fe}+{\text{O}}_2+4{\text{H}}^{+}\to 2{\text{Fe}}^{2+}+2{\text{H}}_2\text{O},E_{cell}^{\circ }=1.67\text{ V}$$

Air then oxidises Fe${}^{2+}$ to rust, ${\text{Fe}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$. Dissolved salts and acids make the water a better electrolyte, so rusting speeds up.

Worked example. For the overall reaction, ${\Delta }_r{G}^{\circ }=-4\times 96487\times 1.67\approx -645$ kJ, so corrosion is strongly spontaneous.

Prevention:

- Barrier protection — paint, oil, grease or plastic keeps out air and moisture
- Galvanising — a zinc coating, which is oxidised in preference to iron
- Sacrificial protection — a block of magnesium or zinc attached to the iron corrodes instead of it

An everyday example. Iron gates and railings in coastal cities such as Mumbai rust faster, because salty air turns the moisture on them into a good electrolyte.

The substance. Galvanised iron stays protected even when scratched, since zinc keeps corroding first — unlike tin plating, which speeds rusting once broken.