KTU S1
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Electrochemical Corrosion, Its Control, and Metal Plating

By the end you should be able to: Explain the mechanism of electrochemical corrosion in acidic and alkaline media, use the galvanic series, describe cathodic protection by sacrificial anode and impressed current, and describe electroplating and electroless plating of copper.

Corrosion is a galvanic cell you did not intend to build

Corrosion is the spontaneous destruction of a metal by its environment. Nearly all of it is electrochemical, and once you see it as a short-circuited galvanic cell, both the mechanism and every control method follow.

Metals are extracted from ores by supplying energy. Corrosion is that energy being released again as the metal returns to a more stable oxidised state. Rusting is smelting run backwards, which is why it is spontaneous and why it can be slowed but never abolished.

Four things are required. Remove any one and corrosion stops — this is the list every control method attacks:

  1. An anode, where metal oxidises.
  2. A cathode, where reduction occurs.
  3. An electrolyte connecting them, usually moisture with dissolved ions.
  4. An electrical path between anode and cathode, usually the metal itself.

Where the anode and cathode come from on a single piece of metal. No second metal is needed. Differences in composition, grain boundaries, stress, scratches, or even differences in oxygen concentration across the surface are enough to make one region anodic relative to another.

That last one explains a common observation: a steel plate under a droplet rusts fastest at the centre, where oxygen access is poorest. The oxygen-starved region becomes the anode. It is why rust starts under paint blisters, inside crevices and beneath deposits rather than on the clean exposed surface — the differential aeration cell.

The mechanism

At the anode, always the same, regardless of medium:

Fe→Fe2++2e−\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^{-}

At the cathode, the reaction depends on the medium — and this is the distinction the syllabus asks for.

In acidic medium (low pH, oxygen absent) — hydrogen evolution:

2H++2e−→H22\text{H}^{+} + 2e^{-} \rightarrow \text{H}_2

The metal dissolves and hydrogen bubbles off. Fast, and it strips the surface rather than coating it.

In neutral or alkaline medium (oxygen present) — oxygen absorption:

O2+2H2O+4e−→4OH−\text{O}_2 + 2\text{H}_2\text{O} + 4e^{-} \rightarrow 4\text{OH}^{-}

The Fe²⁺ and OH⁻ meet and precipitate:

Fe2++2OH−→Fe(OH)2\text{Fe}^{2+} + 2\text{OH}^{-} \rightarrow \text{Fe(OH)}_2

which is further oxidised by dissolved oxygen to hydrated ferric oxide, Fe2O3⋅xH2O\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O} — rust.

Why rust is worse than it looks. Aluminium also oxidises instantly, but its oxide is dense, adherent and impermeable, and it seals the surface. Rust is porous, flaky and occupies more volume than the iron it came from, so it spalls off and exposes fresh metal underneath. Aluminium's oxide protects it; iron's does not. The difference is the structure of the oxide, not the reactivity of the metal — aluminium is in fact the more reactive of the two.

The galvanic series

The electrochemical series is measured under standard conditions with pure metals. The galvanic series is measured for real alloys in a real environment, usually seawater, and it is the practical guide.

Noble (protected) end: platinum, gold, graphite, titanium, stainless steel (passive), copper, tin, lead, cast iron, mild steel, aluminium, zinc, magnesium (active end).

The rule: when two metals are in electrical contact in an electrolyte, the more active one becomes the anode and corrodes, protecting the nobler one.

Two consequences worth carrying:

  • Avoid dissimilar metal contact — a steel bolt in an aluminium plate, or copper pipe joined directly to galvanised steel, will corrode rapidly at the junction. Insulate them.
  • The area ratio matters enormously. A small anode with a large cathode is the dangerous combination, because all the corrosion current concentrates on a small area. Steel rivets in a copper plate fail quickly; copper rivets in a steel plate are almost harmless.

Cathodic protection

Force the whole structure to be the cathode, so it cannot oxidise. Two ways.

1. Sacrificial anodic protection. Attach a more active metal — magnesium, zinc or aluminium — in electrical contact with the structure. The active metal becomes the anode and corrodes; the structure becomes the cathode and is protected. The anode is consumed and must be replaced, hence "sacrificial".

Used for: buried pipelines, ship hulls, water heaters (the magnesium rod inside a domestic hot water tank is exactly this, and replacing it is cheaper than replacing the tank), and underground tanks.

Advantages: no external power, simple, no maintenance beyond replacement. Limitation: limited driving voltage, so it suits small or well-coated structures.

2. Impressed current cathodic protection. Connect the structure to the negative terminal of a DC source, with an inert or semi-inert anode — graphite, high-silicon iron, platinised titanium — connected to the positive.

Used for: large structures such as long pipelines, jetties, and offshore platforms.

Advantages: the driving voltage is adjustable, so it works in high-resistance soil and protects large areas from a single installation; the anode lasts far longer. Limitations: needs a continuous power supply and monitoring, and over-protection damages coatings by generating hydrogen at the surface.

Other control methods in brief: barrier coatings (paint, grease); metallic coatings (galvanising, tinning); alloying (stainless steel); inhibitors; and design — avoiding crevices, sharp corners and places where water can stand.

A distinction worth being precise about. Galvanised steel is zinc on steel and is sacrificial: if the coating is scratched, the exposed zinc still protects the steel. Tin-plated steel is the opposite — tin is nobler than iron, so a scratch makes the small exposed steel area the anode against a large tin cathode, and it corrodes faster than unplated steel would. This is why a scratched tin can rusts through quickly and a scratched galvanised bucket does not.

Electroplating of copper

Depositing a metal coating using an external current.

  • Cathode: the object to be plated.
  • Anode: a copper bar, which dissolves to replenish the bath.
  • Electrolyte: copper sulphate with sulphuric acid to raise conductivity, plus additives.
Cathode:Cu2++2e−→Cu\text{Cathode:}\quad \text{Cu}^{2+} + 2e^{-} \rightarrow \text{Cu} Anode:Cu→Cu2++2e−\text{Anode:}\quad \text{Cu} \rightarrow \text{Cu}^{2+} + 2e^{-}

The copper concentration therefore stays roughly constant — the anode supplies what the cathode removes.

Surface preparation determines the result. Degreasing, pickling and rinsing come first; a deposit on a dirty surface will not adhere however good the bath.

Requires the object to be electrically conducting. That is the limitation the next method removes.

Electroless plating of copper

Deposition by a chemical reducing agent, with no external current.

  • Reducing agent: formaldehyde, in alkaline solution.
  • Complexing agent: EDTA or Rochelle salt, keeping Cu²⁺ in solution at high pH.
  • Catalyst: the surface is activated, usually with palladium.
Cu2++2HCHO+4OH−→Cu+2HCOO−+2H2O+H2\text{Cu}^{2+} + 2\text{HCHO} + 4\text{OH}^{-} \rightarrow \text{Cu} + 2\text{HCOO}^{-} + 2\text{H}_2\text{O} + \text{H}_2

The reaction is autocatalytic: deposited copper catalyses further deposition, so the coating grows on itself.

Advantages over electroplating, and why they matter:

  • Non-conductors can be plated — plastics, ceramics, glass — after catalytic activation.
  • Uniform thickness on complex shapes, including inside holes and recesses. Electroplating follows the electric field, so it deposits thickly on edges and points and thinly in recesses; electroless deposition does not care about geometry.
  • No electrical contacts or racking needed.

This is why every printed circuit board is electroless plated. The through-holes drilled in the insulating board must be copper-coated on their inner walls to connect the layers — a non-conducting surface, inside a hole, needing even coverage. Electroplating cannot do it.

Limitations: the bath is expensive, less stable, and needs closer control of temperature and pH than an electroplating bath.