Grade by Grade
F. Electrochemistry · Electrochemistry

Corrosion and Its Prevention

F3.6 F1.2 A1.1 A1.5 A1.6 A1.10About 75 minutes
Learning goals
  • I can explain the rusting of iron as an electrochemical process with anode and cathode sites.
  • I can explain how water, oxygen, salt, acids and contact with other metals affect corrosion.
  • I can explain how coatings, galvanizing, cathodic protection and alloys prevent corrosion.
  • I can analyse the health, safety and economic impacts of corrosion, such as lead in drinking water.

Words to know

corrosion
The gradual destruction of a metal by redox reactions with substances in its environment, such as oxygen and water.
rust
Hydrated iron(III) oxide, Fe₂O₃·xH₂O, the flaky reddish-brown product of iron corrosion.
galvanizing
Coating steel with zinc. The zinc protects as a barrier and, if scratched, as a sacrificial anode.
cathodic protection
Protecting a metal by making it the cathode of an electrochemical cell, either with a more active sacrificial anode or with an applied current.
sacrificial anode
A block of a more active metal (Mg, Zn or Al) connected to a structure; it corrodes instead of the structure.
galvanic corrosion
Faster corrosion of the more active of two different metals in electrical contact in an electrolyte.
passivation
Protection by a thin, tightly attached oxide layer, as on aluminium and stainless steel.

The orange enemy

Every Canadian winter, trucks spread salt on roads. By spring, car door bottoms, bike chains, bridge railings and parking-garage concrete are a little more corroded. Corrosion is not just ugly: it weakens bridges, causes leaks in pipelines, and can release toxic metals into drinking water. A widely cited 2016 study by NACE International (now AMPP) estimated the global cost of corrosion at about US$2.5 trillion a year, around 3.4% of the world's GDP.

Corrosion is redox chemistry happening on its own, in a galvanic cell nobody built on purpose. Understanding the cell is the key to stopping it.

Rusting is a galvanic cell

Put a drop of water on a piece of steel and leave it. Rust appears, but not evenly: a pit forms in the middle of the drop, and the rust ring forms near the edge. Here is why:

  • Anode (centre, where dissolved O₂ is low): iron is oxidized. Fe(s) → Fe²⁺(aq) + 2e⁻
  • The electrons travel through the iron itself (a metal is a great wire) to the edge of the drop.
  • Cathode (edge, where O₂ from the air dissolves easily): oxygen is reduced. O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
  • The water drop is the electrolyte (the "salt bridge"): Fe²⁺ and OH⁻ ions move through it and meet.
  • Rust forms: Fe²⁺ + 2OH⁻ → Fe(OH)₂, which oxygen oxidizes further to iron(III): 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃. This dries to hydrated iron(III) oxide, Fe₂O₃·xH₂O, rust.

E°cell = E°cathode − E°anode = 0.40 − (−0.45) = +0.85 V: spontaneous. In acidic water (acid rain), the cathode reaction is O₂ + 4H⁺ + 4e⁻ → 2H₂O (+1.23 V), so E°cell is even larger (1.68 V) and corrosion is faster.

iron (steel)water drop: the electrolyteANODE (low O₂)Fe → Fe²⁺ + 2e⁻pit forms hereCATHODE (O₂-rich edges):O₂ + 2H₂O + 4e⁻ → 4OH⁻rust forms near the edge,where Fe²⁺ meets OH⁻ and O₂e⁻e⁻
A water drop on steel is a tiny galvanic cell. Iron is oxidized at the oxygen-poor centre (the anode); electrons flow through the metal to the oxygen-rich edge (the cathode), where oxygen is reduced. Rust forms where the ions meet.

What speeds corrosion up?

  • Water and oxygen are both needed: iron in dry air or in boiled, oxygen-free water hardly rusts.
  • Electrolytes, especially salt (road salt, sea spray), make the water a much better conductor of ions, so the corrosion cell runs faster. This is why cars in snowy provinces rust faster than in dry climates.
  • Acids (acid rain, acidic soils) provide H⁺ for a more powerful cathode reaction.
  • Contact with a less active metal creates galvanic corrosion: the more active metal becomes the anode and corrodes faster. Steel bolts in a copper roof, or an aluminium boat fitting touching a steel part, are classic problems. The Statue of Liberty's iron frame corroded where it touched the copper skin; during the 1980s restoration, the iron bars were replaced with stainless steel.
  • Stress and scratches in a metal become anode sites.

How we prevent corrosion

  1. Barriers: paint, oil, grease, plastic coatings and enamel keep water and oxygen away. They only work while unbroken.
  2. Plating with a less active metal, such as tin on steel food cans. Tin resists corrosion well, so the barrier lasts, but if the coating is scratched, steel becomes the anode of a tin–iron cell and rusts faster than unprotected steel.
  3. Galvanizing (coating with zinc, a more active metal). Zinc is a barrier, and if scratched, zinc is the anode and corrodes instead of the steel. Zinc's own corrosion products also form a protective film.
  4. Cathodic protection with a sacrificial anode: connect a block of a more active metal (magnesium, zinc or aluminium) to the structure. The block is oxidized; the structure becomes the cathode and is protected. Used on ship hulls, buried pipelines, boat motors and the anode rod inside a home hot-water tank. The blocks must be replaced when used up.
  5. Impressed-current cathodic protection: a power supply pushes electrons into the structure, making it the cathode. Used on long pipelines and some bridges and parking structures.
  6. Alloys and passivation: stainless steel contains chromium (at least about 10.5%), which forms a thin, self-healing chromium oxide layer. Aluminium is protected by its own oxide layer; anodizing thickens it electrolytically.
ground surfacemoist soil (electrolyte)steel pipeline (CATHODE: protected)Mg or ZnblockwireANODE (sacrificed):Mg → Mg²⁺ + 2e⁻O₂ + 2H₂O + 4e⁻ → 4OH⁻ happens on the steel,but the electrons come from the magnesiume⁻
Cathodic protection of a buried pipeline. A magnesium (or zinc) block is wired to the steel. Magnesium is oxidized instead of iron, so the electrons for the oxygen reduction come from the magnesium and the steel is protected.
Galvanized (zinc on steel)steelzinczincscratch + waterzinc (more active) is the anode:steel still protectedTinplate (tin on steel)steeltintinscratch + watersteel (more active) is the anode:steel rusts FASTER at the scratch
Why a scratch matters. In galvanized steel, zinc is more active than iron, so zinc is the anode and the steel stays protected. In tinplate, iron is more active than tin, so once the tin is scratched the exposed steel is the anode and rusts quickly.
Worked example

Example 1: Choosing a sacrificial anode

Which of these could protect a steel boat hull as a sacrificial anode: copper, zinc, tin, magnesium? Explain with E° values.

  1. The sacrificial metal must be a stronger reducing agent than iron, so it is oxidized instead: its E° must be lower than −0.45 V.
  2. Copper (+0.34 V) and tin (−0.14 V) are above iron, so they would make the iron the anode and speed up its corrosion.
  3. Zinc (−0.76 V) and magnesium (−2.37 V) are below iron.

Zinc or magnesium. (Zinc is common in salt water; magnesium, with its larger voltage, is often used in fresh water and soil.)

Worked example

Example 2: Analysing galvanic corrosion

A steel screw is used to attach a copper flashing on a roof. Which metal corrodes faster, and why? Write the half-reactions with rainwater as the electrolyte.

  1. Iron (−0.45 V) is a stronger reducing agent than copper (+0.34 V). In contact, they form a galvanic cell with iron as the anode.
  2. Anode: Fe → Fe²⁺ + 2e⁻. Cathode (on the copper surface): O₂ + 2H₂O + 4e⁻ → 4OH⁻ (or O₂ + 4H⁺ + 4e⁻ → 2H₂O in acid rain).
  3. The copper offers a large cathode surface, so the small steel screw corrodes quickly.

The steel screw corrodes faster. Use copper or stainless-steel fasteners, or insulate the metals from each other.

Corrosion and health: lead in drinking water (F1.2)

Many older homes in Ontario (often those built before the mid-1950s) still have a lead service line connecting the house to the water main, and older plumbing may contain lead solder. If the water is corrosive (low pH, low alkalinity, or high chloride), lead is oxidized and dissolves: Pb(s) → Pb²⁺(aq) + 2e⁻.

Lead is toxic, especially to young children and during pregnancy; there is no known safe level. Health Canada's guideline (set in 2019) is a maximum acceptable concentration of 0.005 mg/L (5 µg/L) of lead in drinking water.

In Flint, Michigan, in 2014, the city switched its water source without adding corrosion control. The more corrosive water dissolved lead from old pipes, and many residents were exposed. The lesson for chemists: corrosion control is a public-health job. Water utilities adjust pH and many add a small amount of orthophosphate, which forms a protective scale on pipe walls. Homeowners can have their water tested, run the tap until the water is cold before drinking if they may have lead pipes, use a filter certified for lead, and replace lead service lines (many Ontario municipalities have replacement programs).

Just for fun

Stainless steel is not really stainless: it is "stains less". Its chromium oxide layer is only a few nanometres thick, but it repairs itself whenever oxygen is around. Talk about a self-healing superpower.

Hands-on activity

The great nail race: a corrosion investigation

You need: 6 identical clean steel nails (not galvanized), 1 galvanized nail, 7 clear glasses or jars, labels, tap water, table salt, a little cooking oil, a kettle (grown-up), a strip of aluminium foil, about 15 cm of bare copper wire, and a phone camera.

  1. Question and hypothesis (A1.1): write a testable question, such as "How do salt, oxygen and contact with other metals affect how fast a nail rusts?" and a hypothesis for each jar.
  2. Set up the seven jars listed under Jars below, with each nail fully covered.
  3. Controlled variables: same kind of nail, same volume of liquid, same temperature, same time.
  4. Photograph each jar every day for 5 to 7 days. Rate the rust from 0 (none) to 5 (heavy) and record the results in a table (A1.6).
  5. Conclude (A1.10): which factors sped up rusting? Which protected the nail? Explain each result with anode and cathode reactions.

Jars:

  • A: nail in tap water
  • B: nail in salt water (1 tablespoon of salt per cup)
  • C: nail in water that a grown-up has boiled for a few minutes and cooled with the lid on (less dissolved O₂), with a thin layer of oil on top to keep air out
  • D: nail in an empty, dry jar (with the lid on)
  • E: galvanized nail in salt water
  • F: nail wrapped tightly with copper wire, in salt water
  • G: nail wrapped tightly with aluminium foil, in salt water

What to look for: B and F should rust most (salt; copper making iron the anode). D and C should rust least (no water, or little oxygen). E should show little rust on the steel because zinc is sacrificed (you may see white zinc corrosion products). G often rusts less than B because aluminium acts as a sacrificial anode, but aluminium's oxide layer can make the result vary. Safety: a grown-up handles boiling water. Pour solutions down the sink afterwards and throw the nails in the garbage.

Your turn

1

Write the anode and cathode half-reactions for the rusting of iron in neutral water.

2

Calculate E°cell for the rusting cell in neutral water.

3

Why does road salt speed up the rusting of cars?

4

Explain how galvanizing protects steel even after the zinc is scratched.

5

Why does a scratched tin can rust quickly at the scratch?

6

What is the anode rod in a hot-water tank for?

7

Why is corrosion of lead pipes a health issue?

More practice: warm-up

1

What two substances, besides iron, are needed for rusting?

2

Is the corroding metal the anode or the cathode?

3

Name two metals that can be sacrificial anodes for steel.

4

What metal gives stainless steel its corrosion resistance?

5

What is Health Canada's maximum acceptable concentration for lead in drinking water?

More practice: on your own

1

Explain why the pit forms in the centre of a water drop on iron, while rust appears near the edge.

2

A plumber connects a copper water pipe directly to a galvanized steel pipe. Predict the problem and suggest a solution.

3

An aluminium canoe is said to "never rust", yet aluminium is more active than iron. Explain.

4

Compare a sacrificial anode with impressed-current cathodic protection: one advantage of each.

5

Use E° values to show that tin cannot protect steel by sacrificial action.

Challenge

1

A magnesium sacrificial anode on a pipeline supplies an average current of 0.050 A. How many years will a 10.0 kg magnesium block last (Mg → Mg²⁺ + 2e⁻)? Assume all the magnesium is used for protection.

2

Reinforcing steel (rebar) inside concrete is normally protected because concrete is very basic (pH about 12 to 13). Explain why road salt soaking into a concrete bridge deck or parking garage eventually causes the concrete to crack.

3

Design a quick test to compare two commercial anti-rust coatings for bike chains, including the dependent and independent variables, controls, and how you would measure the result fairly.

Check your understanding

Quiz

1.

In the rusting of iron, iron is:

2.

Which condition would make an iron nail rust the slowest?

3.

Why is zinc better than tin for protecting steel that may get scratched?

4.

A sacrificial anode for a steel ship hull should be made of:

5.

Road salt speeds up corrosion mainly because it:

6.

What caused the drinking-water lead crisis in Flint, Michigan?

7.

Stainless steel resists corrosion because:

For parents and teachers

Big idea (F3.6, F1.2): corrosion is an unwanted galvanic cell: the metal is oxidized at anode sites, oxygen is reduced at cathode sites, and water with dissolved ions is the electrolyte. We prevent it by blocking water and oxygen, by sacrificing a more active metal, by applying a current, or by using alloys that form protective oxide layers. Corrosion has real costs and real health effects, such as lead in drinking water.

How to help: Set up the nail race together and check it each day. Ask your teen to predict which jar will rust most and to explain the copper-wire jar using anode and cathode. If your home was built before the mid-1950s, this is a good moment to check whether you have a lead service line (your municipality can usually tell you or test your water).

Common difficulties:

  • Thinking iron reacts directly with oxygen in one step. Emphasize the separate anode and cathode sites.
  • Mixing up which coating protects when scratched (zinc does; tin does not).
  • Choosing a sacrificial anode that is less active than iron.

Safety: a grown-up should boil the water. Nails can be sharp.