Corrosion is an everyday chemical process that connects directly to IB Chemistry Topic 9 (Redox Processes) and Topic 19 (HL Oxidation and Reduction). Whether it’s rust on iron, tarnish on silver, or the gradual deterioration of metals in the environment, corrosion is fundamentally a redox reaction. Understanding how corrosion occurs helps explain oxidation, electrochemical cells, and methods used to protect metals.
What Is Corrosion?
Corrosion is the gradual destruction of a metal through chemical reactions with its environment, usually involving oxidation.
In most cases:
- The metal loses electrons
- It forms metal ions
- New compounds form on the surface (such as oxides or hydroxides)
Corrosion weakens structures, damages equipment, and leads to economic loss. It is a natural, spontaneous process.
Rusting: The Most Common Form of Corrosion
Rusting is the corrosion of iron. It requires:
- Oxygen
- Water
- An electrolyte (such as salt) to speed up the process
The overall rusting reaction produces hydrated iron(III) oxide—commonly known as rust.
Key reactions involved:
Oxidation (at anode region):
Fe → Fe²⁺ + 2e⁻
Reduction (at cathode region):
O₂ + 2H₂O + 4e⁻ → 4OH⁻
These ions then combine to form rust.
Rust is flaky, porous, and does not protect the metal underneath, so corrosion continues.
Corrosion Is a Redox Process
Corrosion is fundamentally a redox reaction:
- The metal is oxidized (loses electrons).
- A substance in the environment—oxygen or hydrogen ions—is reduced.
This means corrosion behaves like a natural, uncontrolled electrochemical cell:
- The metal surface has tiny anodic and cathodic regions
- Electrons flow through the metal
- Ions migrate through water
- Redox reactions occur spontaneously
This is why moisture speeds up corrosion dramatically.
Factors That Increase Corrosion
1. Presence of electrolytes
Saltwater accelerates rusting because ions carry charge between anodic and cathodic regions.
2. Acidity
Lower pH speeds up corrosion by providing more H⁺ ions for reduction.
3. Oxygen availability
More oxygen increases the rate of reduction reactions.
4. Temperature
Higher temperatures increase reaction rate and corrosion speed.
5. Impurities in metal
Different regions on the metal create miniature electrochemical cells.
Metals exposed to harsh environments corrode far more rapidly.
Why Some Metals Don’t Corrode Easily
1. Protective oxide layers
Aluminum forms Al₂O₃, a tough, stable coating that prevents further oxidation.
2. Noble metals
Gold and platinum resist oxidation due to very positive electrode potentials.
3. Alloys
Stainless steel contains chromium, which forms a protective oxide layer.
Metals that form strong, adherent oxides corrode much more slowly.
Methods to Prevent or Reduce Corrosion
Several techniques are used to slow corrosion, many of which rely on redox principles.
1. Painting or coating
Prevents oxygen and water from contacting the metal.
2. Galvanizing
Zinc coating provides sacrificial protection because zinc is more reactive than iron and oxidizes first.
3. Cathodic protection
A more reactive metal is attached so it is oxidized instead of the protected structure.
Used for pipelines, ships, and underground tanks.
4. Alloying
Adding chromium or nickel improves corrosion resistance.
5. Using inhibitors
Chemicals added to water systems to reduce oxidation.
These methods illustrate how controlling redox chemistry extends the life of metals.
Corrosion in Electrochemical Terms
The corroding metal acts as:
- Anode (oxidation occurs)
The cathodic reaction may involve:
- Oxygen reduction
- Water reduction
- Hydrogen ion reduction
Understanding corrosion as an electrochemical process helps explain why protection strategies work.
Common IB Misconceptions
“Rusting occurs only in wet environments.”
Moisture accelerates it, but corrosion can occur even at low humidity.
“Galvanizing protects metal because zinc is stronger.”
Incorrect. Zinc protects by oxidizing preferentially (sacrificial protection).
“Stainless steel never corrodes.”
It does corrode, but much more slowly due to its passive oxide layer.
FAQs
Why does salty water cause faster rusting?
Salt increases conductivity, allowing faster electron and ion movement in the redox process.
Is corrosion always harmful?
Mostly yes, but controlled oxidation is sometimes useful (e.g., forming patina on copper roofs).
Can corrosion be completely stopped?
No, but it can be greatly slowed using coatings, sacrificial metals, or electrochemical protection.
Conclusion
Corrosion is the gradual oxidation of metals caused by environmental chemical reactions. It is a redox process involving metal oxidation and reduction of oxygen or other species. By understanding corrosion at an electrochemical level, we can protect metals through coatings, sacrificial protection, and alloying. This knowledge is essential for interpreting redox chemistry in IB Chemistry.
