MYP Chemistry Corrosion Notes

What Is Corrosion, and How Is It Different from Simple Oxidation?

Simple Oxidation

Corrosion

Corrosion:

1. Usually occurs at the surface but can penetrate deeper.
2. Leads to loss of strength, holes, and structural damage.
3. Involves a series of redox reactions.

Corrosion as a Redox Process

1. Redox recap:
   1. Oxidation = loss of electrons.
   2. Reduction = gain of electrons.
2. In corrosion:
   1. The metal (e.g. iron) is oxidised (loses electrons).
   2. Another substance (often oxygen or H⁺ in acidic solutions) is reduced (gains electrons).
3. Simplified steps of rusting (iron + water + oxygen):
   1. Oxidation of iron (anodic reaction): $$\mathrm{Fe}(\mathrm{~s}) \rightarrow \mathrm{Fe}^{2+}(\mathrm{aq})+2 e^{-}$$ Iron atoms lose electrons and form Fe²⁺ ions.
   2. Reduction of oxygen (cathodic reaction): $$\mathrm{O}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+4 e^{-} \rightarrow 4 \mathrm{OH}^{-}(\mathrm{aq})$$ Oxygen gains electrons and forms hydroxide ions.
   3. Formation of rust:
      1. Fe²⁺ reacts with OH⁻: $$\mathrm{Fe}^{2+}+2 \mathrm{OH}^{-} \rightarrow \mathrm{Fe}(\mathrm{OH})_2(\mathrm{~s})$$
      2. Further oxidation and hydration produce hydrated iron(III) oxide, the familiar rust.

What Conditions Speed Up Corrosion, and How Can We Investigate Them?

Corrosion happens fastest when the environment helps electrons move and accelerates redox reactions.

Factors That Increase Corrosion

1. Presence of water and oxygen
   1. Both are needed for rusting of iron.
   2. No water → no rust.
   3. No oxygen → no rust.
2. Electrolytes (e.g. salt)
   1. Salt (NaCl) dissolved in water makes the water a better conductor.
   2. This speeds up the electron and ion movement → faster corrosion.
   3. Explains why cars and bridges near the sea corrode faster.
3. Acidic or polluted environments