Corrosion — Revision Notes

⚡ 30-Second Revision

Corrosion — Electrochemical degradation of metals.
Rusting — Iron corrosion ($Fe_2O_3 cdot xH_2O$) in presence of $O_2$ and $H_2O$.
Anodic Reaction — $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$
Cathodic Reaction (Neutral/Alkaline) — $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$
Cathodic Reaction (Acidic) — $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$ or $2H^+(aq) + 2e^- \rightarrow H_2(g)$
Factors Accelerating Corrosion — Electrolytes (salts), $O_2$, low pH, high temperature, dissimilar metals, stress.
Prevention - Barrier — Painting, oiling, plating with less reactive metal (e.g., tin).
Prevention - Sacrificial — Galvanization (Zn on Fe), Cathodic protection (Mg/Zn/Al anodes).
Prevention - Alloying — Stainless steel ($Cr_2O_3$ passivation).
Passivation — Formation of protective oxide film (e.g., $Al_2O_3$, $Cr_2O_3$).

2-Minute Revision

Corrosion is the electrochemical deterioration of metals, driven by their tendency to return to a more stable, lower-energy state. The most common example is rusting, where iron forms hydrated iron(III) oxide ($Fe_2O_3 cdot xH_2O$) in the presence of oxygen and water.

This process involves iron oxidation at anodic sites ($Fe \rightarrow Fe^{2+} + 2e^-$) and oxygen reduction at cathodic sites ($O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$ in neutral solutions). The presence of an electrolyte, like dissolved salts, significantly accelerates corrosion.

Factors such as high oxygen concentration, low pH, and elevated temperature also increase corrosion rates. Prevention strategies are crucial and include barrier protection (paints, oils), sacrificial protection (galvanization with zinc, cathodic protection with more reactive metals), and alloying (like stainless steel forming a passive chromium oxide layer).

Understanding these mechanisms and prevention methods is key for NEET.

5-Minute Revision

Corrosion is the spontaneous electrochemical process of metal degradation, essentially the reverse of metal extraction. It requires an anode (metal oxidation), a cathode (electron acceptor reduction), and an electrolyte.

The classic example, rusting of iron, involves iron losing electrons ($Fe \rightarrow Fe^{2+} + 2e^-$) at anodic areas. These electrons flow to cathodic areas where dissolved oxygen is reduced. In neutral water, this is $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$.

The $Fe^{2+}$ and $OH^-$ ions then combine and further oxidize to form hydrated iron(III) oxide, or rust ($Fe_2O_3 cdot xH_2O$).

Several factors influence corrosion. Electrolytes (e.g., saltwater) increase conductivity, accelerating the process. Higher oxygen concentration fuels the cathodic reaction. Acidic conditions (low pH) provide $H^+$ ions for reduction, speeding up corrosion. Elevated temperatures generally increase reaction rates. Contact with a less reactive metal can cause galvanic corrosion, where the more reactive metal corrodes preferentially. Stress and impurities also create anodic sites.

Corrosion prevention is vital. Barrier protection involves physically isolating the metal (e.g., painting, oiling, plastic coating, electroplating with a less reactive metal like tin). This works only if the barrier is intact.

Sacrificial protection uses a more reactive metal to protect the desired metal. In galvanization, iron is coated with zinc; zinc corrodes first, even if scratched. Cathodic protection involves connecting the metal to a sacrificial anode (e.

g., Mg, Zn) or using an impressed current. Alloying, as seen in stainless steel (iron with chromium), forms a protective, passive $Cr_2O_3$ layer. Finally, corrosion inhibitors can be added to the environment to slow down the process.

For NEET, focus on the reactions, factors, and the principles behind each prevention method.

Prelims Revision Notes

Corrosion: Key Facts for NEET

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Definition — Electrochemical deterioration of metals due to reaction with environment.
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Spontaneity — Corrosion is a spontaneous process ($\Delta G < 0$).
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Components of Corrosion Cell — Anode (metal oxidizes), Cathode (electron acceptor reduces), Electrolyte (ion transport), Metallic path (electron flow).
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Rusting of Iron — Specific corrosion of iron/steel to hydrated iron(III) oxide ($Fe_2O_3 cdot xH_2O$). Requires $O_2$ and $H_2O$.

* Anodic Reaction: $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$ * Cathodic Reaction (Neutral/Alkaline): $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$ * Cathodic Reaction (Acidic): $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$ or $2H^+(aq) + 2e^- \rightarrow H_2(g)$ * Overall Rust Formation: $Fe^{2+}$ reacts with $OH^-$ to form $Fe(OH)_2$, which then oxidizes to $Fe(OH)_3$ and dehydrates to $Fe_2O_3 cdot xH_2O$.

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Factors Affecting Corrosion Rate

* Accelerators: Presence of electrolyte (dissolved salts), high $O_2$ concentration, low pH (acidic), high temperature, contact with less reactive metal (galvanic corrosion), stress/impurities. * Inhibitors: Protective coatings, sacrificial metals, alloying, corrosion inhibitors.

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Corrosion Prevention Methods

* Barrier Protection: Physical separation from environment. * *Examples*: Painting, oiling, greasing, plastic coating, electroplating with less reactive metal (e.g., tin on iron). * *Limitation*: Fails if barrier is broken.

* Sacrificial Protection: More reactive metal corrodes preferentially. * *Examples*: Galvanization (coating iron with zinc; Zn is more reactive than Fe, acts as anode). Cathodic Protection (attaching Mg, Zn, Al blocks to steel structures).

* *Advantage*: Protects even if coating is scratched. * Alloying: Creating corrosion-resistant alloys. * *Example*: Stainless Steel (Fe + Cr). Cr forms a passive, protective $Cr_2O_3$ layer.

* Passivation: Formation of a stable, non-porous oxide film (e.g., $Al_2O_3$ on aluminum, $Cr_2O_3$ on stainless steel). * Corrosion Inhibitors: Chemicals added to environment to slow corrosion.

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Key Distinctions — Rusting (iron specific) vs. general oxidation. Sacrificial protection (more reactive metal) vs. barrier protection (physical layer).

Vyyuha Quick Recall

To remember factors that ACCELERATE corrosion, think of 'SALT-DOPE':

Salts (Electrolytes)
Acidic pH (Low pH)
Less reactive metal contact (Galvanic coupling)
Temperature (High)
Dissolved Oxygen
Purity (Impurities create cells)
Electrical stress (Stress points)