§ C-36 Electrochemical Corrosion of Metals & Sacrificial Anode Cathodic Protection (Grade 9–11 · Corrosion & Protection)
Chapter XXXVI. Microgalvanic Cells (Fe | Zn vs. Fe | Cu), Oxygen Depolarization & Rust Formation. «Attach a small block of zinc or iron to the copper sheathing of a ship, and the sea water will no longer corrode the protected metal.», Humphry Davy, 1824
Theory Summary & Scientific Mechanism
Electrochemical corrosion destroys metals via short-circuited microgalvanic cells beneath an aqueous electrolyte film. In any bimetallic couple, the metal with the more negative standard reduction potential E^° acts as the anode (-) and dissolves (M → M^n+ + ne⁻), while dissolved oxygen is reduced at the cathode (+): O₂ + 2H₂O + 4e⁻ → 4OH⁻.
Because E^°(Fe²⁺/Fe) = -0.44 V, coupling steel with a nobler metal such as tin (-0.14 V) or copper (+0.34 V) makes iron the anode, severely accelerating rusting at any coating scratch.
In sacrificial cathodic protection (galvanized steel or hull anodes), iron is coupled to a more active metal such as zinc (E^° = -0.76 V). Zinc serves as the sacrificial anode (Zn → Zn²⁺ + 2e⁻), pumping electrons into the steel cathode and preventing iron oxidation even if the coating is breached.
Everyday Analogy
Zinc is a bodyguard for steel! Because zinc has a more negative potential (-0.76 V) than iron (-0.44 V), a zinc anode sacrifices itself by feeding electrons to the steel hull. Copper (+0.34 V), by contrast, forces iron to rust faster!
Interactive Question & Derivation
Why does a scratch in a zinc coating (galvanized steel, E^°_Zn = -0.76 V) still protect the underlying iron (E^°_Fe = -0.44 V) from rusting, whereas a scratch in a tin or copper coating (E^°_Cu = +0.34 V) drastically accelerates iron corrosion?
- A: In the galvanic couple, the metal with the more negative standard electrode potential acts as the anode and dissolves: Zn sacrifices itself to protect Fe (cathode), whereas when coupled with Cu, iron becomes the anode (Fe → Fe²⁺ + 2e⁻) and corrodes rapidly
- B: Zinc mechanically seals oxygen out from a distance without any electron transfer
- C: Copper has a more negative potential than iron and dissolves first in salt water
Study Hint: In a galvanic couple, the metal with the more negative E^° acts as the anode: Zn sacrifices itself to protect Fe, whereas Cu makes Fe the corroding anode.
Proof & Explanation: Electrochemical corrosion operates as a short-circuited microgalvanic cell on the metal surface beneath an electrolyte film. The metal with the lower electrode potential serves as the anode (oxidation) while oxygen reduction (O₂ + 2H₂O + 4e⁻ → 4OH⁻) or proton reduction (2H⁺ + 2e⁻ → H₂↑) occurs at the cathode. Because E^°(Zn²⁺/Zn) = -0.76 V < E^°(Fe²⁺/Fe) = -0.44 V, a zinc sacrificial protector oxidizes (Zn → Zn²⁺ + 2e⁻) and feeds electrons to the iron hull, completely suppressing rust formation.
Key Equations & 3D Simulation Parameters
Anode (-): M → M^n+ + ne⁻, Cathode (+): O₂ + 2H₂O + 4e⁻ → 4OH⁻, j_corr ∝ Δ E_galv σ_NaCl
- Contact metal coupled to Fe (1=Cu, 2=Sn, 3=Zn protector, 4=Mg protector): 1 … 4 (default: 1)
- Electrolyte salinity [NaCl]: 0.05 … 1.5 mol/L (default: 0.5 mol/L)
- Dissolved oxygen concentration [O₂]: 1 … 12 mg/L (default: 8 mg/L)
- Medium acidity pH: 3 … 10 pH (default: 7 pH)