Why doesn't gold rust while iron does?
Understand metal 'personalities' scientifically through Pourbaix diagrams
Corrosion is the phenomenon where metals chemically react with their environment and transform from pure metal into other substances (oxides or ions). What we commonly call 'rust' is a prime example of corrosion.
Oxidation: Fe → Fe2+ + 2e-
Reduction: O2 + 2H2O + 4e- → 4OH-
Result: Iron ions combine with hydroxide ions → Red rust（Fe2O3・nH2O）
Key Point:Corrosion is a 'redox reaction.' Metals lose electrons (become oxidized) and turn into ions. Whether this reaction occurs easily depends on the type of metal and the environment.
A Pourbaix diagram (potential-pH diagram) shows in what state a metal is stable in aqueous solution. The horizontal axis is pH (acidic to alkaline), and the vertical axis is potential (redox potential).
The diagram below shows the Pourbaix diagram for Iron (Fe). Click on each region to see details.
Click or hover over a region to see details
Stable as metal. No corrosion occurs in low potential, reducing environments. Cathodic protection uses this principle.
Protective oxide film forms on surface. Stainless steel's Cr₂O₃ film is a prime example. Corrosion progression is suppressed.
Metal dissolves as ions and corrosion progresses. Occurs in acidic (Fe²⁺) or strongly alkaline (HFeO₂⁻) environments.
How to Read:Blue dashed lines indicate water stability region. O₂ evolves above the upper line, H₂ below the lower line. Using pH 7 (neutral) as reference, left is acidic, right is alkaline.
A metal's corrosiveness is expressed by its standard electrode potential. This is also related to 'ionization tendency.'
| Metal | Potential (V) | Corrosiveness |
|---|---|---|
| Lithium (Li) | -3.04 | Very Corrosive |
| Zinc (Zn) | -0.76 | Corrosive |
| Iron (Fe) | -0.44 | Somewhat Corrosive |
| Hydrogen (H) | 0.00 | Reference |
| Copper (Cu) | +0.34 | Corrosion Resistant |
| Silver (Ag) | +0.80 | Quite Stable |
| Gold (Au) | +1.50 | Extremely Stable |
Remember:The more negative the potential, the easier it oxidizes and corrodes. Gold and platinum have large positive values, making them very stable—hence 'noble metals.'
The same metal can have very different corrosion behavior depending on the environment.
Corrosion by oxygen and water vapor. Accelerated by high humidity. Sea breeze containing salt promotes corrosion further.
Strong metals: Gold, Platinum, Stainless Steel
Corrosion by dissolved oxygen and water. pH and dissolved ions have major effects. Chloride ions in seawater accelerate corrosion.
Strong metals: Titanium, Hastelloy
High hydrogen ion concentration dissolves most metals. Resistance to hydrochloric and sulfuric acid is important.
Strong metals: Gold, Platinum, Tantalum
High hydroxide ion concentration. Amphoteric metals (Al, Zn) dissolve. Iron is relatively stable and forms passive film.
Strong metals: Iron, Nickel, Titanium
Some metals form a thin oxide film (passive film) on their surface to protect the interior. This allows metals that would normally corrode easily to exhibit excellent corrosion resistance.
Protected by chromium oxide film (Cr₂O₃). Has self-healing capability.
Film thickness: Several nm
Dense aluminum oxide (Al₂O₃) film. Can be strengthened by anodizing.
Film thickness: 2-10nm
Very stable titanium dioxide (TiO₂) film. Excellent even in seawater.
Film thickness: 1-10nm
Key Point:Metals with passive films automatically repair the film by reacting with oxygen in the air when scratched. This is 'self-healing.' However, chloride ions (Cl⁻) can destroy the film.
Compare corrosion resistance of 188 cards by environment
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