Corrosion Resistance
Also: Corrosion Stability, Chemical Resistance, Oxidation Resistance
Corrosion resistance describes the ability of a material to permanently withstand chemical attack by moisture, oxygen, acids, or other environmental influences without significantly altering its surface or structure.
Corrosion resistance is one of the most important physicochemical properties distinguishing precious metals from base metals. It describes how strongly a material resists attack by reactive substances — oxygen, water, acids, halogens — without showing measurable degradation. The higher the electrochemical standard potential of a metal, the less readily it releases electrons and the more resistant it is to oxidative attack.
Electrochemical Basis
Corrosion is fundamentally an electrochemical reaction: the metal is oxidised (loses electrons) while an oxidising agent in the environment is reduced. Metals with a high standard potential relative to the normal hydrogen electrode (NHE) have little tendency to oxidise — they are considered "noble".
| Metal | Standard potential (V vs. NHE) | Typical attacking agent |
|---|---|---|
| Gold (Au) | +1.50 | Aqua regia, cyanide solution |
| Platinum (Pt) | +1.19 | Aqua regia, hot sulphuric acid |
| Palladium (Pd) | +0.95 | Aqua regia, concentrated nitric acid |
| Silver (Ag) | +0.80 | Sulphur compounds (H₂S), nitric acid |
| Copper (Cu) | +0.34 | Moist air (patina), nitric acid |
| Iron (Fe) | −0.44 | Oxygen + water (rust) |
Metals with a negative standard potential corrode spontaneously under normal conditions; gold and the platinum group metals are so noble that they remain stable even in concentrated individual acids.
Why Gold Is Practically Incorrodible
Gold reacts neither with atmospheric oxygen nor with water or most acids. Even after millennia, gold finds retain their metallic surface. Only aqua regia — a 1:3 mixture of nitric acid and hydrochloric acid — dissolves gold, because tetrachloroaurate anions are formed that shift the equilibrium:
Au + HNO₃ + 4 HCl → H[AuCl₄] + NO↑ + 2 H₂O
Cyanide solution also attacks gold — this principle is used industrially in cyanide leaching for gold recovery.
Silver: Noble, but Sensitive to Sulphur
Silver has a positive standard potential but shows a weakness towards sulphur compounds. Hydrogen sulphide (H₂S) and organic sulphur compounds in the air react with the surface to form black silver sulphide (Ag₂S) — the familiar tarnishing. This reaction is not a classic oxygen attack but a sulphidation, and can be reversed by polishing.
Platinum and Palladium
Platinum is resistant to almost all chemicals. It dissolves in hot aqua regia and in liquid alkali metals but withstands individual acids even at high temperatures. Palladium is slightly less resistant: concentrated nitric acid attacks it. Neither metal forms stable oxides under normal conditions.
Practical Significance for Investors and Industry
For investors, corrosion resistance has direct consequences:
- Value retention: Coins and bars of gold and platinum can be stored for generations without loss of quality. Using a melt value calculator assumes that the fineness is physically preserved — which is guaranteed for precious metals.
- Authenticity testing: Corrosion patterns assist the acid test: counterfeit items (e.g. gold-plated tungsten) react with nitric acid, gold does not.
- Industry: Corrosion resistance makes precious metals indispensable in electronic contacts, catalysts, medical devices, and dental prosthetics.
In Brief
Corrosion resistance is the physicochemical foundation for precious metals retaining their material value permanently — they have been used as stores of value for millennia not because of their lustre but because of their electrochemical inertness. Anyone tracking the current gold price or platinum price is dealing with substances whose physical stability no chemical on earth can challenge — except aqua regia.