Corrosion Resistance
Also: Corrosion stability, Chemical resistance, Oxidation resistance
Corrosion resistance is the ability of a material to permanently withstand chemical attack from moisture, oxygen, acids or other environmental influences without significantly changing its surface or structure.
Corrosion resistance is one of the most important physical-chemical properties that distinguishes 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 gives up electrons and the more resistant it is to oxidative attack.
Electrochemical Basis
Corrosion is essentially 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 standard hydrogen electrode (NHE) have little tendency to oxidise - they are considered "noble".
| Metal | Standard potential (V vs. NHE) | Typical attacker |
|---|---|---|
| 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 single acids.
Why Gold is Practically Non-Corrodible
Gold reacts with neither atmospheric oxygen nor water nor 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 Sulphur-Sensitive
Silver has a positive standard potential, yet 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 well-known 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 molten alkali metal, but withstands single acids even at high temperature. Palladium is somewhat 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 presupposes that the fineness is physically preserved - which is guaranteed with precious metals.
- Authenticity testing: Corrosion patterns help with the acid test: fake pieces (e.g. gold-plated tungsten) react with nitric acid, gold does not.
- Industry: Corrosion resistance makes precious metals indispensable in electronic contacts, catalytic converters, medical products and dental restorations.
In Brief
Corrosion resistance is the physical-chemical basis 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 who follows the current gold price or platinum price is trading substances whose physical stability no chemical in the world can call into question - except aqua regia.