Corrosion Resistance
Also: Corrosion resistance, Chemical resistance, Oxidation resistance
Corrosion resistance is the ability of a material to withstand chemical attack from moisture, oxygen, acids or other environmental influences permanently, without significantly altering its surface or structure.
Corrosion resistance is one of the most important physical-chemical 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 a metal's electrochemical standard potential, the less readily it gives up electrons and the more resistant it is to oxidative attack.
Electrochemical basis
Corrosion is at its core 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 (SHE) show little tendency to oxidise – they are considered "noble".
| Metal | Standard potential (V vs. SHE) | 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 uncorrodable
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 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 extraction.
Silver: noble, but sulphur-sensitive
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 molten alkali metal, but withstands single acids even at high temperatures. Palladium is somewhat less resistant: concentrated nitric acid attacks it. Neither metal forms stable oxides under normal conditions.
Practical significance for investors and industry
Corrosion resistance has direct consequences for investors:
- Value preservation: 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 with precious metals.
- Authenticity testing: Corrosion patterns help with the acid test: counterfeit 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 devices and dental prosthetics.
In brief
Corrosion resistance is the physical-chemical basis on which precious metals permanently retain their material value – 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 in the world calls into question – except aqua regia.