Corrosion Resistance
Also: Chemical resistance, Oxidation resistance
Corrosion resistance is the ability of a material to withstand chemical attack from moisture, oxygen, acids or other environmental influences over time without significant change to its surface or structure.
Corrosion resistance is one of the most important physical-chemical properties that sets precious metals apart 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 surrenders electrons and the more resistant it is to oxidative attack.
The electrochemical basis
At its core, corrosion is an electrochemical reaction: the metal is oxidised (loses electrons) while an oxidising agent in the surroundings is reduced. Metals with a high standard potential relative to the standard hydrogen electrode (SHE) are reluctant 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 | Damp 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 incorruptible
Gold reacts neither with atmospheric oxygen nor with water nor with most acids. Even after millennia, gold finds retain their metallic surface. Only aqua regia — a 1:3 blend of nitric and hydrochloric acid — dissolves gold, because it forms tetrachloroaurate anions that shift the equilibrium:
Au + HNO₃ + 4 HCl → H[AuCl₄] + NO↑ + 2 H₂O
Cyanide solution also attacks gold — a principle exploited industrially in cyanide leaching for gold extraction.
Silver: noble, but sensitive to sulphur
Although silver has a positive standard potential, it has a weakness towards sulphur compounds. Hydrogen sulphide (H₂S) and organic sulphur compounds in the air react with its surface to form black silver sulphide (Ag₂S) — the familiar tarnishing. This reaction is not a classic oxygen attack but a sulphidation, and it can be reversed by polishing.
Platinum and palladium
Platinum resists 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 will attack it. Neither metal forms stable oxides under normal conditions.
Practical significance for investors and industry
For investors, corrosion resistance carries direct consequences:
- Value retention: coins and bars of gold and platinum can be stored for generations with no loss of quality. Using a melt value calculator presupposes that the fineness is physically preserved — which is guaranteed for precious metals.
- Authenticity testing: corrosion patterns aid the acid test: counterfeit pieces (for example gold-plated tungsten) react with nitric acid, whereas gold does not.
- Industry: corrosion resistance makes precious metals indispensable in electrical contacts, catalysts, medical devices and dental prosthetics.
In brief
Corrosion resistance is the physical-chemical foundation for precious metals holding their material value over time — they have served 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 can call into question — except aqua regia.