Non-ferrous metals do not rust, but they do corrode. Rust is chemically specific to the oxidation of iron (Fe), which is present in ferrous metals and alloys like steel. Non-ferrous metals, such as copper, aluminum, and titanium, contain no iron and therefore cannot produce iron oxide, the substance recognized as rust. The corrosion processes these metals undergo are fundamentally different in their chemical reactions and physical outcomes.
The Chemistry of Rust and Ferrous Metals
Rust is the common name for hydrated iron(III) oxide, a compound that forms when iron or its alloys are exposed to both oxygen and water. This reaction is an electrochemical process involving the transfer of electrons, where water acts as an electrolyte to facilitate the movement of ions.
The process begins as the iron oxidizes, losing electrons and forming ferrous ions. These ions then react with the dissolved oxygen and water to ultimately form the reddish-brown substance. This resulting iron oxide is voluminous, meaning it takes up significantly more space than the original iron metal, and possesses a flaky, non-adherent structure.
Because rust is brittle and porous, it flakes off, continuously exposing fresh iron beneath to the environment. This cycle of oxidation and flaking makes the corrosion progressive and destructive, often leading to the complete structural failure of the metal. Salt or acids accelerate this destructive process by acting as a catalyst.
Oxidation, Tarnish, and Patina in Non-Ferrous Metals
Non-ferrous metals are susceptible to general corrosion, a process broadly known as oxidation, which changes the metal’s surface. The resulting compounds are not iron oxide, and the outcomes are typically referred to as tarnish or patina.
Tarnish is a thin layer of corrosion, often appearing as a dull, gray, or black film on metals like silver, copper, and brass. For silver, tarnish is silver sulfide, formed by reacting with sulfur compounds in the air. This surface change is usually self-limiting, meaning the thin film protects the underlying metal from deeper corrosion.
Patina is a specific type of oxidation valued for its stable and protective properties. Copper reacts with oxygen, moisture, and carbon dioxide to form a distinctive blue-green layer of copper carbonate, known as verdigris. This patina is highly adherent, sealing the surface and slowing or halting further deterioration of the copper beneath it. Brass and bronze, which are copper alloys, also develop a similar dusty green patina over time.
The Self-Protecting Mechanism of Passivation
Passivation is a specialized form of oxidation that creates a dense, protective surface layer on many non-ferrous metals. When metals like aluminum and titanium are exposed to air, they instantly react with oxygen to form a microscopic, uniform oxide film. This layer is relatively inert and acts as an effective shield against further environmental attack.
For aluminum, this native oxide layer, which is typically only a few nanometers thick, is extremely hard and non-porous. Unlike the flaky, destructive nature of rust, the aluminum oxide adheres tightly to the metal’s surface, effectively sealing it off from the elements and preventing deeper corrosion. This mechanism is why aluminum can be used outdoors without continuous structural deterioration.
Titanium also exhibits this self-protecting behavior, spontaneously forming a titanium oxide layer upon exposure to the atmosphere. Even stainless steel, an iron-based alloy, relies on the non-ferrous element chromium to achieve corrosion resistance. The chromium forms a stable, passive chromium oxide film on the surface, which imparts the metal’s characteristic resistance to rust.