Rust is a specific form of corrosion, easily recognizable as the reddish-brown, flaky material that appears on older metal objects. It is the common term for the oxidation of iron, and this distinction is important because not all metals can produce true rust. Understanding which metals are susceptible explains why some metal structures deteriorate quickly while others seem to last indefinitely. The chemical process of rusting requires a specific set of circumstances and components.
The Chemistry Required for Rusting
The formation of rust is an electrochemical reaction that requires three components: iron, oxygen, and water. This process is accelerated when the water acts as an electrolyte, often filled by salt or other dissolved compounds. The reaction begins when iron atoms lose electrons (oxidation), transforming the solid metal into iron ions.
These electrons travel through the metal where they are accepted by oxygen and water (reduction). The resulting ions combine to form iron hydroxide, which is ultimately oxidized into the hydrated iron oxide we recognize as rust. Unlike the corrosion products of many other metals, rust does not tightly adhere to the surface and is porous. This flaking nature constantly exposes fresh metal underneath, allowing the corrosive process to continue unchecked until the entire structure is compromised.
Metals That Truly Rust
The capacity to truly rust is limited to metals that contain iron. This includes pure iron, cast iron, and carbon steel, an alloy composed primarily of iron and a small amount of carbon. These materials are known as ferrous metals and are inherently susceptible to the destructive, propagating corrosion of rust.
Cast iron, a common construction material, contains a higher percentage of iron and is prone to rusting. Carbon steel, used in countless applications, will rust quickly when its surface is exposed to oxygen and moisture. Given sufficient time and exposure to the elements, any mass of iron or its alloys will eventually convert entirely to rust.
Metals That Corrode Differently
Many metals undergo corrosion—a reaction with the environment—but they do not produce the destructive, reddish-brown iron oxide that is true rust. Non-ferrous metals, which contain little to no iron, react with oxygen in ways that often protect the underlying metal. This difference in corrosion product makes these metals more durable in exposed conditions.
Aluminum, for example, is more chemically reactive than iron, but it forms a dense, transparent layer of aluminum oxide almost instantly upon exposure to air. This thin, stable layer, known as a passive layer, adheres tightly to the surface and prevents any further reaction. Similarly, copper, brass, and bronze oxidize when exposed to the atmosphere, developing a thin, protective coating called a patina. This patina is typically green or blue-green and consists of stable copper compounds that act as a barrier against deeper corrosion. Even zinc forms a dull, grayish oxide layer that is stable and insoluble, protecting the metal from continued erosion.
Metals Engineered to Resist Corrosion
To overcome iron’s natural tendency to rust, metallurgists developed specific alloys and surface treatments that manipulate the corrosion process. Stainless steel is a prime example, as it is an iron alloy that includes a minimum of 10.5% chromium. The chromium reacts with oxygen to form an invisible, tenacious layer of chromium oxide on the surface.
This thin passive layer is self-healing in the presence of oxygen and prevents the iron from reacting to form rust. Another common strategy is galvanized steel, which is carbon steel coated with zinc. Zinc is more electrochemically active than iron, so it corrodes preferentially, sacrificing itself to protect the underlying steel, a process called sacrificial protection. Noble metals like gold and platinum resist corrosion due to their inherent chemical stability and unwillingness to react with oxygen or moisture.