Rust is the familiar reddish-brown decay associated with metal deterioration, but it is a highly specific chemical process. Not all metals can rust, and not all metallic decay is considered rust. Understanding the chemical composition required for this process allows us to classify metals based on their susceptibility to this specific form of oxidation.
Defining Rust and the Necessary Ingredients
Rust is the common term for the corrosion of iron, specifically the formation of hydrated iron(III) oxide. This chemical reaction requires the simultaneous presence of three components: iron, oxygen, and water. Water acts as an electrolyte, enabling the movement of electrons necessary for the oxidation process to occur.
The process begins when iron atoms lose electrons, becoming positively charged iron ions. These electrons travel through the metal to a cathodic area where they are accepted by dissolved oxygen and water, forming hydroxide ions. The iron ions then combine with the hydroxide ions and oxygen to form the final reddish-brown product, rust.
Unlike many other metallic oxides, iron oxide is flaky and porous. Because it does not form a dense, protective layer, oxygen and water can penetrate easily and continually expose fresh metal underneath. This continuous cycle of oxidation makes rust destructive, eventually leading to the complete degradation of the metal.
The Metals That Rust: Iron and Its Alloys
Rust is strictly limited to ferrous metals, which are metals containing iron as their primary component. These include pure iron, wrought iron, and various forms of steel, an alloy of iron and carbon. Steel is particularly susceptible to rusting, and the presence of carbon can sometimes accelerate the electrochemical reaction.
Common examples of materials that rust are low-carbon steel (mild steel) and cast iron, which has a higher carbon content. The rate of rusting is influenced by specific composition. Increasing the carbon content in steel, for example, generally increases its susceptibility to the rapid spread of rust.
Some specialized alloys, known as weathering steels, are designed to rust superficially but then form a dense, dark-colored patina that acts as a protective barrier. These steels contain additions of elements like copper and chromium to promote the formation of this stable, rust-like layer. The fundamental chemical composition enabling the initial rust formation, however, is still the high iron content.
Why Some Metals Corrode But Do Not Rust
Many metals corrode—a general term for deterioration due to environmental reaction—but they do not form the porous iron oxide known as rust. This difference is due to passivation, a beneficial process where the metal reacts with oxygen to form a thin, dense, and tightly adhering oxide layer on its surface. This layer effectively shields the bulk metal from further oxidation.
Aluminum is a prime example of a metal that corrodes but does not rust. Although highly reactive with oxygen, the resulting aluminum oxide forms a transparent film that is durable and non-porous. This passive layer is only a few nanometers thick, yet it acts as a complete barrier, stopping the corrosion process instantly.
Copper and zinc also display this self-protecting behavior, though with a different appearance. Copper reacts slowly over time to create a distinctive green-blue patina, a combination of copper carbonates and sulfates. Zinc, often used as a protective coating on steel, forms a thin, dull gray zinc carbonate film, which prevents the destructive, flaking corrosion characteristic of iron.
Highly Resistant Metals
Certain metals are inherently resistant to corrosion due to their chemical makeup or through alloy engineering. The noblest metals, such as gold and platinum, are valued for their chemical inertness. They are positioned low on the reactivity series, meaning they have little tendency to react with oxygen or moisture, thus remaining untarnished indefinitely.
Engineered alloys like stainless steel also demonstrate resistance, but through a different mechanism. Stainless steel is an iron alloy containing a minimum of 10.5% chromium. The chromium reacts rapidly with oxygen to form an invisible, self-repairing layer of chromium oxide on the surface.
If the surface of stainless steel is scratched, the exposed chromium immediately reacts with oxygen to reform the protective layer. This tight, durable chromium oxide film prevents the underlying iron from reacting with its environment, stopping the destructive process of rust.