Only metals that contain iron can truly rust. Rust is a specific type of corrosion that forms when iron reacts with oxygen and water, producing the flaky, reddish-brown coating you recognize on old tools, car bodies, and exposed steel. Other metals can corrode, tarnish, or oxidize, but that greenish patina on copper or the dull film on aluminum is not rust.
What Rust Actually Is
Rust is hydrated iron oxide, a compound that forms when iron atoms lose electrons to dissolved oxygen in the presence of water. The process requires both moisture and oxygen. Iron sitting in a perfectly dry environment or submerged in oxygen-free water won’t rust. But expose it to humid air, rainwater, or condensation and the reaction begins. The iron dissolves at one spot on the surface, travels through the water film, and reprecipitates as that familiar orange-brown crust somewhere nearby.
Unlike the protective oxide films that form on some metals, rust is porous and flaky. It doesn’t seal the surface. Instead, it cracks and peels away, exposing fresh iron underneath to continue corroding. This is why a rusting piece of steel can eventually be eaten through entirely if left unprotected.
Metals That Rust
Any metal or alloy with significant iron content is vulnerable. The most common examples are carbon steel, cast iron, and alloy steel. These are collectively called ferrous metals, and their high iron content makes them prone to rusting whenever they’re exposed to moisture. Carbon steel, the material in most structural beams, car frames, and hand tools, rusts readily without some form of protective coating.
There are two notable exceptions among ferrous metals. Wrought iron, despite being nearly pure iron, resists rust better than you’d expect because its purity and fibrous structure limit the electrochemical reactions that drive corrosion. Stainless steel contains chromium (typically at least 10.5%), which reacts with oxygen to form an invisible, self-healing film on the surface that blocks rust from forming underneath. Stainless steel can still corrode under extreme conditions, but it won’t develop ordinary rust in normal use.
Metals That Don’t Rust (But Still Corrode)
Non-ferrous metals like aluminum, copper, titanium, zinc, brass, and bronze cannot rust because they contain no iron. However, they do react with their environment in other ways. Aluminum forms a thin layer of aluminum oxide almost instantly when exposed to air. This layer is hard, transparent, and bonds tightly to the surface, which is why aluminum doesn’t visibly degrade the way steel does.
Copper develops a green patina over time as it reacts with oxygen, moisture, and carbon dioxide. The Statue of Liberty’s green color is copper carbonate, not rust. Titanium forms a titanium dioxide layer that makes it extremely resistant to further breakdown, which is one reason it’s used in marine hardware and medical implants.
Gold and platinum sit at the extreme end of the spectrum. These noble metals are so chemically stable that they resist oxidation almost entirely, which is why ancient gold artifacts look nearly the same as they did thousands of years ago.
What Speeds Up Rusting
Salt dramatically accelerates rust. Saltwater acts as a better electrical conductor than freshwater, which speeds up the electrochemical reaction that breaks iron down. Research on steel corrosion along the Baltic Sea coast found that steel in brackish coastal water corroded at roughly 0.06 mm per year, while steel near a river mouth where salinity increased corroded at about 0.1 mm per year, nearly double the rate. This is why cars in coastal cities or regions that salt roads in winter rust faster than those in dry inland areas.
Humidity matters just as much as direct water contact. Iron doesn’t need to be submerged to rust. A thin film of moisture from humid air is enough. Acid rain, industrial pollutants, and even soil chemistry can all increase the rate. Higher temperatures generally speed the reaction too, though the biggest factors remain water exposure and salt.
How Rust Is Prevented
Most rust prevention works by keeping water and oxygen away from the iron surface. Paint, powder coating, and oil films all serve as physical barriers. Galvanization takes a different approach: the steel is coated in a layer of zinc, which corrodes preferentially. Zinc has a stronger tendency to give up electrons than iron does, so it will oxidize completely before the iron underneath begins to react. This is why it’s called a “sacrificial” coating. Galvanized steel is common in fencing, roofing, outdoor hardware, and structural supports.
Weathering steel (often sold under the brand name Corten) uses a more unusual strategy. It contains small amounts of copper, chromium, nickel, and phosphorus that cause it to form a dense, tightly bonded rust layer when first exposed to the elements. This patina is different from ordinary rust. Instead of flaking away, it stabilizes and acts as a barrier that dramatically slows further corrosion. Architects use weathering steel for bridges, building facades, and outdoor sculptures where the reddish-brown surface becomes part of the design.
Common Non-Metal Materials and Rust
Wood, plastic, glass, concrete, rubber, and ceramics cannot rust. Rust is exclusively a reaction involving iron. Wood rots, plastic degrades under UV light, and concrete can spall or crack, but none of these processes involve iron oxidation. The one exception is when these materials contain embedded iron or steel components. Rebar inside concrete, screws in wood decking, or steel reinforcement in fiberglass can all rust if moisture reaches the metal, sometimes causing the surrounding material to crack or stain as the rust expands.
If you’re choosing materials for an outdoor project or trying to understand why something is deteriorating, the key question is simple: does it contain iron? If yes, it will rust unless protected. If no, it may corrode or degrade in other ways, but rust won’t be the problem.