Steel does rust when exposed to the outdoors. Steel is an alloy of iron and carbon, and rust is the common term for the resulting corrosion product, hydrated iron(III) oxide. The outdoor environment naturally supplies the necessary ingredients for this process. Rusting represents the metal’s attempt to return to its lower-energy, naturally occurring state, such as iron ore. The rate of corrosion is directly influenced by specific atmospheric conditions, making understanding this reaction crucial for effective protection.
The Essential Chemistry of Rust
Rusting is an electrochemical process involving the transfer of electrons between atoms. This reaction requires three primary components: iron, oxygen, and water or sufficient moisture. When water lands on a steel surface, it acts as an electrolyte, creating a miniature electrochemical cell.
The corrosion process begins when iron atoms lose electrons (oxidation), transforming the solid metal into dissolved iron ions. This area acts as the anode. The released electrons travel through the steel to the cathode.
At the cathode, these electrons react with oxygen dissolved in the water, forming hydroxide ions. The iron and hydroxide ions then combine to create iron hydroxide. This compound is quickly oxidized by atmospheric oxygen, ultimately forming the familiar reddish-brown hydrated iron(III) oxide (rust). Unlike the stable oxide films on some other metals, this iron oxide is flaky and porous, offering no protection to the underlying metal, allowing corrosion to continue unchecked.
Environmental Factors That Accelerate Corrosion
The exterior atmosphere determines the speed of the electrochemical rusting reaction. A primary environmental driver is the duration of moisture contact, known as the “time of wetness.” Corrosion occurs only when the metal surface is wet enough to form an electrolytic solution, typically when the relative humidity exceeds 60 to 80 percent.
Areas near the ocean experience accelerated corrosion due to salinity. Chloride ions from sea spray dramatically increase the conductivity of the water film on the steel surface, making it a more efficient electrolyte. This enhanced conductivity allows the electron transfer process to occur more quickly, speeding up the overall rate of rust formation.
Air pollutants, particularly sulfur dioxide and nitrogen oxides, also significantly increase corrosion rates in industrial and urban environments. These gases dissolve in rainwater, forming sulfuric and nitric acids, which lowers the moisture’s pH. The presence of these acid-forming ions acts as a catalyst, further promoting the electrochemical reaction and preventing the natural formation of any stable, protective oxide layer.
Inherent Resistance of Steel Types
The intrinsic composition of a steel alloy dictates its natural resistance to atmospheric corrosion. Standard carbon steel, the most common type, has high susceptibility because its rust layer is permeable and flakes off easily. This constantly exposes fresh metal to the environment, leading to rapid material degradation outdoors.
In contrast, stainless steel exhibits high resistance because of its chromium content, which must be at least 10.5 percent. When chromium is exposed to oxygen, it instantly forms an ultra-thin, dense layer of chromium oxide on the surface. This inert layer, known as a passive film, acts as a self-healing barrier that physically separates the iron from the corrosive elements.
Weathering steel, often sold as Corten, utilizes a unique mechanism to slow corrosion. This steel contains small amounts of alloying elements like copper, nickel, and phosphorus. When exposed, it develops an initial layer of rust, but the specific chemical components cause this rust to be dense and tightly adherent to the base metal. This stable rust layer, often called a patina, dramatically reduces the penetration of oxygen and moisture, effectively shielding the underlying steel from further degradation.
Practical Methods for Preventing Rust
Since standard steel will inevitably rust outdoors, several practical methods are employed to create a protective boundary. One of the simplest and most common solutions involves applying barrier coatings such as paint or epoxy. These materials form a seamless physical layer that prevents water and oxygen from making contact with the metal substrate.
A more robust and long-lasting method is galvanization, which involves coating the steel with a layer of zinc, typically through hot-dip galvanizing. The zinc coating protects the steel in two distinct ways. First, it acts as a physical barrier, similar to paint, by keeping moisture away from the iron.
Second, galvanization provides cathodic protection, often referred to as sacrificial protection. Zinc is a more electrochemically active metal than iron, meaning it preferentially corrodes when both are exposed to an electrolyte. If the zinc coating is scratched, the surrounding zinc will corrode instead of the exposed steel, sacrificing itself to maintain the structural integrity of the iron. This sacrificial action continues until the zinc layer is fully consumed.