Rust is the common name for iron oxide, a form of corrosion resulting from a chemical reaction between iron (or its alloys, such as steel), oxygen, and moisture. This process, known as oxidation, transforms the metal into the reddish-brown compound we recognize as rust.
Rust is a Chemical Reaction, Not an Infection
Rust is not a living organism, like a mold or bacteria, and does not “spread” through contact in an infectious manner. Instead, the reddish-brown substance is the product of an electrochemical reaction. For this reaction to occur, three components must be present: iron, oxygen, and water or an electrolyte.
The process begins when iron atoms lose electrons (oxidation), transforming the solid metal into iron ions. Simultaneously, at a separate location on the metal surface, oxygen gains these electrons (reduction), forming hydroxide ions in the water. These two reactions create a tiny electrical current, effectively making a water droplet on the surface a miniature battery.
The iron ions and hydroxide ions then combine within the water, eventually forming hydrated iron(III) oxide, the flaky substance known as rust. Unlike the oxide on aluminum or stainless steel, rust is brittle and crumbly and does not form a protective layer, which allows the reaction to continue.
Environmental Factors That Accelerate Expansion
The appearance of rust “spreading” is actually the continued progression of the electrochemical reaction into adjacent, uncorroded metal. The conditions that caused the initial spot often persist and expand, allowing the corrosion process to grow outward from its origin. This growth is accelerated because the initial rust spot changes the local environment on the metal’s surface.
The porous structure of rust acts like a sponge, trapping moisture and holding it against the metal surface for extended periods. This prolonged exposure ensures the continued presence of a necessary ingredient for the reaction. The newly formed rust can also act as a cathode, driving the electrochemical process forward and making adjacent areas more susceptible to corrosion.
Certain environmental elements accelerate this expansion. The presence of chloride ions, such as those found in road salt or seawater, increases the water’s ability to conduct the electrical current required for the reaction, speeding up the corrosion rate. High humidity and temperature fluctuations also play a role, as warmer temperatures increase the rate of chemical reactions, and condensation provides a constant supply of moisture.
Halting the Rust Process
Stopping the corrosion process involves eliminating one or more of the three necessary ingredients: iron, oxygen, or water. The most immediate step is to physically remove the existing rust, often through sanding or grinding, to expose the sound metal beneath. This removes the moisture-trapping material and localized areas that conduct the reaction.
Once the surface is clean, creating a barrier is the most effective preventative measure. Applying a quality paint, a dry coating, or an oil-based coating forms a layer that prevents oxygen and moisture from reaching the underlying iron. Another approach is to use a rust converter, a chemical solution containing tannic or phosphoric acid.
These converters chemically react with the iron oxide to transform it into a stable, black iron compound, such as black ferric phosphate. This new layer is stable and paintable, sealing the surface and preventing further reaction. For long-term protection, industrial methods like galvanization coat the iron with a layer of zinc, which corrodes preferentially to the iron, offering a sacrificial defense.