Rust forms on metal in the presence of oxygen and water, creating a chemical process called oxidation. This process corrodes the metal and forms iron oxides that give rust its characteristic color.
Rust is mainly composed of hydrated iron oxides, primarily that good old hydrated ferric oxide (Fe₂O₃·nH₂O). It is this compound that gives rust its typical brownish-red hue. Inside, you mainly find iron, oxygen, and water trapped together, in slightly varying proportions depending on environmental conditions. Besides the oxides, you might occasionally come across traces of other compounds, such as hydroxides and some impurities from elsewhere (pollution, mineral salts...). This little porous and crumbly mixture does not do any good to the metal underneath, as rust protects absolutely nothing; on the contrary, it accelerates degradation by allowing moisture and air to penetrate even deeper through the metal.
Humidity is clearly rust's best friend: as soon as a metal encounters this humidity, oxidation quickly shows its face. Oxygen also plays a central role, as it fuels all these corrosion reactions. If saltwater comes into play, things get significantly worse, since salt accelerates these electrochemical processes. Heat acts as a kind of accelerator: hotter means faster. Other elements, such as certain acidic substances, also attack metal surfaces, widely opening the door to rust. Finally, the prolonged presence of atmospheric pollutants, such as sulfur dioxide, exacerbates the damage to exposed structures.
Rust appears due to a reaction known as electrochemical, where the metal, typically iron, reacts with oxygen and water. At the surface of the metal, certain areas will lose electrons, which is called oxidation. Meanwhile, water and oxygen gain these electrons, which is a reduction. These two combined reactions then form hydrated iron oxide, in other words, rust. This process requires water, which acts as an electrical conductor between the areas that are corroding. No water? No rust.
Humidity is undoubtedly the number one enemy of metals: water clearly accelerates the chemical reactions responsible for rust. Add a bit of salt, like at the seaside, and the situation gets seriously worse—salt facilitates electrical conduction on the surface, thereby accelerating corrosion. Another critical element not to forget is temperature. The hotter it gets, the more the chemical reactions are boosted, resulting in faster rust formation. The presence of certain atmospheric pollutants, such as sulfur dioxide from industrial pollution, also makes the environment even more aggressive for metal. Even the presence of scratches, scuffs, or damaged areas on the metal means multiplying the chances of rusting easily.
To effectively combat rust, the main strategy is to prevent metal from coming into contact with oxygen and moisture. The simplest and most common method is to apply a protective layer such as paint, varnish, or grease to create a waterproof barrier. Another effective method is to use metallic coatings, the famous method of galvanization, where the metal is covered with a thin layer of zinc that corrodes in place of the metal. One can also resort to electrochemical protections, cathodic protection systems, commonly used in industry or on boats to divert electrochemical reactions and preserve the metal. Finally, sometimes the best solution is to use non-corrosive metals from the start, such as stainless steel, which naturally resists corrosion due to its chemical composition.
Modern steel bridges often use a process called 'cathodic protection', which effectively protects against rust by applying a low electric current to the metal, significantly slowing down corrosion.
The Statue of Liberty is green because of rust? No, this green hue actually comes from a patina of verdigris that forms on copper, thus protecting the statue instead of deteriorating it like rust does to iron.
Rust does not only affect pure iron; it can also appear on iron-containing alloys, such as steel, provided they come into contact with moisture and oxygen.
The Eiffel Tower is repainted approximately every 7 years to prevent corrosion. This operation requires nearly 60 tons of specially formulated paint to combat rust!
The speed at which rust forms depends on various factors such as humidity, temperature, the quality of the metal, and the presence of salts or other corrosive substances in the environment. Under favorable conditions (humid and warm environment), rust can appear within a few hours or days. Conversely, under dry and controlled conditions, it may take several months or years before rust becomes noticeably apparent.
Yes, the presence of salt (such as saltwater or de-icing salt) significantly accelerates the formation of rust. Indeed, salt dissolved in water facilitates electrical conduction, thereby increasing the rate of the electrochemical reactions responsible for metal corrosion.
Rust itself is not toxic; however, it can make metal surfaces rough and pose a risk of injury (cuts, scrapes). These injuries can then lead to exposure to infectious agents such as tetanus. Therefore, it is recommended to handle rusty materials with care and to regularly check your tetanus vaccination.
Yes, it is often possible to restore heavily rusted objects. Depending on the condition of the metal, a scraping or sanding step is usually performed to remove the surface rust. Then, it is advisable to apply a rust converter or an anti-rust product, followed by a paint or protective coating to prevent further rust from forming.
Some metals, such as gold, silver, and platinum, do not rust because they do not react with oxygen under normal conditions. Other metals, like aluminum or stainless steel, can form a thin, highly resistant oxidized layer (oxide), which prevents further corrosion. This protective layer allows them to maintain their appearance while being safeguarded against deep rust.
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